I have a 650 Mighty Demon carb. I also run a solid roller setup with a Victor Jr. manifold. My problem started a few days ago when I adjusted the lash on the valves. I found out that the lash was .010" too loose. This made the cam seem small. Once I tightened the lash up, I gained close to 10 degrees duration, now I'm using all of the cam. Its a 236 @ .050". Not huge, but decent. I now have a part throttle stumble. The car will also diesel for a second or two when I shut it off after driving for 30 min or so. The temperature guage always reads normal. The car isnt running hot. I have 20 degrees initial timing, 15 mechanical, and 15 vacuum.

The carb comes with 71 jets in the primaries. I have 69's in there now. Should I go back to 71's or go a little bigger, or do you think my problem is somewhere else? I also have the idle mixture screws set pretty lean. Can this effect it? Should I richen the idle up a little?

I have the same symptoms with my 750 Mighty Demon. The dieseling is because of the high initial advance. Not much you can do about that. I turn off the key with my car still in drive and that solves it. As for the stumble, I upped my jets a little and it helped. My guess is that the real solution is with the High Speed Air bleeds. I hurt my motor, so my testing is over for the year!

I would try upping the jets and see if that solves it. After that, you may want to put in a little smaller HSAB (maybe .004 smaller just to see what it does) and possibly raise the float level some (top line on the sight plug).

I did think about taking 5 degrees of vacuum advance out of it to see if that helps a little if the jets dont. It has never dieseled before adjusting the lash. I cant turn the motor off in park though, as I have a stick. That trick does work though. My brother use to have to do that till I got his timing better.

What about the low speed air bleeds? Is that only at idle, or are they active at part throttle, say cruising at 1800. Thats where I really notice the stumble. At 1800 though I barely have my foot on the gas. It doesnt take much throttle to stay at 1800.

At that low of RPM and throttle, it is probably more the IAB's than the HSAB's. In this instance, a jet change probably wont do much (although you could try it just to see). My stumble has been at more like 2500RPM and transitioning from flat road to a hill (small throttle opening). Still, smaller IAB's and a higher float setting will make the circuit more active and quicker to act.

One of the best purchases I ever made was to buy a pin vice and drill bit set so I could drill all my on bleeds and such.

I cured my part throttle stumble by opening up the secondaries so that about .015 of the transfer slot was exposed.. Have you tried opening up the secondaries yet? Luckially the mighty demon has an external screw for making this adjustment without having to remove the carb..

I have my primaries and secondaries opened equally. I also have the idle eze completely closed. I bought some slightly larger jets, so I'll try that and then richen up the idle mixture screws a little if the jets alone dont fix it. I asked the shop I went to if they sold the screw in air bleeds, but they didnt. So I cant try smaller ones.

Bigger jets made the car much nicer to drive. There was a very noticable difference. I went from 69's to 72's. The stumble is 95% gone. I think I can get the rest out by adjusting the idle mixture screws. I forgot that I havent readjusted them after readjusting the lash on the valves. I do notice a dramatic difference in the car at idle since adjusting the lash, so its possible that the idle mixture does need to be richend up a little for the extra air.

Even part throttle though, the car has a little more power. I had no trouble getting the car sideways going up a decent hill. And the secondaries never opened.

Sounds like you found the cure!

I think so. I didnt get to drive the car for a long time today. It was way too hot outside to do that, so I didnt get to see if the dieseling problem was still there. I'm guessing its not going to though. Monday I'm going to drive the car to work again so I'll find out then.