birdsmith

Hey guys, has anyone out there experienced this? My '70 has lately developed a disturbing tendency to idle around 900-1000 rpm when warm. It starts normally when I set the choke and after a couple minutes the choke pulls off fine and the idle is normal at about 700-750 but as the engine gets up to its normal operating temp the idle begins running away at stoplights. Initially I thought it might be a simple idle stop adjustment so I backed off the stop screw; it's now backed all the way off (throttle plate completely closed) and it still won't idle properly when hot, and also has a tendency to diesel on shutoff. Messing with the mixture screws doesn't seem to help other than affecting idle quality. There are no [known] vacuum leaks (but I will investigate further), the carb is a Q-Jet that I purchased from Zip several years ago that was listed as correct for that engine (base-model 350/300, decompressed to 9:1 with mild Summit cam). Any takers? Lars??

79swtl82

I had the same problem on my 79. Every time i would come to a stop the engine would idle high. It tunerd out to be the power piston in the carb (QJet) would sometimes get stuck in the up position. Rebuilt the carb and no more problem but make sure you check all of your vac. line connections first. Good luck

Disco_Mike

I had that type of idle problem also. It was either too high or too low.
I traced it to a vacuum leak at some of the hoses. Check those out also.

7T1vette

Sounds like your idle circuit is damaged somehow or your carb set-up is overpowering it.

birdsmith

Thanks for the responses, guys. The 'power piston' suggestion looks intriguing; I'll definitely take a look at that one.

65GGvert

Take a look at your timing, too. If it's too high it will affect the minimum idle and also cause the diesel effect. Just a thought.

birdsmith

I already played with the timing; right now it's at 10 deg. initial/ 34 total and had no real effect on the idle speed issue (did affect idle quality though). I'm gonna pick up a carb kit for it sometime in the next few days. Thanks again for the help.

birdsmith

Finally got my carb kit from Summit a few daze ago....usually they're quick but this one was backordered so it took like two weeks to get. It was a disappointment to say the least. All it came with was a few gaskets (and not even an airhorn gasket at that), needle and seat, and some little clips. That's it. I took the carb apart, cleaned it, and put it back together with what few new parts I had but didn't really notice anything wrong except that the power piston had a pretty stiff spring underneath it. I put it back on the car and it ran even worse than before and was leaking all over itself, so off it came again. I reset the float to 3/16", rechecked everything, and put it back together. It seemed a little better this time but was still idling way fast and after a few seconds started pissing gas all over itself again. so this morning back off it came for one more attempt (after that it was call Lars!!). I took the ancient carb that was on the car when I first bought it 11 years ago, took it apart, and started comparing things. When I removed the power piston from the old one and compared it to the bought-from-Zip-and-rebuilt by-Holley one that I was trying to fix I noticed that the return spring on the new one was twice as long and had three times as many coils in it. More disturbing, though, was that my 'NEW' power piston was missing about 1/4" of the plunger from its bottom and looking at it under a magnifier it appeared to have been cut off with a pair of dykes(!!). Since the old carb had a nitrophyl float I swapped that into the new carb as well as the power piston. Put it back on the car and EUREKA!! It idled smoother than ever, actually responded to throttle plate adjustments and once I got it warmed up took it out and stomped on the throttle in 2nd and 3rd and suddenly the thing has MEGABLAST acceleration like never before. In hindsight I could easily have just cannibalized the needed parts from the old carb and foregone Holley's useless 'kit', but oh well. Car's fixed and I'm happy- I've been chasing this crappy-idling, dieseling, gassy-stinking gremlin for the better part of a year now and have finally successfully rebuilt a Quadrajet for the first time in my 50-year life. THERE IS HOPE!!