Vacume Problem?

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mrtires24

mrtires24

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I have a 440 duster that I am almost finished putting together. I seam to be having a vacume issue. I thought I would pick your brains.

When I have been running the car it Idles ok, and you can punch the throtle with short bursts and it wil Back fire a bit ( open headers) but if I slowly bring the rpms up, It won't have any part of it, and all it wants to do is Back fire, and almost die, then come back then almost die, then come back, then....... When It is running like that i have all the ports on the carb pluged except for one, and I have it with a hose conecting it to the distributer, and it back fires when I have it like that. If I disconect it from the distributer it runs fine with no backfiring, I can hold it a 2500RPM nice and smooth, I can't do that if the distributer is conected to the Carb.

I can feal the end of the hose that is conected to the carb and it is sucking more and more as the throtle gets used. I pluged the distributer, and took the hose off and pluged the last carb port too, and it seams to run fine with NO problems now

Is that the vacume advance? if not, what is it?
and do I need to use it?
can I just plug it? seams to work ok that way.
If I don't need it, Why did it come with the distributer???

thanks

Phil
 
both my timing guns are broken, I have got it set by sound. I am sure it is off, but that has nothing to do with the fact that it runs Great one way but bad the other.

Phil
 
It sonds like your vacume pot is adding to much advance. you need to adjust it. There is an allan screw where the vacume hose goes on that you can turn to + or - The advance in it. You may also need a working timeing light. Setting the timing by sound is not very accurate. I have tryed it. then I put a light on it to check it and it was way off of where I wanted to be.
 
mrtires24 said:
both my timing guns are broken, I have got it set by sound. I am sure it is off, but that has nothing to do with the fact that it runs Great one way but bad the other.

Phil
Click to expand...

On the contrary it sounds to me like that has everything to do with why it won't run connected by your description. The vacuum advance affects timing. When you connect the vacuum advance it advances the timing. You probably have it over advanced so when you connect the vacuum advance it advances it more making matters worse. When the vacuum advance isn't connected the timing is closer to being correct. Back the timing off some, reconnect the vacuum advance and see how it does. You might even be better off setting it with the vacuum advance connected that way you'll hear right away when your getting over advanced.

The only other possibility is something is screwed up in the dist and when the vacuum advance pulls on the plate it causes the points or magnetic pickup (which ever you have) to go out of adjustment.
 
nope, It didn't matter where I had the Dist set. even if I had it turned all the way to the left, or the right, and everything in between, it allways backfired. Without the vac adv, it runs perfect.

My timing gun kinda worked today, and I got it at 10*. it still runs bad with the vac adv hooked up, but mint with it pluged.
 
Did you adjust the vacuum advance pot? Also, is your vacuum advance hooked up to source above the venturis?
 
So lemme get this straight. With you vacuum advance hooked up to ported source, you have a stumble at light throttle. You should be able to "step through" the flat spot and the engine pulls fine from 1/4 throttle to full throttle. You remove the hose from the dist and cap it. The stumble goes away.

If this describes you problem, the vacuum advance unit probably has a leak and needs to be replaced. Verify with a mitghty-vac or similar hand pump.

If this does not describe your problem, it's not your problem. Check PVC valve, accel pump shot etc.

What is your carb/cam/intake combo? What is your vacuum at idle? What is your timing?
 
Right now I don't have it hooked up at all, but I don't know whetre a ventrus is. I had it hooked up to a small port in the front of the carb

My carb is a 750 with vacume secondaries. The cam is a crane came, but nothing crazy. I have the dimensions on another post that I have somewhere, but not sure where right now.


That isn't really what the engine is doing. It will idle ( with vac hooked up) you can smap the throtle ( with vac hooked up)
You can't slowly increase the throtle, or hold it at any thing other than idle ( with vac adv hooked up.

With everything pluged it works MINT!

I talked to me mechanic and he said to just plug it, and I don't need it. He is a sertified mechanic for several years. Not just some back yard mechanic.

Phil
 
mrtires24 said:
Right now I don't have it hooked up at all, but I don't know whetre a ventrus is. I had it hooked up to a small port in the front of the carb

My carb is a 750 with vacume secondaries. The cam is a crane came, but nothing crazy. I have the dimensions on another post that I have somewhere, but not sure where right now.


That isn't really what the engine is doing. It will idle ( with vac hooked up) you can smap the throtle ( with vac hooked up)
You can't slowly increase the throtle, or hold it at any thing other than idle ( with vac adv hooked up.

With everything pluged it works MINT!

I talked to me mechanic and he said to just plug it, and I don't need it. He is a sertified mechanic for several years. Not just some back yard mechanic.

Phil
Click to expand...

I think your vacuum advance pot is set too sensitive. When I put a new vacuum advance pot on my 318 it was too sensitive and would add too much advance before the vacuum built up sufficiently. Try sticking an Allen wrench in the nipple for the vacuum advance pot and see if you can turn something inside. I'm not sure about which way does what, but play around with it and see if you can make it better.

Vacuum advance is not crucial, but will save gas when cruising.
 
If it ran bad any where you had the timing set it's either the vacuum advance is bad like Chief said or it could be what I mentioned earlier about something mechanically screwed up in the dist. causing the magnetic pickup to go out of adjustment when the vacuum advance pulls on it. I had one do that once but it's been 20+ yrs. ago so I don't remember exactly what was wrong. I just remember when the vacuum advance pulled on the plate it went cockeyed and caused the points to go from .017 clearance to about .030 and when I fixed what was loose it was fine. It was a points type dist but the principal is the same. Do you have another dist. you can try in it? If not you can take the dist. cap off and manually apply vacuum to the vacuum advance and see if the pickup adjustment changes. It should be approx. .008 if I remember right.

The ported vacuum on a 750 Holley is the one on the passenger side of the primary metering block. That's where you should hook up the dist. vacuum.

Like Mopekid said you don't have to run the vacuum advance but it does help on gas mileage and also gives a little part throttle boost in power.

BTW: The adjustment inside the vacuum advance doesn't change how many degrees it will advance. It just changes the sensitivity level. Screwed all the way one direction causes it to come in earlier because it takes less vacuum to pull it in. Screw it all the way the other direction and it won't come in until later in the rpm range but it still advances the same amount no matter where you set it.
 
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