Vacuum Advance Tubing Question

[Alpine James]

Hi All,

I had taken the piece of tubing from the carbs to the distributor off while doing some work and noticed that it is somewhat difficult to blow air through. It looks like it is the original 1/16" (or so) copper tubing with the threaded fitting at the distributor end and a rubber fitting for the carb end. I am wondering if this may be the cause of some hesitation when accelerating or if it is normal. My thinking is that with only a few oz's of vacuum from the carbs, any sort of restriction would negate its affect on the vacuum advance.

Thanks, James

 

[Pumpkin]

Vacuum advance

Hi All,

My thinking is that with only a few oz's of vacuum from the carbs, any sort of restriction would negate its affect on the vacuum advance.

Thanks, James

YEP!!

Go to your local parts house get the right size and replace it. You can't regret it.
Chuck

 

[husky drvr]

Hi All,

I had taken the piece of tubing from the carbs to the distributor off while doing some work and noticed that it is somewhat difficult to blow air through. It looks like it is the original 1/16" (or so) copper tubing with the threaded fitting at the distributor end and a rubber fitting for the carb end. I am wondering if this may be the cause of some hesitation when accelerating or if it is normal. My thinking is that with only a few oz's of vacuum from the carbs, any sort of restriction would negate its affect on the vacuum advance.

Thanks, James

James,

You should check to make sure that the vacuum unit actually pulls when a vacuum is applied. A bad diaphragm can give the same symptoms.

 

[jumpinjan]

That's the correct size, but its steel tube not copper. It sounds like its blocked. It would be easier to make a new one.

 

[gordonra]

Actually, the flow requirement for the distributor vacuum advance is extremely small because it's a a very small, closed cavity.

You might experience, but not notice a slightly delayed advance if it is mostly pluged, but it would almost certianly still work fine.

I have calculators that will tell the time, typical line pressure drop, and flow required to pull a specified vacuum, and you'd be amazed how quickly the chamber will evacuate when the volume is low.

If in doubt, put a vacuum gauge on it to see how fast it responds.

Rich

 

[sunbby]

I think a stumble is much more likely to be carburetion. A sticking or non-functional vacuum advance would give slightly less power but not cause a noticeable stumble or hesitation.

Additionally a small tube will have more resistance to air flow (more air volume = more resistance). The advance system isn't supposed to be moving any significant volume of air about, just enough to move the diaphram slightly. So even though you have a hard time blowing a lot of air through it, it is probably fine as long as you can blow _some_ air though it. It' similar to the idea of using very small hydraulic lines for the brakes. Basically you want to "transfer" the pressure not the volume, if that makes any sense.

 

[phatt]

vacuum line

If you are worried that it might be blocked, spray some berryman through the line using the tube on the can. If it is clogged then no fluid will come out the other side. When finished, put the line in the sun to dry out the cleaner. Be careful not to get it in your eyes. Wash your hands when done so as to not accidently get carb cleaner in your eyes by inadvertantly rubbing them.

 

[husky drvr]

I think a stumble is much more likely to be carburetion. A sticking or non-functional vacuum advance would give slightly less power but not cause a noticeable stumble or hesitation.

Todd,

Don't forget that when most vacuum diaphragms stop working it is due diaphragm developing a leak. You end up with no advance and also an air leak that upsets the transition fuel mixture.
Yeah, it stumbles.

 

[sunbby]

Todd,

Don't forget that when most vacuum diaphragms stop working it is due diaphragm developing a leak. You end up with no advance and also an air leak that upsets the transition fuel mixture.
Yeah, it stumbles.

Yes I agree, I was almost going to add that exact thought, but James originally asked about the tube being blocked. Thanks for pointing it out.

A leaky vacuum unit could definitely cause a stumble, but I think that stumble is basically a carburetion issue not an ignition advance issue, and an easy test. Just block off the vacuum port at the intake or carb or wherever it is and see if the stumble goes away.

 

[Alpine James]

Thanks all for the input. I think I will leave it as is for now.

I just passed my local gas station and see they have dug up the storage tanks, so I am wondering if I may have some bad gas that is causing the problem. I have heard a few people complaining about thier regular grade, but I havn't had problems with the premium in my Subaru.

Thanks, James

 

[Alpineracer8]

James:

The gas station you spoke of may be having a problem with leaking tanks. But, just as likely, there may be a law in your state that says that underground fuel storage tanks need to be replaced every "howevermany" years, or that they are required to upgrade the tanks to a better material by a certain date. A quick call to the state agency that regulates that business will yield you the answer. All you should need is the name of the station and the address.

How do I know?? I work in the Oklahoma Underground Storage Tank program for the State of Oklahoma. This kind of thing is exactly what we deal with on a daily basis.

Take care,