The Stovebolt Site Forums The Shop Area Engine Shop Duel carb problem

I have two single carbs on a 235 with a fenton intake. The forward carb has tons of vacumm and the rear carb has almost none. This is a newly built engine and im starting it up for the first time. What would cause the one carb to have vacumm and the other none, when they pull off the same intake? My vacumm advance is tied into the forward carb and the rear is blocked off. Does that make a difference? It also seems that forward carb is pulling a lot more fuel, but i can't verify that yet. Cecil.........

Cecil,

My guess would be that the throttle plates are in different positions. The rear is more closed than the front. How are you measuring vacuum....with a uni-syn device.

Is the carburetor linkage nice and tight. I have always disconnected the throttle linkage and backed the idle screws all the way off, letting the throttle plates close in the bores. Adjust the idle screws back up making sure both are the same.

After the carburetors are adjusted, the linkage is merely matched to the carburetors. Good linkage is a must. Poor linkage will throw things for a loop.

Stuart

I think your on it Stuart. The throttle plates are different at idle as you said. I never checked the throttle plates, i was only looking at the choke plate and the throttle linkage. I will reset everything as you recommend and test tomorrow. It will probably purrrr. Thanks, Cecil.........

How is the vacuum being measured? Try disconnecting the linkage between the two carburetors and see how it operates with one of the carburetors completely closed, as if it were a secondary with progressive linkage. Then switch carburetors and see what happens. If the throttle plates are closing properly you should be able to run the idle mixture screws all the way in. Some secondary carburetors have no idle screws or idle fuel passages.

Hoyt

Hoyt brings up a very valid point. So that brings up this question; What kind of carbs are you running? Are they a matched pair? I personally had an issue with my Offy setup when using Carter YFs. Supposedly, according to the re-man company (Tomco), they were the same carb for the same application. However, one of the carbs was originally a ford application (even said "Manufactured by Carter for Motorcraft" ). So it was jetted way too rich for my application, even if it only had one carb on it. I have since switched out to Holley Model 1904s (which my Dad had told me to use in the first place) and am much happier with them. Rebuilt them myself rather than using remans to ensure that they are both the same. I also rebuilt the linkage to ensure it was opening them the same as well. Still need to do some minor tweeks to it, recently aquired a Uni-Syn to assist with that.

Carbking has a new page on his site that may prove useful:

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

Also be sure when you sync the throttle plates at idle,open em up all the way {motor off},and see if they open evenly at wide open,with linkage hooked up between em both.I've seen lot's of em not working together through the whole range.Tom