My buddy has a 82 maro, Here's what it has: - 400 small block - TH400 tranny - Holly 650 -- this is really what the question will pertain too. Problems, car runs great... gets less than 10 MPG Car dyno'd at @200 hp at wheels, 320 lb ft. Now I've never messed with carbs before, but he should be getting better than 10 MPG, right. Choke is broke we were told. He plans on putting vortec heads on there some day, question is should he switch to TBI, TPI, or can we get a carb on there that will handle the 450 + HP the car will see someday with good mileage? I'll get my buddy with the car registered with third gen so he can answer q's about the car you may have.

Yeah, we'll need a bit more info. Like the distributor he is using. Given the power output of the engine something sounds wrong. Laying down 250 to the back end is like falling off a log even with a mild 400. It almost sounds like he doesn't have enough ignition advance. If he's still running the computer controlled carb, then BINGO!!! We have a winner. Gotta ditch the computer controlled distributor when you ditch the computer controlled carb.

Other than that let's hear about what's wrong with the choke. As long as he isn't running around with the choke half closed all the time you should be able to get a little better mileage than that if the carb's in decent running order. I regularly knock down 14 MPGs with 400s and a well tuned carb, even without overdrive- assuming I keep my foot out of it.

OK, I went and looked at the car and this is what I found:

The carb is a Holley, I found 2 different part #'s on it, 6R 6931 and 10569.

Intake was Edelbrock performer

Has headers

We could not get the butterfly to close on the carb, no matter how hard we tried. It was stuck open.

As far as a computer, don't know, Coil was on top the distributor with 2 harnesses coming off it. One harness had 3 wires, 2 red 1 black that went from the coil into the bottom of the distributor cap. The other harness had 2 wires, yellow and pinkish/red.

Heads, looked old. Couldn't get part #, things I noticed were that instedad of being flat on the end of the cylinder head, which I have noticed all aftermarket heads seem to be flat on the end( where edelbrock heads say "Edelbrock"), these have a half inch line around the outside and the rest is indented about an 1/8 inch (sorry, I know that probably makes no sense)

So what do you think the problem is? This is a 400, it was verifed by a mechanic.

The car actually has true duals, so it has no problem breathing.

Thanks for any input.

You really need to get some head casting #'s and what the cam is. Like what was said you should be able to get 250 to the wheels with out even trying. But if you have a lame cam and some ****ty and not in great shape heads it is easy to see why it isn't running right.

That's a GM HEI distributor. Does it have a vacuum advance canister sticking out the side? If not, it's a computer controlled distributor (and should be replaced with a non-computer controlled one).

Check ignition stuff first, then go at the carb. Tuning a carb while the ignition is in question leads to nothing but frustration.

It does have the vaccum advance canister attached to the distributor with a vaccum line going to the carb. I will get back to you on the head casting #'s... where is the number, under valve cover or something?

Thanks!

Get the head casting number. It's under the VCs, between a couple of rockers, 6,7 or 8 digits... will look something like 462624, 3927186, or 14014416.

Those numbers aren't the carb number, they're ... something else. The number we need is on the front surface of the chole tower of the air horn, driver's side, right in front of the choke blade itself. Usually says "LIST" something. That's the number we need, is the "list" number.

Every one of those performance numbers is just pitiful. The gas mileage, the HP, the torque. Something is wrong with that car; it looks too sick for it to even be a tuning issue, looks like it has a blown head gasket or something. That's less HP than my weenie little 305 puts to the rear wheels, and not much more torque... there's a good possibility my Q-Jet carbed California-emissions-compliant 305 would actually beat it in a race. That's just wrong. Wrong, with a capital R.

Right now, he doesn't need to be thinking about "someday" getting 450 HP out of it; he needs to worry about getting at least 250-275 at the wheels, now, today, along with at least 350 ft-lbs. He needs to get his butt in gear and fix whatever is broken because something sure is. Quick before everybody around him gets the idea that 400s are crap.

OK, we got some part #'s.

The heads were part # 333882, we found they were used in the mid 70's on some base 350's.


Now for the carb, part #'s, these should be better:
80508-1 and 1788, these #'s were on the tower with the butterfly. I went to Holley.com and the best I could find was a 750 CFM Four Barrel, P/N: 80508. This is probably it.

So ya think she's running right?


Thanks!

I know for a fact that the heads are huge terds, I have the same. They are old smogger heads, and RB will tell you that they are only good for being used as boat anchors.

Have you checked timing, leak down test, hooked up a vaccuum gauge, or anything yet? Check plugs to see how its running is a good start.

I know I'm not much help, but trying to do something. I'm with the others, that 400 should be putting down atleast 250+ to the wheels, maybe even with those heads, sound like it needs to be tuned a little.

Good luck.

There is only one thing needs at the moment, ditch the heads, - even heads from anything and anything at all are better then what you have.


If you have the money get some AFR's or Trickflow's - even Darts are good.
If your on a budget some vortecs would work well for you also.

Get it running right before you start dropping money on heads. Check your plugs and timing. Does the car bog down or have bad throttle response? These things can help identify problems.