Hi there,

this is a experience report

Had my new zz502 engine installed at a vette-specialist with more than 20 years experience and known for doing good work by many customers.

I was in the shop the whole time, also when the timing was set. But did not verify it. I use the old TI distributor an the engine, i want to keep everything looking like original.

Drove the new engine 1200miles without any problems and carefully like you have to do it with a brand new engine.
The last 200miles with some full throttel but only in lower rpms.
Ok, sometimes the engine started hardly, have new battery, new wiring and so on built in to solve this. (now i know this comes from too early timing)

Then it happened that the engine did not want to turn off (dieseling).
Spark plugs were bright white.
The AFR at idle was already rich, but i richend it up more to eleminate the dieseling. But it did not really help.

Then i checked the ignition timing: 22° at idle without vaccuum.
And the screw to hold it down was good tightened.
At 3000rpm somewhere at 46° I was confused, because the engine was running good all the time. Did not hear any pinging.
(is this possible?)

Anyway, set timing to ~36° at 3000rpm and it still runs. Throttle response is not as good as it was before, and it feels like missing power in lower rpms. But still enough power there at higher rpms.

The zz502 paper from GM says the engine needs 26° at 2400rpm and 32° at 4600rpm.

So, what is the way to go? The guy from the shop recomends me strongly a new MSD distributor.

 

Generally speaking your going to have better throttle response with more initial timing, it looks like your dist has 24* of advance so when you retarded the timing to 36* you dropped your initial down to 12* giving you that "lazy" feeling at low rpm. You can modify your dist and limit its advance so you can apply more initial or get a new dist like an MSD that is adjustable.If the engine was running at 46* without detonation you could bump up your initial until you get the response your looking for but its not going to be optimal for maximum power. GM has figured out the best timing for max hp but how offen do you drive at WOT I would sacrifice some hp for better drivabilty.

 

Which tool you used to check the timing?

 

Hi,
GM says:

Timing Information
Set initial spark timing at 10º before top dead center (BTDC) at 650 rpm with the vacuum advance line to
the distributor disconnected and plugged. This setting will produce 32º of total advance at wide-open throttle
(WOT). The HEI vacuum advance canister should remain disconnected. This engine is designed to operate
using only the internal centrifugal advance to achieve the correct timing curve. The HEI distributor supplied
with the ZZ502 deluxe engine has mechanical centrifugal spark advance with the following curves:
Mechanical Advance:
RPM Advance
1100 0.0º (=10° total)
1600 12.0º
2400 16.0º
4600 22.0º (=32° total)


Currently im running close to the recomendation
900 - 12°
3000 - 36°
despite that the advance comes in earlyer.

Ok i could limit the advance, so i could start with 22°.
But this sounds not normal for me, or whats your opinion?
And i know it starts dieseling, this cant be good. Ok, i could disconnect the vaccuum, this would help against the dieseling.

 

 

If it starts fine with 22* initial you would need to limit the advance to only 10-12*. Since you know its "lazy" at 12* try advancing 2* at a time until you like the way it feels, if it runs on your going to have to retard it. Do you have the HEI ? is it an option to put that in?

 

The tool seems ok, but as you live in Europe like me you should consider a couple of things:
1) the best vette-specialist is YOU after having read many books and worked on YOUR car, don't trust any other
2) the easiest way to solve the problems for these "vette-specialists" is to suggest you to replace parts, only because they don't know how to find the root causes (they suggested me to replace carburetor, manifold, distributor... I fixed all of them with the help of this forum members and some advices from some american experts)
3) maybe your advance timing light is different from mine, but be sure to have it set for four strokes engines and not two strokes engines

If you want to talk to real specialists for tuning correctly your distributor, contact Lars (v8fastcars@msn.com).

Good luck!

 

Iron Dog brings up a good point about the light your numbers might differ from the one the shop used and it may not be 100% accurate but as long as your using the same one when you make change it doesnt matter if its accurate. The fact is the engine would deisel the way it was set before and now after retarding it 10* its lazy and lacking throttle response therefor you need to be somewhere in between those numbers reguardless of the accuracy of the actual numbers. Getting it set to what GM recomends is the best place to start but it doesnt seem you can do that with your current distributor.Maybe the guy Iron Dog recommended will see your post and give you a better idea as to what to do.

 

I dont want to discuss where the mistake comes from. Maybe the guy made a mistake, maybe there is something wrong with the distributor. I dont know.
By the way, he has the same timing light in use like me.

But this could be interesting: sometimes the line on the harmonic damper "jumped"
Maybe 2 of 10 flashes sometimes. The picture is just not really smooth.
Im not sure where this comes from, maybe the pick-up does not like my ignition wires or the distributor is defective. And maybe the guy from the shop had always the wrong reading...i dont know.

I had a look at the spark plugs now, and they are all looking as they should - very dark, due to the very rich idle AFR.

Regarding the weak throttle response in lower RPMs: its not that bad...From my old engine i know how it feels when timing is too late, and its not that hard.
Maybe I have to set it a bit more to advance and it will be perfect. Bad time for
experiments, normaly we have snow here in the winter...

 

I wasnt trying to come across as blaming someone just wanted to explain different timing lights could show different settings and that the numbers you see my not be as accurate as they should be, but your on the right track with advancing it a bit just dont take it for a test drive in the snow!!

 

I wont drive this car in snow, no way.
It would be too dangerous too!

Lars wrote me, that I should limit the advance curve, so that it starts at least at 18-20° and ends at 36° total. Becaus 12° initial is killing the throttle response.

I dont want to modify the original TI distributor too much...maybe i really have to buy the MSD distributor. If it would n´t be that expensive (until it is in germany).

 

what springs do you have in the distributor ?consider a new spring kit for it.check the wear on the distributor drive gear.

 

Big blocks like a little more advance 18-20 initial and 38-40 total is pretty common. Use your Vacuum advance also. The TI dist are finicky if everything is not perfect. Even have their own coil specs.

 

The only reason that thing is still running is the low compression they have (9.6 or less)....

You need MORE initial timing, but the mechanical curve needs to be adjusted to yield less centrifugal advance...

I would set it up to run 20-22* initial, then have the distributor curved with another 14-16* of mechanical advance all in by 2500... Make sure the advance curve maxes out at 14-16* and does not continue to increase past 2500-3000... That should get you a total timing of 34-36* or so

You can also add the vacuum advance for an even better idle/response but you need to limit it's travel as to not add too much timing at light cruise... Limit it to about 8-10* of vac advance... Add that with your 36* total and you'll be around 46* total at light cruise....That is the absolute MAX I would run on a ZZ502, I may not even run that much..

That will give you all of the pep, great idle, and great response you need, yet keep you at a safe level for WOT at 36*...

Good luck.

 

Thank you both!
I decided to modify my TI distributor, because the MSD looks pretty the same and has to be tuned in the same way. So no really advantages, despite everything ist new.

Ok, what is the way to go?
May I limit the centrifugal advance travel to 14° by putting in a thicker bolt?
Or should I use weaker springs, that advance comes in earlyer? There is a mechanical
block for maximum advance, so it will be still 36° total, but im not sure when it will come in with weaker springs.
At the moment I think the best thing would be to lift it up at the lower end with a screw or sth. similar and keep the springs that are already in it, because full advance comes in a bit below 3000rpm, that seems to be correct.

I have a spring kit with several springs and an adjustable vaccuum canister here. I hope the canister will fit, then i could easily adjust it.

 

You made the right choice! Adjust your distr will give you much more confidence on your car. I suggest you to fully disassembly the distr while you remove it, if needed of course. I cleaned it completely and fixed tach gears on mine, now I know it is working ok! I have no experience on TI distrs, I have a point style one and I know you can limit vacuum adv putting a bushing on its pin, you should have this bushing in your spring kit. Ask Lars to send you its papers on tuning distrs, if he hasnt already sent them!
Have a nice day!

 

ok, it ended up in a complete distributor restoration.
Lars sent me 2 documents that helped to answer some questions.

Ok, this is what it looked like at beginning:



And here you can see the pin that limits the mechanical travel.
There was a rubber bushing on it back then. 44 years later only a black piece of grease and mud is left.



Removed the coil:



Removed rust and dirty from the housing and painted it new:



I didn´t trust the old wires, i wanted to have new ones.
So I soldered thicker and high flexible wires to the coil.



To make the adjustable vaccuum canister from moroso fit to the old distributor I had to drill a new hole in it.



works



Adjustable means for a canister that only the spring load can be adjusted, not the advance that is applied to the distributor.
Lars wrote that the canister has to have the full stroke at idle with 2Hg vaccuum left.
But to limit the stroke i had to make a piece of aluminum that slides and can be fixed with 2 screws. So i can limit the advance to the 10° I need. Without it would produce up to 19° advance!



Other parts cleaned and polished



And here i made a new limiting bushing. It has the shape of an egg, because the slot was not made for such big bushings.



Measured it will limit the travel to 8,5° distributor advance, that means
17° crank advance. So it will move from 19-36° later.
(Lars told me that 12° at idle is by far not enough for my engine)



Polished and assembled more parts:



and finaly:



compared with this:

 

I'd concur with that - my 489 has 22 degree 'on the crank' and 16 degrees in the MSD tach drive dissy. Could have used an extra few degrees total but then would have possibly needed to upgrade from Super Unleaded gas. Have vacumn advance connected and it runs great!

 

Very good job!

 

Hi,

today I built it back into the car and get it started.
After a short hestitation it idled better than it did in the past.
I had always to keep it alive with the throttle until it got some temperature, but this time it went noticeable better and needed no throttle help. Run stable at ~800rpm.

After short adjustment I have now 21° at idle without vaccuum and 36° with it. But I shortened the canister stroke a bit after the run, should be at 32° now.

36° total mechanical advance comes in at 2500rpm, like you recomended it to me

Unfortunately its raining, I couldn´t do a testdrive, but cant wait to check out how it works on the street
I will tell you the results later...