Carb pondering

[jdhcth]

I have a 67 LWB with a 283, 3spd, fairly beefy cam, Edelbrock performer Intake and a Holley 4175 Qjet replacement 650cfm carb. I get 10MPG. I want to improve this. If I go with an Edelbrock 1400 or 1406 will I improve my MPG and/or will my performance suffer? Also, is a 650cfm too much carb? Iv'e used some Online calculators and they tell me 500cfm.

I also have one of these early 60s GM 4MV's layin' around, would it be worth it to rebuilt or have this rebuilt?

Thanks,

JD

[Billla]

What is a "fairly beefy" cam? If you have a rumpty cam with tons of overlap then there isn't a lot you can do - the fuel is going right out the exhaust valve.

If it's not a crazed cam, the milage seems low to me. I would do a good quality tune-up including spending some time with the timing and advances then move on to tuning - and possibly rebuilding - the carb. I wouldn't expect a carb swap to fix the problem (unless it's just that the carb you have needs to be rebuilt) and I definitely wouldn't go to a 650 carb - the calculators are right it's just not possible for a 283 to use more than about 500 CFM unless you turn well beyond 8K or have forced induction.

I think you can get this up quite a way with what you have.

My $.02

[stepside454]

I think youd be far better off with 500 cfm.
You said Beefy cam....that may be part of the problem. a beefy cam in a 350 would be borderline radical in a lil 283. small displacement motors are sensetive to over camming & carb. How is it geared ?

[jdhcth]

OK OK It's a radical cam. 292 Advertised Duration. Runs like a spotted a$$ ape. The Holley has just been rebuilt as well as the engine. less than 1000 miles on it. Someone gave me the 650 and I had it rebuilt by a guy that knows his stuff, but it's all race stuff.

Anyway the timing runs very advanced and ignition components are all new. I really love the way the truck runs but, as you guys have stated, I'm over-carbed. Didn't intend on it being a daily-driver but that's what's happened.

Sadly I need something in 500cfm calibrated for economy, not performance.

Thanks,

JD

[stepside454]

If its got a racy cam with alot of duration & over lap, your never going to get much for MPG no matter what carb you use.

[Billla]

Originally Posted by jdhcth

OK OK It's a radical cam. 292 Advertised Duration. Runs like a spotted a$$ ape. The Holley has just been rebuilt as well as the engine. less than 1000 miles on it. Someone gave me the 650 and I had it rebuilt by a guy that knows his stuff, but it's all race stuff.

Anyway the timing runs very advanced and ignition components are all new. I really love the way the truck runs but, as you guys have stated, I'm over-carbed. Didn't intend on it being a daily-driver but that's what's happened.

Sadly I need something in 500cfm calibrated for economy, not performance.

Thanks,

JD

As noted by a couple of folks...the carb has nothing to do with the fuel milage in this case and you're NOT over-carbed. Since you have vacuum secondaries, it's really not possible for you to have more flow than what your engine will use. The milage is strictly related to your race setup - big cam, lots of advance, likely no vacuum advance (BIG milage killer) You like the performance, you pay the price - but don't look for economy with that setup.

[Boog]

[gyjackson]

From my experience with both Holley and Edelbrock carbs...I have both.
The Holley outperforms the Edelbrock hands down..but...The Edelbrock seems to be a bit less "needy" and seems to have a little better response with no "bog" (probably due to my poor tuning skills!). I doubt you will pick up enough mileage to justify buying it. Even if you got an extra 2 mpg you would have to go about 20000 miles b4 you break even...just a thought. As far as a 650 being too big...not really, unless you drive it wide open all the time. 650cfm is the max it will flow, give or take. No matter how big your carb is, your engine will only suck a certain cfm through it as dictated by the rpm and cam. If it is jetted correctly, just a change in the carb's cfm shouldn't make a big difference.(remember holley's predator supposedly would supply 1000cfm on demand, but actually ran pretty good on small displacement engines) If mileage is your main concern, you would be better off dropping your hard earned $$ on an overdrive or a higher set of rearend gears. (my 67 camaro has a 327 with a complete edelbrock performer rpm package heads,cam,intake, and carb, a turbo 350 and 2.92 rear, it runs 13's in the quarter and knocks down 21mpg.)

[pkj]

that old chevy carb is a pretty cool piece. have you tried it? looks clean. what is it off of? pkj

[jdhcth]

Thanks guys for the input.

Thanks gyjackson! I know the gearing is low in it and that may be my answer. Mine may be 3.73 but how do I check that?

I am using a vacuum advance but wether it's working for me who knows. When I put my Vac guage on it it shows nothing at idle then jumps when I throttle it, I believe this is the "timed" advance. I'm just learnin' this stuff. I'll keep the Holley.

PKJ: The old 4Jet carb pic is from eBay but I have one just like it, just didn't have a pic of it. Early 60's Pontiac I think. Mine's not that nice either.

And everyone is right about my setup. You want the ponies ya gotta pay. I'm a Cam Lope-aholic

Thanks again.

JD

[gyjackson]

It probably has 3.73 or 4.11 in it....the best way to find out is pull off the cover and look for two numbers stamped on the edge of the ring gear. That gives you the number of teeth on the ring gear and number of teeth on the pinion gear. Simple division of the two numbers and you have the ratio.

[panhandler62]

With that setup you might want to consider this:
http://www.jegs.com/webapp/wcs/store...55536_-1_10271