I have the stock distributor in my 327 with the petronics electronics installed. I am also running a Holley 600 cfm Electric choke with vaccumm secondaires. My total timing is right at 36 degrees at 2500 rpm. The distributor has lighter springs in place and the total timing is all in by 2000 rpm.

When I have the vaccum advance hooked up the car idles smooth, takes off good andruns good in traffic. But if you want to kick it in, there seems to be no power.

On the other hand, if I disconnect the vaccum advance the car kicks in great and burns rubber in 3 gears. But at idle it's rough, hard starting, wants to backfire through the carb and doesn't sound all that great.

I'm not sure if it's in the distributor (springs?) or maybe a power valve has blown in the carb. Can anyone offer any advice?

Thanks

Rondog

 

Did you set the total timing with the vacuum can disconnected?

What is the vacuum can number and specs?

Is the vacuum can signal line full manifold vacuum or "ported"?

What is the total idle timing - timing at idle speed with the vacuum can connected?

What is the idle speed and manifold vacuum?

Duke

 

SWCDuke is right on!! It's not the carb. If you had a blown power valve, your engine would idle terrible regardless of vacuum hook-ups. Also, your gas gauge would move faster than your tach!!! Check out your timing. By the way, why have the timing all in by 2000 RPM? Unless you're drag racing with a wide open throttle, you're hurting your economy.

 

Duke:

Yes, the vaccum advance was plugged at the carb when I timed it.

WIll have to look up number or specs on vaccum can. I can tell you it is the stock one that came with the car. L-79 327/350HP with the K-66 (?) air injection system that has been removed.

It is hooked up to the ported vaccum on the carb above the base plate.

Total timing I would guess to be about 16-18 with the vaccum connected.

Idle speed is about 750-800 Rpm Do nont know what vacum manifold pull is at this time.

Thanks
Rondog

 

An original '65 to '67 L-79 vacuum can will be stamped "236-16". Specs are 0@4", 16@8". The NAPA/Echlin VC1810 is the replacement - has the same specs.

As far as I know, K-66 used the same vacuum can, but a ported vacuum source, which was for CA emissions compliance. (The 300 HP engine with K-66 had a very different advance map.) If you have the AMA specs that are part of the complimentary GM restoration package, I would like you to post the ignition map specs for K-66 if they are different than 49-state models. If you don't have the AMA specs call GM Customer Service and request the restoration package.

You want a FULL TIME manifold vacuum source for the vacuum can, NOT PORTED!!!

At idle, your TOTAL timing should be the sum of initial, plus full vacuum, plus probably a few degrees of centrifugal since you have a quick curve. Connect the vacuum can to a FULL manifold vacuum source, and verify that the total idle timing is at least the sum of initial plus full vacuum advance. I would also suggest that you check the vacuum can with a Mighty Vac and dial back timing light to verify that it performs to spec. Those internal rubber diaphragms don't last forever.

Once you have completed the reconfiguration, go through the idle speed/mixture adjustment procedure.

Your centrifugal curve may be a bit too fast. If the problem persists with full time vacuum advance slow it down a little so it's all in by about 2500.

If you or anyone else has the AMA specs for your model year, perhaps they could research them and tell us any difference in the timing map for L-79 and without K-66, but you need to state your model year.

When posting a question please state the model year and engine option and whether it is OE or modified, and any other information that may be relevent, such as it was originally built with K-66. Saying "stock 327" covers a lot of model years and engine configurations.

Duke

 

Duke: Thanks for the reply. I will try to be more specific. First my car is a 66 with a 327/350 hp. It has been bored 60 over and has the 350/350 hp cam (hydraulic) I also put 10 to1 forged pistons back in it instead of the 11 to 1. 4 sp PS PB originally came with the K66 Air injector but that has been taken off and all holes plugged. The carb is a Holley 600 cfm replacement with vaccum secondaries and electric choke. It has the stock exhaust manifolds and just about everything else. Hope this is specific enough.

I happen to have the AMA restoration package and the specs. The Air Injection Emission control has the same specs for the distributor as the one without. They are as follows:

Centrifical Advance begins at 900 RPM
Max Degree is 30 @ 5100 RPM
Vaccum Advance Begins @ 4 In Hg
Max Degree is 16 @ 7 In Hg
Initial Timing is 10 BTC with vaccum line disconnected @ 700 RPM

I initially thought about purchasing an Accel Advance can that has the adjustable screw setting inside to see if this would help.

I will have to get all my other vaccum and mighty vac out this weekend to get the other specs you discussed earlier.

Appreciate your help with this matter.

Rondog

 

Thanks for the info.

One other question. Do the AMA specs or CSM show a different carb number for K-66 versus non-K-66. I'm pretty sure K-66 would have included ported vacuum advance, which would be evidenced by a different carb number.

I think all you need to do is verify that your vacuum can is reasonably within spec, be sure it's connected to a full manifold vacuum source, and maybe slow down the centrifugal a hair.

Duke

 

Duke: The Engine with the K66 Air injector shows Carb # 3890499 The engine without shows 3884508 Both Holley for the L79 327/350 hp 3 & 4 speed. Hope this helps

Rondog

 

Yes, except, for the record AIR was K-19. K-66 is Transistor Ign. I missed that in your earlier post.

The different carb for K-19 was probably due to a provision for ported vacuum advance rather than full time vacuum advance w/o K-19.

Duke

 

The K-19 carb was also different in that it had a decel device. The decel device prevented the sudden closure of the throttles which, due to minimal advance and leaner mixture, could cause a stall. An engine that might have a base idle of 650 rpm would see it's idle speed drop to 800 rpm when you let off the throttle and then it would slowly drop tp 650. If you cut the ignition before the idle idle had dropped far enough, the combined effects of momentum (higher rpm), hotter combustion chamber (lean mixture), and minimal advance, you experienced the joys of run-on (aka, dieseling).

 

Actually the "throttle dampers" were there to mitigate the HC spike that occurs when you lift off the throttle. The high transient vacuum pulls excess fuel through the idle ports, which causes a HC spike in the exhaust. This made early manual transmission cars tougher to certify than automatics and is one reason why manual transmissions were not available some years in CA.

GM also addressed the issue by setting curb idle speeds high, which could cause the engine to "run-on (or "diesel") when the ignition was shut off. To solve, this a solenoid throttle stop was sometimes installed that was energized whenever the ignition was on. When the ignition was shut off, the solenoid de-energized and allowed the throttle to close to the low idle stop, which starved the engine for air so it would not run-on.

The throttle dampers are a real nuissance because they cause the revs to hang on upshifts (like a very heavy flywheel) making it very difficult to execute a normal speed upshift, smoothly. The high curb idle speeds also contribute to rev hanging, but not as bad as the dampers.

Modern digital control EFI systems shut off fuel flow at closed throttle overun or when a rapid throttle closure is detected, which usually means that revs drop off quickly for quick, smooth upshifts.

Duke