Topic: Carbs, Spark plugs, coils, points - good. What's wrong???

[rafanomenon]

Hey everyone,

This is my first post on the site and I'm looking to get some help. Checked the FAQ and did the comprehensive search and skim (i'm a search master, btw) and couldn't find any answers to my specific problem.

I have a 1977 Honda CB750F (SuperSport). I cleaned the carbs, new air filter, oil change, new oil filter, new spark plugs.

I found that the float needles for carb #1 and carb #4 were a little 'off' because the spring mechanism in the float needles weren't quite holding the floats as taught as the middle two were. This really wouldn't affect what's going on as you read ahead... After cleaning we reassembled, made sure the idle screws were to factory specs (1.5 turns)  and started it up. 2 of the 4 cylinders were firing. No good. We disassembled the carbs and cleaned once again.







We reassembled and still, the same problem. I gave Chris a call to get a second opinion. Chris is one of the super cool vintage fanatics that emailed me and told me to keep my bike when I tried to sell it. Way to go, Chris. Living in West Seattle, he came right on over. We diagnosed common problem areas, and found out the spark plugs for #3 and #4 weren’t screwed in all the way. Great. A no brainer.



After screwing in all the plugs, we tried starting it back up. Still, nothing firing from #3 and #4. We took the wooden handle of a hammer and tapped the carbs to get any air bubbles that might be hanging out. We also tipped the bike off the center stand, and rocked it far left and far right to increase chances of fuel getting into each of the four carbs. Fingers crossed, we fired up again.

Progress. Using the fingers-on-the-headpipe method, we checked to see if there was combustion/heat coming from all 4 headpipes. #1 and #2 were super hot, #3 was warmer, and #4 is still running a bit cold-to-touch. We pulled the spark plugs out, grounded against the engine and checked for spark. Good. Bright and blue. We disassembled and checked the carbs once more (this is the 4th time doing this), and all the valves, floats, needles - everything - were clean and operational. We even tried switching the carb components from #1 and #4 to troubleshoot if it was actually a carb problem. Still, cylinder #4 didn’t seem to be getting enough spark/gas/combustion/we don’t know. We then did more troubleshooting and looked to the coils, timing possibility, combustion, and even the points. All of which seemed to be a-okay.



In a nutshell, we checked and re-cleaned the carbs 4 different times. Spark plugs are good. Coils seem to be good. Points good. Cylinder #1 and #2 are firing well, and the headpipes are hot. #3 is warmer, and #4 is lagging and doesn’t seem to be running like the others. Could it be a lean/rich setting on the carb? A spark plug problem? Combustion? timing? Points? New coils? Something majorly wrong within the engine?

Somebody. Anybody. Help.

check my blog for in-depth details and progress: www.rjrcreative.com/bike

Thank you all in advance!

[TwoTired]

The slow/idle/pilot jets are pressed into the carb bodies and must be yanked out to clean them and the small passageways in the carbs.  All four exits to this passageway must be checked for fluid flow: jet hole, air bleed, pilot screw and carb bore exist.

Next you need to verify that the slides are in the same relative open position for all carbs.  Vacuum syncing is even better.

Cheers,

[johnyvilla]

Welcome to the forum. I would adjust valves, points, timing (if you haven't already done so) then carb sync, as tootired suggested. Since your other cylinders seem to be firing well, chances are your coils are fine. You can also inspect the condition of the suspect cylinders plug lead. Unscrew the cap and make sure you have a good connection. I have had to cut these down on several bikes due to intermittent spark. Good pic's.

[Soos]

I know this is a really stupid question....


But the wires from the coils, are they on the right plugs?
1 coil for 1&4, the other coil attacked to 2&3....




??


l8r

[TheHun]

I had the same problem but with cylinder 1 and 2 when I first got my bike. number 1 would be warm, but not as hot as 3 or 4 and number 2 was dead cold.

What I did to solve this problem is check and make sure your points gap is correct, and your plug wires. On number 2 cylinder my wire was shot, it would still make some spark at times but thats it no constant spark...so I replaced it my coils with CBR 600 F2 coils...and my bike has been running smooth as silk, plus if you are looking to upgrade your ignition, these coils cost a fraction of what Dyna coils are and they are rated 3-3.5ohms. I got a set of these coils of ebay with 6000 miles for 30 dollars shipped, and its plug and play basically..just have to put 1/4 connector ends on your coil wires.

If you havent done so..check your valves too..that made a huge difference in my bike running smooth

[dragon79]

i would check compression and valves.  if you are checking plug wire ends and trimming i would check resistence in the plug caps. should be 3k-5k ohms i believe. have had that give strange running problems.

[cxtecs]

I'm not an expert on CB iginition system works, but I'm pretty sure that if 1 and 2 are firing, 3 and 4 should also be firing since ignition-wize, they are paired 1-4, 2-3.

One thing that hasn't been mentioned yet as a potential problem is a vacuum leak on the boots for 3 and 4.  Pretty simple to test also.  I belive the test involve spraying WD40 on the boots.  If the RPM's increase when you spray, you've got vacuum leak, and the boot will have to be replaced/repaired.

[rafanomenon]

Man, you guys kick ass.

After a bit of pondering, another diagnosis comes to mind, which I think one of you mentioned: Compression and valve clearances. I'll check those as well.

I must've fiddled with the carbs 5+ times, and with each time came a thorough cleaning. Side note: I wasn't going to switch to pods, but after having to mess with these carbs so many times, pods and rejetting are now a no-brainer to me 

I've got a guy giving me an older set off his 750K. Heard the carbs from post-77 models are different i.e. press in jets, round sliders, different vaccums, etc - which make them a bit harder to clean and tune. Before I do this, I'll check what you guys suggested, and give it yet ANOTHER good cleaning. I'll also take a bit of steel wool to the needles to get rid of anything microscopic that could be preventing them from working. However, I checked with carb cleaner and my mouth on all the valves and everything seemed to be flowing fine. And yes, I snuck in a little tongue. What can I say - I love bikes that much.

I'll start with the carb boots, checking vacuum seals and cleaning the carbs (again) and see if that gets me anywhere. I'll post here with progress.

Thanks again for all the tips everyone 

[rafanomenon]

I know this is a really stupid question....

But the wires from the coils, are they on the right plugs?
1 coil for 1&4, the other coil attacked to 2&3....

??

l8r

Yes, I checked the coils. The firing order is 1, 2, 4, 3. 1 and 4 being on the left coil, and 2 and 3 on the right. I'm having problems with #3 and #4, which are on opposing coils. I also checked the points - good. Sparks are bright and blue.

I did notice that the plug from #4 was dry when pulled out, same as when put in and taken out again. when the carbs came off shortly after, we checked the inside of the mounting boots and there was gasoline. Not as much on #4 as there was as 1-3, so it MAY just be something wrong with the carbs?? mainly #4 pushing enough gas into the cylinder?

[Duke McDukiedook]

Looks like you are running a fuel filter to your carbs, is this clean and oriented vertically?
Check to see if there is rust in your tank, if so you'll probably want to etch and line your tank and cjuck those filters, they only cause problems like vapor lock.
Check for a vacuum leak like cxtecs said on your manifold boots- you'll probably have to buy new ones if they are stock issue.

[rafanomenon]

Looks like you are running a fuel filter to your carbs, is this clean and oriented vertically?

That's actually duct tape attached to a plastic water bottle. It's my jimmy-rigged 'nurse tank' so i don't have to deal with mounting the tank and petcock every time i want to fire it up 

[mutt]

as they say, 95% of all carburetor problems are electrical, but here you have misfiring cylinders off different coils. Original plug wires? Fire it up in the dark & see of the wires are leaking.
now, the carbs: as said, blow out the pilot jet circuit. The needle valves have a tiny preload spring, for checking the float hieght its  when the tang  just touches- not compresses- the spring loaded pin in the float needle.
And I wouldnt rule out bad plugs, no matter how new they may be.
I guess if I was sittin there scratchin my head, id swap the two cool cylinder leads with thier respective hot cylinders & see if the problem migrates. No? swap plugs. No?- its a carb issue.....

[rafanomenon]

as they say, 95% of all carburetor problems are electrical, but here you have misfiring cylinders off different coils. Original plug wires? Fire it up in the dark & see of the wires are leaking.
now, the carbs: as said, blow out the pilot jet circuit. The needle valves have a tiny preload spring, for checking the float hieght its  when the tang  just touches- not compresses- the spring loaded pin in the float needle.
And I wouldnt rule out bad plugs, no matter how new they may be.
I guess if I was sittin there scratchin my head, id swap the two cool cylinder leads with thier respective hot cylinders & see if the problem migrates. No? swap plugs. No?- its a carb issue.....

So I tried some simple diagnostics tonight because I don't have a ton of time to take apart and clean the carbs. I did manage to swap the newer plugs with the ones I replaced, and cylinder #3 picked up a TON, as the headpipes were way hotter than when we tested yesterday. #4 also ran a little warmer, but still not as hot as 1 and 2 - which worries me if they're running TOO hot. I mean, in 2-3 minutes these things are scorching hot to the touch. Still baffled on #4 though...

I'll get to cleaning... Steel wool, carb cleaner, tooth picks, guitar wire, WD-40, Quick Start to test... I'm making this as fail proof as possible.

This Saturday, I'll be trying the new set of carbs, as well as checking the compression. Fingers crossed...

[Spanner 1]

See all my posts on this page under 'Cold #3 etc.,"......dare I suggest your voltage to the coils is weak ??!! Respectfully, I can see a lot of surface rust on your pics. ( Seattle weather ! )..I'm sure all the wiring connections need attention....
IMO.

[rafanomenon]

See all my posts on this page under 'Cold #3 etc.,"......dare I suggest your voltage to the coils is weak ??!! Respectfully, I can see a lot of surface rust on your pics. ( Seattle weather ! )..I'm sure all the wiring connections need attention....
IMO.

I plan on checking all the wiring connections, as I haven't done so yet. I checked your replies on the 'Cold #3' thread, and you and TwoTimer seem to have understandable explanations that are worth a shot at addressing.

I had the same problem but with cylinder 1 and 2 when I first got my bike. number 1 would be warm, but not as hot as 3 or 4 and number 2 was dead cold.

What I did to solve this problem is check and make sure your points gap is correct, and your plug wires. On number 2 cylinder my wire was shot, it would still make some spark at times but thats it no constant spark...so I replaced it my coils with CBR 600 F2 coils...and my bike has been running smooth as silk, plus if you are looking to upgrade your ignition, these coils cost a fraction of what Dyna coils are and they are rated 3-3.5ohms. I got a set of these coils of ebay with 6000 miles for 30 dollars shipped, and its plug and play basically..just have to put 1/4 connector ends on your coil wires.

If you havent done so..check your valves too..that made a huge difference in my bike running smooth

TheHun: your problem sounds strangely similar. Though I'm not ruling out the possibility of some microscopic crud lingering around in those carbs, I made sure they were DAMN clean all 4+ times I took them off. I'm pretty sure my coils are fine as 1-2 are firing hot, and 3 warmer. I didn't check the plug wires, and am even thinking about replacing all 4 because they are OEM, as stated in the reply to Spanner 1. The plug caps (terminology?) were recently replaced by the PO, as they are labeled NGK and appear to be new plastic. I'll also look into the coils, and everything else electrical. I REALLLLY hope it's not a compression problem. We'll find out this weekend.

[devbear12]

I agree with spanner, i would first start with your ground connections for your coil primary,

Also not just because your plugs fire blue outside doesnt mean they fire blue under compression, but your plugs are dry,

so do you have vacuum in those cylinders, is your camshaft broken by chance??

[bryanj]

You tell us you cleaned the carbs several timesbut did you remove the pressed in pilot jets?

[HondaMan]

Also, can you post a picture of your fuel line routing? Recently I've helped "fix" a K6 and an "F" that had misrouted fuel lines, which caused air bubbles to become trapped in them and stopped the fuel flow to one or two of the carbs...it's a subtle thing, but can completely mess up an otherwise fine bike.

[rafanomenon]

Alright folks.

• I tested the compression. 150 (cold) straight across.
• Checked cams: Good
• Spark: Good
• Clipped plug wire ends for better connection
• Checked intake manifold with quickstart/WD40
• Points: Good
• Coils: Good (Firing #1-#3; alternating coils)
• Recleaned carbs. Removed press-in jets

I think that last step is what did it (Thanks, BryanJ!). I didn't want to remove them because of the risk of breaking something, but I went at it with some pliers last night. It appeared to have some microscopic crud lingering, which is enough to restrict fuel flow to the cylinder. I also used carb cleaner, toothpicks, Q-tips and super compressed air out of a narrow nozzle to blow anything else out. This seemed to work. FYI, post '77 carbs are a pain in the ASS. I'll be switching to the 69-76 K-model with air pods!

I reattached the carbs and ran the bike without the air box. Warm head pipes!! There still seemed to be some sort of vacuum or spark problem though, because by the sound of how it was running and the temperature of the headpipe compared to the others was still a little bit different. I'm almost banking on it being a carb problem - in which case I would feel like a dumb ass because they weren't clean enough from the start. 

I'll be swapping the carbs out with a different set this weekend, as well as an extra set of coils my buddy has - just to rule those out. I'll post here with pics and results.

Thanks again for all the input!!

[Soos]

• Recleaned carbs. Removed press-in jets

I think that last step is what did it (Thanks, BryanJ!). I didn't want to remove them because of the risk of breaking something, but I went at it with some pliers last night. It appeared to have some microscopic crud lingering, which is enough to restrict fuel flow to the cylinder. I also used carb cleaner, toothpicks, Q-tips and super compressed air out of a narrow nozzle to blow anything else out. This seemed to work. FYI, post '77 carbs are a pain in the ASS. I'll be switching to the 69-76 K-model with air pods!
 

Thanks again for all the input!!

That goes for the '79/80cb650 carbs IMO as well!
Dropped 'em like a bad habit after finding that cb750 carbs fit the 650.(after dumping $150+ in diaphragms into them however).
Pressed in parts are nonsensical if it ever needs cleaned .. why they ever chose that design over threaded puzzles me still.


Glad to hear it wasn't a small hole in your piston or something seriously wrong!


l8r

[rafanomenon]

a buddy of mine is rebuilding carbs from an earlier K model for trade with a gas tank. as soon as those are done, i'm slapping on some pods, rejetting and fine tuning. good riddance to those stock '77 carbs!

[eldar]

You might find the 77 carbs work better with pods. The 77/78 carbs are very misunderstood. I dare say that once you get to know them, they are easier to work on than the earlier carbs. I find mine are very simple and reliable and responsive. I last cleaned mine about 4 years ago and have not had to do it since. I will probably tear them apart this spring just to check the wear and orings and such.

[rafanomenon]

With pods and aftermarket exhaust (which I plan on getting as well) come rejetting, which the later carb models don't accommodate very well due to the lack of screw-in jets. Drilling is a bit aggressive, which I've read people have had to do when tuning for pods and exhaust. I'll still hold on to these OEM 'just in case,' but I find that tuning and cleaning anything that small is a ton easier if said pieces are removable. We'll see!

[eldar]

Thats the beauty though. Most do NOT have to change the idle jet. You see, with the fuel screw, you can compensate quite a but. Most here that do the pods or 836 kits do not have to change the idle jets. They might change the main but those are exactly like the earlier carbs. PLUS, the accel pump gives a kick the early carbs cant. 

[rafanomenon]

PLUS, the accel pump gives a kick the early carbs cant. 

which was also a pain in the ass to clean!! you're right though.. there is a give and take with each. i'll just tune the stock '77 carbs this weekend, and i'll be doing the swap next weekend. i'll make sure to document the +/- for each