hesitation

[mudman]

I have a '70 307 with rochester 2 barrel carb. Problem I am having is with engine hesitating or stalling when throttle is stepped on from idle. I rebuilt the carb which didn't help. There seems to be a good stream of fuel from the accelerator pump ports. I think that it has to be a fuel problem because it runs good when engine is cold and choke is on. Could it be timing?

[guitarman]

Could be the vacuum advance is doa or the mechanical advance is corroded and sticking.

[jhow66]

Is your pump rod moving at the same time as throttle. Pump could be alright but not in sync with throttle blades.

[mudman]

Pump rod does move at the same time as throttle. Fuel squirts out at the instant the throttle moves. How do i check the vacuum advance?

[stllookn]

Timing light will be required and a tachometer or tach/dwell meter. The specs from your tune-up manual will show you vacuum advance and mechanical advance at different rpm's. Check total timing with vac plugged and then with it just off idle where vacuum kicks in. See if that matches up with the specs. I can't remember what they are for my 283 but probably not the same depending upon transmission, idle solenoid and pollution hardare.

[Truckstr]

mudman,

You can check the vacuum advance by removing the distributor cap so that you can see the point plate. Then take off the vacuum hose that leads to the vacuum advance canister at the carburetor. Suck on the vacuum hose and see if the point plate moves with the sucking action.

If the diaphragm in the canister is good, then the point plate should move. If it's not then you'll just suck in air through the bad diaphragm. It's important that the point plate move. If you think that the diaphragm is good, but the point plate doesn't move then either the point plate is stuck or the arm connecting the canister to the plate is broken.

While you have the distributor cap off, take the rotor off to see if the mechanical advance is working. There should be two cenrifugal "fly" weights that pivot on little pins there, with two little springs holding them there. The "fly" weights should move freely about the pivots and the springs should be attached at both ends.

If thw weights don't move freely, then carefully take them off and buff them with steel wool put on a little dab of oil or WD-40.

Did you know that those little springs determine how quickly the mechanical advance comes in? Lighter springs (available at most auto parts stores and speed shops) can bring the mechanical advance in more quickly, while heavier springs delay it.

Also, Crane makes a neat little adjustable vacuum adavnce canister to help tailor the vacuum advance curve to your driving requirements.

All in all, it's pretty straight forward.

Have fun!

Hope this helps.

Jeff

[Lippyp]

Have a good look around first. My truck developed a similar problem early this week, turned out that a little rubber cap covering the unused timed vacuum port on the base of the carb (Carter AFB 4bbl) had split and I was losing vacuum, which was fouling up the vacuum advance. $0.63 later for a new rubber cap and she is running better than she has for a while. Check all unused vacuum ports for leaks, could be a simple fix.