Engine bucking

[dgkipp]

What would cause the engine to buck on deaceleration (30mph-0mph)?
Engine runs fine otherwise.

[cadillac512]

It can be idle fuel mixture screw adjustment, and it can be ignition timing. I'd make sure the timing (and point gaps) is correct first then adjust idle mixture to see if it helps. A vacuum leak can also upset decel fuel mixture.

[40cpe]

Not to hijack this thread, but I have same problem. I have a 276" with an L100 cam that bucks from about 1200 rpm on deceleration. It has done it since it was built, about 7000 miles. I've had 2 different manifolds, stock and GM distributors, stock (CharlieNY built) and Rochester 2G carbs, all with the same symptoms. The only vacuum hooked to it is the distributor advance, and I've plugged that as a test. Following this thread closely.

[cadillac512]

Originally Posted by 40cpe

Not to hijack this thread, but I have same problem. I have a 276" with an L100 cam that bucks from about 1200 rpm on deceleration. It has done it since it was built, about 7000 miles. I've had 2 different manifolds, stock and GM distributors, stock (CharlieNY built) and Rochester 2G carbs, all with the same symptoms. The only vacuum hooked to it is the distributor advance, and I've plugged that as a test. Following this thread closely.

You may have to live with that. The increased overlap of the cam grind is going to alter the low speed fueling characteristics and may well cause the decel buck. You've tried most fixes already-I'd hazard a guess that altering idle circuit air bleeds in the carb might help but it would be experimental and hard to reverse. It's a challenge for sure.

[40cpe]

Originally Posted by cadillac512

You may have to live with that. The increased overlap of the cam grind is going to alter the low speed fueling characteristics and may well cause the decel buck. You've tried most fixes already-I'd hazard a guess that altering idle circuit air bleeds in the carb might help but it would be experimental and hard to reverse. It's a challenge for sure.

Thanks for your reply. I've found that increasing the idle speed to about 750 helps a lot, but takes some of the "smiley cam" sound away . I can set timing at about 6-8 degrees AFTER tdc and stop the bucking. Using timed vacuum for the advance (I'm using a SBC distributor with adjustable advance limits) brings timing back up upon accelerating but has negative effects on cooling at idle.

[cadillac512]

Sponsored Links (Register now to hide all advertisements)

Interesting. You've done your homework. The retard at low speeds mimics (to a point) a richer idle fuel/air mix but the challenge is to make it idle nice,keep the plugs clean and accelerate well. I'm also running a distributor like yours and really like it. Hmmm....how to retard the timing 10 degrees on decel only?

[40cpe]

Originally Posted by cadillac512

...how to retard the timing 10 degrees on decel only?

That might be a little tricky.


I didn't degree the cam when I installed it and that bothers me. I've wanted to pull the engine and do that but my get up and go has got up and left.

[40cpe]

deleted