CB750F Runs great and then it doesn´t.

[gridlock]

Hi !
Never used a forum before so bare with me.
I have a red CB750F from 1981 that has been sitting for 12 years. Got it running after a while.
I proceeded to get the carbs cleaned, bought a ultrasonic cleaner and had them gone through.
Nothing out of the ordinary, idles great, sounds very nice.
Planned a 140km trip that went super smoothly but quite high revs (5500-7000) all the way.
On my way home, last 30km i turn off the high way and on the stop sign it just dies. Will not idle.
Only starts with throttle.
Ran through the carbs again. Nothing seems clogged or wrong there.
Could i´ve burned valves cause of the high revs for a long time?

Thanks in advance for any answers!

 

[brettp]

When you say the carbs look fine, you checked the slow jet and the pilot?

 

[gridlock]

When you say the carbs look fine, you checked the slow jet and the pilot?

Yeah, took them out, flushed them with carb clean. Saw somewhat of a improvement but none in the running sense.
But I assumed it was the slow jets as you mention, the engine sounds fine as I rev above +3k.

 

[brettp]

Do you still have the vacuum operated fuel diaphragm?
I would inspect the air filter.
I would also check for vacuum leaks by spraying carb cleaner around the rubber boots on both sides of the carb.

 

[gridlock]

1. No I have that deleted and plugged the vac-lines. (I couldn´t find a new membrane sadly)
2. Air filter is brand new but I´ll give it another look.
3. New boots before the test-trip and it didn´t loosen up but will recheck it just because it could be that simple.

I´ve read a similar post on this forum where he ended up having gunked up carbs after all but recommendations were to check the valves?
I figured it couldn´t be a valve problem as it does run and sound well at high RPMs?

(Appriciate your time and answers)

 

[Loose Chain]

As Brett mentioned check for vacuum leaks. What shape was the fuel tank in since it sat for many years? Also don't assume the valves are good. If you have never checked them then now is the time. It is critical that the valves are adjusted properly. It will save you a ton of grief down the road.

 

[gridlock]

Noted on the valves, just never done valves so been something i've postponed.
The tank is in surprisingly good condition. Has original shape, spotrust inside I was gonna attend later on...
and that hits me, I never added a fuelfilter after the vaccum fuel-pump delete... Figured the filter on the petcock was sufficient.

 

[Loose Chain]

That is where I was going with the fuel tank condition. You need a filter. The vacuum valve delete should make no difference when it comes to filtering. Don't postpone a valve adjustment. You need to know where you are at when it comes to a valve adjustment.

 

[gridlock]

Thanks for the answers and pointers, I will attend this matter and postpone no longer.

On another note, If I ever get it running again.
What sprocket setup would you recommend as I found 6000rpms for 110km/h quite high?

 

[Loose Chain]

Do you know what sprocket sizes you have now?

 

[gridlock]

I believe I have 18/46
I'm looking for something reasonable between 110-120km/h ?

 

[Loose Chain]

I see you have an F model so it should be 18-46 in stock form. A C model of the same year has 18/43. So that would lower your rpm a little from where you are at now. These bikes would have been awesome with one more gear in the transmission.

 

[brettp]

Good call on the filter, Loose Chain. Check those carbs again. I hate to say it, but... yeah.