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Well, once I convinced myself that I had botched my 350 project to the point of no return, I decided to buy a ready to run 383 from a local builder. I had it built, and also had it dynoed. When I received it, it was supposed to be bolted in and go.
How did that work out for me? So far, not too good. Here is what I have.

383/TH400, Holley 670 Truck Avenger carb, Proform HEI.
All of this is in my Jeep Wrangler. Today, after it was installed and running, I took it on the maiden voyage. It stumbled like it was running on 6 cylinders. I drove it right to the shop, since I was out anyway. They checked the timing, and it was spot on. He suggested hotter plugs, and gave me a set. I drove it home and swapped them out. No change.

He then said that he wasn't too happy with the way the carb acted when they dynoed it, and suggested a new Road Demon (a shop favorite). I told him that I was not really that anxious to run a 339.00 dollar experiment without some sort of proof that my current carb was defective. I should mention that about half the plugs looked good, and the other half were black.

He also suggested that maybe my 12V suppy to the HEI might not be delivering enough current, and that I should run it from the starter, and put it on a switch. (has anyone done this?)

So, I have a motor with about 30 miles on it, and 10 gallons of gas run through it on the dyno. It runs terrible, and I do not want to start guessing at the problem to the tune of 339.00+ bucks to find out what is wrong here.

Help.
 

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Sounds to me like the engine builder/shop is not being very helpful for the money that you paid them. When I had my 350 rebuilt the shop was very helpful in how to prep/set it up, and certainly did not suggest I pay them more money. Did you get to see it run on the dyno, or did they just give you a printout? Did it run smooth and strong for them, or did it stumble for them and they just passed it off as your carb? I'd find this out and want to see them make it run very well before I gave them another penny.
What kind of carb are you running? Is it new or used? It definitely could be a carb or ignition problem but I would want to know without question that the motor ran well for them before I spent money anywhere else.
 

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nice roach.
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Does it stumble all the time... only on acceleration? only when cracking the throttle? Didja check the firing order? make sure plug wires arent burnt.... check for loose connection on the hei wires? Is the accelerator pump too tight and flooding the engine?

good luck

-j-rOd
 

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Timing?

When you say the timing was "spot on", what do you mean? Spot on stock specs? Or what they told you to run? If you're running any kind of non-stock cam or heads or manifold, the stock specs may not be the best place to end up for ignition timing. Mess with it a little. Most likely, you'll end up a few degrees advanced from the stock setting. On my 355 SBC in my YJ, a few degrees makes a huge difference in how the engine runs and it doesn't run all that well at the stock setting.

The deal with half the plugs being black and the other half not is kinda weird, too. Don't know what to suggest about that.
 

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Dude we are having the same trouble you are. Steve has a 60over 350 with rv cam flat tops performer intake and Avenger carb.

I have a 40 over 350 with a comp cam extreme cam flat tops mild valve train upgrades performer intake and avenger carb.

Let me guess the rear plugs are black and the front plugs are ok or white on both sides of the engine. My engine now has 300mi on it and the plugs are all black. My guess is that it is running to rich, I am sitting at 5000ft above sealevel and only stepped one jet size, acording to holley I should be between 2 & 3 jet sizes leaner than the carb comes for my altitude.

Both of us are having trouble at low RPM the engine starts bucking and sputtering untill you get above 2000 or so RPM.
After that however the engine sounds good and realy takes off. Steve had an older holley 500 on his engine before (5000mi on engine now) that worked prety good. Since he only swaped carbs he is betting there is something funny with this new holley carb.

Steve is a prety good engine builder (used to do it for a living) so I think we should have it sorted out after a bit. I'll post what I find works when we get it sorted out.

ROB
 

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well as far as timing, your running a high performance motor. Just loosen the distributor advance it till it pings then retard it about 1/4 turn. If it still stumbles then you probly have a fuel delivery problem. Your said that half the plugs were black, what cylinder numbers where they and were they all on one bank.
Yeah your motor shop should be more helpfull, but then they guy that does all my motor work my dad had as a pit chief when he drag raced.
 
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