Hey guys,

While I've done work on Chevys before, this is the first time I'm doing some diagnostics on my 'Vette so I'm not sure if any of these conditions are pre-existing or have recently developed. It still runs nice and strong (or it seems so to me). I've been trying to track down an oil leak, and while I was at it I checked some other stuff.

1) A bit of blue smoke on start-up (valve seals?) Should I worry about this?

2) I hooked a vacuum gauge up to the manifold port, and at idle when it's warm I'm pulling about 13" of vacuum. It seems that is pretty low, shouldn't it be pulling around 18"-22"?

3) I assume this is related to 2, when I shoot the timing it jumps between 8 and 12 degrees before, I'm not sure what this means and a forum search was unsuccessful.

I haven't tried adjusting the timing to see if it would pull more vacuum or make the timing mark stay still- thought I would post first and see if there is anything I should be aware of or any other tests I should do.

I have also read about setting timing not using a gun, but just using a vacuum gauge and setting the timing so that the engine pulls the most vacuum. I actually did this on my el camino and it worked really well.

Thanks.

 

If the timing marks are jumping around at idle, I would think the cam chain is wearing out and jerking the cam forward & then back with too much slack. If it's smoking a little at startup and you suspect the guide seals, then I'd think it had enough miled to have a worn out chain as well.

 

Have you read Lars' paper on timing? I would highly recommend it. I can email it to you if you like, also you can search, and probably find it posted. I think BarryK has it in a sticky in Tech. I think he also has a vacuum paper.

 

You need to shim the distributer.

BigBlockk

Later.....

 

Joe,
Have you tried moving the timing lead on the plug wire, down next to the plug, (away from all other wires) i"ve seen that stop it from jumping around before.

 

 

Sounds like you have a vacuum leak at idle; 13" Hg is too low for a stock GM engine. Whether that has anything to do with the timing problem is dependent on whether you disconnected the dist. vacuum can [or not] as you took the timing reading. If the can was disconnected, the dist. could need shimming (as noted above) or you could have a defective harmonic balancer (rubber shot). {The latter is not very likely.} Before you spend any more time adjusting your idle quality, you need to diagnose your vacuum leak(s).

 

What does shimming the distributor mean ???

 

You can put shims in or out of the distributor to change the heights of the weights. Not sure how that will help, but don't forget to mark where it is before you take it out or your timing will be way off and it won't be directly on the oil pump.

 

If you take the distributer cap off and lift up on the distributer shaft you will notice that it rotates as it moves up and down. This is what is causing the timing to jump around on you at idle. When you shim the distributer gear you remove the up and down motion and thus the jumping advance. I believe the correct number is something like .007" but if you can get it down to .010" it will surely help.

BigBlockk

Later.....

 

Excellent, thanks for the suggestions. I'll see if I can find the vacuum leak and then also shim the distributor.

Re: 7T1vette I did test my vacuum with the dist can disconnected, so that seems to fall in line.

 

Vacuum leaks won't cause a low manifold vacuum reading. Your reading is normal depending on the cam you may have in the engine. A vacuum leak would have to be truly massive in order to change manifold vacuum readings. To demonstrate, the PCV is a "calibrated" vacuum leak. Plug and open your PCV while you read the manifold vacuum: the vacuum won't change as you plug or open the "leak" of the PCV...

3 things will cause the timing to jump around:

1. Loose timing chain. If there is a bunch of slop in the timing chain, you'll see some "scatter" of the timing.
2. Excessive distributor shaft endplay. As BigBlockk states, you want to have about .007 - .010" of enplay (vertical movement) in the shaft. You shim the shaft by installing shimes between the distributor gear and the distrubutor body (Moroso part number 26140).
3. Slop in the breaker plate. The breaker plate in the distributor is positioned by a hole which attaches it to the vacuum advance control unit rod. If there is slop in this hole, the breaker plate will rattle back and forth, changing timing by twice the amount of the slop. Fix it by removing the breaker plate, laying it on a flat hard surface, and giving the hole a good whack with a hammer. This will tighten up the hole and remove the slop.

Lars

 

Awesome technical advise. Truly 'fall out of your chair' awesome.

 

A 5-pound hammer is my #1 Tuning Tool - any Vette can be tuned with a hammer... ask anyone who has attended a Tuning for Beer World Tour Event...

 

Check the light first. I think the idle would be unstable if the timing was jumping around like that. You didn't indicate it was having that effect.

 

What about turning the timing light pickup that clamps on the #1 plug wire, I read somewhere that jumping timing marks can be caused by this.

 

You read wrong

 

Well there was tons of end play in my distributor. I got the shim kit and got the end play down to a bit below .010. I put the distributor back in again but couldn't get my marks that I had made on the dist and the intake before I pulled it out to line back up.

The only way I could get the distributor to go back in and seat properly would have the rotor pointing to #8 after the rotor would rotate when the dist got seated. I did get it to start like that and it was pulling more vacuum than before, but the timing mark was nowhere near the tab, I couldn't even see it. But it did run, although the manifolds seemed to be getting really hot really quick so I shut it down.

This was probably a mistake, but I figured I should pull the distributor, find TDC again, then point the rotor at #1 on the cap, mark it, and put it back in. I found it using a compression gauge in #1, lined up the mark (well, it was about 1.5 degrees after off on the timing mark). Got the distributor back in and still, the only way I could seat it was to get the rotor pointing at #8 just like before.

Now I can't get it to start and it's backfiring out of the exhaust.

Was putting the dist back in while it was slightly off TDC that big of a deal? Do I need to pull it again and get it exact? Think maybe the balancer is slipping? Something else I did maybe?

Thanks.

 

The reason you couldn't seat the distributer is becuase of the oil pump drive shaft. The slot in this shaft didn't line up properly with the distributer gear key. Do your TDC drill on #1 cylinder again. If the distributer doesn't seat you will need to rotate the oil pump drive shaft a little with a screw driver. Keep making small adjustments until it drops into place.

BigBlockk

Later.....