Engine a bit "thumpy" during warm up

[boybach]

Its a '77 403, carburetted, 43,000 miles.

On a cold start I can only describe the sound as "thumping" at idle, but if I increase revs slightly the motor smooths out as it does once engine is warm. This is the only time it sounds like this, otherwise engine is perfectly smooth running. Subsequent (warm) starts are also smooth.
Is this normal at cold start?

TIA

Larry

 

[Christo]

Maybe a misfire? Check plugs/wires. If that checks out, a compression test might not be a bad step.

 

[RoadTrippin]

Mine does that when the engine is cold and the choke doesnt engage and it idles low. Once warmed up, its back to normal smooth self.

 

[boybach]

Mine does that when the engine is cold and the choke doesnt engage and it idles low. Once warmed up, its back to normal smooth self.

it's always done it btw ...so maybe a choke issue or just leave it?

 

[RF_Burns]

Its a '77 403, carburetted, 43,000 miles.

On a cold start I can only describe the sound as "thumping" at idle, but if I increase revs slightly the motor smooths out as it does once engine is warm. This is the only time it sounds like this, otherwise engine is perfectly smooth running. Subsequent (warm) starts are also smooth.
Is this normal at cold start?

TIA

Larry

Mine does that from a stone cold start-up. No amount of fooling with all the EFI settings fixes it. I just need to rev it up and it clears and runs perfect the rest of the day.

I believe its because I have a blocked crossover on my intake manifold. When stone cold some of the fuel condenses on the cold manifold runners. There is no exhaust to quickly warm the intake manifold. Revving it up clears the condensed fuel and then it's a happy camper for the rest of the day. On subsequent starts the manifold is still warm enough to prevent the fuel condensation.

 

[boybach]

So my buddy saying "they all do that when cold" wasn't BS ing me after all eh?

 

[RoadTrippin]

it's always done it btw ...so maybe a choke issue or just leave it?

I know when the choke works right, it starts and idles a little over 1k RPM. I let it sit for a few minutes then quick blip of the throttle and it settles down nice and smooth at 900ish.

Ive had cars with Holley carbs w/ no choke that would just do the 'thumping' until they got some heat into them. All those emission devices/methods to get heat into the top end were there for a reason I guess