I'm embarrassed to ask this question, but I need a little help on carb/fuel adjustment. I'm running a little Ford 302 in the '27 roadster, and currently it has a Holley 750 Double Pumper on it (too big). This engine was (I believe) from a '74 Maverick, and originally a two-barrel intake. I am quite sure 750 CFM is way too much...the little stock carb was a two barrel at something like 400 CFM I imagine. So, what is a decent carb that can be well adjusted to minimize fuel dumping, i.e. get the car not so ritch running? I was thinking of finding a little Holley 600 CFM carb, or maybe an AFB. Is one more adjustable than the other? Is it possible to "de-tune" the existing 750 Holley and get it to run not-too-rich on the little stock 302? I'm not worried about gas mileage too much, but I don't want to foul plugs and fill my baby exhaust with soot every month. Thanks for the help, and I realize 750 CFM is not needed...I just don't mind leaving it there if it can be adjusted to the proper air/fuel ratio.
Thanks dirtbag! I found one in my little garage! I didn't remember owning it, and it looks decent...I'll rebuild it and see how she works.
Yep the holley 600 list 1850 should do just fine. I had one on a stock '68 302 in my '66 mustang and it ran great with good gas mileage. Used the same carb on a mildly camed 289 in the same car, which ran great also. Am putting the same carb, only a new one, on another 289 for my 23 t-bucket.
stock 302?.....390 holley jmho. summitt has a formula to determin what cfm you need. i think its on their web site
that can be well adjusted to minimize fuel dumping, i.e. get the car not so ritch running Again, again, again: engine too small = carb runs LEAN. There is no rich, there is no fuel dumping, does not exist.
that 570 holley avenger sure is good .i run it on a 54 with a 302. good power and plenty good mileage.
Look for a carb with the smallest primary side you can possibly get, I'd guess you can probably find carb cfm info on all carb brands on the net.
Dudes, keep the info coming. I'm learning a lot here... So is it true that an over-sized carb makes your motor run too lean? I guess because it is more CFM...but does not that mean more gas also? And don't I sound dumb?
750 is to big It can dump gas (rich) into motor if power valve and or gaskets are shot or not enough manifold vacume at idle, don't waste your time with it. Even if you got it to run it would be a total dog without proper stiff gears, etc. Find a nice 400-500 CFM for better throttle responce and drivability JMO
Here's a bunch of vague generalizations about carbs. Oversize carburetors result in poor vacuum signal where the fuel is vaporized. End result it pulls droplets instead of a mist. Those bigger particles don't burn as efficiently as equivalent small particles. End result, lean mixture because what's there is not burning all at once on the power stroke. And dumping what's left in the exhaust, making a gassy stink. Generally this is low engine speed only, once a small engine is going down the highway, it should pull plenty of air to satisfy an oversize carburetor's needs. Once you get past that concept, different carburetors are set up to have different cruise and acceleration mixtures. Double pumpers are set notoriously rich across the board. That little 600 holley is set up to run much leaner, and will deliver better mileage (and less power) but this is really independent of CFM. If you scaled the little 600 up to 750 CFM, it would still get better mileage than the double pumper, because of it's internal calibration. Generally speaking, you size the carburetor's CFM for 2 reasons, that's responsiveness and WOT power. If you're looking for mileage, then you look at different models of carb, not size. Make any more sense?
c.i.d. x rpm / 3456 = cfm for 100% volumetric efficiency No factory motor approaches 100% VE. That's NASCAR and NHRA ProStock stuff. Figure 85% VE on a Mustang 302, less on a truck motor. You won't turn 6000 rpm routinely on a stock motor, so let's skip that discussion. 302 c.i. x 4500 rpm / 3456 = 393.29 x .85 = 334 cfm at 4500 rpm You can have throtle reponse and economy if you choose wisely. If you want every HP out of that stock motor, you MIGHT use all of a 600 Holley at the Friday night bench races, but not at 6000 rpm with the baby cam gasping for air and the valves getting ready to float. (in case you are wondering, a 302 at 100% VE would pull 524 cfm, assuming that the heads, intake, exhaust, air cleaner, etc offered no restrictions) Try a small AFB or Holley, maybe even a 2bbl., oh wait, that would mean Ford had it right to begin with....
I ran Holleys for years and the 1850 is a good carb, if you have a good one run it, you can get it dialed in to work on the 302. My personal favorite is the QuadraJet once you learn your way around one they're the next best thing to fuel injection but you have to have the patience of Job and a big selection of metering rods, hangers and jets. Not for everyone. Or if it's 3 days before a cruise you've been waiting all year for, your carb floats decide to do their imitation of the Titanic and you find out after tearing the carb down Primitive Pete has been there first and it's FUBAR you get out the plastic and order a new Edelbrock Performer 500 with electric choke. My Studebaker 289 loves it.
try an Edelbrock 500 or 600 cfm with vacuam secondries....you can change the springs to make the secondries open sooner or later and you can change the metering rods to tune it to match your engine....
THANKYOU!!! These are wonderful, helpful education classes! And yes, the engine would be probably happiest if it still ran the factory 2 barrel...I mean, it is not modified at all. I feel dumb needing to ask these questions, since I got some extreme brain injuries in May from my 44K electrocution. These replies are like me getting my memories back, including realizing I am still an amateur car guy! Very helpful Shifterton and scottybacus!
everyone is saying 500-600 CFM....i say add another 750 on it. if more is better , too much is just right seriously... a 500 CFM edelbrock will be about right for you. i have one on the mild 283 in my `36 and it runs great
You might also look at the ignition system and idle voltage at the coil. A lot of the cars I troubleshoot with "rich carburetors" are running 10.5V (or less) at idle and that's really to blame for stink and soot. But then again short pipes, open cars, and double pumpers are just generally a recipe for burning eyes too. Good luck
Even using 5500 it still comes out less that 500 CFM. I remember reading an article way back that the 351C-2V carb is rated @ only 350 CFM. They put a holley 2 Bl carb (targetmaster?)on one that was also rated @ 350 & got better performance & milage that with the OEM 2 bl.
Don't overlook that answer. A bad power valve is the #1 most common problem for rich idling. If you can lean your idle screws all the way and the engine still runs, most likely your power valve has failed, and that's very common on Holley carbs. If the carb was allowed to dry out, then was filled with gas again and used, the PV is probably bad. Some Holleys have a PV on the secondaries, which doubles the problem. Double pumpers come jetted about 10% rich out of the box, and that contributes to the rich idle as well even though lots of people will say that jetting doesn't affect idle mixture. If you want to start with what you have, that 600 should be able to be made to work OK. Think of it as a 300 cfm 2 barrel when running on the primaries, the secondaries may never open on that engine with the stock cam. Put a new PV and gaskets in it before you install it.
My roadster had a stock 302 in it for over 20 years. Ran a small (525, I think) AFB (currently made by Edelbrock). Never leaked, always worked fine. New motor has more fur (aluminum heads, pretty big hydraulic roller cam, headers) and the next step bigger Edelbrock. I guess you could say I am a satisfied customer.
Thanks yoyodyne, this is more very helpful info. I plan to do as you stated, since I have a Holley 600 laying around. And I did have power valve trouble...just as you said.
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