The Stumble That Just Won't Die

13 lbs of vacuum is low!!! Unless your running a big cam, your vacuum should be in the range of 16-20 lbs at idle. Your timing might be retarded. If you advance it, your vacuum may go up. I'm curious? When you had the 770 Holley on the engine, did you still have stumble with that carb? Have you checked your float level on the front and rear bowls? Sometimes the float may be set a bit higher causing fuel to leak into the motor. But your claiming your plugs are running brown which is another good sign. If your running that low of vacuum, you might check what size power valve you have in the 650 Holley. The normal is 6.5, but that is for vacuum in the range I stated before. There is a formule you can use to find the right power valve setup for your carb. I believe it is half of your vacuum. Someone else might give better info on what power valve to use. Could be the powervalve is faulty as well. The diaphram on them don't last long. That will cause the stumble as well.

Do you have a choke on this car. If so how is it activated.

You vacuum seems low at idle for your setup. Perhaps you have a vacuum leak or as suggested a poorly seated intake manifold. If it runs better when cold and you have an electric choke I will almost bet you have a vacuum leak somewhere. The base gasket is a good place to start looking. I would also temporarily disconnect and plug all vacuum ports on the manifold and carb and see if the vacuum goes up. You could have a leak somewhere that is not obvious.

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Chet

Yea, that vacuum reading was taken when I had my initial timing set to 8 initial. It is probably higher now because I have it set to 12 initial now. I'll have to check it again. I did have the stumble with the 770 also. All the new parts I have put on this engine have been trying to get rid of this awful stumble. However, nothing I have changed so far has effected the stumble at all. The float level on my old carb was good and I was able to see the level through the clear site it had. My current carb does not have a float site, so I can not see the level. My current carb also does not have a choke and is mechanical secondaries.

Thanks,
Jason

Sounds like you are to fat, a cold engine needs more fuel. Usually this is accomplished with the choke. If it runs good when cold an gets worse when warm, it is to rich for sure

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Chet

I am pretty sure this problem doesn't have to do with being to rich or lean, in the carb jetting anyways. I have gone rich, I have gone lean, and everything in between with two different carbs with no effect to this god awful stumble.

Thanks,
Jason

Just thinking, if it was too rich, wouldn't it have a bog instead of a "cut out" stumble? I think I might break down an by one of those wideband controllers to see what my real fuel ratio is if the next few things I try don't help. I'll try all of the cheap stuff first and move my way up to the more expensive. Tuning an engine at 5000ft is proving not to be an easy task
I am originally from Kansas BTW

If it runs good cold with no choke it is to rich.

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Chet

Chet has a good point here. The best way to find out if the carb is running rich is to pull a spark plug and check it. If it's black and sooty looking, your running rich. If it's light gray or white, your too lean. Timing has a lot to do with the stumble as well. The stumble comes on when you have either too much air or fuel entering the combustion chamber. The engine and ignition can't ignite it or the timing, etc is off causing the engine to drown itself at that point. It may just be too big of a carb in CFM. I'd try borrowing another carb with small CFM's and see if that cures it. I started my truck this morning and it ran great for about 2 minutes, suddenly started smoking and studdering like you don't know what. I couldn't keep her running. The darn rear float needle wasn't working again. (had the problem a while back and adjustment fixed it). Took the carb apart, pulled the needle and noticed it was a little loose and worn. Had a couple needles laying around, got a new "o" ring and gaskets. Works great again. Actually, I'm running a bit on the lean side now since I did that. I also went from .31 squirters, back to the original .28 squirters and now the carb runs hard, no stumble, studder, etc. Sometimes those float needles seem like they are OK, but at times, they'll stick on you and cause the stumble when too much fuel enters the bowl. Just a thought!
Ed.

That's just it though, the spark plugs are always that nice tan color and are never white, grey or black. Also, I have never seen any color of smoke come out of the exhaust before and timing has no effect on this stumble.

BTW, I put in a fuel regulator today and set it to 5lbs, it didn't help any.

Thanks,
Jason

You are running 1.6 ratio roller tip rockers. The exhaust may be opening too soon, with the engine at temperature.... Try switching the exhaust rockers to 1.5 ratio.

Hmm, never thought of that. I may try that if the next few things I try don't work. Thanks.

Some carb idle circuit are calibrated at sea level. At 5000 feet the lack of air causing the carb to run rich. The air bleed in the idle circuit may have to be open up to lean the idle circuit. You may have to take the carb and bring a shop and have it drilled. I just put in my 2 cent.

What's the rear end ratio. ON a perfectly tuned motor, a mis matched rear will create a stumble.

Perfect, just Perfect. Now excuse me while I go and bang my head against a wall, for not even thinking about this.

Gamester, These are words of wisdom, from someone who had the same problem. His stumble did not sound as bad as yours, but your car came with a taller rear gear, than what his did.

During a phone conversation, I pointed out that a 3.73 rear gear, would get his rpms up into the power band of his engine quicker.

Read this thread.

http://www.hotrodders.com/forum/new...nce-160101.html

I do know that it couldn't be the rear end for 2 reasons.

1. When the engine is cold it takes off like a bat out of hell with no stumble at all.
2. When the engine is all warmed up the stumble is so bad that I can easily feel it when just revving the engine in neutral.

BTW, I do have 3.73 gears

Thanks,
Jason