Vintage and Classic Hondas

78 CB750F hesitates / cuts out when revving form low RPM

_‹ Previous Topic Next Topic _›

Classic

List

Threaded

5 messages Options

Rockwork

78 CB750F hesitates / cuts out when revving form low RPM

What would cause my engine to hesitate and even cut out when I rev from low idle (~1,000 RPM)?

Based on recent counsel for signs of carbon fouling, I adjusted my carbs to run leaner. Runs great on the road, but I have to idle at 1200 - 1500 RPM, otherwise it cuts out when I rev from a low idle.

"To strive, to seek, to find, and not to yield."

Re-run

Re: 78 CB750F hesitates / cuts out when revving form low RPM

Administrator

Clean the plugs, start and idle the bike at low rpm for a few minutes then check the plugs again. Look at the insulator for color.

Otherwise, it sounds like the accel pump isn't working.

The ride IS the adventure. The destination is just to get gas!

TOOLS1

Re: 78 CB750F hesitates / cuts out when revving form low RPM

Administrator

In reply to this post by Rockwork

The F-2, F-3 were really lean from the factory. If anything, you needed to go richer, especially with today's ethanol gas. I just re-jetted one of these engines, and had to go to 118 mains, and raise the needle 1.5mm to get it to run correctly on this crap gas we have today, and that was with the stock airbox, and exhaust. I would also be checking for a vacuum leak as that would also cause the problems you describe.
TOOLS

Life is not about the number of breaths, you take, but the moments that take your breath away.
I don't have an anger problem. I have an idiot problem. Hank Hill
Never confuse education for intelligence.
Happiness is a belt fed weapon.
I just can't imagine what could go wrong.
No fire? No explosions? So whats the point of your story?
Those who are too smart to engage in politics are punished by being governed by those who are dumber. ~Plato
It couldn't be done, but the darn fool didn't know it, and did it anyway.
We all got problems. Ksharp
I like vintage bikes because they take me away from the clutter of technology that I work with everyday and back to a simpler time of mechanical elegance and simplicity.. "ninadm"
Darkwing Duck: The worst part of public transportation is the Public.
"That is awesome shit there" Re-Run
"Fear nothing, attack everything" Eric Berry
" Oh, you read that on the internet? Clearly it IS a massive problem. Of course it CAN’t be normal operation."

1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)

Rockwork

Re: 78 CB750F hesitates / cuts out when revving form low RPM

Thank you for the feedback. I'm still a little richer than the recommended setting in the manual. Vacuum leak sounds plausible, since I've experienced this before. I'm very confident with the boots on the engine side since I boiled them in wintergreen per suggestion last year (great advice), but the connection of the boots on the airbox side might not be as tight. Are air leaks on the airbox side as significant? I may need to get some new clamps.

Also, is the the timing affected by the change in mixture?

"To strive, to seek, to find, and not to yield."

TOOLS1

Re: 78 CB750F hesitates / cuts out when revving form low RPM

Administrator

The airbox side is not that important, and no the mixture cannot affect timing.
TOOLS