Help! Vacuum advance distributor


Been working on Dad's Gehl 3510 skid steer with a Ford Industrial motor. Don't have a timing light but I thought I should hear the difference when I pulled the vacuum tube off with it running wide open. There is vacuum on the tube and everything is free in the distributor. So should I hear a difference or will I need a timing light to know if it is working?
 
Vacuum advance is "advanced" when vacuum is high like when idling or low speed operation. If you have a vacuum tester, the easy solution is as follows. With the dist. cap off, look at the points. as vacuum is applied, the points should rotate in opposition to rotor rotation. Jim
 
Depends on what system engine has, is the vacuum line to the intake manifold, for a full signal at idle or to a carb port which may or may not be ported vacuum signal, meaning none at idle, vacuum just above idle, then no vacuum as the load increases? Best use a vacuum pump and a timing lite to see if it's working after checking all the hose connections.
 
On them OLD FORD engines ya set the curb timing with a light at the slowest ideal you can get . Then you hook up the vac line and IF the Vac advance is working you should get about 9- to 11.5 degrees more when the carb vac over comes manifold vac. Now don't ask me what the total should be as the old feeble mind has lots all the spec.'s many years back , that is why they make BOOKS for us old farts that can't remember anymore . The big thing here is to make sure that the diaphragm is not leaking as with time they get dry rotted and leak and will not hold vac. and they will not preform .
 
I always just suck on the vacuum hose. Of course you always sterilize it with a red rag first! LOL

But you should be able to hear the engine speed up a little as the timing advances, and tell if the advance will hold vacuum by holding your tongue over the end of the hose.

If yours has points, those old Ford distributors were notorious for the plastic hinge points on the advance plate to wear out or heat bake and crumble. Once that happens the points won't stay set. The plate is easy to replace, used to be readily available.
 
Easier yet - I take off the cap and rotor and suck on it. You can see it advance and then put your tongue on the end of the hose to hold pressure and see if its losing it through a leak.
 

With the engine running wide open I would not expect much difference - you should be on centrifuge advance schedule at high RPM. The vacuum advance schedule operates at lower RPM. Check at idle for a manifold vacuum system or just off idle for a ported vacuum system. You should note a difference there.
 
Vacuum advance is going to be greatest when vacuum is greatest: at idle. Assuming the diaphragm isn't leaking, there isn't much to worry about. If it fails to advance the engine will use a little more gas at low power settings. If the operators I've watched are any indication, the vacuum advance on a skid steer doesn't see much use.
 
Just for the sake of argument, is it manifold vacuum advance coupled with centrifical advance. I will assume it is , but some systems used venturi vacuum and no centrifical. Manifold vacuum is high at low idle, light load, where venturi vacuum is highest at wide open throttle. Old Ford flat heads use venturi advance but IH used it on one of their large truck engines also. It was a weird system on IH as they used two vacuum chambers inside the distributor and you needed a vacuum pump to check timing advance. Regular manifold vacuum advance systems are pretty easy to test if you have the specs what vacuum it should start and end advance but just sucking on it will usually get you by.
 

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