Question for all you Holley 390 gurus
 

I have had a stumbling- bogging problem since I have owned the truck. I have rebuilt the carb twice, but have been reading more and more to try and understand the problem. When under load, even when warm, there is a part throttle region where it stutters and generally has no power. If you back off and slowly throttle, you can somewhat overcome it. At full throttle, it almost stalls, and then roars off, with LOTS of power.
To try and answer my own question: I think it is a wrong size squirter on the accelerator pump system. It seems that with too much throttle, there is more air than fuel, and it stutters because not all cylinders get enough fuel to adequately light. Is this correct? I rebuilt it following every detail in the SA holley book, but it didn't correct this.

To get mine to work right I ended up putting at .038 squirter on it, and running a 50cc accelator pump with the yellow cam that came with the 50cc kit. Obviously, there are a thousand ways to skin a cat, this is just what I did to mine to get it to work better, I also dropped the jets down to 49 in the front, put in a conversion plate and put 51 jets in the back, and changed the power valve from 6.5" to 3.5", and put a plain colored secondary spring it. This seemed to make it run alot better for me, but every truck will be different. I'm running a 4wd with a NP-435 xmsn and 3.50:1 gears, with the efi exhaust manifolds dumping in to a 3" single exhaust.

My truck is having exactly the same problem. Does anyone know of any good books that tell what different jets and boosters will or should do when installed on a 4160? Someone please help!!!!

Zod, thanks for the reply. I have tinkered with mine, and put in a 28 squirter, which helped a little. When I tried to change the jets, it ended up that it ran better with the stock ones, so I went back to them. I guess I will keep tinkering with it.
Peppermint, I got a holley carb book by SA. I think I bought it at barns and noble, but I'm not sure. Any of those big book stores will have a bunch of books on it. I just chose the SA because I was told it was one of the better ones. It really isn't bad, but I am completely self taught, so that is a big reason I don't have tuning down quite yet.

I'm working on the self taught thing, doing pretty good so far, only things i can't rebuild are carbs and trannys

I can rebuild manuals, but haven't tackled an auto yet.

I've never been accused of being a carb expert, but my buddy has. His advice to me was check the basics and start over with tuning when you run into a problem like this. Make sure your float levels are right, downsize the squirters, check the accellerator pump cam and secondary spring tension.

Ted, you said it chokes at full throttle..is that from a stop or while cruising at 50+? When I put my 390 on and was tuning it, there was a stumble from take-off. I had to downsize my squirters and the pump cam. I just couldn't believe that my old motor could run so well on a leaner fuel mix, but it does. If it's happening at speed, it sounds like your secondaries are not opening at the right time, maybe too soon. Change that spring! (There's a chart out there somewhere that outlines what vacuum is required to actuate the secondary with a particular spring.)

That's my 2 cents! :P

Hey, thanks for the tips! I will take any advice. My problem was most noticed from take off, but any time you went from steady state to mashed throttle, it ran like it was only firing on 5 cylinders. If you roll into the throttle to wide open it is ok. I was reading up on it, and switched my 25 squirter to a 28. I also went with the brown spring, which is next to the highest, for the secondaries. My timing is at 8 BTDC. As far as the pump cam, I have the stock white one on there. When I went to the 28 squirter, it actually seemed to do better, but still isn't right.
If you would, ask your buddy how this setup sounds. I am stumped.
Oh, I tuned the mixtures using a vacuum guage for max vacuum, like my Holley book said. I got about 18.