| Car is driving me crazy: Engine tuning troubles (with a carb) |
Subject: Car is driving me crazy: Engine tuning troubles (with a carb) by JLeatherman on 2009/11/25 21:21:43 I'm gonna repost the first section of this from CF, and follow that up with the new info from today. Posted 9-14 Having some trouble with my new 383 engine. It's in a C4, but since I carbed it I figure this is the better area to go for tuning advice. Engine is a relatively mild 383. 9.8:1 compression, ported stock 462 iron heads, cam is .465/.488 with 1.5's and 224/234 @ .050. Distributor is a vacuum advance HEI MSD from Summit (just a basic distributor, no MSD box or anything like that). Base timing is at 10*. So here's the deal. I had a Barry Grant carb on this engine and had nothing but problems with it, so I changed to the Holley StreetAvenger 670. Vac secondaries, with the heavier springs in the package (the yellows instead of the blacks). Got the engine barely broken in at this point, maybe 500 miles. It fires right up no problem. Choke kicks off and idle settles in at around 850. Seems just a tad rich at idle right now, but I'm not done fiddling with that yet. The engine produces good vacuum and all that at idle. But the preformance between idle and ~2000 rpms is terrible. When leaving a stop the engine is very sluggish to respond at lower rpms, and you really have to feather the clutch and be quick with the gas pedal to keep it from stalling. As it accelerated from off-idle to 2000 rpms the engine stutters and bucks a bit, almost like it's too lean, but it doesn't backfire. At around 2000 rpms the engine clears up and runs great from there to 6000. Once above 2000 rpms it cruises fine too, with very light throttle the engine maintains 2000 no problem and runs nice and smooth. From idle-2000 you gotta give it a lot of gas and it still bogs terribly. If you try to cruise at 1400-1800 rpms is when the stuttering is worse. It gets a bit better under heavy throttle at low rpms, but it's still pretty rough until 2000 rpms. So, I'm wondering where to head first here. I'm thinking either the distributor isn't advancing to where it needs to be until the higher revs, or the carb isn't opening the secondaries early enough. Like I said, it feels lean based on the stuttering, but it doesn't backfire. Fuel pressure is a rock-solid 6 pounds because it's an electric pump with a return-style mallory regulator and a gauge. Anyone got a suggestion? The car is down at the moment to have the trans rebuilt, so I can't hit the road again for a little while. Anyone had a similar problem? The Barry Grant had a similar problem of very poor performance down low, so I'm thinking it might be the distributor curve? Possibly the vac advance mechanism? It is adjustable, but I'm not sure how to set it statically since it's based mroe on engine load. I guess I could unhook it and see if the car behaves the same way? |
