Why no change in RPM's??

**Patrick H.** · June 20, 2017, 02:22 PM

Re: Why no change in RPM's??

Ed,

Are you actually getting vacuum at the end of the hose that you're plugging into the canister?
I know that sounds obvious, but you don't mention checking that as well to be sure that what's plugging into the canister actually is providing vacuum.

Patrick

**Edward B.** · June 20, 2017, 02:33 PM

Re: Why no change in RPM's??

Yes, that's correct Pat. The hose that's going to the Mityvac is coming from the VAC so I know that works. I even used the Mityvac to engage the VAC while the engine was running, but again, no change in RPM's! I'm completely stumped on this one.

I also forgot to mention that the distributor is the original 1111954 and the VAC is the stock 1115360 (marked "360") canister that's all in at 12" Hg.

Ed

**Duke W.** · June 20, 2017, 04:20 PM

Re: Why no change in RPM's??

The balancer may be suspect. I recall that the timing notch should line up with indexing pin that you can see if the pulley is removed, but need someone to verify.

**Edward J.** · June 20, 2017, 05:31 PM

Re: Why no change in RPM's??

Ed, could it be that the mechanical advance is stuck, or already advancing @ 700? so you may not see any RPM change? Just a thought.

**Michael W.** · June 20, 2017, 05:50 PM

Re: Why no change in RPM's??

Originally posted by Edward Bertrand (58273)

Over the weekend, I tried to change my L36 4 speed TI distributor from ported vacuum to full manifold vacuum. I tee'd off the hose coming from the intake manifold to the vacuum canister for the headlights and the windshield wipers since I know for sure this is full vacuum.

In theory it should be full vacuum but rarely is due to leaks. The headlights will raise and lower with very little vacuum, far less than the advance canister requires.

I'd choose another source in any case as the check valve in the headlight system will keep the system at maximum depression, not the varied signal that the ignition timing requires.

**Jim D.** · June 20, 2017, 06:14 PM

Re: Why no change in RPM's??

With the engine running, disconnect the line AT THE VACUUM canister and hook it to a vacuum gauge. If you do indeed have full vacuum, then the problem lies in the distributor. If you don't, the issue is with the way it is plumbed or possibly the "T" you used is defective/plugged.

**Edward B.** · June 20, 2017, 07:12 PM

Re: Why no change in RPM's??

Ed, could it be that the mechanical advance is stuck, or already advancing @ 700? so you may not see any RPM change?

The weights are nice and loose, so I don't think it's that. They open and close with finger movement.

I'd choose another source in any case as the check valve in the headlight system will keep the system at maximum depression, not the varied signal that the ignition timing requires.

Yes, I thought of that too, which is why I used the Mityvac to advance the VAC instead of the engine.

And another issue, but I don't think would have any bearing on this, is that when I had the engine rebuilt, the builder showed me the balancer and it was shot! So I went out to Autozone and got a Dorman 594-030, a "direct replacement" for the 69 Corvette 427 (except the special high performance engines). Take a look at the TDC marker in the picture below. It's 30 degrees off!! (The rotor is pointing to the number one spark plug wire on the distributor cap in the second picture below).

There's NO WAY it could be 30 degrees off or the engine wouldn't run!! If I try to move it toward the zero marker on the balancer, the car ends up stalling and won't restart unless I put it back to "30 degrees". I just looked online at the Dorman catalog, and they say this balancer comes with two keyways, so I'm thinking the engine builder installed the balancer on the wrong keyway. Even though I don't think this has anything to do with the RPM's not changing, it still needs to be resolved, so this weekend I'm going to have to pull everything out (AIR pump, pulleys, fan/clutch, shroud, etc...) and see if I can get it right.

Ed

Attached Files

**Jim D.** · June 20, 2017, 08:23 PM

Re: Why no change in RPM's??

While a balancer issue will affect your timing readings, it has nothing to do with the issue you have described. You still have to find out if the problem lies with the vacuum source or the distributor. I hate to simplify this but it's not rocket science. It's one or the other.

**Edward B.** · June 20, 2017, 08:35 PM

Re: Why no change in RPM's??

It's not the vacuum source. I tried ported vacuum, full manifold vacuum and even the Mityvac and the RPM's didn't change. I can't see how it can be the distributor since the pole piece is moving when vacuum is applied to the VAC.

Ed

**Jim D.** · June 20, 2017, 08:47 PM

Re: Why no change in RPM's??

Originally posted by Edward Bertrand (58273)

It's not the vacuum source. I tried ported vacuum, full manifold vacuum and even the Mityvac and the RPM's didn't change. I can't see how it can be the distributor since the pole piece is moving when vacuum is applied to the VAC.

Ed

I wish you lived close so I could fix it for you. The bottom line is - if your idle doesn't increase substantially when the vacuum line, that is hooked to MANIFOLD VACUUM, is hooked to the vacuum can (vs. not connected), there are only two possibilities. It's either the vacuum source or in the distributor, including the vacuum can. Again, it's not rocket science. A comparable is - my engine uses 1 qt of oil per 500 miles. It's either burning it or leaking it. No other possibilities. Sorry if I sound rough but it really shouldn't be hard to diagnose.

I had a buddy with a 59 that would idle all day long and not overheat but when he drove it, it got hot. He took it to a local "expert" that replaced the radiator, thermostat, hoses and water pump. It still overheated. He gave me a call and I fixed it in a couple minutes with some steel wool and WD40. His advance weights were rusted and he had no advance above an idle. Don't over think a simple problem.

**Mark E.** · June 20, 2017, 09:49 PM

Re: Why no change in RPM's??

Have you verified with a timing light that the timing advances when vacuum is applied to the VAC?

My bet is it does not.

If this is the case, connect the Mityvac directly to the VAC and open the dist. cap window. While applying vacuum, you should see the breaker plate move. If it does not, the VAC is bad or improperly installed, or the breaker plate is frozen.

**Duke W.** · June 21, 2017, 10:01 AM

Re: Why no change in RPM's??

I think the damper is the prime suspect. When Ed sets the initial timing at the OE specified 4 BTC, it's actually more, so adding vacuum advance increases total idle advance far beyond what is ideal, which is why revs don't increase and the engine may run worse.

BTW, Ed, since the damper is calibrated you don't need a dial back timing light to set the total WOT advance above the speed where centrifugal is all in. Once you get it installed correctly, just adjust the distributor so the 38 degree hash mark is aligned with zero on the tab, which should yield 8 initial if the OE max centrifugal of 30 degrees has not been modified.

Duke

**Michael W.** · June 21, 2017, 12:07 PM

Re: Why no change in RPM's??

Originally posted by Edward Bertrand (58273)

I used the Mityvac to advance the VAC instead of the engine.

I take this to mean that you were running the engine at the time and used the mityvac to activate the advance mechanism. Did the spark timing advance?

**Timothy B.** · June 22, 2017, 05:01 PM

Re: Why no change in RPM's??

Why not find out for sure where TDC actually is in the damper. That will tell you right away if the idle timing is so far advanced that the idle speed will not change. Have you used a timing light and looked at the damper marks when you pump down the miti-vac vacuum.

I agree with Mike about another source for the manifold vacuum.

Why no change in RPM's??

Why no change in RPM's??

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