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Vapor Canister

1K views 7 replies 5 participants last post by  Mike Romain 
#1 ·
So I was over in the Weber tune up thread and we were discussing the finer points of tuning a carb with the vapor canister attached. In my case, I was able to get the Weber tuned up as the OP described and all was well with 0" of vacuum at the S port. Than Matt drops the bomb, is your canister working???? So I went to the Sprynet Jeep Adventures Under the Hood site and checked out the vapor canister page. The test listed in that site was to pinch the main purge line and if the idle stumbles then the canister need to be replaced.

I little about my setup, I have the main purge hooked to the PCV valve and the Purge Signal runs off of the 5 port CTO that is tied to the S port of the Weber 38.

I completed the pinch test and the idle took a nose dive. SO it got me thinking, why does this happen??? What exactly is happening in the canister that causes the idle to dive when the main purge line is pinched. I opened up the 1983 FSM and read the following: From 1J-6

The canister has a staged dual purge feature (fig 1J-8). Two inlets are provided, one for fuel vapor and one for carburetor bowl vapor. The outlet is connected to an intake manifold vacuum source. The fourth nipple is connected to the carburetor spark port (ported vacuum).

When the engine is operating, fresh air enters through the inlet filter in the bottom of the canister and purges the stored vapor.(emphasis mine) When the ported vacuum reached 12 in. Hg (41 kPa), the secondary purge circuit is opened, and the canister is purged at a much higher rate.
Seeing that the main purge line is hooked to a manifold vacuum source it is drawing in air through the canister from the moment the engine is started which is confirmed in the above listed paragraph (see emphasis). Would it not stand to reason that if it is supposed to be drawing in fresh air from the canister from the moment the engine is started that if you pinch the line it would be restricting airflow that it expects to be receiving causing the idle to fall.

It would be different if the FSM did not state clearly that the canister is venting while running through manifold vacuum. Although it does not state it that clearly, one can infer that since it is hook to manifold vacuum then by default it is always "purging" and that when it is presented with the appropriate amount of ported vacuum that purge rate increases.

Using that reasoning, does it not render the pinch test invalid? Seems to me that it does but that is me using my walnut sized brain trying to wrap my head around exactly what happens in the canister. To me it is always drawing air through the canister because of the manifold vacuum.

Those of you that really understand this, please chime in and tell me what I am missing or I don't have laid out correctly above. I attached a snapshot of the FSM for review.

Gary
 

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#2 ·
Gary,

You know, I've never given it that much thought. I suspect it has to do with the relative size of each circuit in the canister.

I broke two of the nipples off my original canister when I swapped engines, so it needed replacing. I never paid much attention to how it operated.

I suspect you could use a vacuum gauge and some compressed air to more thoroughly check whether yours is working.

Matt
 
#3 ·
When the canister purge has failed, it makes a huge vacuum leak that really affects the idle or causes surges in the idle as it raised the revs and turns on the S port.
When I have done the pinch test, I expect a little drop in idle maybe, but it is real little from what I have seen, but if there is a huge drop, it is bad.
There are also different flavours of canisters. It has been posted before that some of the stricter emissions ones have the dual purge and others, line mine are a single. I live in a country where an 86 CJ was a utility vehicle. It only has one air filter snout flap with no tubes to the cat or cat even, no overflow bottle on the rad, basic canister, etc.
 
#4 ·
I may be missing something myself, but it sounds as though the "pinch test" is to determine whether the secondary purge is working properly. If manifold vacuum reaches 12 in, the secondary circuit opens. If you close off the main line and your idle takes a dive, then in my mind, the secondary circuit is not working properly.
 
#7 ·
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