I took my 66 stock Hemi out today for the first time in a while. Fired up good and set into a fast idle and I let it idle like that for a few minutes. After it started to warm up it has this stutter in the fast idle of about 1500 rpm that vibrates everything and feels like a severe imbalance or something. It’s had this since I got it a few years ago but it seemed worse today in the cold weather. Kick off the fast idle and it idles smooth. It continues when driving off but doesn’t feel like a miss as there’s no bucking, just a constant stutter. As the engine gets good and warm it smooths out and feels fine. As far as I can tell, all stock Hemi, stock carbs, stock jetting, stock dual point recurved for about 16 degrees base timing, stock intake, stock choke with tubes in place.
I also have a 427 Corvette I’ve owned for over 35 years with an old school L88 headed 427 and a big cam. It also used to do the same thing upon a cold start, throttle up with a stutter. I learned to idle it a little bit and turn it off to heat soak a little while and that eliminated some of it when I finally drive off. In any case it also goes away after the engine gets warmed up. Last year I took the open plenum aluminum L88 intake off it (center divider in plenum milled out by GM and put a stock L72 (427/425) intake on it which is the same aluminum casting as the L88, but has a full height divider separating the upper and lower plenum tracks. This seems to have greatly, if not completely, eliminated the stuttering after a cold start. It’s at least much better.
This has me thinking it’s something to do with aluminum intakes, engine heat, and maybe poor gas vaporization and distribution on cool passages, and maybe plenum design too. However the Hemi intake has separate throttle bores but with the 2 carbs the combined inner, upper and lower plenums are probably pretty large. I have the heat tubes hooked up to the intake but somewhere along the line the heat riser valve was removed and shaft bores welded up so it takes a while to get heat up there.
Is anyone else with a Hemi dealing with similar behavior? Is there some other reason for cold rpm stuttering? Maybe choke tuning? Right now it’s probably set a little rich as the motor has to warm up pretty good before it comes off fast idle.
I also have a 427 Corvette I’ve owned for over 35 years with an old school L88 headed 427 and a big cam. It also used to do the same thing upon a cold start, throttle up with a stutter. I learned to idle it a little bit and turn it off to heat soak a little while and that eliminated some of it when I finally drive off. In any case it also goes away after the engine gets warmed up. Last year I took the open plenum aluminum L88 intake off it (center divider in plenum milled out by GM and put a stock L72 (427/425) intake on it which is the same aluminum casting as the L88, but has a full height divider separating the upper and lower plenum tracks. This seems to have greatly, if not completely, eliminated the stuttering after a cold start. It’s at least much better.
This has me thinking it’s something to do with aluminum intakes, engine heat, and maybe poor gas vaporization and distribution on cool passages, and maybe plenum design too. However the Hemi intake has separate throttle bores but with the 2 carbs the combined inner, upper and lower plenums are probably pretty large. I have the heat tubes hooked up to the intake but somewhere along the line the heat riser valve was removed and shaft bores welded up so it takes a while to get heat up there.
Is anyone else with a Hemi dealing with similar behavior? Is there some other reason for cold rpm stuttering? Maybe choke tuning? Right now it’s probably set a little rich as the motor has to warm up pretty good before it comes off fast idle.
