I recently purcahsed an Offy 3x1 intake from Josh (SinisterCustom). It's going on my stock 250 Chevy inline. (It does have a split exhaust if that's any help.) My question is, will 3 stock carbs be too much for this motor? One option would be to block off the center carb and run just the outside ones, but Josh pointed out, in doing that the middle cylinders might be starved for fuel. He suggested running 3 carbs from a 216 as they have smaller bores. Has anyone had experience with this type setup? I'm not in a postion to make any internal changes to the motor right now so I'll have to work with what I have.
a stock 250?? or do you mean a 235? You can run all 3. You get a big increase in power by adding a second, but a miniscule increase (over 2) by adding a third. It won't be too much
[ QUOTE ] One option would be to block off the center carb and run just the outside ones, but Josh pointed out, in doing that the middle cylinders might be starved for fuel. [/ QUOTE ] No more than the end cylinders with the stock set up. A progressive linkage with 3 carbs would be my first choice.
Donzie- I think "Unkl" has the right idea. I have the very same manifold for the 292 I'm getting ready to put together for my RPU, so I'd be interested in hearing what you finally decided on - and how you like it. The progressive linkage that came with it is a bit lame. I'm hoping to figure out a system that's a bit more slick. I can't imagine that any three carbs on a stock 250 would be anything but big-time overkill. I'm expecting to have to do a good bit of "massaging" to get my set-up to run just right - and I'll have a hopped-up motor to work with. I had dual carbs (Edmunds manifold) on a stock 235 (straight linkage) and it was seriously over-fueled. A real dog until it picked up speed and cleared itself out a bit (even then, it was overkill). BTW - If you want to block off any of the carbs, I'd keep the center one. That's how the stock manifolds are, and they seem to "wet" all the cylinders equally. My 2-cents.
I have a 235 inline chev, just put dual 1bbl rochesters on it and now have a lot of popping and backfiring. I think its just over carberated and im wondering if anyone has a link showing how to build a progressive linkage setup for these carbs. I havent changed the timing because it ran so well with the single carb.
Does it have a vacuum advance? If so, that could be part of your problem. More Carbs=Less Vacuum Drew
When putting multiples on stock or near stock engines, keep in mind the CFM of the original single.You should choose carbs enough smaller to equal the original CFM. The stock engine can't draw properly with 2 or 3 times the venturi area, no matter how you jet it down. Gemini EFI
3 small carbs set up together (non-progressive) will give you the best atomization and fuel distribution. back in 1980, emissions standards went up. The only inline six to survive was basically the port injected Jeep. Why? Center carburetted inlines run the outer cyls lean at low RPM and rich at high RPM, due to inertia of the air/fuel charge. Factory settings were averaged to accept this, but not good enough to pass stricter emissions. What I'm trying to explain is that at high RPM, the fuel gets "slung" out in the intake towards Cyls 1 and 6. Check your plugs to verify this.
when i was reading califonia bill's book on 216's they said while experimenting with dual and triple carter intakes the difference was minute when going from 2 to 3 carberators with a stock cam. but when going from 2 to 3 carbs with a different streetable cam the difference was much larger. maybe putting a larger cam in that 250 would do a bit of difference? Just thinking out loud........
It seems to me that the best atomization would be on a 2 carb setup. There is no direct runner to a cylinder. Therefore all get the same amount of gas. Am I wrong?
Grafter, what year/model rochesters? It does matter. My wife ran 2 Rochester B carbs with a straight (not progressive) linkage on her 51 and it ran great. Dickster on the HAMB built the carbs for her with the right jetting and was a tremendous help with the troubleshooting.
On an engine with siamesed intake ports, I'd run two, connected on a log manifold, syncronized. That's what works the most evenly with most sixes firing order. Spaced sorta like this: O:O:O:O:O:O (O=cylinders) -P-O-P-O-P- ( P=intake port, O=carbs) Chevy 6 firing order 153624153624 132312132312 port order Notice the syncopated order of the port order relative to the firing order With three, the linear unevenness of the firing order "ignores" some carbs some of the time, slowing down any constant ram effect venturi pressures and flow. With two the distance between the ports allows the carbs to alternate "duty" better more of the time during the firing order. (Tuned and timed port injection is better...)
If your considering a multi. carb setup please check out inliners on moding 1bl. carbs for this app. Different carbs but the same idea's will work. I had 3x1s on a 261 in a '55 chevy. My dad moded the carb along the same way and it ran REALGOOD. Just a thought.
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