'69 427/390 Engine Run-On / Dieseling

[Floyd B.]

The car had a rough and slow idle so I decided to check the dwell, idle speed and timing. Using the procedures in the '69 Chassis Service Manual, I:

1) Adjusted the dwell. It was set at 20 degrees at idle and I set it to 30 degrees. The engine did start idling a bit smoother but it would stall after about 15 seconds.
2) Adjusted the idle speed. Before the dwell adjustment, it was idling @ 600 rpm. After the dwell adjustment, it was idling @ 500 rpm and stalling - as mentioned above. I adjusted the idle speed to 800 rpm with the M21 in neutral.
3) Checked the timing and it was at the specified 4 degree BTDC.

After making those adjustments, the engine was idling really nice but it would run-on almost indefinitely after turning off the ignition. I suspect the adjustment to the idle speed screw was not allowing the forward throttle plates to close completely, thus providing fuel mixture for the run-on. Based on other threads that discuss the run-on problem, this could be due to an improper or malfunctioning vacuum advance unit. The unit currently on the distributor is stamped B22. Documentation I have been able to find indicates: Starts @ 7-9 Hg, and provides 8 degrees advance @ 14-16 Hg. It does work when I apply vacuum to it. I measured the vacuum at the end of the vacuum advance hose to be 9 Hg @ idle. I don't know how to test if it is a ported or un-ported unit. Also, the spark plugs are new AC R43N's.

I would try replacing the vacuum advance unit, but I don't know which one it should have. For now, I have backed off the idle to to 650 rpm and it isn't dieseling. But I would eventually like to get the idle set to the specified 800 rpm w/o the run-on problem.

Thanx

[Mark M.]

Re: '69 427/390 Engine Run-On / Dieseling

Floyd one thing I would check is vacuum issue from the headlights and wiper parts. Just block off the port on the fitting behind the carb. which feeds all of it. It has the yellow stripe hose on the little disc filter. After blocking that off, if the idle goes up, there is a vacuum issue to resolve. The original 1111926 distributor came with 1115360 vacuum can. The 360 = to echlin vc1765 B20 . If I remember correctly a 69 L36 I checked at the carb. vacuum port for the distributor, showed manifold vacuum. If that is so, it should be the same Hg. at the port I said to block on the manifold.

[Floyd B.]

Re: '69 427/390 Engine Run-On / Dieseling

Thanx Mark. Checking the idle Hg reading at the manifold port (with nothing else attached) is a good idea. I'll try that today.

[Duke W.]

Re: '69 427/390 Engine Run-On / Dieseling

Just about all emission controlled engines have ported vacuum advance. With only 9" at the VAC signal line is it probably ported. With full time idle behavior should be close to 15" @ 600, so you need a 12" B26 VAC.l Find a full time vacuum source like the choke vacuum break.

Set initial timing (VAC disconnected) at achieve 36-40 total WOT advance with full centrifugal . Specs are in your CSM and AMA specs. Lighter centrifugal springs to bring full centrifugal advance in earlier is also a good idea. The only limiting factor is detonation.

Look at the following thread post #27 and download the link to a pdf of my tuning seminar. Pay particular attention to the page on emission controlled engines.

Duke

[Floyd B.]

Re: '69 427/390 Engine Run-On / Dieseling

Thanks to the suggestions and information from Duke, I think I have a plan to resolve my idle issue which, in turn, causes my run-on issue. I performed the test suggested by Mark and obtained the following results:

• I warmed the engine to operating temp
• I disconnected the vacuum lines from the manifold fitting that is aft of the carb. And plugged both ports. One goes to the power brake booster and the other to the headlight actuator, wipers, heater controls, etc.
• This made no perceptible difference in the engine idle
• I then attached the vacuum gauge to one of the manifold ports and the needle constantly danced between 12 and 16 Hg
• I then removed the gauge from the manifold port and replugged the port. I disconnected the hose that connects the distributor vac advance to the carburetor port and attached my vacuum gauge. It was now reading only 4 in Hg.

I cannot explain why I was getting 9 Hg at the carb on Saturday and only 4 Hg the next day but I didn’t spend a lot of time worrying about it since it was obvious that the vacuum advance wasn’t getting nearly enough vacuum.

I then used one of the hoses to attach the vacuum advance directly to the manifold fitting. The RPM’s immediately jumped from 600 to 850 rpm. Mystery solved, but now what to do about it?

On Saturday, I had read the TIB on Vacuum Advance Units authored by Lars and Duke. This provided good background so I could understand how a properly functioning vacuum advance unit relates to idle rpm and run-on.

I think the current B22 vac unit will work fine when connected directly to manifold vacuum. The intake manifold and carb are not original to the car. The intake is from a ’68 and the carb is a rebuilt service replacement Quadrajet. I have the correct intake and carb but will wait until I perform the full body-off resto and engine rebuild to install them. I'll eventually get to correct VAC unit for my restoration.

Next weekend I’ll replace the two-port manifold vacuum fitting with a 3-port fitting as used on C60 equipped cars. I’ll also cap off the port at the base of the carb. This identical set-up is pictured in the AIM on Sheet C60-C8 for RPO L36. I can then set the timing as suggested by Duke.

VAC_Fitting.jpg

Two remaining questions:
1) Should I be concerned with the dancing vacuum gauge needle at idle? Is this any indication of a vacuum leak at the manifold?
2) When I search for 1115360 or B20/B26 VAC units, all I have been able to find are units stamped B10. Is B10 similar in performance to a B26? It is not included in the Vac Advance TIB authored by Duke and Lars.

Thanx

[Duke W.]

Re: '69 427/390 Engine Run-On / Dieseling

Typical L-46 idle behavior converted to full time vacuum advance is 14" @ 750 with a slight lope and vacuum variation. So you need a 12" B26 to pass the Two-Inch Rule. The NAPA number is VC-1765 and any parts store should be able to cross that to whatever "brand" they sell.

Pick a vacuum source that is full time to connect the VAC to. Also get a Mr Gasket 928G spring kit and start out with the black springs. Find the RPM for max advance and set it to 40 (VAC disconnected ). Then connect the VAC and go through the pre-emission idle speed mixture adjustment procedure.

Take it out for a test drive. Load it up at low revs and if you hear any detonation, back off Total WOT advance in two degree increments until it goes away, but not less than 36. If if persists install one next heaviest spring (silver), then silver-silver, silver-gold, and gold-gold as necessary.

At some time you will want to readjust fast idle speed as it will be higher with full time vacuum advance.

Duke