Please Help...high rpm's rough running

[bellison80]

Hi guys I will try to provide all detail and keep as short as possible. 79cb 750f all stock. I have rebuilt carbs, new rubber between airbox and carbs, kreemed tank, new fuel line, new plugs and wires, new choke and throttle cables. The mixture screws are out 2 1/2 turns plugs look good and last night I used my new sync gauges and dialed in the carbs. Running really smooth now at idle, cleaned up some noise coming from engine.

But I am still having the same problem. It is losing power at about 5-6000 rpms. When I took it out for a test run after the sync it runs great until you really try to get into the powerband. I also watched the rpms continue to climb at a couple of points while the power did not.

So its got to be either electrical or a main jet issue right? I also checked the charging system and battery is good and system is charging.

Where do I start again? Is there anything that I should do first. I had a friend tell me that the mains that came in the rebuild could be the culprit. I still have the originals and was thinking of pulling the carb bank, cleaning thoroughly again and putting those back in. Also I am having a hard time being convinced that it would be the main jet or the size being that everything is still stock?

What have your experiences been with this guys? I feel like I am missing something so simple, and so close to having a strong running bike.

I have done some other body modifications to it and working on getting it painted. I will post up some pics real soon in this thread.

[TOOLS1]

Check to see if the ignition is advancing properly.
TOOLS

[rrgunslinger]

I would say either jets or timing. Check the timing at high rpm to see if it is holding. 

Harvey

On Apr 6, 2012 8:57 AM, "bellison80 [via Honda CB750&apos;S]" <[hidden email]> wrote:

Hi guys I will try to provide all detail and keep as short as possible. 79cb 750f all stock. I have rebuilt carbs, new rubber between airbox and carbs, kreemed tank, new fuel line, new plugs and wires, new choke and throttle cables. The mixture screws are out 2 1/2 turns plugs look good and last night I used my new sync gauges and dialed in the carbs. Running really smooth now at idle, cleaned up some noise coming from engine.

But I am still having the same problem. It is losing power at about 5-6000 rpms. When I took it out for a test run after the sync it runs great until you really try to get into the powerband. I also watched the rpms continue to climb at a couple of points while the power did not.

So its got to be either electrical or a main jet issue right? I also checked the charging system and battery is good and system is charging.

Where do I start again? Is there anything that I should do first. I had a friend tell me that the mains that came in the rebuild could be the culprit. I still have the originals and was thinking of pulling the carb bank, cleaning thoroughly again and putting those back in. Also I am having a hard time being convinced that it would be the main jet or the size being that everything is still stock?

What have your experiences been with this guys? I feel like I am missing something so simple, and so close to having a strong running bike.

I have done some other body modifications to it and working on getting it painted. I will post up some pics real soon in this thread.

Brian E
Ann Arbor Mi
Current Bike: 1979 cb750f supersport
Previous: 1978 Kz650
              1993 Yamaha Fzr600
              1978 Suzuki gs550
              1984 Honda Elite 125

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[bellison80]

I will check the timing. How can I tell if it is advancing properly? Last time I checked it it was on but when I increased throttle to check the other mark it got there but it didnt seem as smooth as Ive remembered on other bikes. That had been a thought of mine also that the advancer is in need of replacement.

[bellison80]

Also I had trouble getting the rubber float bowl gaskets to stay put the last time I put the carbs together so I used black silicone, very neatly but non the less am wondering if this is always a bad idea to do, and if this material could cause problems in the carbs.

[TOOLS1]

The best way to check the advance is with an adjustable timing light. Set the light to whatever the total advance is supposed to be, then rev up the engine, and the timing mark should line up.
TOOLS

[bellison80]

Thanks Tools, much appreciated.

[Re-run]

Remove that silicon! Much of it is not fuel proof and will flake off and clog the jets. I used superglue and that will just kinda dissolve.
As for timing, tools said to use an inductive timing light, full advance happens are 2500 rpm, it should NOT happen sooner though. If it does your springs need replacing. If it happens much after that, new springs again.

It seems though the mix is leaning out though. What clip position are you using on the main needles?
If it is the top setting, drop it down 2.

[shinyribs]

Ditto on the silicone with gasoline. It won't last and it won't dissolve. It'll come off in chunks and most likely get stuck in a place you don't want it. I know you dont want to hear that,but better now then later.

[bellison80]

Re-Run this helps out a lot. I think it was advancing way after 2500rpm. I will be tearing back into this thing in the next couple days and will keep you posted. I knew it was probably a bad idea to use silicone, live and learn. I did however find a set of wide paper type gaskets which should work much better for me. I also found a handfull of used advancers on ebay, my thoughts are to clean the carbs and then see whats what. As for the clip position, I dont believe these carbs have that adjustment, I could be missing that but I dont think so, thanks also Shiny.

[bellison80]

Re-run if your willing and have the time, here is a link to the carbs that are on my bike, take a look and correct me if I am wrong or missing something about the clips you describe.

http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revD.pdf


Thanks

[Lucky 1]

If it is running good on the road in the lower gears as far as accelerating then it is NOT the timing.
Never use silicone on the carb float bowls. If the bowl is actually leaking replace the gasket.

You did not tell us which exhaust you have.
Please tell us if you have the accelerator pump system
 on those carbs and if it is working properly.

If it is running lean at top end and you keep trying to make it accelerate it could seize up and throw you right over the handlebars or just slow down like the brakes are on and that is the end of the piston rings.

Syncing the carbs will not help with this problem.

[Lucky 1]

If it is running good on the road in the lower gears as far as accelerating then it is NOT the timing.
Never use silicone on the carb float bowls. If the bowl is actually leaking replace the gasket.

You did not tell us which exhaust you have.
Please tell us if you have the accelerator pump system
 on those carbs and if it is working properly.

If it is running lean at top end and you keep trying to make it accelerate it could seize up and throw you right over the handlebars or just slow down like the brakes are on and that is the end of the piston rings.

Use the proper rubber o ring on the float bowl !!!

[bellison80]

Has stock exhaust and the accelerator pump diaphram was replaced when I rebuilt the carbs.

[bellison80]

Not sure how to know if it is working properly

[Piute]

                               I just pumped it with my finger and watched the gas squart ,
                             Should be able to see with airbox off also.
                                             Get yo head down in there

[Re-run]

Sorry, you do not have a clip system. You have CV carbs and a piston with a needle and a diaphragm. So no clips on the tops of the needles. I am not sure why I was thinking that. Your advance is also going to be different as well.
I am not sure why I was on the sohc model but you have a cdi ignition and it should advance differently.
Look on page 3-6 of the manual and that will have you rev to 6000 and there are timing marks to look for. Your timing at 6k should be right in the area.
I would look at your air cutoff valves and make sure there are no cracks or holes and that they function right.
Page 48 of the manual shows some possible causes of a lean mix.
I would double check the spark plugs to make sure things are actually lean and not rich instead.

[bellison80]

Thanks Re-Run I will check that. I had a thought. I may have made a rookie mistake that could be contributing to this problem. Spark plugs are supposed to be properly gapped right out of the box correct? Well I think I may have misread the spec and gapped to big. I remember having to open them up when I put them in. I just got in the mail today a set of NGK Iridium plugs so after the holiday tomorrow I am gonna see what happens. From what I have read online to large a gap would be most noticeable at higher rpm's. If anything I have been learning a whole lot more about this bike and a bit of patience haha. I just want to get out and ride.

[Re-run]

Another thing on high rpm, cv carbs are controlled by the throttle butterflies. When you set these, take a strip of paper about 3/32nds wide or about 3mm wide or so. Set the carb with the idle knob so the paper just slides in and out, like a feeler gauge, on the side the opens OUT. Once you have that carb set, then continue. I think the manual denotes which order this is done in but I am not sure.
Once the plates are set right, idle should drop as long as you do not have an air leak.

[bellison80]

Update:

I must have gapped the plugs to big. I put in the NGK Iridium's and it started on the first push of the button, it never did that. Engine smooth all the way up. That was the most enjoyable ride I have taken since I got this bike late last summer. I didnt realize it was as fast as it is. I went through the city some and then took it out on a stretch of the highway, engine is sounding great. I still plan to pull the carbs back off and get the silicone off that I used on the float bowls and check a few other things. Now I can get some new tires and begin working more on the look of the bike. I am going to paint the tank flat black and put the chrome honda badges back on it. I am also thinking/planning a way to make a cafe style cover for the rear seat something that can be taken off with ease so I can carry a lady around. I have cleaned a lot of carbs this was the first bike in which I rebuilt the carbs and re-tuned it. I learned a lot.

Thank you, thank you, thank you to everyone that helped me in the past few months with my posts in diagnosing various problems.