Vacuum Advance question

[suburble]

All-

I am missing the "Distributor delay valve" shown in the vac diagram below.

My understanding is that this valve will have the vac advance running from ported vacuum until the engine gets up to temp and the thermal switch kicks it over to direct manifold vacuum.

Is this correct? Or do I have it backwards and it runs on direct until the thermal switch kicks it over to ported?

If it is running from direct vac after warm-up, I should be able to cap the two lower ports of the "EFE / Dist TVS" as shown in the vac diagram and run the vacuum advance directly from manifold vacuum without seeing any difference once the engine is up to temp, correct?

A little experimentation has shown that, with the base timing at factory spec, the 'burb runs / drives MUCH better with the advance connected to manifold vacuum than when connected to port "H"on the vac diagram.

Thanks in advance (no pun intended),
-Josh

 

[bucket]

I personally would run the base timing advanced a bit and hook the vacuum to ported. Or just locate the correct vacuum valve and run everything stock.

 

[Boone83K10]

Do you really need to hook it all back up to pass emissions?

 

[suburble]

Do you really need to hook it all back up to pass emissions?

No, they will only check the hoses on the Vapor Can and the EFE, if I remember right. But I do need to hook the vacuum advance up somewhere.

When I bought the 'burb the P/O had hacked a bunch of stuff and the vac advance was not working at all. he had advanced the base timing to compensate. It had been hooked up to port "H" per the vac diagram on the air cleaner, but was not functional.

-Josh

 

[Boone83K10]

Thank god there is no emissions where I live, only safety inspection.

 

[Driver4r]

Are you still running the Q-jet?? If so, this is where the vac advance goes. (the small hose)

 

[suburble]

Are you still running the Q-jet?? If so, this is where the vac advance goes. (the small hose)

Thanks.

 

[suburble]

Passed emissions this morning!

It passed the test but was running like a dog. It would stall out unless I drove like there was an egg under the pedal.

I'll start isolating things this weekend and see what the culprit is.

Thanks for the help everybody.

-Josh

 

[Driver4r]

Thanks.

Your welcome.

What was it running like?? Surely it wasnt missing or it would have failed emissions. Is there to much initial advance mabey(weights sticking). Or mabey some crap in the carb?

 

[suburble]

Your welcome.

What was it running like?? Surely it wasnt missing or it would have failed emissions. Is there to much initial advance mabey(weights sticking). Or maybe some crap in the carb?

Feels like it's pulling an anchor. Sounds like it may be pinging under load. I know the carb is shot- I've got another I'm working on rebuilding if I can just find the time. I'm just about positive the accelerator pump is leaky or completely shot.

I'm thinking the Harmonic Balancer may have slipped the outer ring and my timing may be off. I'll find true TDC when I can spend some time on it. Also planning on running a compression test just for kicks.

I unhooked the EFE butterfly vacuum line and that seemed to help a little.

I also should probably check the fuel filter.

 

[suburble]

Your welcome.

What was it running like?? Surely it wasnt missing or it would have failed emissions. Is there to much initial advance mabey(weights sticking). Or mabey some crap in the carb?

I guess I didn't answer your questions, really-

-Not missing, but non-existent acceleration, and pinging under load.

-I'm sure the initial advance is OK, as checked with the advance unhooked.

-Could well be crap in the carb.

Thanks,
-Josh

 

[rich weyand]

Use manifold vacuum for the vacuum advance. Ported vacuum is nonsense. No such thing back in the day. Why? Because that's not the way to do it unless you are trying to finesse idle emissions numbers. Ported vacuum will give you an off-idle stumble until the advance catches up.

Read this:
http://www.corvetteactioncenter.com...ported-vs-manifold-source-vacuum-advance.html

 

[350runner]

You very well could be running lean. I'd get that other carb rebuilt.

Sent from the dust in front of you!

 

[suburble]

Use manifold vacuum for the vacuum advance. Ported vacuum is nonsense. No such thing back in the day. Why? Because that's not the way to do it unless you are trying to finesse idle emissions numbers. Ported vacuum will give you an off-idle stumble until the advance catches up.

Read this:
http://www.corvetteactioncenter.com...ported-vs-manifold-source-vacuum-advance.html

Yeah, it was running even worse when I had it hooked to ported. I'm running it off manifold vacuum and still having these problems. I'm going to verify that the TDC mark on the balancer is correct before I try to tune it any more. It ran MUCH better with base timing set at about 12-15degrees or so (according to the marks on the balancer, anyway).

-Josh

 

[rich weyand]

I'm running 18* BTDC base timing. More is better, until you get knocking, then back off 2 degrees.