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I recently replaced an old, incorrect Solex carburetor that was on my M38 when I bought it with a correct rebuilt YS637. When I installed it on the engine it fired up quickly and idled great. Before I installed the carb I set the idle mixture screw at 1-1/2 turns out as an initial setting. Once the engine was warmed up some I tried to do a final setting on the idle mixture screw, but when I started turning the mixture screw futher out there was no changes in the engine idle quality or speed. I eventually screwed the mixture screw completly in to a point where the needle seated and was completely shut off, with still no change in the engine idle speed or quality; the engine idled fine. I then revved the engine to evaluate throttle response and it revved quicky and strong. However, when I released the throttle and let the engine slow down quickly from being revved, the engine died. It started right up again and idled fine, but when I revved it again, the same thing happend, it died when I released the throttle back to idle. I also noticed that if I revved the engine up and then slowly redcued the engine speed, it returned to a steady idle. I took the carb off the engine and verified that the idle mixture opening in the carb body is not blocked.

Since it idled fine I then took it for a test drive and under acceleration the carburetor worked great, but when operating under at a steady load / speed, there was some "surging".

Any thoughts on what might be going on with those two issues?

If you examine this illustration closely it will be clear that if the needle's adding or subtracting of additional fuel just below the throttle plate has no affect on the idle speed then there is already too much gas there. Two common reasons for that are:

1-throttle plate is open too far causing idle mixture adjustment to be ineffective.

2-Bowl float level is too high causing an overflow making the system too rich.

Thanks for the input Wes. When I took the carb off to verify the idle circuit passages were not blocked I had also looked at the throttle plate as a possible culprit, but the setting appeared to be ok. The surging at slow steady speeds made me suspect the float level might be incorrectly set, so thanks for the confirmation. I'll check the float setting next.

I like to make sure the idle speed adjustment screw does not hold the plate open at all when starting the the idle mixture adjustment.