Flat Spot
 

I have a French built flathead V8 with the Zenith carburettor it runs perfectly on tick over and above 20 mph but is hesitant at low speeds, I’ve overhauled the carburettor but this hasn’t cured the problem, I’ve made an adapter and fitted a Holley but it’s just the same , it’s timed to the crank mark . Any ideas?

Re: Flat Spot
 

Sounds like what you need to do is a add some more initial timing. The timing mark on the crankshaft may be only 4 degrees advance I would go up to 10 degrees initial advance then see if that helps the flat spot go away. More initial advance is not a bad thing even with a stock flathead.
Ronnieroadster

Re: Flat Spot
 

I was a smog mechanic. We had the advantage of running the cars on the dyno while they were hooked up to an oscilloscope scope. It is strange that it has a misfire at lower rpm and then it goes away at upper rpm. One thing we would do is run the car up to the point it started to misfire on the dyno and then give it little shots of propane to richen the mixture and see if it straightened out. That was a way of determining if it was fuel or ignition. We used small propane bottles with a momentary pushbutton valve on the top of it. It was a State mandated tool for finding vacuum leaks during the smog tests. You could also extend the hose from inside the car to the carb and use it while you drove around to see if it had any effect while driving.

It's an expensive tool but it's really neat for finding vacuum leaks. It's a lot cleaner to spray propane around your carb looking for air leaks than it is to spray a liquid spray. Plus, you can use it while driving down the road. You might be able to make something up for a lot less and go for a ride, give it some propane and see it straightens out. If that don't straighten it out, then I would assume it's an ignition problem. You're at a huge disadvantage without a scope to watch the pattern.

https://www.toolpan.com/OTC-Tools-71...3wmyvqUfxT04Gx

Mart has a good idea, check the ignition advance throughout the rpm range.

Re: Flat Spot
 

The French distributor is vac advance only, no mech advance. I had one where the vac advance had failed. So I had no advance. Make sure yours is working. I'm not sure where the vac source is on a French motor but it may not be correct if running a Holley. I have plenty of French Flatty experience but all mine have been converted to 59A clones. Mart.