The Stovebolt Site Forums The Shop Area Engine Shop Dual intake - Choke & Vacuum Advance

Got a few questions for installing an offy dual carb intake to my 235.

Two carbs means also two chokes and two vacuum advance inputs.
How can I connect both choke valves to one manual pull - opening and closing in sync?
Should I split the vacuum line with a T-fitting and feed both vacuum inputs at both carbs?

On a past thread w/pic, I saw a setup, where only one choke was connected to the nearest carb and the vacuum line was connected only to the other carb. Can't see if the vacuum input of the other carb was plugged or left open.
Don't both carbs have to be in sync at all times? :confused:

You can generally get away with choking only one carb, as long as the passage between the two is big enough, but you will want to tie the vacuum advance together. Get a 'tee' fitting, and connect the vacuum advance of the carbs together and then tee off to the distributor. My motorcycle has four carbs with the chokes all tied together but it doesn't have balance tubes between the cylinders so it needs it. If you have balance tubes the engine will suck the rich mixture from the choked carb. It just may not be quite as effective as two choked carbs, but it should work in most cases.

Josiah's advice is good.

My dual Offy came with fuel and linkage kits. These were made for Carter carbs not Rochester carbs but I got over that. One set included a plug for one of the carbs' vacuum "outlet". I plugged the outlet in the carb nearer the firewall and connected the vacuum advance line to the carb nearer the radiator. It works fine.

I do not know what is correct but I have only one carb choked (the one nearer the firewall) and I have no problems starting the engine. This was convenient for connecting the original choke cable.

The throttle cable is connected to the accelerator linkage rod (similar to how it is connected in a one-carb, stock setup). This resulted in not as smooth a movement as in the stock setup but it works OK.

Tim

Interesting.
I haven't purchased the offy plus linkage kit yet and didn't know it came with a plug for one vacuum fitting.
I wasn't sure if there was a big difference in plugging one or having it connected in parallel.
If there is one, it must be negligible.
Why else would the kit come that way, since 50-some years?

Using only one choke makes sense, since the fuel vapor of the two carbs gets mixed before the cylinders anyway.

Thanks for the info.

The only reason to connect the vacuum advance in parallel is that it improves the response time a bit and evens out the pulsing action from each carb more or less only feeding 3 cylinders.

Thanks - I really appreciate, when somebody knows that extra bit of info that actually makes a difference.
Now I just have to contact my vacuum line supplier to see if he can make me the two extra lines with the proper endings.

Siegmund,
This is not strictly relevant, but is a similar situation. I once had a tri-power setup on a 348.(3xRochester 2 bbl.) The center carb was the only one with choke, curb idle and vac.advance. The front and rear carbs did not have any of that and were on a progressive linkage for power only. I guess that backs up what the guys have been saying, although it's a bit different with twin carbs on a 6.

Most all 235 multi carb manifolds are like the Offenhauser and are a common plenum design.Only one carb needs to have a line run to the vacuum advance.Most guys use only one choke if using a manual cable.