Idle issues times 3

[DaveMcLain]

Am I the only one who thinks 6-7 inches of vacuum at idle for this engine is WAY low? probably a valve adjustment situation or am I missing something here?

I doubt you are the only. Something is amiss, timing retarded or a bunch of valves are too tight, crazy camshaft, etc??? Doubtful the original engine is still in the car and still in original condition.

The Pontiac engines of that era suffered from severe timing chain wear such that the chain would get so loose if the engine dieseled and kicked back on shutoff the chain could hop a tooth, saw that several times back in those days. One, I recall it was a '62 389 Catalina, chain so loose you could pass a piece of paper between the chain and crank sprocket. It ran OK until the last time it dieseled.

My dad used to put in two or three Pontiac timing chains a week back in the 1970's some of them with as low as 50,000 miles. If you saw one with 75,000 and the chain had not been changed it was very unusual. At least they were an easy one to change.

[statsystems]

Am I the only one who thinks 6-7 inches of vacuum at idle for this engine is WAY low? probably a valve adjustment situation or am I missing something here?

I doubt you are the only. Something is amiss, timing retarded or a bunch of valves are too tight, crazy camshaft, etc??? Doubtful the original engine is still in the car and still in original condition.

The Pontiac engines of that era suffered from severe timing chain wear such that the chain would get so loose if the engine dieseled and kicked back on shutoff the chain could hop a tooth, saw that several times back in those days. One, I recall it was a '62 389 Catalina, chain so loose you could pass a piece of paper between the chain and crank sprocket. It ran OK until the last time it dieseled.

The Pontiac method of locking the adjusting nut by tightening it to the shoulder on the stud confuses most younger guys who have only experienced the Chevy with self-locking nuts.

Pontiac engines require a different Poly Lock than the Chevy because the original stud has the shoulder.

I'll bet very few know why Poly Locks are called Poly Locks, how they got the name Poly Lock.

Well, I don't know. But I always thought it a weird term for a lash adjusting nut. So what's the scoop?

[MadBill]

For some reason I'm thinking; "Isky"...

[Tuner]

I have to ask if I back of the valve adjustment and gain vacuum but not enough (guessing 18'' with a stock cam) what is the engine telling me?

If you backed off the adjustment and the valves did not get noisy, the nuts were turned down too far, and it is probable they have been damaged by over tightening.

This all depends on if the engine is original and the heads have not been modified with screw-in studs of the Chevy type, which is a possibility because of the age of the car, unless you absolutely know the history of it. In the '60s cars like that got a lot of attention and a camshaft change and all the valve train folderol that went with it is a real possibility.

To diagnose this, be sure of the ignition timing is reasonable, near 10 ~ 20 degrees or so for now and the valves just tight enough to not make noise.

Is the vacuum gauge steady or is it jumping around, twitching? At 1500 ~ 2000 RPM and timing at 30~40 degrees it should have close to 18" vacuum or more with a stock cam. Hot rod cam might not be as high but even a crazy radical cam should have 10 ~ 12 inches vacuum at 2000 steady RPM no load.

Do you know what cam is in it? More information is necessary.

It is possible a plugged exhaust will cause low vacuum and the engine still run somewhat. Some GM cars of the 60s - 70s era had double-wall exhaust pipes and the inside pipe would collapse but the outside pipe still looked fine. That fooled a lot of folks. Low vacuum makes the transmission stay in gear too long or not upshift because the vacuum modulator thinks the engine is near WOT. My neighbor tranny shop guys got pretty good at diagnosing that with a giant pair of alligator jaw pliars.

[Tuner]

I'll bet very few know why Poly Locks are called Poly Locks, how they got the name Poly Lock.

[
Well, I don't know. But I always thought it a weird term for a lash adjusting nut. So what's the scoop?

For some reason I'm thinking; "Isky"...

Yep, starting in the '50s Isky's ad campaign explained how his cams were designed using a computer (to think, today you can put a freight train full of punch cards in a thumb drive) and mathematics method known as "Polydyne", short for "Polynomial Dynamic", to describe the ramp shape, lift profile, acceleration rate, etc. You can read about it on their website. Isky had a parrot "Poly" in his ads and various products were Poly- this and that for a while, several years.

[notagn]

I have to ask if I back of the valve adjustment and gain vacuum but not enough (guessing 18'' with a stock cam) what is the engine telling me?

If you backed off the adjustment and the valves did not get noisy, the nuts were turned down too far, and it is probable they have been damaged by over tightening.

This all depends on if the engine is original and the heads have not been modified with screw-in studs of the Chevy type, which is a possibility because of the age of the car, unless you absolutely know the history of it. In the '60s cars like that got a lot of attention and a camshaft change and all the valve train folderol that went with it is a real possibility.

To diagnose this, be sure of the ignition timing is reasonable, near 10 ~ 20 degrees or so for now and the valves just tight enough to not make noise.

Is the vacuum gauge steady or is it jumping around, twitching? At 1500 ~ 2000 RPM and timing at 30~40 degrees it should have close to 18" vacuum or more with a stock cam. Hot rod cam might not be as high but even a crazy radical cam should have 10 ~ 12 inches vacuum at 2000 steady RPM no load.

Do you know what cam is in it? More information is necessary.

It is possible a plugged exhaust will cause low vacuum and the engine still run somewhat. Some GM cars of the 60s - 70s era had double-wall exhaust pipes and the inside pipe would collapse but the outside pipe still looked fine. That fooled a lot of folks. Low vacuum makes the transmission stay in gear too long or not upshift because the vacuum modulator thinks the engine is near WOT. My neighbor tranny shop guys got pretty good at diagnosing that with a giant pair of alligator jaw pliars.

The cam is stock. At idle the vac reads 7 to 8. Fairly steady. If the idle is increased to 1500 rpm the vac will increase to 17 or 18 steady. Haven't increased the timing because of run on after shutting of the car after an advance of 2 degrees.

[Tuner]

Run on occurs with retarded timing because ...... retarding the timing slows the idle speed ....... to get the speed back up the throttle must be opened more than it would require if the timing were advanced ......... the larger throttle opening causes higher intake pressure (lower vacuum) ..... the higher intake pressure causes idle air mass consumption to be greater...... because of the retarded timing and larger throttle opening ...... higher intake pressure (lower vacuum) gives higher compression pressure .. higher compression pressure causes the "dieseling" and run on.

Advancing the timing should speed the engine up, so then slow the idle speed back down by closing the carburetor down to the required idle speed. The 1966 Motor's Auto Repair Manual says 6 deg. BTDC and 590 RPM.

The compression ratio is 10.75/1 so today's gasoline is not going to cut it. Using octane booster that says "Contains MMT" on the bottle will help, Avgas would be nice. Today's ethanol laced fuel is more pre-ignition sensitive than old school gasoline (and Avgas) and that ethanol is part of the problem. If the spark plug is too hot it will encourage this problem. I would use a NGK B6S plug.

You need to know if the distributor is advancing like it should, both mechanical and vacuum advance. Retarded timing will cause it to run hot and the heat will cause run on. When these Pontiacs of that era run on and kick back on shut off, that is what causes a worn timing chain to jump a tooth.

[induction apprentice]

reset the valves with half a turn past zero lash. Make sure you are on the basecircle when you adjust them.

stick a compression gauge in one hole. If it reads 65 psi or less. Install a new timing chain set.

install vac gauge and confirm repair.

If vac is still low. Repost and we will figure it out from there?

[notagn]

Awesome information to all! Thanks will report back when time becomes available.

[enigma57]

How did this work out for you? FWIW...... When I rebuilt a 400 Pontiac back in the '70s and swapped it into a '56 Chevy......





...... I found that Dormans had a replacement rocker nut specifically made so that you could set valve lash in the same way as a Chevy. I installed a new Sig Erson hydraulic cam along with matched valve springs, a double sprocket roller timing chain setup and the Dorman rocker nuts. Set lash at 1/2 turn past zero lash with engine running and at operating temp. No problems with that following initial setup.

Built a tri-power setup using Holley carbs (the high perf. replacement carbs having metering blocks and accelerator pumps on end carbs that MOPAR sold over the counter to replace the OEM vacuum actuated end carbs). No issues there.

Once I swapped in a large enough radiator to keep the Pontiac engine cool...... And reworked the fuel supply to the carbs so the engine wouldn't starve for fuel when the end carbs kicked in......

The one area that gave me problems was that the stock Pontiac distributor would cause the engine to ping under load. Finally had a fellow recurve the distributor and set it up with an adjustable vacuum advance can and no more issues. Ran 12 degrees initial timing and full advance was limited to 34 degrees. He brought it in slow from 1,200 RPMs to 3,200 RPMs for mechanical advance.

This was a low mileage '74 400 low comp. smog engine to begin with and I was running reworked '68 428 cop car heads with small chambers to boost compression and 2.11" / 1.77" valves for better breathing. Standard comp. for the 428 was 10.5:1 so I believe the 400 with 428 heads was very close to that. I ran the stock 400 flat top pistons along with composite head gaskets and the heads were milled 0.060" so it probably came out the same as uncut heads and steel shim head gaskets.

Hope this helps,

Harry

[My427stang]

I am not familiar with the distributor on the 64, but I did a 67 recently and it used manifold vacuum at idle. I didn't like it, recurved the distributor and used ported, but it would have been a ton of initial that you are missing if it is the same. Just an idea

After that, I'd figure out which carb it idled on, make sure the others are closed and focus on that one. However 6 degrees initial keeps poking at me, sounds real late