High rpm sputter

[Pwnypower]

Hey guys i have a cb750c from 81 which has the dohc and when i ride it it sputter from 7k upwards. When i give it full throttle it cant get higher then 7k but when i turn down the throttle it again starts increasing beyond 7k.

I cleaned the carbs and checked the spark plugs. Can you guys point me in the right direction of which i need to search for the problem to get solved. I suspect ignition, because syncing isnt an issue on full throttle??(because all the throttlebodys are wide open?)

Thanks guys!

 

[amc49]

Set the valves.

I know it sounds crazy but what commonly does it.

If you do not have the OEM airbox on it or the carbs have been jetted up then that will do it too.

'...when i turn down the throttle it again starts increasing beyond 7k.'

The slides are not lifting quite fully and then it runs rich even if jetted slightly lean. A CV carb phenomenon. Either condition above can do it. When you slightly close the throttle then the increase in vacuum lifts the slide a bit more to pull out better. When the valves are not set many of them can be at close to zero to make you lose compression, that shows up as dropping the slides too.

I've been there a hundred times. The throttle bodies are not the limiting factor on CVs, the slides are, the slides can be half closed when the butterflies are 100% wide open.

You'll swear the sputter is ignition but the over rich condition from slide not fully open does it.

Run a compression check and it will show you the valve leak issue, looking for 170 psi, most will not get there.

 

[Pwnypower]

Thanks amc49!

I checked the compression and they were all around 170-175 psi. The lowest was 167 psi with throttle full open and a couple of starting turns.

How do i get the slides to open better? I cleaned them and checked them on how smooth they are.

I do suspect that the rings in the slides(those round black ones) are way to stiff to even work as an oring or something.

I could take off my valve cover and check the clearanced of the inlet and exhaust valves, but to check them i need to make some time and i dont have much of that rip..

Thanks for the reply! Hopefully i will get the bike to run better again!

 

[amc49]

I'd instantly suspect the compression gauge quality if I got all numbers like that, they just don't really do it unless the engine is under 5000 miles. I've seen defects at 2000. But whatever. If it IS the compression you will not get the issue to stop with ignition. BTDT.

The black rings you talk about have nothing to do with slide 'sealing', the slides seal at the OD of the slide top biggest diameter, the grooves cut into it do it. The seal there is differential air, and why the slides on those are so much more sensitive to engine condition that rubber diaphragm sealed ones are. The black rings are wide open bumper stops. They change color (thickness) depending on the carb bore to match wide open setting on the different carbs (28,30,32,33).

If nothing is messed with on the bike including the airbox then the slides only improve opening with engine condition, the engine MUST be sound and sealed tight to pull them open, it cannot be over-emphasized. Engine draw is what yanks them open all the way. Messing with stock airbox can kill that as well. Gone to use pods I guarantee the slides will not open fully. I take note of no word on that, a common messup that puts people where you are now. The rolling off of throttle to run higher rpm screams it.

It COULD possibly be the plugs but it would be the odd bird to do so, they have to be pretty bad to do it. On other engine types I always go to plugs first but not on these. Far too many never ever touched the valves at all and then throw $$$$ at why they won't pull out to 9500 and wasted. Most engines the valves get looser but on these they tighten up generally with use and based on the whopping cam looseness and crap valves that were only barely surface hardened at all. They then recede into head to leak with any use and then the engine misses both due to compression leak and slides too rich from not opening up. They if not adjusted then burn exhausts at the drop of a hat. The lead indicator of that is the top rpm not pulling out, it will get lower and lower. How most of them die. Honda sped that up by saying in the service manual that the valves set at .002"-.004", and many of them burn valves at .002", been there myself. The clearance has to be opened up to 4-6 to make them live. I've seen brand new ones set at .003" be at .001" by the 600 mile initial new bike check and those already burning. Why the Honda spec sucks.

 

[Pwnypower]

I will check the valves out because they could maybe need new clearances. Will get back on that and the results!

The compression gauge is checked yearly(is from motorcycle shop) they checked them when the bike was hot though.

Dont know if that gauge is right.

Thanks for the help and i will post results soon!

 

[Pwnypower]

@Amc49,

I did detect some leakage from the boots that go from the airbox to the carbs, could these be the culprit along with maybe the valves? I haven't found the time to check them though but will get to it soon!

 

[amc49]

The airbox cannot leak up to the paper filter. Even boot leaks allow vacuum drawdown that opens the slides quicker to fall off. The individual intake tracts share vacuum in an accumulative fashion to all together raise the slides faster and further. Why pods instantly screw that all up, the pods flow too easily and that very sensitive vacuum ramp-up disappears. The paper used in the filter even aids the slides opening, it is a needed restriction and even a different say foam filter in the airbox can make less power.

The engines are really sensitive to intake length too, the boots give roughly a 2 inch extension to carbs and what the engines like. Boot leaks will affect that too. Look at all the racebikes, they all use pretty much 50 mm. stacks on the carbs if they run hard.

 

[Pwnypower]

Yes but the filter is leaking from the airbox to the carb boots. I sprayed starter fluid there and it ramped up, i am going to seal that, check the valves and then take it for a last spin when the valves are at around 0.005" or between 0,10 and 0,15 mm in and ex.

We will post an update when i did it!

 

[CollinDAP]

i have had the exact same issue with my 81 cb750k has about 7k miles. the previous owner said he rebuilt, synced and tuned the carbs, and it honestly ran great except for breaking up around 7K rpm. i found the boots from the filter to the carb were cracked and leaking so I replaced them. it ran the same as before for a little while, then started to idle very lopey and did not want to accelerate at all. almost like it was running out of gas, only it wasn't. plugs are fouled now, weren't before. not really sure where to go from here... keep us posted on what you find... I'm lost...

 

[amc49]

Flooding maybe from ethanol in the fuel. Flooding anyway. The ethanol leads to floats sticking. Need new plugs now, once good and wet they tend to miss after that.

 

[CollinDAP]

There isn't really a way to avoid ethanol in fuel though, is there? and if the carbs were cleaned recently, how could they get stuck that quickly? i can replace the spark plugs but wont they just foul up again?

 

[lafona93]

My 1977 cb750 was sputtering at mid to high range throttle. Ran like shit. But fine at idle. It didn't give me issues tell it was under load. I went through the carbs. That wasn't the issue. I've had them off and gone through so many times in the last 2 years, chasing issues, I'm ready throw the kick stand up and drop the bike on my face... Carbs end up turning out fine. Carb sync is good. Air is good (I had issues with that but fixed it. Keep your original air box on!! Don't drill to many holes it ). I checked the petcock and screen in the fuel tank. They're fine. Ended up being my ignition system. First time, it was my points. Replaced them with new ones off of the cb750 website. Worked great for a day. Then it started happing again. They were cheap shit " made in Taiwan" points. One started sparking off the top. Replaced them again. No difference. Was told it was probably the condensers. But they were replaced a year ago. So I didn't know wtf... I just ended up ordering a Dynateck ignition system for 250 bucks and wham bam afghanistan! Not only did the problem go away all together but the bike has never ran better!
Anyways. Check your ignition system.