Big block rich idle - Team Camaro Tech
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post #1 of 12 (permalink) Old Jan 19th, 03, 04:16 AM Thread Starter
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I have been having fits with a brand new 464 that runs rich at idle. I have replaced the distributor, checked the charging system to ensure proper voltage and replaced three carbs along with the sixth set of plugs. The engine runs rich at idle and I can not lean it out no matter what on the idle mixture screws. I am at my wits end. It is a brand new engine and all new components. The first two carbs were new Holley 750 vac secondarys. I tried changing the front power valve to a 3.5 but that did not help. Yesterday I installed a 750 double pumper and a another set of plugs. Same thing - it will run you out of the shop. Help
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post #2 of 12 (permalink) Old Jan 19th, 03, 04:37 AM
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Hello there, we need a little more info: timing at idle, cam specs, how much vacuum, idle speed, and what the plugs look like. But just as a shot in the dark assuming everything is adjusted as it should be. There are several things that could be going on. First, you could be spilling a little fuel out of the boosters either because of incorrect float adjustment or too much fuel pressure (how much are you running BTW). Second, Some cam, intake, and head combinations require a small hole (like 1/16th of an inch) be drilled in the primary butterfly to lean it up a bit. Third, your air-bleeds for the idle ciruit might be a bit restricitve, whether by actual orifice size or dirt that has gotten in the holes. The last thing that comes to mind is that we have gotten so used to these new cars that are essentially "fragrance free" when we start working on the old ones we forget that with no emission equipment on a high performance big block there are certain "aromas" present. Sorry if that got a little long winded but I hope it helps. Scott
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post #3 of 12 (permalink) Old Jan 19th, 03, 06:19 AM
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How about too much fuel pressure? It seems like if you have put three carbs on it you might want to check the fuel pressure.

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post #4 of 12 (permalink) Old Jan 19th, 03, 09:04 AM Thread Starter
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It was built to the specs of a LS6454 by John Lingenfelter with the exception of open chamber heads instead of closed chambers. I set the initial advance at 4 degrees at 750 rpm's. It is pulling 15 inches of vacuum at idle. The plugs are wet with fuel and eventually will not even start.
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post #5 of 12 (permalink) Old Jan 19th, 03, 07:12 PM
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check your throttle blade allignment. it should be at a 90 degree angle with the bore of the barrel. there is a little slot engraved in the bore like the intake on a 2 stroke cylinder. it is an air feed for idle. if it is running too rich, then there is not enough air volume traveling through it and it needs to be increased. is it smoking black? or smoking out the tailpipes at all? an easy check for this is to blow on the throttle blades until the smoke goes away. if it goes away, then i would take the carb to the speed shop to have them tune it or do it yourself if you have the right stuff. or, as psalm69 said you can drill a hole into the blade and it will fix it just as well.
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post #6 of 12 (permalink) Old Jan 20th, 03, 12:50 AM
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Instead of drilling holes in the primary throttle plates, I'd open up the secondary plates with their stop screw on the pass side of the base plate.

One could insert feeler gauges between the sec plates' set screw to set the opening, then adjust the stop screw later.

This would allow the primary plate to close futher allowing more mixture adjustment. With 15 inches now, he should probably end up with 17-18 inches when done.

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post #7 of 12 (permalink) Old Jan 20th, 03, 03:24 AM
 
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Have you looked for vacuum leaks? I always go with the obvious and work my way down the list. My car was running really, really rich when I first got her started and it turned out to be a vacuum leak at the intake runners. Just my .02

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post #8 of 12 (permalink) Old Jan 20th, 03, 04:49 AM
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<BLOCKQUOTE>quote:</font><HR>Originally posted by Everett#2390:
Instead of drilling holes in the primary throttle plates, I'd open up the secondary plates with their stop screw on the pass side of the base plate.
<HR></BLOCKQUOTE>

The problem with that is when the throttle blades are tipped in the secondary the idle transfer slot is uncovered too much (excess of .020) and you become like a dog chasing his tail.

As for the problem with your car Mark, if the engine is pulling that much vacuum the motor should not be that difficult to tune. Assuming again that everything is set up occrectly I would take a look at the fuel pressure, 7.5 lbs maximum, first; and here's why. You've been through 3 carbs and the odds that 3 new carbs are way off on their adjustment is really remote. To solve your problem I would first put in the correct power valve. A 3.5 with 15 in/HG at idle is going to cause a bad stumble off idle. The correct PV really needs to be a 6.5 at the smallest to accomplish a smooth transition from idle circuit to the low speed circuit.

Next, when you get the carb back on turn the idle adjustment screws all the way in until they just seat then back them out 1.5 turns. Plug in a vacuum guage to the unported vacuum port (it's the one that is on the baseplate and is connected directly to the throttle bores) to begin tuning. Fire up the engine and let it warm up completely. This is very important otherwise you'll end up running rich.

After the engine is up to temp take note of your vacuum reading then turn in or out the idle mix screws on the metering block in 1/8th to 1/4th turn increments until your vacuum guage registers the highest reading. Very important adjust all screws equally and one at a time. After you get everything set from a high vacuum standpoint take the car for a 5 min drive to let the motor really respond to the changes and clear out any excess fuel that may have accumulated in the runners. Pull it back in the driveway and repeat the process one last time. Your idle mixture screws should now be set. If your plugs are still wet with fuel take a look and see if perhaps you are getting excess fuel pressure at idle. Your goal here is to be about 7.5 lbs maximum at idle. I hope this helps. Let me know. Scott
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post #9 of 12 (permalink) Old Jan 20th, 03, 11:13 AM Thread Starter
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A buddy of mine brought a new race carb to my shop last night and we bolted it on and it ran like a scaulded dog. The only problem with that is there are no vacuum ports on the carb what so ever. I will need ported vacuum for the distributer and I have a 700R with a lock up torq converter that requires ported vacuum. I really like the way the car responds with this carb but in the long run it will be difficult since there is no choke either.
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post #10 of 12 (permalink) Old Jan 20th, 03, 04:27 PM
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Can you swap baseplates?
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post #11 of 12 (permalink) Old Jan 20th, 03, 06:15 PM Thread Starter
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If I swap the base plates then it would be pulling full vacuum at idle. I have other vacuum sources on the tree behind the carb to run the R/S headlights, power brake booster and the pcv valve. The ported vacuum is on the metering plate on the carb normally and this one is not there.
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post #12 of 12 (permalink) Old Jan 21st, 03, 04:04 AM
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Hmm quite the quandry you have there. You've probably already come to this conclusion but I'll mention it anyway. I would try to duplicate the settings on the race carb and if that fails take it to a shop that will professionally rework the idle circuit. Good luck and let us know what transpires. Scott
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