'74 Mav I6 250 getting 10mpg. HELP!!

[Anonymous]

Ok all. There's a '79 Fairmont with a I6 in it in the local U-Pull-It.

What carb would it have used? I think it's a 200.

Here's the story. - Fiance has a '74 Mav, I6 250 1bbl (carter RBS), that's getting about 10mpg. It's eating gas like there is NO tomorrow.

I'm pissed at the points, and am converting it over to HEI (tired of resetting points!) this week. The carb on the '74 Mav is a rebuilt Carter RBS with a 'Rebuilt by Holley' sticker. I'm 99% sure it's running super-freaking-rich as if I cause a vacuum leak on the vacuum tree, the idle comes way up (too much gas, not enough air).

Questions - Is there some way to jet this thing down a couple sizes? What's teh stock size on it? Will the '79 Fairmont's carb work on it, any better after being rebuilt?

I want maximum mileage out of this 250. My 354 Pontiac '77 Firebird gets 18-20mpg with a 4bbl Qjet. There is no reason this Mav can't match it.

 

[MustangSix]

The RBS has an internal power valve that often wears out and sticks. If all the normal adjustments are set properly and it is still rich, bet on either that or a stuck float or fuel inlet valve. There is no way to jet it to compensate for a worn out carb.

The later carbs might work and will probably bolt up, but compared to a 74 Maverick, they are usually laden with emissions devices that are hard to remove or bypass. If you don't mind fiddling with those, they are more sophisticated carbs and my actually produce better mileage.

Electronic iginition will help a lot.

 

[Anonymous]

The '79 Fairmont 200 has the same head as the 250 engine, so the carb will fit - it's likely a Holley 1946 carb.

Check this on the Mav - make sure the carb is well seated, well sealed against the intake. Low vacuum to the bottom of the carb will cause the power valve in the carb to drop open and it will run exactly as you describe. Also, make sure the top plate of the carb is tight - Holley's screws tend to come loose after the carb has 1,000 miles on it (or so) and the power valve's vacuum signal leaks away, causing the same problem.

Hope this helps.

 

[Anonymous]

doward: wow you got here quick!

 

[Anonymous]

rofl... yes, I did.

I'll check the power valve, and all... this thing is just giving me such a fit. Give me a Quadrajet any day