The Stovebolt Site Forums The Shop Area Engine Shop Dual Rochesters b

Having an issue with a stumble of idle, lean condition feels like. It bogs down then takes of. Does drilling the idle circuit help with this ? It's on a offenhauser intake. Both carbs rebuild, Acc pump working as it should

Sounds like the carbs are too big.
If you go from one carb to 2 without changing bore or stroke, you need to use a step smaller carbs (in CFM) or you just give the engine too much air.

I think Jim is right on this one, the air is moving so slow when both throttles open, it can't pull any fuel in with it. Try opening the idle mixture screw some more. If you can get it to idle really rich, you might over come the off idle lean. If it does work, then you are going to have to figure a way around the rich idle. My dual mono-jet carbs were way to big for the 250, one was plenty, it ran rich all the time. Full throttle felt really good, as did normal driving, problem was you could almost see the fuel gauge moving.
I now run dual W-1 Carters, a much smaller carburetor. Fuel mileage went way up along with all the problems of the Mono-jets.

What type of linkage are you running? Maybe you could go with a progressive type with the back carb slightly behind the front with a slightly quicker opening ratio. At highway speed both would be open the same or close to it, but off idle one would lag behind to off set the airflow of both opening at once.

Buy a good fire insurance policy.
Jerry

Here's an easy way to size your carbs; find out the CFM rating of the original stock carb. Take 2/3 of that number and find 2 carbs with that rating. (if the stock is 300 cfm so get 2 200 cfm carbs = 400cfm.)
With headers and 2" pipes the engine will breath well and the carbs will function properly.
I used an Offenhauser manifold on a 235 speciffically because they offer manifolds with either small mounting base or large mounting base, that way I bought the small base and could put 2 W-1's (much better than Rochesters, sorry)that were sized for the 216 and they worked fine.

Thank you for the replies. It's a stock 235, Fenton headers. The linkage looks like the kit you buy with the offenhauser intake. I will try and play with some more.

Easiest way is to install 1(one) carb on the engine.

Ed

This link may help:

http://www.thecarburetorshop.com/Dual1barrelcarbs.htm

Jon.

Thank you Jon, I looked at that. I found a lot by searching and tried couple of things. I ended modifying the carbs. It's getting better, ran out of time. I will do more tomorrow. The reason I messed with the carbs is that if I do damage them then I will try and get 2 216 carbs or zenith carbs. I'm trying to use what came with the truck. I bought it like this. I don't think it ever ran correctly. I found couple issues so little by little I'm getting there.

Silenpro slow and steady wins the race. Sounds like your on your way to making it right. Can't beat that.

This is true Martin. I almost have it bog free. It was my idle / throttle that was off. So one carb had diff vacuum then the other. Set the linkage so that both are equal and started again with a vacuum gauge. It's so much better. Then I picked up edelbrock uni-syn. Tomorrow is another day