TOPIC: 1964 Merc 650 surging at idle

Rebuilt/cleaned carbs a few weeks back... made a big difference - it's now starting right up after sitting for a while (thanks guys). But I'm surging at idle - making it hard to set a good consistent idle speed and will sometimes stall in the water running at idle. Runs fine while cruising/WOT.

What should I looking at? Fuel pump issue (bad diaphragms)?

Thanks.

If it runs well at all rpm ranges & WOT I wouldnt worry about fuel delivery system, pumps bad diaphragms etc

this rpm surge is it erratic or rythmatic

I assume you have adjusted the idle mixture (richen) as best you can get it to run?

were these carbs soaked overnite in a carb tank, then blown out with compressed air?...

run the motor again till it dies, then read the plugs see if there is a color difference between plugs to help isolate the problem

it may also be a weak spark at idle from resistance in the cap, a single plug wire, plug wire end connections or even a bad plug

check the continuity of the plug wires or do you know how to bench test your mag with a drill motor?
if you connect a drill motor to the mag shaft rotate it fast and find a great spark, then slow down the rpm, you may infact see 1 or more plugs not firing as well or consistently at idle

I just rebuilt 3 mags this weekend, drill test showed 2 had a weak/intermittent spark at idle...which solved poor idle

I want to say it's both erratic and rhythmic... it definitely cycles on a few seconds, off a few seconds... but those few seconds can vary if that makes any sense?

test your mag first for strong consistent spark at low rpm

also ck the scavenger tubes for cracks etc. fuel will puddle in the lower cyls and surge at times as the vacume over takes the air leaks, also note when you set your low spd idle jets you must do it Idleing in gear not attached to anything,like a doc or trailer etc.