1978 L-82: Rough Idle. Timing mark jumping around
 

Hello all. A while back I discovered that the mechanical advance
on my car was stuck in a fairly "advanced" position. Discovered this after a lot of troubleshooting and talking with BarryK on this forum. I was able to free the stuck mech advance and properly tune the car according to the emissions sticker.

Here's the problem: My idle is fairly rough, and it's worse when the A/C is on. Also, the timing mark jumps around a bit when you are checking it with the timing light. (vac adv plugged or unplugged) The RPMs seem to stay pretty close to their proper setting..... although they are not rock solid like they were when I bought this car in 97. At that time it only had 9800 miles.

Questions:

Can a faulty mech advance cause a rough idle?

If so, where can I send my distributor to have it rebuilt by some one who really knows what they are doing?

This car only has 40,000 miles and is rarely driven.

I forgot to add that when you are sitting in the car at a red light, engine idling, you can really feel the rough idle. I can watch the fat on my leg jiggle!! I know you didn't really want to hear that, but I wanted to get my point across.

Any help would be greatly appreciated.

Its saying "get off! your too heavy.:rofl:

this may seem silly, but are you sure you got all the plug wires back on snug and correct in the posts? Maybe check the plugs to see that none of them is fouled.

You say the car is not driven much, I wonder if a little varnish has formed and fouled the idle circuit in the carb?

Yes. All the wires are in the right place and are snug. I connected each one to the timing light. They are all firing.

AND, when you rev of the motor it smooths out. It also accelerates as it should. It runs down the highway just fine with a nice low even hum. (duals with Magna Flows)

It's just the idle that is crappy.

My mechanic put a kit the in carb a couple of years ago. He also soaked all the parts in carb cleaner. I think the carb is OK. I noticed no real difference between the performance of the car before or after my mechanic rebuilt it. He was working on the A/C at the time, and we though it might be a good idea to rebuild the carb since the car sat up a lot.

At this point, I think other good questions might be:

- What are all the symptoms of a faulty mechanical advance?
- What faults in the distributor could exist that would cause a rough idle?
- If the idle circuit in the carb is at fault, what's the fix? Rebiuld? Can it be cleaned with an additive?

Maybe a small vacuum leak somewhere

Sean

Since you don't or shouldn't have any mechanical advance in at idle, you could lock out the advance by putting a rubber bank on the weights so they can't move. Put the rotor and cap back on and see how it idles. If it's the same, you know it isn't the distributor advance causing the rough idle. I know it sounds kind of shade-tree, but it's just to test at idle. Don't drive anywhere with it like that.

If the bushings in your distributor are worn the timing will appear to jump around. Same for the breaker plate or pickup coil. Timing chain too. 40k miles is not a big number, but it's also 30+ years old. The original timing chain has an upper gear with nylon teeth.

75vettehelp - I'll block off everything except the kitchen sink and see if my vacuum changes.

PeteZO6 - Thanks! This is a good idea. I will try this tomorrow!

TimAT - You make a good point. I really have no love for the idea of replacing a timing set. BUT, I can check the slack by hand-turning the engine and observing the timing mark and the rotor. How many degrees of slack am I allowed? I believe I was told one time that 4 degrees was too much.

Not only will a stretched timing chain cause your problem, but also excessive up and down distributor shaft freeplay - pull it up and down with the rotor off - if you have more than ~.015, it's too much. Similarly, a worn distributor gear and/or cam gear will also cause sloppy timing.

Did you check if the timing is more stable at a higher RPM ( free revving engine ) ?
A bad timing chain / timing gear can give the symptoms you describe... and however be more stable at higher RPMs.
If you use vacuum advance, just be sure it is fully engaged ( or disengaged ) at idle ( Ok you checked it :thumbs: ).
Also check if your distributor shaft doesn't have too much vertical free play or doesn't have a worn gear.
All of these are possible causes of a jumping timing mark.
If you want to be sure the mechanical advance is not the cause of your problem, remove distributor cap & rotor, use adhesive tape and stick the weights together,
then try again with the timing light... And don't forget to remove the tape after...

:iagree:

The description of the OP exactly describes an issue I had with my 78 back when I bought it.. I chased all of the suggestions made in this thread - even swapped out the dampner thinking maybe the ring had separated.

In the end it was a stretched OE timing chain and several severly worn vinyl gear teeth on the original set. If youve chased the rest - check the timing chain for stretch

Wow! A lot of good advice here. Thanks! Tonight I was able to check the vacuum and I think it looks good. I posted a video of it on youtube.

http://www.youtube.com/watch?v=YHcYmmIhmR8

If you can't watch the video, the vacuum at idle hovered around 16 inches on my gauge.
It bobbed around a bit between 15.5 and 16. Occascionally it would dip to 15. When I rev it up, the gauge smooths out. Idle for this 4-speed car is 900 rpm and is pretty steady. It does dance around a tinny bit however.... less than 25 rpm I would guess.

As for the distributor tests and timing chain tests, I'll have to get to those in the next day of two.

Just one last thing I was thinking about :
When you check timing, always pick-up the signal near the plug, away from the other wires, to eliminate cross talk. This can also cause jumping timing marks. And very often, the inductive pick-up has a polarity, with an arrow on it indicating most often the plug ( check your timing light manual ). :thumbs:

Make sure your carb mixture screws are adjusted properly.

Also, do you have a stepper motor that increases your RPM when the A/C is turned on?

Looks like I have a lot of work to do. But it's fun! :)

About the mixture screws: I completely forgot about this. Some one else mentioned the idle mixture. So two days ago I turned each one outward (enrichment) a turn at a time. The tach gradually increased in idle revs from 900 to around 1000. The engine did start to run better. Then I ran out of mixture adjustment, or so it would appear. I then adjusted the idle rpm back down to 900 with the regular idle adjustment.

The mixture screws seem to be out too far, although the engine runs good (except for the idle issue.) Now they are quite loose. You can touch them with your finger and move them slightly from side to side. I expect there is a mechanism to keep them from screwing all the way out and falling out? Is this correct?

This is a 4-speed car with A/C. Chevy did not put a solenoid on the 4-speed A/C cars in 78. Instead they just set the idle at 900, and when the A/C compressor kicks in, the revs drop to 700.

I think we are getting close on this one.

With the mixture screws out that far, they're leaking air. And they may fall out. Run them back in until they seat lightly, then back them out 1-1/2 turns. Hook a vacuum gauge to a good source on the carb and start the engine, then adjust the idle mix screws for the highest vacuum reading. Keeping your idle speed at the recommended speed on the emissions label. (Probably 750?)

:iagree::iagree::iagree:

I will re-check the mixture screws as you state above. Keep in mind that this car's emissions sticker indicates proper idle is 900 RPM since this is a 4-speed car with A/C.

Has anyone seen the video I posted on youtube? Is 16 a good reading on the vacuum gauge at idle? Note the small fluctuations in the gauge at idle. Or they normal. This video was made while the car was idling at 900 RPM.

Please note that this is a bone stock 78 L-82.

http://www.youtube.com/watch?v=YHcYmmIhmR8

These small fluctuations are pretty normal, I think. Nothing to worry about... :thumbs:
Try to blip the throttle quickly and see if the vacuum varies from 2 or 3" ( acceleration ) to 23 or 25" Hg ( deceleration ). This is an indication of a good engine condition.
You can try to adjust idle mixture screws to achieve the highest vacuum reading ( but keeping the same idle RPM). Sometimes it also makes the indication more stable.
Note that engine vacuum readings will decrease in higher altitudes ( 1" less every 1000ft ). More infos here :

http://www.earlycuda.org/tech/vacuum2.htm

and here too :

http://autospeed.com/cms/title_Using...3/article.html

900 RPM is reasonable- without going back and looking at the specs I have no way to tell, other than the usual 650 in drive works out to about 750 in park. Manual trans 900 is what the sticker says then that's what it is.

16" at idle is not too bad. See if you can get it to 17" and then 18". Don't go for a big jump all at once. Small little careful adjustments can pay big rewards. Keep the idle mix screws together- if you give one a 1/4, give the other that same 1/4 turn. That keeps the intake charge balanced between both sides of the engine. Not a real big deal, but if you're looking for that glass smooth idle, that's how you get there.