So I'm curious...for setting my idle mixture on my 7040207 the procedure says to "plug the vacuum hose from the distributor" or similar. My vac advance line goes from the canister on the distributor to a fitting high on the front of the carb. Do I plug both the line and fitting on the carb when tuning, or just one of them?

 

You plug the port on the carb. If you leave the hose connected to the carb, plug it. If you remove the hose from the carb, cap the port.

 

Something doesn't jive to me....


When you're setting the ignition timing, you disconnect/plug the line to the distributor's vacuum canister, so you don't have any vacuum advancing the static timing. When you're setting the fuel mixture, as far as I know, your only concern is that there aren't any vacuum "leaks" altering the mixture settings.

 

leadfoot4 is correct - timing is set by omitting vacuum advance, fuel/air mix should be set with car idling correctly, vacuum advance connected.

What really does not jive for me is the comment of the vacuum advance port being 'high' on the carb body. Does that port have vacuum at idle? if not, that advance canister ain't doing diddly. The correct port on my carb (7040205 - a '70QJ regardless) was located adjacent to the divorced choke assembly.

Regardless, the port it's connected to MUST have vacuum at idle. It must suck big time!

The original OP question; block off the carb side, not the canister side if you want to disable advance but not cause a massive vacuum leak.

 

Thanks guys.
Spinadog...port is in the same location on my 7040207. That's all I was after...making sure I didn't have a vac leak when trying to set my mixture and idle.

 

There's a few things I don't understand here. The vacuum port "high on the front of the carb" is ported vacuum, so it's not drawing vacuum at idle, anyway, so why would anybody bother to disconnect it to set idle? Also, even if it were drawing vacuum at idle, why would you want to set your idle mixture with the ignition timing different from how it will be when running normally? I think that if you want to set your idle mixture, just pop the hood, take off the air cleaner, and adjust it. Am I way off base here?

One thing's for sure, if you email Lars, and ask him for a copy of his paper on how to tune a Q-jet, you'll have that thing purring like a kitten in no time flat.

Scott

 

Brcmpbl, no worries, sounds like it's the correct one - a longer than normal port on the passenger side of the carb, right?.

The point all are making in this thread is still valid; no need to disconnect the vac advance if all you are doing is adjusting the fuel/air mix. A vacuum gauge is a good investment here, to tune for the highest reading possible at idle. Plenty of experts in this forum who can advise better than I on this subject, Lars for sure.

scottyp99, the port 'appears' located fairly high on the carb body but is not ported vacuum. it's a bit of an illusion too due to the intake manifold being recessed away from the carb at that viewpoint.

 

As you can tell I'm new to carb tuning...I did in fact buy a vac gauge, and I have Lars' tuning paper for reference. Can I hook the vac gauge up to the fitting on the carb? Or should I hook it up to the vac fitting on the centre / rear of the manifold? My stock L-46 manifold only has the one fitting...has manual brakes, etc.

 

"Ported" (or timed) vacuum is a relative term. The intent of a ported vacuum signal is that there is none at idle...due to the venturi port being just above the throttle plates when the carb/engine is at idle.

But, if the throttle plates are just a little bit open, then there will be a small vacuum signal going to the distributor--or to the solenoid switch which should prevent it from going to the distributor, if you still have that contraption hooked-up.

Anyway, with the hose connected to the vacuum can, you can't be absolutely sure that there is NO vacuum signal getting there. So, you are to BLOCK that signal from getting there by plugging off that line. This also prevents a vacuum leak, as you have disconnected a line that could be connected to manifold vacuum (or something less).

Bottom line: You don't want ANY vacuum signal getting to the dist. advance can for the timing adjustment (assuming that you do it per GM's recommended method). Once set, you can then hook it back up and test that it works properly with a bit more than zero throttle.

Personally, I would never choose to operate my C3's engine with NO vacuum on that advance can when at idle. It causes the engine to run too hot (the purpose of ported vacuum...so that excess hydrocarbons are burned at idle), it uses more fuel, and it causes unnecessary wear for the engine. But, that's just my opinion....
________________

P.S. For those choosing to set up timing per Lars Grimsrud's paper [for best engine 'performance'], none of this is important...as setting the timing at idle is not included in the process.

 

I must have missed something, I thought he was referring to the port, high up on the carb body, on the driver's side front. I'm pretty sure THAT port is timed. Anyway, I still say don't worry about it, just make sure your vacuum advance is hooked up to manifold vacuum and set the idle mixture in the car's normal, running state, and you'll be all set. If I remember right, manifold vacuum at idle to the vac can will advance ignition timing at idle, which allows a leaner mixture at idle.

Scott