1980 CB750F running terribly!

[spangers]

Hi guys,

I recently purchased a 1980 Cb750f which ran beautifully when i test ride it but on the ride home I noticed a huge drop in power and the bike would no longer rev above 5k and would miss fire and backfire when applying throttle near that rpm. Also below 4k rpm the engine would run rough and would stall if I let the rpm degrade further. I had to ride the clutch to keep it at 4k rpm and the bike had so little power that it could barely pull it's weight up my drive way riding the clutch!

I pulled the plugs and they were badly fowled but in otherwise good condition. I replaced them to make sure but the problem remains.

The bike also starts cold with no choke when cold and seems to run very rich from fuel consumption and smell (as well as the plugs).

I read on a forum that carb balancing could be the issue and took it to a mechanic today but he didn't think that would cause such severe symptoms. He also didn't even want to look at the bike because of its age and bitched the whole time a about not wanting to get "stuck with the job" if more things kept cropping up. I would have walked out of the shop then and there but I don't have the equipment to balance carbs, although I am regretting not buying them now!

I also put fresh fuel in the bike (98 octane) and checked the tank for contaminants.

Does anyone have any idea what could be causing this? My next step is to get the carbs cleaned in a ultra sonic bath.

Sorry for the wall of text, if there is any cb750 experts in Melbourne, Australia I would much rather give you the money than this mechanic!

Cheers,

Spangers

 

[Travis]

Dirty carbs usually cause a lean condition with clogged jets. I don't know what could have happened between when you bought the bike and when you brought it home to cause it to run extremely rich. Extreme altitude change? Maybe a couple of the floats got stuck? Something restricting the air? Hopefully someone will chime in that has a better idea.

By the way, most mechanics are like that with any old bike... I can hardly blame them.. You start messing with one thing and find problem after problem. Pretty soon their labor bill alone is more than you paid for the bike.

 

[tommycarl]

I'm not an expert by any means, but here's what I thought of as I read your post:

1. First thought was that maybe your gas tank wasn't venting correctly so you lost fuel flow as you rode the bike a long distance. Probably not the case since you opened the tank to add gas, and the problem didn't go away. When riding a long distance and that type issue arises I generally open the gas cap to immediately vent the tank and eliminate that as the problem.

2. Dirty tank or gas and the carbs are now dirty... is this a bike that has been sitting and the previous owner recently got it back on the road, and you bought it?

3. Fouled plugs... running way rich. Are you running stock air box or pods?

4. Carb synchronization... I believe that bad carb synch would show most at idle, not higher RPM. I could be wrong, but if the bike starts and idles well, I don't think synch is the issue.

5. Spark... stock coils or aftermarket? Is the voltage at the coils with 1 volt of whatever the battery voltage is?

6. Electrical system... is it charging correctly? If the battery was getting weak as you rode, performance would suffer. Do you get about 14 volts at the battery when the bike is turning 3,000 RPM?

The thoughts just ran through my mind and I had more questions than answers. I would personally want to know more about the bike, then ask about the easy things to check. Like... are you running stock carbs and air box, or something different like pods? Has it been ridden regularly or just recently brought back from the dead?

Travis is a pretty sharp fella, so pay attention to what he says. This is a great opportunity for you to learn about your bike and how to make it run. Most "current" mechanics really don't want to mess with these old bikes, and when they do, you better bring your check book. Do it yourself and after the frustration you'll feel a whole lot better and glad you did.

 

[spangers]

Thanks for the replies guys, I am still working to find the problem, tommy is helping me out also.

The carbs have been cleaned a synced, spark plugs checked and they are all strong sparks. The bike runs a million times better but there is still hesitation on acceleration when coming off power and reapplying it. Sounds like fuel starvation but it could be air also.

My next step is to check the air box for holes or cracks and the fuel tank vent (which I cant find!), if anyone knows where it is on the 80's cb750s could you please let me know!

Thanks everyone for your help

 

[tommycarl]

I suggest that the easiest way to check a problem with gas tank venting would be to just open the cap. If you have no flow with the cap closed but it returns when you open it, the vent could be an issue. It's something that would take at least a little riding to develop, which is why I considered it when you mentioned the problem getting worse as you rode the bike home. Sorry I can't tell you the exact location, but it's surely built into the gas cap somewhere.

The accelerator pump would be my first target for lack of acceleration as you describe it. The only way I know if to check it visually while the carbs are installed and with stock air box would be to remove the caps and slides and look inside the throat from the top... at least that's how I checked my 900 last year. It helps to know where to look because you can't see the little nozzle inside the throat when the carbs are installed, but it'll be toward the rear of the throat. On mine, I couldn't see much of a spray, but I could see gas running along the bottom of the throat when I twisted the throttle. If you don't see anything you might try a mirror, or maybe svn put a piece of paper towel on a wire bent so you can get the paper towel toward the rear of the throat (I've never tried that, but it might work.) With the air box on you just can't get to the back of the carbs, and even if you could get the air box off with the carbs installed, it's way easier to get the carb caps and slides out on at least the two outside carbs. In case you haven't figured it out, I avoid taking carbs and air box off whenever there's another way.

Maybe someone else has a better way to check the accelerator pump and will chime in.

 

[spangers]

I suggest that the easiest way to check a problem with gas tank venting would be to just open the cap. If you have no flow with the cap closed but it returns when you open it, the vent could be an issue. It's something that would take at least a little riding to develop, which is why I considered it when you mentioned the problem getting worse as you rode the bike home. Sorry I can't tell you the exact location, but it's surely built into the gas cap somewhere.

The accelerator pump would be my first target for lack of acceleration as you describe it. The only way I know if to check it visually while the carbs are installed and with stock air box would be to remove the caps and slides and look inside the throat from the top... at least that's how I checked my 900 last year. It helps to know where to look because you can't see the little nozzle inside the throat when the carbs are installed, but it'll be toward the rear of the throat. On mine, I couldn't see much of a spray, but I could see gas running along the bottom of the throat when I twisted the throttle. If you don't see anything you might try a mirror, or maybe svn put a piece of paper towel on a wire bent so you can get the paper towel toward the rear of the throat (I've never tried that, but it might work.) With the air box on you just can't get to the back of the carbs, and even if you could get the air box off with the carbs installed, it's way easier to get the carb caps and slides out on at least the two outside carbs. In case you haven't figured it out, I avoid taking carbs and air box off whenever there's another way.

Maybe someone else has a better way to check the accelerator pump and will chime in.

Hi tommy, would it be worth me just taking the carb bank off and rebuilding the accelerator pump with a kit from ebay? Its not to expensive and I need to replace some rusting bolts on the carbs anyway.

 

[dirtdigger]

If you take the carbs of the bike you can easily check the accell pump by simply making sure the bowl is full and operating the throttle manually. Each carb does have a spray nozzle but there is only one pump. They should make a decent fuel squirt toward the engine side of the carbs. If they dribble fuel or you only get fuel out of 1 or two carbs you have plugged spray nozzles and or a bad pump diaphram and check balls. If you get a rebuild kit make sure you get the kit that also has a new pump cover. There is a check ball in the cover and the almost always stick and get full of crud because it is the very bottom of the fuel system in the bottom of a bowl. I have freed some up but 75% of the time you will never get the check ball to work and seal correct.

 

[tommycarl]

If the accelerator pump isn't working correctly and you want the acceleration you're now missing, then replacing the accelerator pump is the only option I know of. However, if you've never removed and reinstalled these carbs, you're in for an experience... likely including a good bit of frustration and you'll probably even work up a sweat. Since the carbs were just rebuilt I assume that the insulators aren't completely hard and brittle, so that hopefully won't be an issue. Before you begin, save up some patience. It's not an overwhelming job, but it IS frustrating and if you have no patience.... well, just be prepared. The first time is really the worst... last year I had mine off and back on my CB900 6 times before I got my problem fixed... the 6th time was a breeze, But let's hope you're better than me and get the job done the first time.

Also before you start, I suggest that you go to www.cb750c.com and find seang in that forum... he has a GREAT carb rebuilding manual that's downloadable and free. You can find it in the documents section of that site. He also sells a complete set of replacement allen head hardware for your carbs, and a complete set of o-rings... and the price is waaaay reasonable. I'm not suggesting that you replace the o-rings since you just had the carbs rebuilt and won't be taking them apart to fix the accelerator pump, but if you end up buying the hardware, it'll be worth the cost of the o-rings (maybe $6) just to have them in case you ever need them. If you have any problem finding him or the manual let me know and I'll post links for you.

Besides the diaphragm in the accelerator pump, there are also diaphragms in air cutoffs for each carb that can cause issues. Those can easily be bypassed if you want, and while you have the carbs out would be the time to do it if you want. That will be covered in Sean's carb rebuild manual.

I mentioned before that I always avoid removing these carbs if there's another way... I just suggest that you get the issue isolated to the accelerator pump if possible before tearing into them. I'd even go back to that mechanic and ask him if he checked the pump at any time just in case he can confirm that it worked okay.