Stumble and Backfire

[tartan18]

For the past week my engine would stumble every once in a while -- just a quick and occasional slight "hic-up" while driving steady at 35 mph. Today it stumbled again but this time there was a single backfire (exhaust) then everything was normal. I have a 1975 with Mark's Computronix ignition system and a Empi DFEV 32/36 carb. Until now the engine just purred. Any suggestions on what I should do to diagnose this problem?

[SLOSpider]

Check to make sure your carb is still bolted down tight. While its idling you can grab the carb and try to wiggle it around and pull on it straight up. If the idle changes at all its loose somewhere at the base. check plugs and make sure they are not fouled.

[tartan18]

Carb is tight (no leaks) but I did replace the plugs. Old plugs were heavily sooted which indicates a rich mixture. I've adjusted the idle mixture (a bit more lean) so hopefully this corrects the problem. Unfortunately I will be away from home for the next week so I won't be able to do anything more until I return.

[So Cal Mark]

you may need a smaller low speed jet. Adjusting the idle mixture will only help at idle speed

[tartan18]

Is the lower speed jet also called the Primary Jet? I can't find a low speed jet on my DFEV diagrams. What size jet should I order? I bought it from you, Mark so I don't know what size is in the carb now. Here is a pic of the plugs I pulled and replaced yesterday.

[So Cal Mark]

#3 doesn't look too good with those deposits. It may be pulling some oil past the valve seal/guide