Found out the hard way (as usual with most things hot rod) that dual quads are a little tough to make run right. I'm running a pair of rebuilt, small 350cfm (or something like that) 4bbls - originally used on Buick 215 aluminum V8s - with progressive linkage on a vintage Offy small runner manifold on an early 283 with power pack heads & Isky 270 HL cam. I was going for a late 50s/early 60s authentic set up for my '40 ford pickup. The problem is, the thing is a dog. Actually I knew it would be, in part due to the trans/rear end I'm running (turbo 350, ford 8 inch with highway gears). Both are due to be replaced soon with parts more compatible to the engine, specifically a s-10/camaro t5 highbrid trans and a 9 inch with 3:55s . . . but the 301/triple deuce engine I had previously ran with this same drive train pulled harder than this new engine with the dual quads, and it had a worn out cam. After putting about 1000 miles on the new engine, it starts running really rough. It seemed like an ignition miss, so I went through everything - actually even replaced the distributor and coil as they were old anyway. No improvement. I know part of the problem is too much fuel pressure - I was running an electric fuel pump straight to the carbs (use of an old Hurst cradle style engine mount precludes using a block mounted mechanical pump). I hadn't had problems doing this with my old set up (Rochester 2gcs), but these carbs were a whole different story. So I installed a Holley fuel pressure regulator and firewall mounted fuel block, but somehow some little metal shavings got into the carbs and screwed up the needle/seats . . . maybe from the fuel block? No idea where they came from, but obviously a big part of my rough running issue. I ordered a cool little fuel pressure gauge from Marshall Gauges so I'd have a clue what I was setting the regulator at. By the way, their customer service was great, and I litterally had the gauge at my front door the next day - man, I was impressed! So today, I'm going back through the carbs again to make sure needle and seats are good, floats are ok and set correctly, and that there aren't any foriegn artifacts anywhere in the fuel system. So, beyond fixing my obvious mistakes with fuel pressure, trans and rear end gears, what does it take to make these set ups pull? I would really appreciate any input you guys would offer - Thanks in advance . . .
Caution: those gages (those little liquid filled ones) read WRONG when they're HOT - so depending on where you mount it - you may want to ignore it after the motor's running for any length of time. Multiple carbs can also let to much air in too fast depending on all your variables - make sure you have enough PUMP SHOT to deal with that and it should do wonders.
Any Offenhauser dual quad sbc intake I"ve ever seen was a single plane design. A dual plane manifold may be better suited to your setup.
Carbs alone should not make a motor run rough. Running rough is usually a sign that one or more cylinders are not firing properly. If you think the carbs are too much, try blocking one off. Make sure that you block it off from fuel and air. If it runs better, you need to tune the carbs to your engine. I ***ume that the carbs are Carter AFB style, these should only pull enough air/fuel as needed once you are off idle. If the motor just started running rough, I would be looking elsewhere for the problem. If you have stiff valve springs, it is possible you wiped the cam out-very common lately as most oils do not have nearly enough Zinc in them (do a search on oils/zinc) I think most engines should be able to run ok with two small 4bbl carbs especially if run progressively. Although often a single 4bbl will outperform 2x4s, they should work fine and not cause serious drivability concerns.
Thanks for the input guys: I was wondering about how engine heat would affect gauge accuracy, and also was thinking that a dual plane design would work better for the RPM range I drive at, but it was a compromise (and for $75 for the manifold, I couldn't beat the deal). As for the cam, the thought that it may be whipped out was in the back of my mind, but I guess if I can't get it to run right after eliminating all the other variables, then it will be all that is left. I wouldn't be surprised, although I did put a special additive in the oil during break-in to help prevent trashing the cam, but you never know . . . Thanks also for the carb tuning tips. I appreciate all the info and advice I can get.
I run a stout 302 Aluminum head motor in the 32 with dual 500cfm Edelbrocks with progressive linkage on an Edelbrock manifold, and other then a little cold blooded (no chokes) it runs and drives great. Pulls very hard, no stumbles, no drive ability issues.
My friend that built them (AFBs) said they were something like 350cfm each, so I'm still getting 700 cfm even with small carbs, which is more than the 283 needs, but they were about the smallest carbs I could find for a dual quad set up. At this point, I think other issues are causing most of my problems. Hey Shoprat, that is a absolutely beautiful Ranchero, brother. You are a lucky man
Okay, now I'm confused. Buick/Olds 215s never came with AFB carbs from the factory. Sure they're not Rochester 4GCs?
built a t-buckett for a customer, not your average t, high zoot. anyway dual plane two 500 edelbrocks, 302 mostly stock. little ******* was a dog, black smoke at idle, would load up and foul a set of plugs a week. so we cut the idle air off on the front carb, didnt help with the dogness but it did solve the loading up problem. several months after the engine developed a faint poot from the left bank header just above idle. we did every thing to try and find the problem, first the obvious, plugs, wires, msd module, cap ****on.... worse poot same plug. adjusted valves, new springs, nothing. i even tried to move the poot to another cyl. by moving #1 on the dizzy. nothing, we tried a points dizzy and a stock coil and wires. nada. finally we adjusted the front carb back to the original setting and set the progressive linkage to work both carbs the same. BINGO. still loads up a little (1000 cfm) but the loss of poot and the unbeleiveable boost in power is well worth the little bit of idle smoke. no more progressive linkages on this car.
They're definitely Carter AFBs, not Rochesters. I'm no expert on the 215 v8s, it was just what the guy I got the carbs from told me. He rebuilds vintage carbs for a living and has been hot rodding since time began, so I tend to listen to him. Regardless of where they came from, the idea was using the smallest 4bbls I could find for the 283 to try to keep throttle response decent and hopefully not over-carb. Exactly what I've been going through - actually my situation was worse than a "poot", but still same idea. THanks for the input - I'll look at a linkage fix, too. I rebuilding the carbs tonight, so we'll see what happens tomorrow . . .,
sounds like you have several things to "fix" until you really know what you need to do to the carbs. Bad needle and seats will make you pull your hair out. I had three bad. Thing sounded like a pro stock idling but wouldn't run right. Make sure you run enough timing, get a big gulp of fuel and air...
This has become just freakin' humiliating. Here is the latest: Replaced the needles and seats in both AFBs (the carbs only had about 2000 on a complete rebuild, but I'm getting desperate), placed a Holley fuel pressure regulator inline (adjusted way down until I could get a pressure read) and installed a new fuel pressure gauge on my firewall fuel block to see what I had going to the carbs, then turned on the pump (can't use a mechanical pump with the vintage Hurst engine mounts I'm using)... within seconds of the pump starting up, I've got fuel dumping out the secondary carb (but not the primary), but a read of 0 lbs pressure on my gauge! Back to square one. I know these are simple carbs, but they are frustrating me to no end! Ideas? Thanks again in advance!
Did you try blowing on fuel inlet of the offending carb with it off the car drained of fuel and upside down. You should not be able to blow through it. If you can then there is no sense putting it on because it aint going to hold. Havent done a AFB in a few years but they work the same. Are you sure your float drop is right and the floats are not bent or hitting the bowls?
First, thanks for the compliment on my 57. It had dual quads most of its life. 30 some yrs ago it had 500 cfm Carter Comp series on a 390. It ran 12's back then. Your car should run good on those. Talk to your carb guy. I have a hunch who he is. My Bro lives up your way. o fuel pressure sure won't work. Does sound like more problems than just carbs. I always liked 2-4's myself. Keep us posted. Ron
Thanks for the ideas to try, and also for the link to the Carter carb set up PDF! I really appreciate it. I've had this dual quad set up running great in the past (although not as well as the 3x2 Rochesters I had on my last engine) so I'm just not willing to give up on them just yet. I'll try again tomorrow . . .
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