Strange tuning response on my engine?

[patrickt]

Originally Posted by DanEC

Hmmm - is it that simple? I have to admit that makes sense. I'm so accustomed to engine rpm dropping when I pull the vacumm advance line off for my gage due to the timing fall out, I was kind of startled when rpm jumped up on my non-vacuum advance distributror.

So, much ado about nothing.

Thanks

OK, last year I was at a Ford festival and was chatting with a Ford technician who was deep in to his seventies. He said the way they adjusted the carbs at the dealer on the original 427 4-V cars was to adjust the idle mixture screws the exact same amount on each side so that when you pulled the cap off the manifold vacuum the idle did not go up, nor did it go down. He said "try it and see." I've used a vacuum gauge to adjust my carb for decades, but last year I did just as he suggested -- and I have to say my engine has never run better.

[DanEC]

Patrick - I'm not sure I understand how that would work? If the idle is jumping up because of additional air pulled in through the vacuum circuit below the butterflies when uncapped - how would it ever be able to be equalized? What did you do other than just back your mixture screws out equally by 1-1/2 turns, or 2 turns, etc?

[patrickt]

Originally Posted by DanEC

Patrick - I'm not sure I understand how that would work? If the idle is jumping up because of additional air pulled in through the vacuum circuit below the butterflies when uncapped - how would it ever be able to be equalized? What did you do other than just back your mixture screws out equally by 1-1/2 turns, or 2 turns, etc?

My traditional vacuum gauge method was to adjust the mixture screws to achieve the highest vacuum at idle. That worked reasonably well. But, with a large overlap cam, vacuum is low and the response of the gauge needle is not instantaneous.

The old Ford service tech's method is, at least on the internet, almost totally undocumented. The only doc I could find that said the same thing was Tip #3 here: 12 Carb and Fuel System Tech Tips - Muscle Car Review Magazine

Now, I will say the "pull the vacuum plug" method gives you instant feedback. It was exceptionally easy, it resulted in different idle mixture settings from what I had with the vacuum gauge, and my car runs astoundingly well adjusting it his way. For me, and my setup, that is the way I will always do it from now on. It may not work for others. The only reason he mentioned it to me was that, after we chatted for a while, he remembered the specs of my original Ford "K" cam, along with a single four barrel, and how they dealt with getting the carb set just right when the cam was causing a lumpy, low vacuum idle.

I must admit that, at the time, I did not think his technique had any merit. But testing it out was as easy as pie....

[DanEC]

No. 3 goes back to my original question - is the idle mixture too rich? I guess I can experiment with leaning it down and see if the rpm jump lessens when I uncap the vacuum port.

[patrickt]

Originally Posted by DanEC

No. 3 goes back to my original question - is the idle mixture too rich? I guess I can experiment with leaning it down and see if the rpm jump lessens when I uncap the vacuum port.

If you pull the cap off the manifold vacuum, and you hear it sucking clean air through the little tube, and the RPM jumps up, then "yes" your mixture was not at the optimal level for idling, it was a little rich -- but maybe your engine might like a little rich mixture -- I don't know. Now remember, my car doesn't have a vacuum advance but, obviously, if you're changing your timing by pulling the vacuum feed off the distributor that runs to your manifold vacuum then that would have to be taken in to account. If I had a vac advance distributor, I would just isolate that out of the equation by plugging it off while I adjusted the carb. I guess they didn't have exhaust gas analyzers at the Ford dealerships in 1963? Anyway, this is not a tough test to perform. And if it doesn't work for you, just go back to whatever method it was that you were using before. Of course, you should perform all your testing and adjusting with the air cleaner on.

[DanEC]

I'm using an old 427 dual point distributor - it doesn't have vacuum advance. But with dual quads getting everything dialed in at idle is a bit more of a chore. But overall it seems pretty responsive and running fine - nothing really wrong with it.