The Stovebolt Site Forums The Shop Area The HiPo Shop Holly 390 jetting baseline

I have 1948 3/4 ton with a '56 235. A Clifford intake with 4 barrel adapter and a Holly 390. Just going thru break-in, and find the new setup to be running rich. Can anyone provide a jetting baseline for this carb on a 235?
Thanks

How rich is it? Puffing black smoke out the exhaust? Spitting fuel out the tailpipes? Or dark plugs? Rich at what rpm? Stinky at idle? All fixable, but more info is needed. You're probably not even into your main circuits until 2500-3000 rpm, meaning a jet change is not going to affect anything below this.

Devin

Did you get the carb new? If so, they come with a really good CD that takes you through the tuning. Make sure that the choke is fully off, too.

Just my 2 cents.

best wishes,

Les

I don't have much experience with this carb. I have been working primarily with Weber carbs for the last several years, It is a rebuilt carb with no tuning guide. It idles nicely and was running rich at higher rpms during the cam break in. Blackened plugs, rich burn smell, no smoke. There is no exhaust other than the Fenton manifolds.
Thanks

First off, get some exhaust on there. Those fumes can kill you. Having said that. there's nothing like the sound of open headers. I don't like them on break in since you can't hear something before it becomes too late.

In any case, it does sound as though it is jetted too big. I'm assuming you've set the mixture screws to highest idle speed or highest vacuum reading, either will work.

Is the truck driveable? I would suggest jetting down 4-5 sizes and going for a 20 mile drive. Should clear the plugs and give you an indication of how total air/fuel ratios are doing. If it starts stuttering and stumbling immediatly, go home since it's probably too lean.

Holley makes a nice assortment of jets from 60 or so to 99, ive used it quite a bit over the years to tune cars in.

Devin

Yes, As 55Tractor states you need an exhaust on the vehicle to tune. The back pressure from an exhaust will effect the how your carb is tuned. Tune it the way you are going to run it (in relation to your exhaust) otherwise you are wasting time.

Yes, blowing black smoke is running too rich. Black sooty plugs is running too rich. If you are running rich on an open header, I would assume that it will be rich when you get an exhaust on it too.

I've never used one, but this sure looks like it would be a slick way to dial in the mixture....

http://www.summitracing.com/parts/PHP-15001/

Might check here for your list number and original jetting. http://www.mortec.com/carbs.htm

The original jetting should be relatively close, as the air and fuel need to be in the appropriate ratio regardless of what engine it is mounted on. There is also a tuning section as well.

Devin