I installed a new distrib gear. What a difference.
When I removed the cap i noticed that the rotor was very very loose.
I was able to rotate the rotor back and forw for several degrees before to stop against the next thoot on the dist gear.
AT the most external part of the rotor i can say a full 1/2" of slack...
Removed the distributor I noticed that the gear was with 1/2 of the thooth eated....here the slack on the rotor.
Installed the new one this slack is reduced to a minimal amount...
I think that now the timing is much more accurate. Before this I had problems with some strange knock counts. Probably a commanded advance of 30 deg was sometimes 25 or sometimes 35. On the higher side the knock occurs(timing removed by the computer) and on the lower side was cause of lack of performance too.
I connected the scanner and I have no more knock retard.
Now I have to do again the advance tables on the chip because this strange problem with the knock counts has convinced to me to change (reduce) the timing advance where the knocks happened.
I gained seat of the pants power and torque an I have for sure a margin to work on the spark tables.
...So if you have some strange knock and reduced power... remove the cap from the distributor and look for rotor slack..
-Beppe-

 

Great advice, I never even thought about that.

 

So how much slop is too much? If you count slop in the timing chain plus the distributor gear, it adds up I guess. Mine stays pretty steady with the timing light though.

 

Nothing wrong with replacing gear but you can get same result by removing gear and adding a shim to the shaft between gear and body. Takes out slack in gear mesh. Trick from the old days.


Larry

 

I couldn't agree more , I think both should be done espacially on a fresh build. Stock, mine must have checked with a popsicycle stick.

 

Ok, I thought that to cure a worn dist gear with a new dist gear is the best way to correct the problem
-Beppe-

 

...my timing light also now shows a steady mark on the damper. Before it was a bit undefined bouncing from 8 to 4 advanced.
About the slack ...now I have a real minimal slack on the rotor, just the fisical slack of a tooth in the teeth of the cam gear.
Do you think that shimming the dist can zero-out also this slack? There is no the danger to have a "too tight" contact on the teeth??

-Beppe-

 

Shimming the distributor won't change the mesh any. Well, maybe a little if you wear a newer part of the gear, but if you're pulling it you might as well replace the gear.

 

OK, probably I don't understand the advice of Vetracr when he say: " .....you can get same result by removing gear and adding a shim to the shaft between gear and body. Takes out slack in gear mesh..."
Could you explain??
thanks
-Beppe-

 

It really should not change the mesh, shimming it will allow for more stable timing.

 

Yepper that is one source of bad timing. There are are actually a whole slew of things in the conventional timing setup that can add error.

First off is the timing chain, slack in that will give you errors.

(2) harmonics in the crank, can increase or decrease that slack

(3) Cam twist or tortional effects

(4) Cam gear to distributer gear - as you saw.

(5) oil pump chatter

(6) Cam shaft end play.

These ALL happen in every motor. Some more than others. The more power you make, the more it if there. These will give you inconsistant timing, and you CAN'T control it, all you can do is try to minimize it.

The only way around it, is use what is known as a crank trigger. This is a wheel that is sandwiched between the crank pulley and the balancer. It has magnets in it, that will do the same as the pickup in the distributer. These tell the ignition when to fire. Since you don't have all the coupling points, chains etc you get dead nuts timing.

I have one of these on my car, because through logging the RPM feedback if you get a logger that has a very high sample rate, is truely VERY poor, and this is because of all these events happening. Now my curves that used to be VERY jagged, are VERY smooth.

ETs picked up nicely as did consistancy.

 

I would be concerned that the gear was severely worn. I would also be wondering if the worn gear ate the gear on the cam.

 

Not true. Conv90 had a distributor gear that was severely worn. Although there is normally a lot of end play in the distributor shaft, there is not enough to move the distributor gear into a totally different wear area, by shimming it. Because the distributor gear is helical, up and down movement, caused by end play in the distributor shaft, induces rotation, into the distributor shaft. As the distributor shaft turns due to end play, the ignition timing is altered. Shimming the distributor shaft to a reasonable amount of end play WILL reduce timing variances, whether the gear is worn or not, but it won't compensate for a worn gear.

RACE ON!!!

 

And whether it is one or both gears that are worn, I'd be concerned about the whereabouts of all the wear particles.

RACE ON!!!

 

His engine is a rebuild 383 ect that he has recently finished. The wear particles are most likely in the old engine that he took the distributor out of.

 

I dunno, I pulled my stock dist out of my stock motor at 52,000 miles and the gear looked really good.
For his new 383 to eat a dist gear makes me wonder about the new cam.
On my rebuild, I pulled the dist out within a few hundred miles and the dist gear showed lots of new wear.
I still don't quite know what material to use. Bronze? Hardened? Poly?
Depends on the cam....call the cam manufacturer, I know.

Beppe, how did the cam's gear look? Any wear like the dist gear?

 

Take it from someone who's walked this road before.

I've been through three distributor gears before I finally solved the problem. In fact, on one occasion, the teeth on the distributor gear were so worn that the car actually cut off as I was driving down the street.

Did a lot of reseach and even tried using an aluminum/bronze gear that was RECOMMENDED in one of the magazines as a cure for the problem. Nope, same thing. In less than a couple thousand miles, the teeth on that gear looked like it had been hit with a grinder.

The cure, GM melonized gear. Installed one of those many, many thousands of miles ago and I've had the distributor out countless times since then, either just to check the gear or to replace the oil pressure sender.

Gear looks just like it came out of the box. PROBLEM SOLVED!

According to the article I read, the Tech Guy from, I believe it was Accel, said not to worry about the metal that gets worn away from a failing gear; it's so fine it won't cause a problem.


Just my personal experience.

Jake

 

I too have been down the distributor gear/hurt the motor road. ALWAYS check the distributor shaft to see if it is bent(as mine was) and wear in the bushing that guides the shaft (as mine was due to bent dist shaft). I would not take this lightly. It could be an expensive issue if left go.

 

Conv90, would it be possible to see pictures of the dist gear?

BTW - wear surfaces are usually selected so that one is relatively
hard while the other is relatively soft for best longevity. If two
hard surfaces or two soft surfaces are mated together, they tend
to fail prematurely.

Select the appropriate distributor gear for the gear on the camshaft.

.

 

If there is a concern about the condition of the cam gear, a
boroscope like the ProVision PV-636 would be helpful for inspecting
the gear condition while the cam/engine are in the car.

.