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wrightk20 
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Joined: 04 Jul 2010
Location: wisconsin
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 Topic: Tuning with a Vacuum Gauge
Posted: 27 Dec 2012 at 11:18pm |
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I have heard about tuning with a vacuum gauge and i'm wondering what the procedure is. I understand tuning in the idle mixture with the gauge to try and get the highest vacuum, but are you guys using the vacuum gauge to tune wide open throttle under load as well? I would think that at wide open throttle under load there would be 0 on the gauge with a properly sized carburetor, so i would assume tuning would not work with a vacuum gauge. I could very much so be wrong. Just wondering if anyone would enlighten me with the proper procedure. Thanks, Kevin.
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D-17_Dave
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Posted: 28 Dec 2012 at 2:06am |
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Your spot on. As soon as you open the throttle the vacuum drops. Automotive carbs have a port that comes off the venturi, this will give you 0 on idle and increase as you open the butterfly. But this does you no good for tuning, it;s an accesory port for selected attachments on the engine like the EGR valve.
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Yea, I can fix that.....
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CTuckerNWIL
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Location: NW Illinois
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Posted: 28 Dec 2012 at 6:20am |
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How could the vacuum drop to 0? If it did, the engine wouldn't be able to draw air or fuel in to keep running.
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mlpankey
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Posted: 28 Dec 2012 at 7:39am |
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you have to be reading actual manifold vacuum and no ported vacuum. Ported vacuum as already stated rises with butterfly opening. warm engine adjust idle mixture for the highest reading then adjust timming to highest reading then pull it back 1/2 to 1 inch to keep from detonating. Ctucker the answere is venturi effect . I dont adjust timming this way . Any time you advance timing the idle comes up. I adjust timing by sound and timming tape and light.
Edited by mlpankey - 28 Dec 2012 at 7:46am
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cotncrzy
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Posted: 28 Dec 2012 at 7:48am |
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At an idle for both?
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C, WD45,WD puller, 185, 200, 7060 Red Belly, 7060 Black Belly,8010, and a R52 Gleaner, AND PROUD OF THEM!
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mlpankey
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Posted: 28 Dec 2012 at 8:02am |
 cotncrzy wrote:
At an idle for both? |
at idle sorry should have said it but forgot to.
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Mrgoodwrench
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Posted: 28 Dec 2012 at 8:23am |
i've used timing tape on v8's but where do you install it in a tractor 4cyl like a 226...also what kind of timing you running, obviously it will be different with every set up just looking for a guide line
one other question when you say pull it bact 1/2 to1 inch i assume you mean retard the timing?
Edited by Mrgoodwrench - 28 Dec 2012 at 8:24am
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There are 3 ways to do job GOOD, FAST, CHEAP. YOU MAY CHOOSE 2. If its FAST & CHEAP it won't be GOOD, if it's GOOD & CHEAP it won't be FAST, and if its GOOD & FAST it won't be CHEAP!!!!
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mlpankey
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Posted: 28 Dec 2012 at 8:34am |
Mrgoodwrench wrote:
i've used timing tape on v8's but where do you install it in a tractor 4cyl like a 226...also what kind of timing you running, obviously it will be different with every set up just looking for a guide line
one other question when you say pull it bact 1/2 to1 inch i assume you mean retard the timing? |
retard the timming so the vacuum falls. with a degree wheel you can transpose degree marks with a hacksaw blade into the pulley once you 0 then you have the base line for tape you also need o fabricate a good non moveable pointer . the degree of modification will depend on how much advance initial timming needed. ofcoarse my initial is also my total.
Edited by mlpankey - 28 Dec 2012 at 8:37am
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wrightk20
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Posted: 28 Dec 2012 at 9:08am |
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So then this is only used to tune idle then?
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mlpankey
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Posted: 28 Dec 2012 at 9:15am |
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pretty much in my opinion
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Hudsonator
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Posted: 28 Dec 2012 at 9:47am |
About any method of tuning is futile these days if you are running "pump gas". The ethanol content and/or additives are so inconsistent, as to drive a carbureted/distributor engine crazy. Modern computer controlled engines don't have these problems, as they constantly "re-tune" themselves to accomodate the variable fuel quality. Its dang near impossible to tune a carb/dist combo to an "optimum" state these days, as that "tune" is only as good as the particular tank of fuel being run. At best, you can hit a happy medium, a low-medium at that.
As far as fuel goes, even if your pulling engine doesn't need it for octane reasons - find a good, high quality racing fuel. For no other reason than consistency. I don't pull anymore, but this fuel problem is driving me nuts with my gas-burning farm tractors, antique cars, and carbureted motorcycles.
A vacuum guage is great for tuning, particularly lean-best-idle and timing. What it does for your timing is compensate for altitude and atmospheric pressure. If you adjust your timing until maximum vacuum is reached, this is the best timing at idle for your altitude and weather conditions. The generic backing off 1/2 to 1" mmHg is to compensate for crappy fuel and crappy advance curves.
Theoreticly, if your distributor is curved properly to your engine and fuel - the vacuum gauge is the quickest and best timing tool you have, simply adjust to max vacuum. Even if your distributor is not curved so good, you can adjust your timing to get max power at whatever rpm you are running at max power, then back the engine down to idle and observe what your vacuum is - bingo - that's your vacuum setting at idle.
Lean-best-idle is simple as well. Adjust your timing to max vacuum first (regardless of whatever your final desired mmHg is), then adjust the idle mixture screw lean until you notice a drop in vacuum. Go rich until it reaches max again, then adjust your idle speed down. Repeat. You are looking for the highest vacuum reading you can get, as lean as your mixture screw will allow, at the lowest idle speed your engine can maintain. Once this is done, return your idle speed to whatever you want and return your timing to whatever mmHg you want to run.
About the only time I use a timing light anymore, is to correlate vacuum readings to actual distributor timing or on a "first fire-up" after having the distributor out.
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There isn't much a WC can't do.
WD's just do it better.
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Larry(OH)
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Posted: 28 Dec 2012 at 3:02pm |
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I was using it in my Jeep in the manifold runner to get readings, then tuned the power valve in the holley off of it. Also played with the timing some as stated, but not that much
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