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vacuum gauge

October 3 2017 at 12:05 PM
lee  (Login calcoral)
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does anyone make a more sensitive vacuum gauge than the regular vac/fuel pressure that are commonly available ? and or a larger dial? sometimes its hard to get the mixture just right on my carb but when you do dang its good....lee

 
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AuthorReply
eric
(Login wwyin62)
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search...

October 3 2017, 12:33 PM 

stewart warner motor minder vacuum gauge on ebay

 
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Gerry
(Login gerryproctor)
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Sure. You want a digital gauge.

October 3 2017, 1:49 PM 

You could probably find some analog gauges if you search. I know what you mean with the resolution of the standard size gauges. I use an 8" Robertshaw scientific gauge, but they don't make them any more. I also made an electronic gauge using a MAP sensor feeding my oscilloscope. I don't recommend that route because sometimes the resolution can be too much and there's also math involved.

 
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A70
(Login a70eliminator)
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Tuning the carb

October 3 2017, 2:43 PM 

I have to ask only because I'm not sold on the whole tuning by vacuum.
When your turning the mixture screws to achieve highest vacuum. Which I have done, it always comes down to being abe to hear the engine labor if vac drops and smooth out when vac increases the gauge really just doesn't help much imo.
But I'm always learning stuff here.

 
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Gerry
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You're basically correct.

October 4 2017, 6:34 AM 

It's best to tune to what the engine wants. Instruments and data are great to help you establish a baseline and to accumulate tuning data. The one mistake I see being repeated here and elsewhere is when someone gets an AFR meter and tries to tune the engine to reach a data point, like 14.7 idle AFR. Well, what if the engine just doesn't like that? Are you going to stick with that even though the engine is hard to start and has a lean misfire? Probably not. Yes, I have an AFR and yes, I tried to tune the idle circuit as lean as possible using the gauge. The 390 just does not like anything above 13.9 AFR at warm idle.

As far as the vacuum gauge; In my experience, leaning out the idle mixture to get highest vacuum usually leaves you a little lean on the idle circuit. I normally go to highest vacuum leaning it out, then go rich until the gauge drops around a quarter point. This seems to make the idle circuit rich enough to assure easy cold starts and a clean warm idle.

 
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Falcon67
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Should not be that hard

October 4 2017, 7:43 AM 

I use a old vacuum gauge about 2 1/2" in diameter, had it for years. Turn in the screws until you see an RPM drop, then open up to smooth/best vacuum reading more or less, then another 1/8~1/4 turn to stabilize. If you try to nit pick it, you'll never get to drive it. It's not that critical. As soon as you pull away, the idle circuit become a footnote to transition and the mains.

 
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A70eliminator
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Re: Should not be that hard

October 4 2017, 8:07 AM 

Sight travels faster than sound so you'll hear the rpm drop about a milisecond after you see your vac or rpm drop, only if you have your sense of hearing.
Imx never have see a drop in vac or rpm without hearing a change, even the slightest ventury whistle.

 
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lee
(Login calcoral)
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vac gauge

October 4 2017, 6:37 PM 

thanks for all the good advice...lee

 
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