[69237]

I have been chasing my tail with an issue for the last few years and wanted to see if anyone has experienced this.

I have a pontiac 350 engine I rebuilt. It runs great when warm, but is sluggish when cold. I looked into this and found the initial timing is around 2-4 degrees when cold and 14degrees when hot.

Yes this is true initial with vacuum advance unplugged and before centrifugal advance comes in. Verified on multiple timing lights.

Engine has maybe 2000 miles on it. New balancer that was verified and has not changed, new DUI HEI distributor with proper endplay, crower cam, comp cams double roller timing chain.

I've also noticed that timing will decrease by the same amount ~10-12 degrees when to shifting to park. Vacuum advance hooked up to manifold amd is fully engaged at idle in gear at 16inHg of vacuum.

My only thought is bad cam end play allowing the timing to shift, but I don't know if thats feasible.

Has anyone heard of this before?

[nUcLeArEnVoY]

What year is your car? There are ported vacuum switches for emissions purposes that will add in additional timing if the engine temp gets high enough, which allows the idle to speed up and the water pump to spin faster - the additional advance itself also causes the engine to run cooler. Maybe that's it? Otherwise, that one confuses me, too. Do you notice an accompanying drop/rise in RPM and smoothness with each shift of timing?

My only other guess is maybe your distributor advance springs are far too light, wherein revving up the engine will add more timing that will not go back down due to a positive feedback loop caused by the weak springs maintaining the weights in a slightly extended position.

[RocktimusPryme]

Someone probably more qualified than me will jump in, but I would say that just sounds like its RPM based to me.

When the engine is warm the RPM is probably higher and you are getting some mechanical advance out of the distributor vs when its dead cold and probably idling lower.

Same with the park to in gear thing. Your idle speed is going up in park and giving you a couple extra degrees of advance based on the RPM.

Cheap test I can think of off the top of my head would be to buy some heavier mechanical advance springs and see if that changes how much extra timing you see between your hot and cold idles. If Im right you are just seeing mechanical advance before you want to due to wear or super light springs.

[69237]

Good questions and thanks for the replies

Engine is a 68 in a 69 lemans. I have no switches or ports for vacuum advance. Its straight to the manifold vacuum and fully engaged. No major changes in rpms, its just gradual improvement until fully warmed up. Almost chokes out when cold in gear because timing is so low.

I've verified the centrifugal advance doesnt come in until 1000rpm. I remove the fast idle so that I can check timing at 700 rpms cold and hot. Same thing happened with a points distributor with a ton of different spring combos...

Good point to note is that the timing goes DOWN in park... I agree with the logic you mentioned, but its the opposite.

[jhein]

FYI, if you decide to change springs and/or weights, I'd call DUI. There are lots of complaints about the quality of the aftermarket kits. When I needed to change mine, DUI sent me the parts free of charge. I don't know if they are as good as OEM, but maybe better than generic repos??

[RocktimusPryme]

Shot in the dark but sloppy timing chain? Something expands slightly when it warms up and she tightens up a touch.

[Cliff R]

I've ran into this issue multiple times but only with stock distributors.

When they get a lot of age on them and the factory grease has long dried up they can and will "stick" when cold and show a different base timing setting than fully warmed up and heat soaked. So basically they will stick with the timing advanced slightly not allowing the weights to pull the timing all the way back to the base setting so you show a higher number fully warmed up vs cold.

Not sure if any of that applies to the DUI never messed with one and don't plan on it.

I'd also mention that the Comp double roller timing set is a pile of bovine excrement. They sold two last time I looked, both with cast iron sprockets. Even their "higher end" model is nothing but two rows of imported bicycle chain that actually roll on the pin with cast iron sprockets with 3 useless keyways cut in them. Even with that said aside from seeing WAY to many of them stretched way out and loose as a goose tens of thousands of miles before they should have been, it's probably not the issue here.........

[steve25]

All I can say right now while I take time to rap my brain around this is if like you posted the same timing change takes place even when you have swapped in a points Dizzy, then it must be related to something mechanical within the motor.

[Goatracer1]

You said you have HEI? Borrow another control module and see if the temp is affecting that. I've seen them do some strange things.

[69237]

I can check the timing set for play. Check by turning engine back and then seeing how much it takes to turn the distributor rotor, right?

I'll also chek the rotor to make sure its free. The distributor is new, nice, and less than 1000miles on it so I'm thinking it fine.

Can also look into a module, but I didn't think timing was affected by them?

Has anyone had timing issues from camshaft endplay? I know the distributor has 0.015 endplay so to me I don't understand how it can even more the timing more than a couple degrees regardless.

If the timing chain has enough wear to change ignition timing that much the cam timing is probably way off too. I dont think thats the case though.

[steve25]

Well the cam drives the oil pump through the distributor gear, so the load on these two gears is dependent on how thick the oil is and in turn how fast it can flow thru the motor.

If the camshaft thrust plate is worn what would happen to the timing when at the rear of the motor the gear to gear clearance goes from being very loaded and small to a wider clearance when the pump is not as hard to drive?

What happens if the cam gear is worn?

He has had two different distributors in the motor, so it’s not that gear most likely.

Just thinking out loud here.

[Cliff R]

Since it appears to be internal pull the timing cover and check the slack in the timing set. Also check the keyway that's engaged in the crankshaft key key as I've seen several of the double roller timing sets with the cast iron bottom sprockets split at the keyway. They are NOT good parts unless you went to the "top shelf" offering and bought the 9 keyway variety with the steel billet sprockets and much higher quality true roller chain.......

[69237]

I haven't had time to do a lot digging yet. I did take the distributor cap off and check things out.

The springs and weights move freely and there aren't any spots that hang or bind. I noticed that it installed the arm of the distributor turns some. Without the rotor on, and measuring at the end where the rotor screws down, itll turn around a sixteenth of an inch before hitting resistance against the springs. I know they'll move some, but is that normal?

I checked endplay with it installed and it only moves roughly 0.020. That is close enough with my measurement method.

[Cliff R]

Did you do the test mentioned about turning the engine in both directions while watching the rotor and see how much "slop" is in the timing set?......

[69237]

Ok so I rocked the crank to see how much play there is in the timing chain. If I simply load one direction and turn the crank the opposite way, it says 12degrees which is rough.

I was thinking about the rotational play that I found in the rotor and it seems that this is taking into account how much it takes to reverse the slop in the chain and then overcome the play in distributor to turn it.

Then I tried by turning in one direction to load the chain to get to TDC. Then I turned the rotor the opposite way to take out the play. This resulted in 5 degrees..

So it seems there's 5 degrees of slop due to the chain and there's rotational play in the distributor worth 7 degrees...

[steve25]

So now the question is what takes place once the motors operating temp is reached.

Do the gears expand more and eat up some of the chain slop, or does the expansion of the chain do the opposite?
Considering that the chain is taking a valve spring hit 16 times per every crank revolution my moneys on your chain being the main source of your issue.

[69237]

Took the car out today.

Checked timing at 700rpms when cold. 0 degrees without vac advance, 13degrees with it. No difference between park and drive. Checked it when it was heat soaked. 14degrees initial, 27 with vacuum advance. Verified by a couple year old innova timing light and same measurement with my usa craftsman light.

The timing is rock solid when cold in drive and park. The timing is rock solid in drive when hot and shaky a couple degrees when in park. I think that is only because it's at the edge of too much timing. It does start perfectly when hot though.

[steve25]

And this is with the HEI in the motor, or the points dizzy in?

[69237]

Using the HEI. It runs so much better than the points distributor I have it wouldn't even be a fair way to diagnose.

[69237]

When you all have seen stretched chains, are the timing problems consistent when hot and cold?

I've been reading some various threads to see what timing fluctuations have been observed. I do use 15w50 oil as the engine was built with looser bearing clearance. Hot oil pressure is about 22psi and 60-65psi when cold.

The oil pump driveshaft ears are worn. I had a nice surprise after the engine was installed and my distributor brought the driveshaft up with it.

If the distributor is clamped down, any oil pump driveshaft movement would be limited to the camshaft endplay, right?