The completely stock 1970 LS5 454 was initially idling erratically.I removed and plugged the vacuum port at the rear of the manifold to check for vacuum leaks in the larger vacuum system and I noted no difference in idle performance. I also placed a rag over the carb at idle to check for the idle smoothing out or rpm increasing, and the engine rpm dropped as would be expected if no vacuum leaks are present. The idle mixture screws will shut down the engine when screwed all the way to the stop. I also checked for bad spark plug wires and they were found to be good. I next checked the timing. After removing and plugging the hose, I noticed no drop in rpm. I re attached the hose and checked the timing, then removed the hose and checked again and noted no difference in either timing or rpm. When the rpm is increase above idle to the 2000 to 2500 rpm range the engine runs normally. I also recently installed new R43TS plugs.

I then attached a vacuum gauge to the manifold vacuum source for the distributor and noted 15'' of vacuum. I then removed the cap and rotor and attached a hand held vacuum pump and observed the vacuum advance canister move the pick up c;il starting at 7" and achieve full travel at 15". This is in accordance with the specs which I have access to.
The distributor is modified to utilize an MSD magnetic pick up coil and a remotely mounted GM 4 pin ignition module. I have check the pick up and it shows 550 ohms. I have also swapped out modules with no change. The module currently on the car shows dwell at idle of 10* and about 28* at 2500 rpm.

I am scrapping the bottom of my shallow bag of tricks and will be very grateful for any help that anyone can offer.

Thanks

 

Odd that the vacuum advance is functional with a hand held vacuum pump but not with a known 15" manifold vacuum source. Is it possible when the dist'r cap is put on and clamped down that there is some not so obvious interference preventing the vacuum advance from advancing? Strange. Are the weights and springs for the centrifugal advance moving freely and snappy?

 

The dwell moves from 10 to 28 or the timing?

 

As best as I can determine, the vacuum advance is moving with the engine supplied vacuum. I say this because I can hear it snap back after I remove the vacuum hose from the cannister.

 

It is the dwell. While looking into how to determine if a partial failure of the module was possible, checking that the dwell incresed with rpm was one way to test the module.

 

The dwell needs to be 30 at idle. Set it...then reset your timing.

Jebby

 

Is there a way to adjust the dwell when the distributor is equipped with a gm 4 pin electronic ignition module?

 

My bad....I assumed points, that is what I get for not reading the whole thing.
Will closing the throttle blades kill the engine?

Jebby

 

No. The module changes the dwell automatically depending on RPM and alternator voltage.

 

No, the module is working correctly.

Are you sure vacuum advance is connected to the manifold vacuum nipple on the carb and not the ported vacuum nipple?

 

I have not tried that yet. I have only been able to kill the engine with the mixture screws.

 

Yes, I did confirm that the source is manifold vacuum.

 

Put a timing light on it, disconnect the vacum advance and see if the timing changes.

 

Centrifugal advance sticking? Or kicking in and hunting at idle?
Check timing at idle, with vac adv plugged.
Raise idle slowly to 1000-1100 rpm, adv should not change yet, but should start soon.
Needs to be stable for 200 rpm above idle.

 

I will try that. Thanks.