Topic: '82 CB650SC cylinders drop out and fuel smell

[Depco]

I have an 82 Honda CB650SC Nighthawk with 23k miles on it. I have owned it for 20+ years and the previous owner had it since nearly new. The bike has sat off and on for most of those years, getting service every few years as interest in riding it came and went. The bike was largely garage kept so the connectors and fasteners are still in decent shape and very little rust is present anywhere on the bike.

When I first got the bike it was fairly peppy, but liked a bit of choke to run smooth. Over the years I have had various issues with it, resulting in the on and off riding and maintenance events.

Fast forward to last year and my son is using the bike to learn how to ride. I have since moved onto another bike for my riding pleasure.
Recently the bike has been having an issue where it will fire up, run “ok”, but then once warmed up it will have 1 or 2 cylinders stop firing or run significantly rough, only to start running fine again within 10-30 seconds. When this rough running event occurs, the bike will usually stall unless I feather the throttle. This issue has been with the bike for a few years now but is more pronounced now. The bike idles fine apart from the drop outs, has terrible low RPM performance, and begins to smooth out and pull hard after about 5-6k RPM.

Making the problem worse is the fact that while the bike is running, I am starting to get white smoke from the 1 and 2 cylinders as it warms up. I can also smell a strong odor of gasoline while riding and while riding behind the motorcycle. It is burning through fuel at an alarming rate.

Before the rebuild I was getting some fouling of the 1 and 2 cylinders over time. There is no current active leaking from the carbs, but I did have to replace some o-rings on the fuel rails a couple of years ago due to a fuel leak. I partially busted the carb pack apart to do this but never had the carbs resynced. Could this cause the cylinder drop out?

I have replaced the ignition coils, spark plugs, spark plug wires and boots, Battery, and recently completed an entire top end rebuild. I have not done any maintenance on, or replaced the pulsers, or CDI and don’t know how to test these components reliably.

During the rebuild I noted that the cylinders and valves were not nearly as gunk coated as I had expected. However, I had some oil leaks at the cylinder head gaskets as well as pressure loss in cylinders 1 and 2. Valves were lapped as the seats were not as well matched as I would have liked. The pistons, rings, and cylinder walls were in very good shape, and I only did some light honing to the cylinder walls.

As far as the cylinder drop out, I am still leaning toward an electrical issue so now I am asking if there is a common problem that someone may be aware of that might present itself in this manner.

For the gas smell I suspect the carbs are dumping in way too much fuel and the bulk of it is going out the tail pipe. There has been no noted backfiring.
My next step is to remove the carb bank again and completely clean them again hoping that the slow jets are gummed up.

Sorry for the long post but I have been dealing with these problems for a long time and am finally deciding to seek outside help. Thanks for any assistance you can offer.

[newday777]

Welcome to the forum Depco

Is your bike a Double Overhead cam motor like the one pictured? If it is a DOHC then it should be in the Other Bike section. A moderator might be along and move it from this SOHC Single Overhead Cam section.
I don't know anything about the 650 so will leave it for others to come along.

[kerryb]

Welcome to the forum Depco, this is a great place with good folks.
I have an 82 cb650sc, so I have experience with those issues.  Is the stock airbox still in place?  These carbs need the intake restrictions in the airbox to make the carbs pull fuel correctly. For example mine came with pods and would only run till it warmed up!  It sounds like your problem is the opposite…you have too much fuel.  Probably from leaking float valves. I suggest you start by cleaning the float valve tips and polishing the seats with a q-tip and brass polish (brasso).  If the float valves are not genuine keihin parts (oem honda) you may never get them to seal.  Best way to sort this out is to use the “clear tube method” to see where the fuel level is in your carbs.  There are several discussions on here about that.  IF this is the cause of your problem, that would explain the excess gas smell and two flooded carb/cylinders that won’t fire.  Diagnosing the problem is the hard part, this step will be a good start in my experience.  Your bike may still have the factory vacuum petcock, you may have to splice the hoses together around it for testing purposes.  I took mine off to use a standard petcock.

[Depco]

Thanks for the replies guys. The bike in bone stock. I've never modified any of the components, other than  to replace o-rings, gaskets, carb boots/insulators, drive chain, tires, Etc... Im attaching a photo of the old girl for verification. Ive always put in high octane gas in it and fuel stabilizer most off-seasons.

I plan on taking the carb bank off the bike and giving a good cleaning. I have been contemplating getting a complete rebuild kit to replace all the o-rings, but I had the carbs off for the top end rebuild and they looked pretty clean. I did not do as good an inspection on them as I probably needed to, but I was trying to get the bike back up and running so my son could use it to take his cycle endorsement test.

As far as the low speed poor performance I imagine the slow jets are probably gummed up, I have not had the carbs completely apart in a few years so anything is possible. Ill let you know if I get any improvement once they are cleaned again.

The funny thing about the 2 problematic cylinders is that they fire fine at idle until warmed up and then it is sporadic non-firing. It's the damndest thing. Based on the some of the research I've been able to do I think maybe the CDI is having issues.

I'll keep working on it. I was hoping there was a clear indication of the problem.

[fizzlebottom]

If you think you have a problematic ignition module, then just swap them and see if the issue changes to the other 2 cylinders. Can do the same with ignition coils.

My money is on carbs though. You very well might have out of spec floats, worn valves, clogged slow jets, or some combination of everything. If you're fine taking them off then I'd suggest doing so to give a good cleaning, check those float valves, and measure floats to see if they're out of spec.

[newday777]

When you clean the carbs are you using just spray carb cleaner or how are you cleaning the carbs?

[scottly]

The funny thing about the 2 problematic cylinders is that they fire fine at idle until warmed up and then it is sporadic non-firing. It's the damndest thing. Based on the some of the research I've been able to do I think maybe the CDI is having issues.

Check the resistance of the pulse generators. They should measure 530 ohms, +/- 50 ohms.

[Depco]

When you clean the carbs are you using just spray carb cleaner or how are you cleaning the carbs?

I have had the entire bank off the bike and pulled the jets out. They were cleaned with carb cleaner and a small wire for the tiny holes. This was last done a few years ago, but I really did not get any actual improvement in the power drop outs.

[fizzlebottom]

When you clean the carbs are you using just spray carb cleaner or how are you cleaning the carbs?

I have had the entire bank off the bike and pulled the jets out. They were cleaned with carb cleaner and a small wire for the tiny holes. This was last done a few years ago, but I really did not get any actual improvement in the power drop outs.

Just for peace of mind, do this: Make sure you spray the hell out of the accelerator pump circuit including the check valve in the fuel rail. Once you do that, blow through EVERY single hole using an air compressor and a rubber-tipped blow gun. 30psi is plenty.

While they're out, check your float height specs just to see if they're out. I really do think you've got something gunking up your slow jet circuits. Blast every orifice with cleaner and compressed air and then test the bike.

[Depco]

The funny thing about the 2 problematic cylinders is that they fire fine at idle until warmed up and then it is sporadic non-firing. It's the damndest thing. Based on the some of the research I've been able to do I think maybe the CDI is having issues.

Check the resistance of the pulse generators. They should measure 530 ohms, +/- 50 ohms.

I'll have a mini vacation in a week so I'll try the suggestions here and report back. Thank you again for the help.

[scottly]

Just for peace of mind, do this: Make sure you spray the hell out of the accelerator pump circuit including the check valve in the fuel rail.

I wouldn't recommend dousing the accelerator pump diaphragm with carb cleaner, especially since the symptoms don't indicate a problem with the pump.   

[fizzlebottom]

I wouldn't recommend dousing the accelerator pump diaphragm with carb cleaner, especially since the symptoms don't indicate a problem with the pump.

The rest of the circuit inside the carb body itself is fair game though. The only thing really needed to test the pump diaphragm itself is to press the rod (with the orifice facing away from your eyes) and see if fuel squirts out, or to put your finger over one of the holes and see if it creates solid suction.

[Depco]

***Update***

I took the carbs out and began the process of cleaning them. Right away I noticed that the main jet in carb #2 (for cylinder 2) was loose. Like, not even a thread holding the jet in the port loose. Also, a metallic seat that sits in the main jet's port dropped out as I tipped the carb over. I got that placed back where it belonged and continued to spray the bejesus out of everything (except the rubber diaphragm in the fuel accelerator) and then followed that up with the recommended 30 psi of air. I noticed that when I first sprayed the carbs a thin layer of varnish came off of every surface. I was not entirely surprised to see this. None of the jets has any blocked holes

I also realized that the cap on carb #2 was pinching the top of the slide. Compared to the other 3 carbs, the slide was not moving up and down smoothly. I was able to see the specific spot where the slide was catching on the cap and took to the cap with a piece of sandpaper and then some scotchbrite. It still wasn't as smooth as the other carbs but was able now to move up and down without catching and stopping.

One other possible problem is that the last time I had the carb bank off the bike I was not able to get it to seat into the rubber intake boots. These were replaced with new boots about 3 years ago so I was not expecting them to have such a hard time seating. While putting the carbs back this time I used some lube and a ratchet strap to try and assist in seating the carbs into the boots. I'm not sure they completely seated, but they are a lot further into the boots than they had been.

I didnt find any other obvious problems while cleaning the carbs.

After getting the bike back together I took it for a ride. Upon first firing it began to smoke from Cyl 1 and 2 the same way it was before the cleaning. It also started dropping out again once it warmed up. It was doing all the things it had been doing prior to the carb cleaning, with the exception of the gas smell.

I came home and found that there was a little bit of varnished gas coming from the petcock. This got me to wondering if the petcock has some varnishing in it that is restricting fuel flow occasionally. I have never cleaned the fuel tank, but it has little to no rust inside. Something to pull and clean during the next cleaning session. Is it possible it is starving the engine?

I then decided to test the compression since I had not tested it after the top end rebuild. 85psi in Cyl 1, 125 in Cyl 2, 100 in both 3 and 4. Better than it was prior to the rebuild. I put a little bit of oil in cyl 1 just to see if the compression improved. It improved to about 100 psi.

I then decided to try a ride again. This time it fired up and idled really well. No smoke. It also had better throttle response. I went out and it pulled hard through the entire rpm range. The low end hesitation was completely gone. Once the engine was hot it died at two separate stop lights, but fired back up without any problems and idled well and continued to run strong through accelerations and decelerations.

I think the #2 cap catching on the slide may have been a part of the problem. Im not sure what may have caused this deformation, but it has been rectified. The carbs not being seated in the boots may have also contributed to the issues. The #2 carb main jet being loose was likely causing the gas smell problems.

I still have some work to do, but I am pleased with the results so far. Ill take the bike out in a couple days and see of I can put some miles on it to empty the tank and then attend to the petcock.

Again, thanks for the help. If you think of something that might add to getting this bike running well again, let me know. I want it to be in top notch shape so my son can be confident while enjoying his rides.

[Depco]

***Update #2***

So I took the bike out after work today to verify that it wasn't a fluke it was running so well. It started like a champ. This was odd as it has always been very cold blooded and takes a good long time to start and run smooth. It warmed up fairly quickly and I was able to get on the road without the usual warm up shenanigans. I put on a couple of miles to see how it was responding, and it was running really well. I stopped to see the missus at her place of work. I then went to go on a longer ride...

Before getting on the road I noticed again varnished fuel coming from the petcock. I was not able to tell where in the petcock it was coming from, only that there was a drip of varnish forming on the base of the petcock. I wiped it away with my hand and smelled the oily residue on my fingers. I then left the parking lot and almost immediately the bike stalled at the roadway entrance from the parking lot. It restarted without issue and I continued to try a longer ride. I noticed that the low speed hesitation was back. I turned the petcock to reserve and the performance increased a short in a few seconds. Long story short. I made some adjustments to the petcock mid-ride as I made my way back home. The performance did not initially respond to the adjustments, but some changes did eventually happen. I don't know if these are related or not.

Once I got home the bike was idling fine so I decided to see if the petcock was actually working. I turned the fuel to the off position, but the bike continued to run. It never seemed to run out of fuel and I eventually turned the bike off. After this experiment though I had difficulty getting it to idle again. I could feather the throttle and keep it running but it was rough. as soon as I let go of the throttle it died. This occurred no matter where the petcock was set, on, off, or reserve.

Tomorrow I plan on dumping the fuel from the tank and removing the petcock for a good thorough cleaning. I am now more convinced that the petcock is either letting too much fuel through or that it is having occasional obstructions that are effecting its fuel flow and operation. There is no fuel filter in line from the tank.

If any of this sounds familiar and you think I am off base here let me know.

[fizzlebottom]

That sounds like some very thorough troubleshooting.

Regarding a few things you mentioned:

• A loose main jet is weird for sure. I hope you confirmed that it is the proper #118 Keihin jet and not any aftermarket variety. Same with slow jets (#35)
• The metal bit that dropped out when you removed the main jet holder is the needle jet. So long as you got it back in place, you're all good.
• Replace your carb insulators if they're hard. New soft ones make seating the carbs a breeze. The trick I've found to getting them in place is to brace myself against the motor and PULL until I feel one side pop. Clamp that side in. Then do the same with the other side.

I think now would definitely be a good time to remove the petcock and clean the every living heck out of it. You might have some gunky fuel jamming things up in there, and it may have made its way back into the carbs.

Once you have all of the old junk out, go through and re-sync your carbs for good measure. Sounds like you're on the right path.

[Depco]

That sounds like some very thorough troubleshooting.

Regarding a few things you mentioned:

• Replace your carb insulators if they're hard. New soft ones make seating the carbs a breeze. The trick I've found to getting them in place is to brace myself against the motor and PULL until I feel one side pop. Clamp that side in. Then do the same with the other side.

I got new boots about 3 years ago and they are still pretty soft. I am surprised I needed so much force to get them to mostly seat. I am wondering if I would be better off putting the boots on the carbs first and then trying to seat them on the intake manifold.

[fizzlebottom]

I got new boots about 3 years ago and they are still pretty soft. I am surprised I needed so much force to get them to mostly seat. I am wondering if I would be better off putting the boots on the carbs first and then trying to seat them on the intake manifold.

I never had luck doing it that way at all. Definitely make sure they are clocked correctly. The '81-'82 CB650 carbs are spaced in a way that required offset boots, so 1&2 and 3&4 are clocked closer to each other (I think?).

Other than that, if they're still soft then they should pop right in with a little bit of force.

[Depco]

***Update #3***

I emptied the tank of fuel and removed the petcock. I pulled the filter tube and noticed some debris on it. However, the screens were still in good shape and did not show any signs of degradation or tears. The interior screen in the petcock itself was clean as a whistle. Attached is a photo of the filter tube.

I then went to town spraying the petcock with carb cleaner. I did not see obvious debris or gunk. I also did not see obvious discoloration of the cleaning spray. I did however see varnished fluid buildup in the area between the fuel selection dial and it's housing. I continued to spray that, but was not able to open the valve assembly due to the rivetted enclosure. After cleaning the petcock and filter tube I reassembled the tank and bike.

After putting everything together I went for a ride. The bike fired up well again and ran without problems for a few miles. It then started to sputter and die again once warm. Initially I could get it started without a problem, but the rough idle and poor performance continued until I was on the side of the road an unable to start it.

Thinking it might be a heat issue I sat on the side of the road to let the engine cool. While sitting there I got to thinking about the petcock again. I started to turn the fuel dial a few times back and forth. The engine fired up again and ran without issue. I ran it home and parked it. I will pull the petcock again and see of there is more debris on the fuel tube.

Again, the tank does not have much, if any, rust in it so I don't think there is rust getting in the way. It is possible that there is some amount of varnished sludge in the bottom of the tank that is causing issues. How would I best clean the tank of this type of sediment or varnish? I have acetone and mineral spirits on hand but can get as jug of something else if it will be more effective.

Also, any advise on opening the valve portion of the petcock? I am thinking that drilling out the rivets and replacing them with screws should do the trick. I would rather not replace it if I can open it and clean it out completely instead.

[Depco]

Some other thoughts that have gone through my brain:

Are the carbs running out of fuel and becoming starved?
Floats getting stuck closed, causing the starving of fuel until the bowl begins to get fuel again?
Fuel not being pulled from the gas tank appropriately? I dont have a complete understanding of how fuel moves from the tank to the carbs, either through straight gravity, or a vacuum system.
Does the heat of the engine have an effect on the running of the engine? It seems so, but I am having a problem determining if it is a fuel or electrical system that would be prone to this type of issue. My inclination is that it is still fuel related.

[Depco]

I see the Petcock on this bike is a vacuum demand fuel valve. is it possible that I am loosing vacuum when the bike gets hot and that is causing the petcock to stop flowing fuel, thereby starving the carbs? I read somewhere that I can bypass the vacuum hose and operate the petcock without it. I just need to turn off the fuel valve when the bike is not running. I might try this as a way of verifying the vacuum leak.

[HondaMan]

First: the bike looks to be in GREAT shape! I don't see those in that nice condition very often.

About the fuel delivery: check that in all of the off-on trips for the carbs that the fuel hose still drains DOWN all the way to the carb's fuel rail. If it goes down, then back up, then down again, it can actually pump hot air back against the meager fuel head pressure when the engine warms up the carbs, pushing the fuel back up into the petcock. It looks like a bubble happening if you get to see it, but it's REAL quick-moving and only visible with clear fuel lines (which usually leak, because they are not the right diameter for these pipes). I recently had to remove an inline fuel filter from a 1979 CB650 because it went horizontal on the way between the tank and carbs, and as soon as it got warm it would hydraulically lock the fuel line unless the bike was tilted toward the right (a lot), then it would 'burp' the bubble and run OK until it was parked and the fuel line went dry again (then it repeated the cycle). The bike would run about 8 minutes or 1.5 miles thru the neighborhood each time before it would die from a cold startup.

In the CV-type piston carbs, if the slide hangs up ANYWHERE in its travel, that MUST be freed up. Sometimes the cover of the carb warps a bit, and the tiny (VERY tiny) clearance between the cover and the fast-moving slide causes 'stiction': they must be completely free, clean, and clear of any debris. If not, that piston will stick in the 'tight' spot, whether it is up high or down low, and that cylinder will suffer. If it sticks HIGH then that one will be rich, if LOW then that one will be lean. Most of the time I have seen them stick HIGH. This also makes the bike hard to start when hot. When it cools off the CV piston shrinks and drops back down, then the engine starts better, and the cycle repeats.

The warpage that can happen to the CV carbs' cover is purely due to the type of metals used at the time: Honda didn't think these bikes would be here now, so it isn't the finest aluminum alloy. If you can determine for sure which cylinder is suffering the stuck piston, remove that cover and try polishing the inside of it with a jeweler's rouge like Simichrome: you will find the "high" spot as the area that looks different from the rest of the inside. This rouge won't alter the tiny piston clearance required while you do this, but can point out that spot(s) that needs to be 'pushed back' a tiny bit for smooth operation of the piston.

Now, about the engine: in your 'top end rebuild' did you replace the piston rings only, or did you have the cylinders bored and new pistons & rings installed, and with a piston clearance of LESS than 0.001"? It must be that tight. I use 0.0008" maximum. If they are more than 0.0015" then it will not seat the new rings enough to ever stop smoking: have seen this too many times.

If you just honed it and replaced the rings, they will not seal again. Ever. Nature of the SOHC4 engine. They may get close to sealing, but they will not seal because the bores are not round anymore after about 8k miles, but the outer 2 are egg-shaped while the inner 2 look sort of like a cam lobe with the narrow part being the original diameter and the wide part nearest the cam chain tunnel. The metals are now cured: this means that boring them an oversize WILL ensure forever-round bores, and they won't shift again. But, they MUST be bored for a TIGHT clearance, and the top of the cylinders must be decked down 0.010" to compensate for the too-thick head gaskets that we get in the kits today: the OEM ones were made with asbestos, and this ended in the 1990s. The head gaskets all got thicker by about 0.8mm (0.008" or so) with the new materials, so any oil ports that pass thru the head gasket need correspondingly thicker O-rings to seal them. I got a 650 (1979) about 2 years ago that was an oil-soaked dribble in my garage to fix, and this was the problem with it.

[Depco]

First: the bike looks to be in GREAT shape! I don't see those in that nice condition very often.

Thanks. That photo is from a few years ago. Since then I had a tree fall on my garage and break a glass table stored in the rafters. The glass rained down on the bike causing a lot of paint chips in the tank as well as a small dent. Other than that (and a missing right cover) it is still in decent enough shape for a bike of this age.

About the fuel delivery: check that in all of the off-on trips for the carbs that the fuel hose still drains DOWN all the way to the carb's fuel rail. If it goes down, then back up, then down again, it can actually pump hot air back against the meager fuel head pressure when the engine warms up the carbs, pushing the fuel back up into the petcock.

The bike would run about 8 minutes or 1.5 miles thru the neighborhood each time before it would die from a cold startup.

That sounds very familiar. Ill have to check the fuel lines and how they are routed.

In the CV-type piston carbs, if the slide hangs up ANYWHERE in its travel, that MUST be freed up.

I had this happen in the #2 piston. It was sticking at the bottom. I was able to see where it was sticking (dark area in the cap) and went to it with some light 120 grit passes finishing with a scotchbrite pad until the piston was no longer sticking. I did noticed significantly better throttle response after correcting this problem.

Now, about the engine: in your 'top end rebuild' did you replace the piston rings only, or did you have the cylinders bored and new pistons & rings installed, and with a piston clearance of LESS than 0.001"?

I did not replace the rings. When I took the top end off everything looked fantastic with the exception of the carbon buildup on the valves and piston tops. That got cleaned to the best of my admittedly limited abilities and then the piston cylinders got 4-5 passes from a harbor freight honing tool just to put new cross hatching on the walls. Everything got put back together with new gaskets and seals and I set the rocker arm clearances to the cam. Oddly enough, it seemed I needed to set the clearances in a different order from the manual. I set it according to the manual the first time and there was a horrible knock in piston 3. When I opened the top back up it appeared that the cam lobes were opening and closing in a different order than what was shown in the manual. Once I set it "correctly" for what I was physically seeing with the cam, the bike ran very well. It is possible I was mis-reading the manual, but I read it over and over, coming to then same conclusion each time.

In any case, thanks for your response. You clearly have a lot of experience that you draw from and are willing to share with those of us who are still learning. I will let you know if any of your suggestions help.

[HondaMan]

Oddly enough, it seemed I needed to set the clearances in a different order from the manual. I set it according to the manual the first time and there was a horrible knock in piston 3. When I opened the top back up it appeared that the cam lobes were opening and closing in a different order than what was shown in the manual. Once I set it "correctly" for what I was physically seeing with the cam, the bike ran very well. It is possible I was mis-reading the manual, but I read it over and over, coming to then same conclusion each time.

Yeah, I've noticed that about the Honda 650 manual (mine is for the 1979-80 model). It's not very clear, and it's not the Jinglish problem we used to have, either. It seems to be lacking some good editing, or something...

[Depco]

***Update #4***

With more f'ing around comes additional finding out.

I cleaned the tank with a can of carb cleaner sprayed in and swished around. No debris or other junk came out. This tank is as clean as the rest of the bike. I also removed and inspected the petcock again. As before there was nothing to suggest it was blocked or otherwise operating incorrectly.

I removed, inspected, cleaned and reassembled the vacuum valve between the petcock and carbs. It was clean as a whistle and the rubber valve inside was in like new shape. I then bypassed the valve completely and went straight from the tank to the carbs to make sure this was not the cause for the problems. 2 miles into the test ride the same power issues and cylinder drop out reappeared. Fwiw, I destroyed the hose going from the vacuum valve to the carb so now I need to find a suitable replacement and the fuel is running from the tank directly to the carbs until then.

I have completely addressed the fuel concerns of this problem and am confident that the issue is not gas.

I now turn my attention to the electrical system. With the bike is idling I waited for the drop out to occur. I then pulled the plug wires, one by one and noted that there was no significant change when I pulled the plug wires for cyls 2 and 3. When I pulled 1 and 4 the bike died instantly. This suggested an issue with the ignition on the cyl 2 and 3 circuit. The plugs were still sparking on all 4 plugs but I suspect the spark for 2 and 3 were poor.

I replaced the ignition coils 3-4 years ago during the last major revamp of the bike and chasing this ongoing problem. I decided to swap the ignition coils to see if the cylinder drop out changed. It did not. Cyls 2 and 3 continued to have problems. I then traced the wires back to the Spark Units. Again, I swapped the connectors for 1,4 and 2,3. No Change. Off to the pulse generator.

Check the resistance of the pulse generators. They should measure 530 ohms, +/- 50 ohms.

I recalled the above information from Scottly. I indeed tested the resistances. On the yellow circuit it tested at .583 K Ohms, on the blue circuit it tested at .7 to 1.1 M ohms. That blue circuit seems a bit out of spec. Interestingly, the reading was dancing all over the place. The yellow circuit was pretty steady as I continued to watch the ohms meter over time (15 seconds or so). Swapping the probes did nothing to change the wonky readings either.

I think I have my next avenue of replacement. I have begun to look into picking up a new pulse generator. I am surprised that most of the replacement generators dont look like the ones on my bike. I have included a picture of mine for reference. If you have any advice on the replacement parts or where would be a good place to source one let me know. Ill update the post once I get the new part here and on the bike. It may be a bit though. Fall is springing on us and cold weather is settling in. I am a fair weather rider so I will be unlikely to do significant amounts of riding and testing unless I get the part quickly.

I hope this thread has been as entertaining (and maybe educational) to read as it was for me to work on this problem and post. Thanks for all the help and suggestions.

[Depco]

I found a used pulse generator that appeared to be in good shape on ebay for $125. All other sources I have found with new old stock wanted north of $400-500. I am hoping this part will work. I will get it in a week or so, slap it on, and then let you know.

[bryanj]

Recently had to change the pg's on a gl1500 that wouldnt start and run right, all these coils are getting old now plus heat and wet is not friendly

[M 750K6]

Are you certain the fuel tank is venting?

[PeWe]

Are you certain the fuel tank is venting?

+1 on that.

[Depco]

Are you certain the fuel tank is venting?

I'm not sure how I would test that. I certainly don't have any indication that the fuel filler cap is having issues.

[scottly]

Are you certain the fuel tank is venting?

I'm not sure how I would test that. I certainly don't have any indication that the fuel filler cap is having issues.

Besides, you've already found a smoking gun with the obviously faulty pulse generator.

[M 750K6]

Are you certain the fuel tank is venting?

I'm not sure how I would test that. I certainly don't have any indication that the fuel filler cap is having issues.

Besides, you've already found a smoking gun with the obviously faulty pulse generator.

True, but it's easy to test. When you experience the fault, open the gas cap. If not venting, it will resist opening and you may hear air being sucked in as it opens. Start it up. If the fault has disappeared, suggests your tank's not venting.

[Depco]

True, but it's easy to test. When you experience the fault, open the gas cap. If not venting, it will resist opening and you may hear air being sucked in as it opens. Start it up. If the fault has disappeared, suggests your tank's not venting.

Ill have to get another key. The gas cap needs the ignition key to open. I only have one key. LOL.

[RAFster122s]

It has an actual lock on the gas cap? Lots of them are able to open with anything the fits in the slot.

[Depco]

***Final Update... hopefully***

I received the replacement pulse generator in the mail today. I came home from work and went directly to the garage to get it on the bike. It is cooling off here in the midwest and the todays top temp was in the upper 40's F. I tested the new pulse generator and found it to be showing resistances within anticipated tolerances. The boy and I put the new generator on and fired up the bike. It was humming right along as expected. While pulling the old one off I tested it again and it was also within tolerances. I was not surprised since it had not warmed up and usually the bike was good while cold. I may hit it with a heat gun and see of the readings fall off as it warms up, but that is a post mortem for another time.

After letting the bike warm up I decided to go for a spin. I got the cold weather gear out and bundled up. To my surprise the throttle was much more lively and it was responsive through the entire rev range. From a stop to red line it was pulling. It has been many a year since this bike has run this well, and I had a stupid grin on my face the whole time. There was part of me that kept waiting for the bike to have the 2 cylinders drop out, but nothing of the sort happened. As an FYI I ride an '01 Road King so this bike is a completely different experience. Nimble and compact. The sense of speed is significantly more pronounced. It is a great bike for a young man learning to ride.

I rode for about 12 miles and was satisfied with the result, if not a bit cold as my gloves were not as warm as I had expected them to be. Luckily I had a warming machine between my legs and I was able to wrap my hands around it at stop signs. (I mean the engine, you degenerates) This weekend promises to have temps back in the low 70s F. Me and my father-in-law are planning on taking the boy out for a ride on Saturday to let him feel the bike running as it was intended to be. Without hesitation or lack of power. I am hoping this will erase his previous poor experiences with this bike, and get him into enjoying riding without anxiety of being stuck on the side of the road on a busy highway.

It has an actual lock on the gas cap? Lots of them are able to open with anything the fits in the slot.

Yup the gas cap uses then actual ignition key. No other slotted tool will work without breaking the cap.

I would like to thank everyone here who responded and helped troubleshoot this problem. I have been chasing this issue off and on for nearly a decade, in some form or another. Although this is the first real serious attempt at getting the bike back to its original running condition, it had been a long frustration every time I took the bike out to try and see if I could fix it. I doubt highly that I would have come across the solution without the help of the knowledgeable posters here.

Now with the bike running like new I am inspired to finish finding the replacements for the missing/damaged parts for this bike to bring it back to newish condition. I still have to replace the Mirrors, Right side cover,  and rear tail lamp cover, as well as try to rehabilitate the paint on the tank from the glass shower it took a few years ago. I have found a source for some of the parts but I suspect it will be through the winter to get the parts together and on the bike. I will make sure to put up a photo of the bike on the road as soon as I can get one of the boy on it. Again, Thank You, and keep up the good work.

[scottly]

We all love to hear when a problem finally gets fixed!!

[fizzlebottom]

Wild. I've never actually seen one of these fail. I'm super curious about what actually failed internally in one of the pickups. I think they're basically just hall effect sensors, which just take a magnetic signal and amplify it via a transistor. They're probably sealed from the elements, so taking one of the pickups apart to investigate would probably destroy it. The engineer in me sure wants to though.

[scottly]

I think they're basically just hall effect sensors, which just take a magnetic signal and amplify it via a transistor.

No, the pulse generator is a coil of wire like an electromagnet, but in reverse.

[Depco]

Wild. I've never actually seen one of these fail. I'm super curious about what actually failed internally in one of the pickups. I think they're basically just hall effect sensors, which just take a magnetic signal and amplify it via a transistor. They're probably sealed from the elements, so taking one of the pickups apart to investigate would probably destroy it. The engineer in me sure wants to though.

I want to heat it with a heat gun and see if the readings start to fall off as it warms up. If they do I then want to see what's inside... But I also want to salvage the working circuit in case It is needed to repair another bike down the line.