Carb tuning problems...recomendations welcome
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Safety Car
Joined: Jul 2002
Posts: 3,770
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From: St Louis MO
Carb tuning problems...recomendations welcome
I've exchanged my old leaky Holley 650 dp for a new Holley 850 dp. The problem is, I can't seem to get it tuned right. For example, I set the idle last night at 800-900 rpm and this morning on the way to work it started idling at 1200-1300 rpm. Also, when taking off from a stop, it will puttputt until about 1500 rpm (thats the exhaust note: puttputtputt), then sound right from 1500 on up. If I punch it when cruising at about 2500 rpm, it will buck and cough a few times and gradually catch up and go. If I punch it at 3000 it's a toss up, sometimes it'll take off and other times it'll buck one time then go. If I lean on it gradually from any rpm there's no problems.
I thought this coughing and bucking was a low float level but the problem persists even after I adjusted it. Plugs are AC R45s with bout 100 miles on em. I haven't changed the timing since the carb swap. The 850 seems to idle best with both idle mixture screws out 3/4 of a turn. I have the same K&N on it that I had on the 650. I have an electric fuel pump that is putting out about 6-7 lbs. The engine is a 468 with a slightly larger than stock cam (no rumpity-rump but a healthy sound), torker 2 intake, msd 6, headers, true duals with what I think are 40-series Flows.
I thought this coughing and bucking was a low float level but the problem persists even after I adjusted it. Plugs are AC R45s with bout 100 miles on em. I haven't changed the timing since the carb swap. The 850 seems to idle best with both idle mixture screws out 3/4 of a turn. I have the same K&N on it that I had on the 650. I have an electric fuel pump that is putting out about 6-7 lbs. The engine is a 468 with a slightly larger than stock cam (no rumpity-rump but a healthy sound), torker 2 intake, msd 6, headers, true duals with what I think are 40-series Flows.
Race Director
Joined: Mar 2001
Posts: 11,322
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From: Living in the Hartland
Re: Carb tuning problems...recomendations welcome (Brettmc)
you might need to adj your acc pumps, sound like you are not getting enough fuel from them when you first hit it. Take a look at Lars' tuning paper in the tech section and follow it, worked great for me after I rebuilt my 75 dp. One thing I did to make it easier to tune was disconnect the sec linkage so I could deal with just the primaries to start. Then once I got the primaries dialed in I hooked up the sec's and tuned it again. I also was using a Edelbrock O2 sensor which helped a ton. I also had a nice Holley tuning book which helped with getting the pump shots adjusted properly, you can get diff cams and use diff holes on them to aid in getting the pump shot timing, volume and duration correct. I like to tinker with stuff so it was fun but truth be told I am switching to TPI so I can tinker some more. :)
