Topic: My first complete build 72 CB750 - completed for now...

[algophobe]

then fab a little bracket for the idle screw extension for the CR carbs. something strangely satisfying about make simple little tid bits like this. which brought me back to the frustrating issue at hand... which much advice is needed from you all.

I took davez134's advice and check the spark advancer shaft to see if it's true with a dial gauge. its only about 0.3 mm off.

so to recap. cylinder 1/4 runs great when using the timing gun. but when used to evaluate the 2/3 cylinders off the spark plugs, the firing is very erratic. I fired her up today, and the 2/3 pipes are barely warm. in fact the timing gun doesn't even pick up anything. so I decided to swap the coils and same thing happens. therefore it can't be the coils nor spark plug wires. I took out the spark plugs to check for spark. the spark plugs from 1/4 sparks well. and if I took these spark plugs and hook it to the 2/3 coils, it sparks well too. therefore it couldn't be the magnetic pick ups nor wiring leading up to the coils. but when I took the spark plugs from 2/3 and hook it to 1/4. they have a trouble firing. THEEEENNN!!! I notice a spot of what appears to be oil on the ground.. Thinking "Oh great another issue I have to deal with, a leak from the case". but all engine mating surfaces are clean as can be. apparently it was coming from the seam between the exhaust down pipes and the mid pipe coming from cylinder 2.

it smelled more like gas mixed with exhaust carbon and not oil specifically. and after some air dry time and minor cleaning.. the spark plugs taken from 2/3 fires well. I put a camera down into the head and 2/3 looks clean (in fact it barely looks like its been working). 1/4 looks like it's been working with minor carbon build up. I decided just for the hell of it to do a compression test and all cylinder produce about 140 to 145 psi.

now i'm all confused. HEEELLLPP!!! is it that the 2/3 is running too rich and fowling the parks? I set the air screw the same amounts of turns. I remember reading somewhere that the jets for 2/3 usually is a little bigger than 1/4 because they tend to run hotter being in the center. could that be it? that's why then ran for a short period then stopped working? any input would be greatly appreciated.

[Godffery]

 Those budget pod filters are going to mess with your air / fuel mix. Your best to go with K&N RC-1820 before even trying to tune it.

[algophobe]

Those budget pod filters are going to mess with your air / fuel mix. Your best to go with K&N RC-1820 before even trying to tune it.

hmm. you really think that's the base of my problem? why would it only affect cylinder 2/3 and not 1 or 4? but it would explain why it ran initially then gradually became erratic and then stops firing all together.

[Godffery]

Those budget pod filters are going to mess with your air / fuel mix. Your best to go with K&N RC-1820 before even trying to tune it.

hmm. you really think that's the base of my problem? why would it only affect cylinder 2/3 and not 1 or 4? but it would explain why it ran initially then gradually became erratic and then stops firing all together.

Its not your current problem as to why 2/3 aren't firing, but as Jeff points out, they will hinder you long term in getting your bike running well.

Yes, it will help to start with a good base line by eliminating all known week links right down to the basics.

[algophobe]

Those budget pod filters are going to mess with your air / fuel mix. Your best to go with K&N RC-1820 before even trying to tune it.

hmm. you really think that's the base of my problem? why would it only affect cylinder 2/3 and not 1 or 4? but it would explain why it ran initially then gradually became erratic and then stops firing all together.

Its not your current problem as to why 2/3 aren't firing, but as Jeff points out, they will hinder you long term in getting your bike running well.

Yes, it will help to start with a good base line by eliminating all known week links right down to the basics.

ordered!  ... I guess you get what you pay for. I took this opportunity to take down the carbs and boots as well and try the mod as suggested earlier about putting vacuum ports into the boots to hopefully help with the carb balancing once I figure what the hell is wrong with my 2/3 cylinders.

[algophobe]

so I happened to run into an old friend at work and he asked about the bike. long story short, I told him the current situation. he recommended checking the valve clearance. I may come loose after a few times running the motor, especially when new valves and guides were placed. so I check first thing when I got home, and lo and behold. they were waaaay off. in fact all 8 valves. it may not explain every thing... so I proceed to adjust the valve clearance back to 0.002 and 0.003 " for intake and exhaust.

my question that with the compression test I ran yesterday producing 140-150 psi on all four cylinders. a loose tappet wouldn't affect compression.. is my thinking correct? because loose tappet would not open the valves wide enough to produce efficient intake and evacuation of ignited fuel/air mixture?

[Terry in Australia]

No mate, a loose valve clearance won't affect a compression reading, only if you have no clearances, as is sometimes the case with valve recession etc where the clearances are so tight that (especially when hot) valves can't entirely seat. Cheers, Terry.

[algophobe]

Took a needed break from the cb750 build and worked on my running 05 Kawasaki 636.  a little more brainless work on the known vs. the unknown of a custom bike. a while back I had install the Dyno Jet quickshifter. I committed myself to the "pull" sensor type and like to shift GP style. some clearance issues required that I raise the lever up.. which makes for a tired shin after a few hours of riding. started work on fabricating an extension tab for the lever. works pretty well. except I have to make another one to extender it a little more for more anatomically correct forefoot positioning. I mocked it off using an older crashed Sato rear set.

[algophobe]

so I decided to get new updated 5 ohm coils for the ignition system. I was concerned maybe the old type of screw connectors from the coils to spark plug wires were weak or loose. the engine fired right up. ignition was timed to +10, all cylinders firing consistently. and timing gun picked up all four wires. check full advance at 2500 rpm, it seemed off. but I thought I had solved the problem... went back the next day to see if I advance it to 15 deg @ 1000k and recheck 2500k full advance. and guess what I find. cylinder 2 was not firing again! I'm at my wit's end. I'm about to pull my hair out. from an electric standpoint the system runs fine, but fails after some time running. could the problem be coming from below? I'm out of ideas. do I need to pull the motor and take it apart?

[algophobe]

Did you install new condensers? Have you checked the resistance of your plug caps? Your thread is pretty much all over the place, so pardon me for re-asking these questions.

You say #2 isn't firing, is there no spark at all, or no combustion? Is the pipe at all hot? There will be some heat from the spark going off even if the fuel isn't being combusted.

no worries. I took a little hiatus from this issue due to frustration. I'm running PAMCO ignition so no condenser. new plug caps, new plug wires.. so I don't think its a plug cap resistance issue.  2 fires when the new coils were installed. I checked for spark before placing back the spark plugs into the engine. but after running for 10-15 minutes to adjust advancement. combustion does occur initially and pipes get hot. but the next day, 2 won't ignite, the pipe is barely warm compared to the other 3. I swap the wires for 2 and 3 and again 2 doesn't produce combustion.

[algophobe]

Fuel issue on #2

so why would it combust initially but not the next day? any chance that there's something wrong from the piston and rings down that would cause it as well? btw the carbs are new as well. CR specials.

[Godffery]

 Any chance that plug is fouling out?  Have you tried swapping it out with one from another cylinder after it happens?
If you do that, and it fires back up in #2, then fails again after a while, I would consider a possible fuel issue. (If I recall correctly, you had some question about that in the recent past?)
 Also; have you done a compression test on all 4 cylinders?

[algophobe]

Any chance that plug is fouling out?  Have you tried swapping it out with one from another cylinder after it happens?
If you do that, and it fires back up in #2, then fails again after a while, I would consider a possible fuel issue. (If I recall correctly, you had some question about that in the recent past?)
 Also; have you done a compression test on all 4 cylinders?

you are correct sir! yes compression test were repeated on 2 separate occasions, consistent across the board. my initial suspicion was it fouling out. but are we talking about fouling from excess fuel or oil leak. I thought maybe there was an oil leak from the rings. but would you see a drop in compression? i'll have to try swapping the spark plug next chance I get.

also you refer to fuel issue, are you thinking the actual gas itself or the quantity coming from that particular carb?

on a side note, this may show my lack of knowledge but could using the wrong oil foul a spark plug sooner? I've traditionally used mobil  automotive oil for years on all my bikes without issues, as long as it is compatible with the wet clutch.

[Godffery]

 Yes fouling from excess fuel or oil.
If the oil was getting in from the rings; That would likely effect the compression (unless it was swimming on oil) But if it's coming from a valve guide, that would not effect the compression.
If it is from fuel; Then I would not say the fuel itself, but an excess of fuel that is coming from that #2 Carb. (possibly a float needle not seating properly?)

[algophobe]

Yes fouling from excess fuel or oil.
If the oil was getting in from the rings; That would likely effect the compression (unless it was swimming on oil) But if it's coming from a valve guide, that would not effect the compression.
If it is from fuel; Then I would not say the fuel itself, but an excess of fuel that is coming from that #2 Carb. (possibly a float needle not seating properly?)

the spark plug do look wet and black like an oil foul, but then all the sparks looks like that, and yet the other three runs still. the piston definitely not swimming in it. however if its coming from the valve guides, what can you do? these were new valve guides as the head was redone with Kibblewhite guides. the repair manual suggests running hotter spark plugs which doesn't seem to make sense to me. currently using NGK iridium IX plugs. you think running heavier weight oil would help?

I also check the needle, on the CR specials you can take the top off and pull the needle, there is retention clip that was slightly off... I thought that this may be the cause, but one other carb had the same situation as well and its running fine. i'll post some pics later and show you that I was referring to.

[Terry in Australia]

Hey mate, just for a test, ditch those Iridium plugs and try some NGK D7EA's. I've got a set of Iridiums in my garage that I yanked out because they made my bike run like sh1t, I can't explain why, they were the right ones for my bike, but ditching them and replacing them with the D7EA's (also recommended by Hondaman) was like night and day on my bike. I remember Mike Reick had the same issue. I'm sure I'll be shouted down, but for the sake of a few bucks, just give it a try, and report back? Cheers, Terry.

[Godffery]

Yes fouling from excess fuel or oil.
If the oil was getting in from the rings; That would likely effect the compression (unless it was swimming on oil) But if it's coming from a valve guide, that would not effect the compression.
If it is from fuel; Then I would not say the fuel itself, but an excess of fuel that is coming from that #2 Carb. (possibly a float needle not seating properly?)

the spark plug do look wet and black like an oil foul, but then all the sparks looks like that, and yet the other three runs still. the piston definitely not swimming in it. however if its coming from the valve guides, what can you do? these were new valve guides as the head was redone with Kibblewhite guides. the repair manual suggests running hotter spark plugs which doesn't seem to make sense to me. currently using NGK iridium IX plugs. you think running heavier weight oil would help?

I also check the needle, on the CR specials you can take the top off and pull the needle, there is retention clip that was slightly off... I thought that this may be the cause, but one other carb had the same situation as well and its running fine. i'll post some pics later and show you that I was referring to.

It is possible that one of those guides was damaged or didn't seat properly during installation.  That requires dissemble to check, so I would look at all other potential issues B4 that.   

As for the Float needle; that is at the bottom of the Carb. The needle you described up top is the slide needle.

[algophobe]

thanks for the input gents, I thought about changing the sparks as well. supposedly the iridiums are "more resistant to fouling out".  i'll put that on my list of things to do before any dismantling.  was thinking of draining the oil and see if there is any strange metal shavings that could indicated something going wrong on the bottom end that would push me to pull the engine. if no improvement, then i'll probably be taking down the carburetor once again after that and check the float needle as suggested. then go from there to check the head and work downwards.

just curious, when you're hand cranking the crank, should there be somewhat of variations resistance? I know there is some difference depending on the location of the piston, but I do notice some spots that are "rubbery" (for a lack of a better word).

[Terry in Australia]

Yes mate, it gets "rubbery" as the piston is traveling up the cylinder compressing the fuel/air mix, so one complete turn of the crank will feel easy, the next will have a lot more resistance.

[algophobe]

Yes mate, it gets "rubbery" as the piston is traveling up the cylinder compressing the fuel/air mix, so one complete turn of the crank will feel easy, the next will have a lot more resistance.

yes I sound like a total dumbass  but you can read and watch all you want but hands on experience to feel, see, and hear what "normal" is supposed to be like only comes with doing it yourself I guess. once you know normal then you can kind of develop your antennae for what is "abnormal".  I've never actually heard one of these cb750 run in person, some of these extraneous noises freaks me out. like hearing a dry clutch in person for the first time. one can only hope to have a knowledgeable friend near by to learn from... till then, you all will have to put up with stupid questions.

[Terry in Australia]

That's OK mate, we were all dumbasses once, but that's OK, you've gotta start somewhere! Cheers, Terry.

[algophobe]

well, some parts came in. I ran her yesterday with the hotter new spark plugs D7EA using the velocity stacks that came with the Keihin Carbs just to see how it runs. I drained the oil and put in 20w50 (since california gets hot these days anyways). all four cylinders fire like a champ. but we'll see if it fouls out today again.

I've order four of the K&N RC-1820 pod filters as Godfrey suggested from Bikebandit. apparently there is a shortage and is on back order. they were only able to ship 2 and the order automatically cancels after 28 days. does anyone have any supplier for these things? i'm using the 51 mm ID filters. just need to more to complete the set.

[Terry in Australia]

I'm glad you're trying the D7's mate, I'll be surprised if you foul a plug. Ebay is full of listings for those filters, but I wouldn't know if they're cheaper or more expensive than BB. Cheers, Terry.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050890.m570.l1313.TR0.TRC0.H0.TRS4&_nkw=K%26N+RC-1820&_sacat=6000

[algophobe]

Well terry.. fine advice u gave all the way from over there. The D7 have consistently fired and was able to time the engine to +10 for a fraction of the cost! New pod filters placed... turns out K&N's manufacturing plant is near my work in Riverside California. They had like 20 something on hand and we weren't able to figure out why Bike Bandit couldn't stock it. Pretty much the same price and was able to get it to my door in 24 hours. My plans for this weekend is to take out the spark plugs and check to see how they actually look and balance the carbs.

[algophobe]

I balanced the carbs using this old motion pro mercury gauge I've acquired many years ago. the throttle response seemed snappy and crisp as one would want to have, the rpm quickly return to idle when throttle is released. I proceed to disconnect the vacuum lines to the mercury gauge and re-run the engine. I notice now that when I rev the engine to by about 1/4 turn it doesn't snap back to idle as crisp as it did during the balancing process with the gauge hoses on. the engine only gradually returns to idle when the throttle is release, is this an issue of carb balance or air/fuel mixture in general? I find that the Keihin CR specials, the air screw for 1/2 turns out, anymore or less it would sputter and shut off. I've looked various sources on carb balancing and tuning, nothing really mentions this problem.