Flat spot hesitation when accelerating?

[melhilburn]

292 with Rochester single barrel, HEI ignition - Starts fine, idles perfect - Under hard acceleration will hit a dead flat spot for a split second - After that no problem - Carburetor is new rebuilt unit - I don't have a timing light so I can't make an accurate statement on timing - Tuned it be ear - Could this be a result of a bad vacuum advance?

[i82much]

These days I would strongly suggest getting an O2 sensor. So much easier to see if you have a fuel curve issue. My guess is a lean condition, although that is based largely on experience with Holly 4150's so perhaps the Rochester has other tendencies.

[TJ's Chevy]

Originally Posted by melhilburn

292 with Rochester single barrel, HEI ignition - Starts fine, idles perfect - Under hard acceleration will hit a dead flat spot for a split second - After that no problem - Carburetor is new rebuilt unit - I don't have a timing light so I can't make an accurate statement on timing - Tuned it be ear - Could this be a result of a bad vacuum advance?

If your definition of Flat Spot hesitation is the same as mine I would say either timing or vacuum. What RPM does it hesitate at? Might need to check your gaskets like the carb gasket and intake/exhaust manifold to head gasket. Make sure you snug everything down when the engine is cold. Have someone rev the engine for you and see if the vacuum advance is working right by seeing if the little arm outside the distributor is moving towards you when reved and will move back when left at same rpm or dropped back to idle. But does it do this at only say like 3,000 rpm? That would help if you know.

[TJ's Chevy]

Could also be a lean factor. A good way to test if you truck is running rich/lean is to fire it off when cold and DON"T warm it up. Right after you fire it up drive it as hard as you can. Stop and shut off the motor and pull one of the plugs. If their black yer running rich if their like new and maybe even a slight bronze like discoloration yer running waaaaay to lean.

[melhilburn]

Thanks for the reply - I will check to see if the vacuum advance arm is moving this afternoon and let you know what I find - Also, I will also give it run and take a look at the plug.

[melhilburn]

If I have a lean condition what would my options be for dealing with it

[i82much]

Originally Posted by melhilburn

If I have a lean condition what would my options be for dealing with it

Not sure on a Rochester, but on a Holley probably the most common problem is improper float level. Set it a little higher than you think and see if it changes any thing.

[Sillyoldman]

Loaded question. It could be Quadbog, vacuum leaks, bad or incorrectly gapped plugs or wires, deteriorated fuel lines or worn out vacuum advance weights and pins among other things. I'd start by setting the timing correctly and inspect the internal workings of the distributor.

[TJ's Chevy]

Originally Posted by melhilburn

If I have a lean condition what would my options be for dealing with it

There is an idle mixture screw on the carb...turn it clockwise till the engine starts to die(do this at operating temp) then turn clockwise till engine frees and gains max rpm, once max rpm gains turn a little more and that will richen it just a tad. But make sure the motor is at operating temp before you do any adjusting.

[Wrenchbender Ret]

T.J. means counter clockwise till engine gains max rpm.
This is a good suggestion. Try advanceing the timing a little.
If everything else is good you may need for someone familiar with carbs to check the float level & metering rod adjustment.
Most "reman" carbs are not right.

[davbowen]

check the float levels and accelerator pump.

[TJ's Chevy]

Originally Posted by Wrenchbender Ret

T.J. means counter clockwise till engine gains max rpm.
This is a good suggestion. Try advanceing the timing a little.
If everything else is good you may need for someone familiar with carbs to check the float level & metering rod adjustment.
Most "reman" carbs are not right.

Thanks for correcting me! lol!