Manifold vacuum

[70spider]

I have the little guy running pretty good and decided to fine tune him. I have never used a vacuum gauge to set the carb but thought I would give it a try. My question is what kind of intake manifold vacuum should I expect? At the moment I have the carb jets set at stock and I am getting a reading of 15". The gauge I have has 18" to 22" as normal idle vacuum. I live at 6000 feet above sea level so I was going to adjust the jets 1 size lean. and then recheck the vacuum. Any suggestions?

[18Fiatsandcounting]

Many years ago on my '69 spider, I had a vacuum gauge installed in the passenger compartment, and reading the vacuum off the intake manifold. I seem to recall that I could never get it higher than 17" at idle, and I also recall that the gauge listed 18" to 21" as normal. However, I think this was for a larger American engine, and so I figured 17" was about right for a Fiat DOHC with its particular camshafts. I was at 1000' or less of elevation, and so it wouldn't at all be surprising if you see less vacuum at 6000'. In other words, 15" might be normal.

As for jets, yes, at 6000' feet you probably need to re-jet the carb. One, possibly two sizes smaller. For instance, if you have a 55 idle jet, you might need a 50. If you have a 135 main jet, you might need a 130 or possibly 125. Although I understand the air corrector jets, getting these sized properly is something of a black art, and so I just left them close to stock. Same with the emulsion tubes. Getting the right ones almost certainly involves having an engine on a test stand with all the appropriate diagnostics. You might be able to sense some very small differences in driving around, but I doubt that the trouble is worth it.

-Bryan

[70spider]

However, I think this was for a larger American engine, and so I figured 17" was about right for a Fiat DOHC with its particular camshafts.

that was my thinking as well. My 351 Cleveland that has a moderate cam only makes 14" @ idle. So, I'll shoot for 17.
I had the carb leaned out to much (plugs read a lean condition) so I returned the jets back to stock and was going to use the vacuum to dial it in better. I will go with Primary main @ 130 and the Secondary @ 135. I agree on the air jets I'll go with 170 and 165 and see what the plugs tell me.

[Nut124]

The idea that one could jet or tune a carb using a vacuum gauge is new and strange to me. Where does that come from?

[70spider]

Where does that come from?

https://www.youtube.com/watch?v=aBbD_Jl ... %27sGarage
So when I leaned out the air mixture screw according to Uncle Tony, I went for 10" to 12". So I thought to myself if I lean out the jets one step at a time and repeat the procedure until I get the best possible vacuum I'll be good. So after jetting it to better match my altitude of 6000 ft. above sea level, I have 16.5" . The idle is good and throttle response is good, so I am happy for the moment, after all it is a Fiat.

[Nut124]

I'm glad you are happy but one cannot jet a carb at idle nor based on manifold vacuum. Manifold vac can be used to set idle mixture but one can do that by observing idle speed just the same. Best mixture = highest idle speed.

At idle, only the idle mixture screw and the idle speed screw are active. The jets do nothing at idle and you could swap them all day and not see any difference at idle. Jets must be adjusted based on dyno or road testing.

[70spider]