Tuning question-Increasing air:fuel ratio and its affect

[John S.]

I have my '71 bus with a stock rebuilt 1600 running well; I am happy but have a question. I have about 1,100 miles on the rebuild, new 34 PICT-3 from Volkzbitz and a newly-rebuilt (by me) 205Q with both vacuums working.
I have the mixture screw out four turns and the bypass out two turns; idles steady at 800 rpm.
Question: when revved it advances as it should and comes back to 5 degrees ATDC.
If I richen up the mixture even half a turn, when revved it takes quite a while for her to return to 5 degrees ATDC. Same with turning the bypass out even half a turn, it advances when revved but fights to come back to an idle at 5 degrees ATDC. When either screw is turned in it does have the expected affect and idle drops, so the mixture seems correct.
Have I just hit the sweet spot on the somewhat finicky 34 PICT-3?
She runs like a champ as it is set now.

Thanks for your time

[modok]

yes, I have noticed overly rich setting does take longer to return to idle.

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Normally lean will be slow to return to idle speed. If it does it when richer there is likely an undiscovered vacuum leak.

I'd also look for a sticky cent advance.
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[John S.]

Here is the WebMD syndrome again...
One guy says yes, that's all it is...another says no, you've got yet another problem...the joy of forums. No wonder the ancient Roman Empire didn't work out too well in the end.
I just completely tore down and rebuilt the distributor. I think it's good; and it only does this when the screw is turned out just a bit further from what appears to be the sweet spot.
I have checked for vacuum leaks everywhere.
The only place it maybe (maybe) could have one is from the somewhat dodgy brake booster line leading up front out of the engine compartment.
Where would I get that type of line so I could replace the whole thing?
It does run well and the carb seems to be set correctly, judging from "turns out from completely in" thought train.

[Alstrup]

Throttle plate most likely not entirely closed. That's why it is slow on the return when a tad rich

[John S.]

Thanks, I backed the throttle screw off about 3/4 of a turn.
We'll see how that goes.

[John S.]

Seems a touch better.
She is running the best she ever has.
I think I'll ride like this for now and monitor the situation.

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an engine cannot idle fast without air. So a fast idle is automatically an air leak (or throttle not closing) somewhere, at least.
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[John S.]

Agreed, the throttle plate adjustment seems to have helped.
I have checked and rechecked for vacuum leaks; haven't found any.
I would also think you would have to back out the mixture screw quite a bit more (seven turns or so) to compensate for an air leak, if one were present.
It runs and idles well. The only issue I had was the slow return to idle.

I'll run like this and see how it goes.

Thanks again, people.

[mark tucker]

many engines were made to do this. if you have hit the sweet spot stop effing with it, as you already have it licked and it's sweet.