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vacum gauge guru's please help
i'm tyring to get a good vacum reading in order to dial in my air mixture screws. i plug it into my constant vacum port on my demon carb, after i plug the pvc port. the needle jumps back and forth radically and i am not able to get good reading. anyone know why????.. thanks
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A camshaft with a lot of overlap (i.e. a big lope at idle) will cause a vacum gage to bounce back and forth.
Why are you plugging the PCV port? You can also use the tach to adjust the idle air screws. Just adjust the screws until you get the max RPM reading, this will be the same point that you will get max vacum. You will need to go back and repeat this procudure as you fine tune the carb in. |
thanks tom, i thought i read somewhere its good to plug pvc. ill try it left alone. the cam is 248 at 050. overlap 28.
race demon 750. #75 front #76 rear + power valves front and rear. the fastest idle is with 4 corner screws almost seated!!! i ordered smaller idle feed restrictors in order to lean out idle. but that is a couple week order. the local speed shop claims that they are just holly emulsion jets and will fit as idle feed restrictors also in my demon.. hmmmm. so i should set idle mixture where idle is highest?? |
on my 650 Demon there are 2 vacum ports on the passenger side front of the carb, I think in the base plate, I have one plugged and the other connected to the dash mounted vacum gauge......... I do use a hand held vacum gauge under the hood to adjust my carb, works great, gauge doesn't lie, can see a vacum change with as little as 1/4 turn sometimes.................
David |
well, it seems to be reading low. fluxuates between 4" and 12"... new engine. i must be doing somthing wrong.
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I have never worked on a Demon carb, but if it is like a Holley you might want to check for a vacum leak. An easy way to check for a vacum leak is to spray starting fluid around and see if the engine changes the way it runs. If the engine is running lean it will speed up, if rich it will slow down.
Yes you want to set the idle screws where the engine is at the highest RPM. However, if the RPM gets to high you will be out of the idle circuit and will need to turn down the idle speed (by closing the throttle shaft). How well does the engine run? How does the engine idle? |
it runs really great, new motor built by tom at fe specialties, especially due to switching to the 750 with smaller jets. it did have a 850 on it. the egt's got a little higher with the #73 jets all the way around and 750cfm. i was a little concerned about that, but water temp stayed about 180-190'. we also added a power valve to the secondaries.
the idle is strange, being that air mixture screws so close to seated. demon recomends 1-1 1/2 turn out. its almost seated as is now. now after reading the vacuum that is 4"-12", i'm wondering if the power valves are open during idle, not sure if that is possible. the valves are probably 6.5's. wouldnt they be open if idle vac is 4"-12"???? thanks for your help... |
Please explain "the idle is strange"
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because i have to screw the air mixture screws in so far. demon says that the screws should be set at about 1-1 1/2 turns from seated. they say anything under 1 turn out means a rich idle condition. so i guess this will cause richness in just more than idle. now i wonder if it has something to do with manifold vac being low, mabey causing a power valve to stay open. thanks for your help. ken
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Low manifold vacuum
With 4" to 12" of vacuum and a 6.5 power valve the power valve will be opening when the vacuum goes below 6.5" of vacuum thereby making the idle screws ineffectual.
Go lower on the power valve with a large overlap cam. Been there done that. Cheers, Bryan |
My Demon is a little better....3/4 of a turn.My off idle is still kind of rich. I dialed in 18 deg of initial timming which helped a bunch.
John |
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1) Change power valve so it does not open at idle
2) To reduce the vacuum gauge swings, increase the volume between connection on motor and the gauge. Make a vacuum can and insert it in the line. |
well, i did a few things to my car that made a big difference. just about there i think.
first, i re-curved my msd dist. i put the black bushing instead of blue that was in there. i put two silver springs in it, instead of one blue one silver. the old setting was initial timing at 17, total at about 38 all in at about 3600 rpm. this new setting is 19 initial, and 34-35 total, all in at 2400-2500 rpms. the 19 degrees allows my transition slots to be squared like they are supposed to be. i also drilled 1/16" holes in all four butterflies. you were right about having all timing in at about 2500 rpm's. the power band is there for all of the pedal(except for tiny flat spot at end of primary pedal.). it seems like max torque for my engine is about 34-35 degrees, as opposed to 38 degrees as before. it pulls real nice in all gears with secondaries opened. at 1000 rpm's idle i'm slightly in advance already, but doesnt run on during shut down. with the new holes and advanced timing it runs real smooth. the vaccume jumped up to about 14-16 inches also. the increased vaccume allowed me to use a gauge to dial in four corner adjustments. i wonder if i could jet up now with dist re-curved... |
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