Technical pop or stumble in exhaust

I am running a 1940 flathead with electronic dist, Isky max 1 cam, dual carbs and Edlebrock heads. New dist, coil, plugs and wires. I get a light stumble and exhaust pop at cruise. It accelerates smoothly but the problem is only at cruise speeds. At first I suspected the timing but have played with the timing and the problem still exists. I even did a compression test to make sure that I had no stuck or burned valves? HELP

 

Constant speed and throttle cruise?

If the cruise air/fuel ratio is too lean it can misfire.
Do you have a manual choke ? I'd pull the choke slightly to see if the problem goes away

 

Flat cam lobe possibly

 

One single pop, or more than one. If it is more than one, is it rhythmic?

 

New Cam. Rhythmic pop. I tried the choke thing. I found something interesting. It popped less when cold. When I pulled the choke it seemed to get a little worse. I checked the tail pipe and found a little black. I pulled a plug and found the electrode a medium grey but the edge of the metal on the plug was black. Too rich maybe? Could too rich cause a pop and stumble?

 

Look for an exhaust leak by the block.

 

Also observe down the carburetor throats, especially on the one near the blackened plug.

Look for a drip of raw fuel.

 

I actually just repaired an air cooled engine that was popping out of one bank and it turned out that it was leaning out. Busted intake gasket. Sounds *** backward.

It wasn't idling well and when I tried to adjust the idle jets and couldn't. I checked the plugs and found a white one.

That doesn't mean that it is the problem with the engine in question but it is something to think about.

 

Yup, them internal combustion engines are funny things.

Lean mixtures take longer to burn, and could still be burning when the exhaust valve opens. POP!

An overly rich mixture, couple with the air introduced by an exhaust leak can lead to light-off in the exhaust. POP!

Fun, huh?!

 

Check for lean , spark plugs are hard to read on pump gas without a O2 gauge , gas on the pump is different than it was 20 years ago ,they always look rich
Also make sure your oil has zinc in it, really only race oil and few others have it now ,IF NOT it will wipe out your brand-new camshaft

 

I'm going to say you're leaning out. Also if you have the distributor that mounts to the front of the timing cover those have to be set up off the car, they work a bit differently than a conventional distributor.

 

Lean

 

Slightly rich at part throttle will have a gurgley-blubbery sound in the pipe, and of course a dark plug if it is too cold a heat range.

Does it go away when you step on it a little? If it does, try one step leaner on the main. The new cam may have a stronger vacuum signal at part throttle requiring a leaner mixture.

 

It runs great when I step on it. Only stumbles at cruise. The cam is an Isky max 1 (3/4 cam). I will check for exhaust leaks at the manifold and probably lean it a couple jet sizes. Thanks

 

Diagnostics, is something that takes a while to get right. Some have natural talent and others learn it, but it is something that still takes a while to perfect. Most of what I know I learned my accident, I am afraid.

 

Yup.

Blow it up, and then do failure ****ysis.

Repeat until lesson has been learned.

 

Just be aware that lean mixtures take longer to burn. You might have to add a little advance to get max performance out of it.

And, watch your temperature. Leaner means hotter.