uum at Carburetor for vacuum advance

[Arl37]

Trying to check out everything on my newly aquired 1969 C-10 307. Vacuum at carb has very little pull on my finger. Timing mark dosent move when I increase speed. Trying to figure whether it is the vacum advance mechanism at distributor or something plugging up carb. Tried to blow out vacuum port at carb ( on vehicle) but didnt produce any change. Any ideas? Thanks

[camrycurt]

You might be looking at two related issues here: ported vs. manifold vacuum and a faulty vacuum can. Are you checking this vacuum source only at idle or at varying speeds? Does the vacuum change at different speeds?

If there's no vacuum at idle it may just be a ported vacuum port on your carb and you might want to look around and find a manifold vacuum source. There's tons of debate one way or the other, but I switched my vacuum advance can from ported to manifold vacuum and it improved my ride tremendously.

You may also just have a faulty vacuum advance can. You can check to see if it will hold vacuum with a hand pump. But if it's hooked to a good vacuum source and isn't adding advance, it sounds like the diaphragm may be bad. You can pull it out of the distributor and hook it back up to the vacuum line with the engine running and watch to see if the plunger moves.

[Arl37]

Thanks . Thats very helpful.
Posted via Mobile Device

[geezer#99]

Even easier way to check vac pot. Hook up hose to manifold vacuum. When you hook it to vac can feel under the can. the rod from the can into the dizzy is on the bottom side. You should feel movement. Or with motor off hook vac hose to can and suck on the hose. You should feel the rod move and when you release your lung powered vacuum you should hear the vac can snap back.
Ain't trucks fun! LOL!!!

[54blackhornet]

Geezer I sent you a pm concerning manifold vacuum gauge hook up…Jack

[Mnguy]

The vacuum advance should bump the timing advance as soon as the engine starts. Your centrifugal advance isn't working either if the timing doesn't advace from idle to 2500 rpm or so.

[Steeveedee]

11 yo thread. I sure hope that the OP got it figured out.

[YellowK5]

Originally Posted by Mnguy

The vacuum advance should bump the timing advance as soon as the engine starts. Your centrifugal advance isn't working either if the timing doesn't advace from idle to 2500 rpm or so.

You are assuming the vacuum hose is connected to manifold vacuum ..if it is on venturi vacuum it will only advance when you open the throttle.