The Stovebolt Site Forums The Shop Area Engine Shop Spark advance port

Hello, I have a siple question, I think. I have a reman Rochester 1 barrel for my 66 C20 with a 250, and I can't seem to find which port I can use for my spark advance. The model is 7040011, it has three ports on it, one in the base plate and two(one on top of the other) on the side of the throttle body. My understanding is that the port should have no vacuum at idle and should then increase to something above 16 inches once opened.....it has a manual choke and no other control devices on it. Which of the three should I use? Does anybody happen to "know" this carb and what each port is for? Thanks, Josh

Sounds like you have a Monojet. It didn,t come out till 68 but is a good carb. The lower port is manifold vacuum & the other 2 are probably ported vacuum. You can tell by putting a vacuum gauge on them. You have it right about ported vacuum but you may want to use manifold vacuum for dist. advance. If you are running a manual trans. try it. If it idles good & doesn,t surge when you rev it just off idle I would run it that way. Cap off all unused outlets.

Thanks for the reply. I wonder though, why I would use manifold vacuum if I have a manual trans? I do, however, have a manual trans. I didn't think I wanted manifold vacuum at idle because I would then get full spark advance at idle. Isn't that what I don't want? Thanks, Josh

Many engines use full advance at idle. The only time you don,t want vacuum advance is at full or near full throttle. That,s when vacuum drops off. That is why they use vacuum to control spark advance. It is basically for economy although it helps with driveability.

I always thought that most distributors used both mechanical and vacuum advance, and that the vacuum advance kicked in first because the distributor wasn't spinning fast enough to throw the weights out. My understanding has been that, any given engine spinning at idle needs X degrees of advance to ignite the mixture at the proper time in the cylinder. When the same engine accelerates up to a higher rpm, X degrees of advance was no longer the right time because the engine was spinning faster essentially making X degrees retarding the timing. And that's where mechanical and or vacuum advance kick in to rotate the distributor internals to fire earlier in order to iginte the mixture sooner. I understand that vacuum is high at idle because the throttle plates are closed, decreases as they are opened and basically the same as outside at WOT, but the engine hardly ever reaches WOT. Why set base timing then, if you are going to use full advance at idle? Would full advance then become the "base" timing? I guess I just don't understand why the manufacturer would recommmend base timing settings and have mechanical or vacuum advance if it's possible to just go with full advance at idle....why the two if you can just go full advance until vacuum falls off? It's just totally backwards of what I've always read, heard, and been told. I'm not saying it's wrong at all, just confuses me. Is there an article on this particular subject I could read, or is it an easy explanation? What's the thinking there? I appreciate any more light you can shed on my darkness. Thanks, Josh