1981 CB750C If you have had this issue save me the headache!

[samcraig182]

Hey everyone I am having trouble getting this bike to the finish line and after researching the manual and other posts on the internet I can't seem to find what's my exact issue. There is a potential list but if anyone understands the issue or has had it before then it would be much appreciated!

Main Issue: Bike starts but stalls after throttle is released.

Starting procedure: Choke open or closed. Jump start or normal start(when battery is charged) with throttle 1/2 to 3/4 turned. Throttle is held in this range after bike starts and idles from 1000 to 2000. Increasing throttle to full maxes out at 3000. When or if throttle is released bike dies. Bike does not rev high and dies immediately if put in first. Tried to adjust idle knob with a small increase in idle after turning clockwise. But after the knob is maxed out CW bike still dies eventually. Simply put I have to hold the throttle to keep bike at 1000-3000 rpms.

Bike status: Carb clean twice and float re-adjustment. One float has bad seal and leaks 10 drops every minute. Battery is very weak but working condition. Can attempt to start the bike 15-20 times if it is charged, but after attempts battery is 20-40%. Automatic fuel valve/vacuum was removed and there is now a direct line to carbs with a capped vacuum line between the carbs. Petcock was leaking but does not after clean and rust removal. Tanks has some rust inside.

What it could be according to other's:
1. Vacuum leak from carb boots. Tightened and adjusted all 8 boots 3-4 times but did not spray/rev check for leaks yet
2. Spark plugs. Not checked but will soon.
3. Pilot screw
4. Improper jetting. Accidentally put #100 main jet instead of #102. Basically the 80' CB750K jetting. Realized after carbs were put back in and will change soon.
5. Not enough fuel not reaching carbs. Routing may be causing issues or removal of automatic vacuum.
6. Exhaust. Working with air from all openings so far.
7. Compression
8. Ignition timing

My plan: Check all these issues but find out from others what is causing the problem or if anyone has experienced something similar. I'm not too experienced but I am learning so I can use all the help I can get! Thanks

 

[amc49]

All of those except 4 & 5 could do that. #4 is not enough to hurt you with that problem. Not #5 if you having dripping fuel which can do it too. Tightened boots mean nothing if they are cracked from taking them on and off. You're talking an almost 40 year old rubber part there. You cannot adjust plastic floats if that is what they are.

Compression test first or then find out all of the work you do has been wasted. Nobody ever sets the valves and they then burn and no way will it ever idle after that. Why everybody tears the carbs down over and over for nothing.

 

[samcraig182]

Had a few broken pilot screws so pulled the tips and replaced those. Same issue. I checked the plugs and they seem fine. 2 times now the bike has gone full throttle and worked at 5000-7000 rpm before dying!

Also the exhaust afterwards was interesting. 2 are cold, one is warm and the other is hot.

Still only getting about 20 seconds max run time but I definitely will check the compression.

 

[samcraig182]

Had a few broken pilot screws so pulled the tips and replaced those. Same issue. I checked the plugs and they seem fine. 2 times now the bike has gone full throttle and worked at 5000-7000 rpm before dying!

Also the exhaust afterwards was interesting. 2 are cold, one is warm and the other is hot.

Still only getting about 20 seconds max run time but I definitely will check the compression.

Cylinder 2 and 3 are the warm and hot. 1 and 4 are the cold. Checked the spark again and I have a definite weak spark on 1 and 4 and strong spark on 2,3.

Pull/replace the plugs or check the ignition/coils is what I am reading mostly. But could this still be a fuel issue? The bike is now consistently revving high before dying as well instead of just twice.

Thanks for the help amc you know your cb750's very well.

 

[amc49]

Compression check, with the way it sounds you may have a dead motor. Only running on one cylinder pretty much.

 

[samcraig182]

Compression check, with the way it sounds you may have a dead motor. Only running on one cylinder pretty much.

Damn. I shouldn't even waste my time checking anything else and just compression to confirm it's dead once the battery is charged again. You've probably seen this before so am I likely dead in the water or is there a chance anything else is holding it back?

 

[samcraig182]

Also 4 got warm today after being ice cold yesterday don't really understand that.

 

[samcraig182]

PO only had leaking carb issues as well but never had engine trouble. I've only put the carbs back to working condition so if the engine is toast it's a huge surprise how that could of happened.

 

[amc49]

People say that all the time, and the number of them that have not a clue as to what running 'good' is is very high. Expect the BS to 2X at sale time.

How I got my Kawasaki 750-3, the guy kept bragging how utterly fantastic it ran until I tuned it up for him, then he got so scared of it he sold it to me quick. He had been running around on only 2 out of 3 cylinders the whole time. Once right the bike terrified him.

The number of people who will swear their car or bike runs absolutely fantastic........................I used to build race car engines too, you could take them for a ride in a 'real' car and they wanted no part of that after doing so. We built AMERICAN MOTORS cars that with 390 inch engines commonly outran like big motor Cobrajet Mustangs and 454 Camaros all the time. Those guys usually got pretty p-ssed off about it.

You can only relate to what you have always known...............

Cylinder 4 getting warm now could easily be the valve clearance off enough to simply barely leak, at that point it will hit sometimes and sometimes not.

Look, it's silly to do scads of whack-a-mole work on a dead engine, assuming they are great after almost 40 years is the classic mistake most make. Based on the guy before you that had no clue either. Basic mechanic rule #1..........you make sure the basic motor is sound before wasting any other time on it. Or join the guys in sentence #1 that think their stuff runs great.

A proper running early DOHC should be able to light the tire up all the way through low gear and part of 2nd at 9500 rpm. Or, in the case of a heavier C model, wheelie hard enough to throw you on your -ss.

Not saying it's dead but it's VERY counterproductive to assume it's 100% right enough to tune carbs out and such as the CVs on these will break your -ss if the motor is not right. Most people who do not check the compression go after the carbs and then how the bike gets sold, you get NO result doing it, so they screw it up more, like mixture screws broken from getting mad. It's a male self abusive testosterone thing. I have been there to see the result hundreds of times. Carbs may now all be junk if the screw forced into the hole enough to widen the discharge port, now you can't set them all the same and any thing you do there is wrong.

Read again what I put at the end of post #2, the valves can burn in less than 5K miles if they were not set right at the first checkup at 600 miles. Some stay on pretty close but some are already getting close to burning by then. The service manual spec showed later to be too close in the real world too, that burns them even faster. Why the compression check.

Looking for at least 130 to max and book spec of 170 psi. Anything lower and you get progressively worse running issues, any cylinder at 100 is considered effectively dead.

 

[samcraig182]

PO was my friends dad trustworthy source but you never know like.you said how he maintained it.

It wouldn't even start before carbs so that's why I worked on it to get it far enough to do things like a compression.

Could only.get the bike to start three times(20 seconds) before battery died. It sounds like the starter is shot too. for 20 seconds each. So bike was not very hot but all the exhaust are heating up now. Only could get one compress reading as I have to charge the battery back up for the next.

120psi and rising before bike gave way on cylinder 4.

 

[samcraig182]

Don't know how close to 160-170psi should it be at that point of 60 seconds off/on run time but it looks like it is definitely missing the mark somewhere.

 

[amc49]

You don't measure compression while engine running if that is what you are doing. Use starter cranking only and all plugs out, throttle held wide open. A car battery works better but do NOT!!!! have the car running doing it. If the 120 taken while engine running then a big indicator of crap compression as still rising at that kind of rpm screams compression is leaking. It should firm up to level off in maybe 5 seconds max at only starter cranking speed. Not leveling off quick to continue to try to increase means leak.

FYI, the carbs DO make issues of their own but you must get the compression issue out of the way to determine further direction.

 

[samcraig182]

New Compression test: 90-95psi on all 4. Think the reader is slightly under as well because first test showed 70psi and after adjusting it went up to 90. Carbs in, choke closed, full throttle and all plugs out.

I can confirm that at least 2 cylinders are running as well which isn't saying much.

 

[dirtdigger]

Choke needs to be open or you are still choking the motor for air when doing the compression test. Like said max compression should be reached in 5 revolutions of the motor. First one will be the biggest jump followed by a couple shorting increases in compression. On a good fresh engine you will reach compression in as little as 3 revolutions. If you gotta spin the crap out of it to get compression then you got big issues. What kind of compression tester you using? Has to be a tread in gauge not a push in or you will never get accurate test.

 

[samcraig182]

Thread in good and tight. Max of 90 was reached on third revolution. It's a Pittsburgh auto from harbor.

 

[samcraig182]

That is a thread in on the spark side not the guage. The guage side is a push and lock type thing.

 

[samcraig182]

Also did it with choke open and closed for each test. Not much or any difference. But now I know for next time.

 

[dirtdigger]

Sounds like you got some mechanical issues. Normal healthy engines will make 75 to 90 on the first revolution, some can hit even higher.

 

[amc49]

Now it all comes out.

Double and triple check, compression testing always has flukes. You may want to rent another gauge say from auto parts place for backup readings, you're judging whether to maybe give up on bike or not. I say that after multiple issues reading compression for a living, it's hard to find good equipment for that. Battery must be hot as well, low gives low numbers and why I prefer a car battery to last through all the spinning.

Next is to run the valve clearances, odds are you will find some at less than .002" which is effectively zero on these. Valves are being held open or possibly burned after being like that too long.

What's the mileage? Also, all 4 cylinders being really close to same number argues that gauge itself may be lacking but not a guarantee. Why the backup. You just always gotta suspect the numbers until they really prove out.

Plugs coming out black backs low compression up more, when you get around 100 psi the plug temps drop off because compression is what makes heat, the plugs then begin to foul like a flooding engine because they are not hot enough to keep burning clean. Actually a gradual thing but after years fixing car and bike engines I found 100 psi is a number at which you quit any work with tuning and then go to engine rebuilding or whatever it takes to get the compression back up. 2 strokes can run that low but they fire on every revolution other than every other one like a 4 stroke does, the extra heat keeps the plug clean on the former.

 

[amc49]

You're not the first one to be here by a longshot, most early DOHC end up that way, nobody ever sets the valves and perfectly good engines go to crap over it. Almost every one you buy will have the carbs apart over this issue, the owners commonly telling you it ran fine. Well, it did, and likely in recent memory, they run fine then begin to lose their edge and then suddenly it's running like crap.

The cams even on a brand new bike are super loose in the head and the valve springs then push the cams all over the place to change the effective valve clearances, which are commonly at .003" if by the book and .002" is considered useable. But it's not, .002" is pretty much zero and valve held open and why if you can recover the engine you loosen them up to a setting of .005" instead, then the engine goes whoppingly longer with no issues. A Honda mistake that they knew of but rectified it by changing up the valve setting marks on cams, that did nothing to fix the problem.