Holley 390 CFM Sea Level Jetting????

I know we've touched on the issue of what size carb to use on many occasions but I haven't found "sea level" jet sizes. I bought a Holley Street Dominator 4 bbl intake for my 225. I'm looking to buy a 390 cfm. I live in Michigan, just about as close to sea level as you can get, never go into high altitudes, just looking to get some jetting numbers that would work well with where I live. Thanks for your help............Anyone have any dislikes with the Holley 390cfm, I'd be glad to hear it, pro or con.

 

Also keep in mind this is a stock engine, rebuilt a short while ago, but stock, no RV cam, no headers, nada...........

 

At sea level you should probably be fine right out of the box. I bought the Holley 2 bbl somewhere around 350 cfm and living on long island I don't get very high either . I was looking at the 4bbl too, but to be honest, I would have to change the intake manifold as well as put on some headers to make the upgrade worth it. Plus the 2bbl is a bit easier to setup. But it is all up to preference.

 

The Holley 390 cfm should work perfect right out of the box. You'll have to adjust the idle a little because of the rough idling Dauntless but that's it. I originally had the 390 on my 225 but once I rebuilt it, I went to the 650 and jetted down. No complaints whatsoever. The only problemb is at altitude. I went above 7000 feet this year and wouldn't run at all. I mean it just sputters and dies.

 

There are three mixture controls on a Holley that you may need to adjust: the idle screws, the main jets, and the power valve. There's also pump-shot tuning, but that's separate from "mixture."

The idle jets are straight-forward. Adjust them out, each the same amount, till you get a good idle. Them start adjusting them in, each the same amount, until the idle starts to stumble. Back out about a 1/4 of a turn and that's your best lean idle.

I've always adjusted the main jets by trial-and-error, and looking at the spark plugs. Good mixture will give a tan-colored plug. If the plug is brown; too rich. If the plug is white; too lean. Jets for Holleys are available at your local speed shop. The size differences are significant; if you're close, a change of 2 size steps will be too much. The secondaries have a metering plate that's also adjustable, but the plate size won't affect your drivability much unless it's way too lean.

The power valve is tuned by which engine vacuum it will open at. You need a richer mixture under low flow - low speed conditions. Generally you wouldn't mess with this too much; you can try going a step lower (ie opening later = lower vacuum) and see how the drivability and mileage are affected.

You can alcos adjust the accelerator pump shot with a kit of cams that comes from Holley. If the carb is ok, try going a step leaner and see if you get a stumble at when you open the throttle all the way.

 

Also, Holleys work great on the street, but have a less-good reputation off pavement. They tend to flood pretty easily on up/down slopes or side-hills. They can be made better - member Schardein (thanks, Greg) has an article about tuning the Holley for off-road; maybe he'll send you a copy. Take a look at that, ask some questions and we can comment further.

 

Actually Holley has an off road version of the 390. It has all of the typical offroad mods and then some already done to it. I have an older 390 that has some mods for offroad and it works fine in telico.

the best mod I saw in the one that comes on the off road 390. The bowl vent tube bends over top of the carb. This helps prevent fuel from sloshing out of the vent into the carb making it run rich.

 

On the Holleys sea level jets will be what new ones come with. You go up in jet size about 1.5 for every 1000'.

Stan

 

The biggest problem with Holley's is their tendency to blow power valves, causing idle problems. There is a check ball fix for this problem that is probably a good idea. I've never liked Holly's (for what thats worth lol) on offroad vehicles and have always had minor tuning problems with them. I've run just about every 2bbl-4bbl carb known to man on my 225, and have found the Q-jet to be my carb of preferance. I have also had good luck with the Carter AFB, with the originial Rot.2GC on a stock manifold running a close third. I do a lot of rock crawling and technical woods wheeling. Your application might be different, and this may effect your choice of carbs.