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Old 11-05-2005, 04:18 PM
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Holley 650 / Edelbrock performer combo

I've got a rougher idle than I should have. And a frequent hesitation just off idle. Seems to clear up some with a good carb cleaning, but comes back too soon. Tank and 2 filters are fresh. I buy the High Grade Gas from a good source.

Here's the specs: 1968 307 small block, stock exhaust and cam.
holley 650 spreadbore 6210-3, no mods. manual choke. [ setup for economy, primarily ]
edelbrock performer with egr capped.
Stock ignition and coil, not HEI yet.
I'm using a 3/16 thick gasket under the carb.

This is set up on a '78 pickup frame, standard tranny, '78 pickup rearend. 1/2 ton.

[ misstated #5 wire as #3 the first time I posted ]
While working with a rough idle problem, I noticed that I can pull off #5 or #8 wire at the dist cap, and it makes no difference in the idle. I can hear the spark OK when seating the wire. But when the carb is up off idle, or under load, #5 and #8 are hitting more normally. The plugs aren't dirty. Coil is good. Complete tuneup with all new parts & wires didn't change the symptom. Timing is 8 degrees, advancing is normal. NO vaccuum leaks evident. Tried more rich & more lean settings. Air bleeds and jets are clean. Idle is about 600. Symptom Doesn't change if I set the idle to 800.

A friend mentioned that edelbrock manifolds sometime manifest this problem. Some branches aren't getting good mixture at idle. But this problem builds up over time. If I tear everything down, regasket, soak the carb, etc, I won't get the symptom for a year or so. Could be the new gas is getting to some gaskets or something else. I use good kits.

So, does anyone remember an edelbrock manifold that just didn't want to deliver at idle ? [ on two cylinders ]. #5 and #8 are opposite each other in the firing order, so the idle isn't as rough as it would be if two adjacent cyls were not hitting good.

I would like Spark Plug recommendations, too. I've tried autolite 74s, champ j12y, and I'm running AC 44T right now. Believe it or not the champs run a little cleaner, but any of these plugs will exhibit the rough idle.

the idle is a bit rougher when cold. The idle speed goes up only a little when warm, but still rough, with a hesitation.

Any thoughts ? I have thought about dropping down to a smaller carb. I don't open the secoundaries very often.

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Last edited by Andy in SD; 11-06-2005 at 07:40 PM.
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Old 11-06-2005, 12:03 PM
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You have a carb problem, not a manifold design problem.
Your installing kits makes it go away for a while.. a clue.

WWaaaaaaayyyyy too much carb. Try a 500.

I guess that's a 305 Chevy.

3 and 8 are NOT opposite each other in the GM firing order....
If yours are, that might be your problem.

but they are in the same v4.

The left side of the carb could be bad and has a fuel delivery problem. Is the throttle shaft to the carb body worn out and sucking air? That is the throttle linkage side with all that pulling on it.

3 and 8 plug wires could be bad. It takes more power to spark a running cylinder than sparking to ground.

You also need to check 3 and 8 for mechanical problems before you go any farther. Compression, cam lift, exhaust valve leakage, etc.

Overall I would say that you have a bad carb on the left side, and since you have been chasing this so long, that the excessively lean mixture on the left side has burned out your exhaust valves.

As an afterthought.... you might try one range hotter plugs in those 2 holes. I had one Pontiac quadrajet that hated the recommended AC plugs, but stepping from 45s to 46s made it run perfect.

Last edited by xntrik; 11-06-2005 at 12:27 PM.
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Old 11-06-2005, 03:32 PM
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Errors in Original Post have been editted - it's #5 and #8

You are correct, it is #5 and #8 that I'm having trouble with. The Holley 6210-3 really likes this engine out on the freeway, and for passing. I've never gotten the greatest mileage around town, but didn't foul the plugs or have dark exhaust. The idle problem shows up after a few months of driving, between carb rebuilds or cleanings. I posted this thread to see if someone could explain why #5 and #8 don't seem to be getting mixture the same as the others. Just curious if it was something else besides the carb being a bit too much for average street use.

And this is a 307 small block, not a 305. 305 came out later. It's .030 over now. The cam is close to stock, being new at the last rebuild. Napa recommended it. Don't have those numbers handy. Even though the 650 carb is probably too much for this motor, I didn't have problems with it when I first installed it. It is the holley replacement for '68 period Q-jets. I haven't made any changes to it. I have been getting a set of 350 dogbone heads ready for this motor. Wonder if that is good idea? Maybe the carb will like this setup better. I run premium gas.

The carb shafts are tight enough. I get no indication of a vacuum leak when I spray WD or other tests. They feel good. Secondaries are closing tight. The compression all around is 160. No indication of burned valves, but that has occured to me also. The last time I rebuilt the carb, the problem cleared up for a while, but it repeats after some time. Will probably toss the carb soon. I just posted pictures of the truck in this gallery.

Thanks for the answer to the post. Any other ideas? Andy

Last edited by Andy in SD; 11-06-2005 at 07:47 PM.
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Old 11-06-2005, 11:50 PM
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Andy, 5 and 8 are still feeding off the left side and toward the rear. Feeding the same V4. It could be that the left side is getting leaner. Years ago we used to hear more about staggered jetting. Maybe the left side needs a jet one or two bigger in the primary and maybe the secondary too.. try that and let's see if that helps.
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Old 11-07-2005, 09:25 AM
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Holley Carb 650

OK, I'll try richer main jet on the left side. Thanks for the tip.
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