impalaman

Ok. This makes sense. Thanks for the distributor education. I'm a novice when it comes to digging down into the motor. This will be the first time
I've taken the distributor out. I've read Lars' procedure and several others. Looks like we'll see whether or not I can translate "book learnin" into practical use.

I will order the shims, and do this as soon as I can!

PeteZO6

I wasn't trying to get you hurt impalaman, I was just trying to make the point that a fully warm engine will give you the best or truest readings. Since yours are so consistent and pretty high, it really isn't an issue.

Pete

impalaman

Thanks Pete. I was pretty wound up about this annoying problem. My appologies for flying off the handle.

HOWEVER!!

I think I have found the problem.....

While going back through every single vacuum line (they were all good by the way), I discovered that the !@#@#$% !@#!#$@# EGR valve would not hold a vacuum. In other words, it simply doesn't work. Vacuum goes to it, but when you put a hand-held vac pump on it, the valve will not hold a vacuum. I am assuming that the EGR valve intake side port on the manifold is getting outside air through the valve in some fashion or that the exhaust port is slightly cocked open due to carbon.

ALSO, the vacuum switch on the A.I.R. system will not old a vacuum, so I think it is probably not working either. I have not read all the test procedures for the A.I.R. system yet, so I will be doing that while I wait on my ACDelco EGR to arrive.

This car was sold in PA. PA is strict about emissions, so it has all the emissions stuff on it. I'm the third owner.

fauxrs2

If I'm not mistaken the Corvette uses a posistive backpressure EGR which will not react to vaccum applied when the engine is not running. My 78 I know has a positive backpressure model and I'm pretty darn sure thats what its supposed to have. I made the mistake of replacing the original for the same reason you cite - it wouldnt react to the hand pump...this is when i learned about the positive backpressure type.

The positive back pressure EGR valve can be identified by the letter "P" stamped next to the part number and date code.

This applies to original parts only I suspect - mine has a painted on POS on the top of the can - POS - how appropriate

A back pressure valve is easy to spot because its pintle shaft is much thicker than the single diaphragm type. This is so because the shaft is hollow. The hollow design allows exhaust gases to flow into the shaft and push up on it. When positive back pressure in the exhaust system is sufficient, the shaft raises up and seals the built-in control valve.

Once the control valve is closed, it allows applied vacuum to pull up on the diaphragm. Without back pressure to lift the hollow shaft and close the control valve opening, the EGR valve will not hold vacuum. It is bled off to the atmosphere. This design thus modulates EGR flow by modulating the applied vacuum. As engine load increases, so does engine back pressure, which causes the control valve inside the EGR to trap vacuum and open up. To test this valve, bring the engine up to 2,000 rpms to create back pressure, then apply vacuum. EGR should open and cause a 100 rpm drop or more. Exhaust leaks or a modified exhaust system can create havoc here. Adding dual exhaust or headers on a car designed for a single exhaust will reduce back pressure and set a Code 32 on GM cars. Positive back pressure EGR valves are used in simple vacuum controlled systems, as well as more complex pulse width modulated applications.

impalaman

Nice. Man you guys love bursting my bubbles.

I'll check the valve tonight and see what happens.

I do recall reading in my Haynes manual for the 68 - 82 C3 that both regular and positive back pressure valves were used.

impalaman

Well, you were right about the positive back pressure EGR. I bought a new one anyway just to try it. No luck. Next, I'm going to take a bottle of propane and a hose and see if I can find a mysterious leak anywhere around the intake manifold.

Regardless of the outcome I going to shim the distributor since it has too much play.

7t3shark

If adding the shims doesn't eliminate the "jumpy" timing, check the shaft side play to see if the bushings are worn excessively. That was the problem on mine.

Buddy1980

I just installed a double row timing gear and chain on a 69 Camaro with the original 350-300 engine that had 72,000 miles. It had a terrible idle but seemed to run ok at cruise speed. The original nylon tooth cam spocket was worn and the chain had a lot of slack. With the new chain, which is fairly snug, the engine idles really smooth and the car feels like it has another 25-50 horsepower. The throttle response is also a lot stronger. Just something to think about. I think maybe it had a little chain stretch and a little nylon sprocket wear which created a very loose chain. And by the way, I checked the compression on the engine and all cylinders checked between 191-200 lbs. I thought this was unreal for a original engine that had never been apart that is 40 years old. 69 SS-350 Camaro Rally Sport 72,000 original miles.

impalaman

Thanks for sharing Buddy. Yes, a timing chain replacement is rolling around in my brian. I'm just sort of dredding it since my one-car garage is pretty cramped and doing jobs like that require some space. I've got an 81 Coupe Deville that has an enormous amount of slack in the timing chain in its 368 engine. (yes, the infamous V864) The slack on that car was easy to check however due to the ease of getting to the balancer to turn the motor manually while observing the rotor. The result was 7 degrees of slack. It's loose as a goose, and runs like crap......BUT it keeps on runnin so I haven't messed with it. I digress....

Tonight I took a propane bottle and checked all around the manifold. There was no difference in the idle. I even put the propane hose down the carb primaries and there was no difference in the idle. Just to make sure I was getting propane, I went out into the yard and checked it with a lighter. I got blue flame.

Is propane a good test for a vacuum leak?

One thing I did find tonight was that the carb bolts were (in my opinion) loose. They were less than 5 ft lbs. Probably 3 or 4. I got the inch-lb torque wrench out and slowing torqued all four bolts down to 11 ft lbs. I did this 1 lb at a time just to be safe. I read in Doug Roe's Q-Jet Book that they should be 10 to 14 ft lbs. This is a stock aluminum manifold so I didn't want to push it.

I took it for a test drive and the car seemed to be more responsive. Also, I there was some back pressure rumble and gurgle from the exhaust when you let off the pedal after a rev. I assume that I tightened up the carb to manifold connection and the mixture is now a little too rich. I'll have to re-adjust the mixture tomorrow. Does this sound reasonable?

After titghtening the carb bolts my vac gauge was hovering around 16 to 16.5. I think it made an improvement. I believe that if there were any vacuum leak, this was the only one.

fauxrs2

Dont worry about the 1 car garage, I rebuilt my engine, did an engine swap, rebuilt the rearend and swapped in a TKO 5 speed all in a one car unlit garage.

A timing chain really only requires that you pull the waterpump and fan to accomplish - the radiator can stay in. and you can rent the proper tools for removal and installation of the balancer. - The only real issue is getting the bottom (crank) gear off. I got lucky with a 3 jaw puller.

The timing chain was teh culprit on my car like I said early on in this thread, but you are going about this in the proper way I think, eliminating other possibilities and in many cases resolving other minor issues.

Keep it up

Carl79

I hate to say this but your problem sounds very similar to mine a few years ago.......I hate to say it...Close your eyes.....nylon timing gear and chain.. Okay you can look now !

dctonka

Just got a 78 this last Saturday, my first vette, and am also having the rough idle issue. Mine is not an L82, but I am thinking there isn't too much difference in some of the troubleshooting things you guys have been talking about. Mine idles rough, but you get it on the road and it smooths out and seems to go pretty good. I do know there is a vacuum leak in the A/C temp controls, cause you can hear a hiss coming from the console during running and especially when you turn off the engine. Is there an easy way to get the console off to get the the temperature controls?

I am a novice myself and have really enjoyed reading the forum so far to see what I can learn. I also have an 84 Cadillac Sedan DeVille, so I spend some time with her also. Seems like the older ones always need a little TLC.

Thanks in advance for any info.
Dan

jdp6000

I think the vaccum supply comes from the same connection that feels the headlights, If you want to test it. Unplug the hose after there orfice at the manifold and plug the line coming from the block with your figure.

If you want to do a visual on the switch remove the passengerside side panel. Sometimes the connection to the switch with all the vaccum hoses comes loose or falls off. There is a lock ring the holds it.It may have fallen off and you will see it liying in the area of the switch.

If you want to remove the switch remove both side panels. You can easily get a the bolt the holds the switch on the driverside. The passengerside is a little trickier if you have a defrost switch. Its in the way. But if you can get at the bolt the switch will drop down.

To remove the conosole cover entirely. Pull the side panels off. Undo the two bolts that hold the console to the guage cluster....one on each side. Don't miss these or you will break them off. There are also two bolts that hold the plate where it meets the arm rest cushion. Remove these there is one on each side. There is also a bolt that holds the plate near the shifter you have to look for it its underneath too. Unscrew the shift ****(its an automatic??). Then move the plate backwards. As you move it backwards pull the shifter from the park position towards the L1 position and lift the panel up and off at the same time.

You sure its not the hose that goes to the heater core box thats leaking? You need to take off the passengerside interior panel to get a visual of this....pretty easy.

Jim

jdp6000

Your problem could be caused by many things.

Sounds like you need to make sure the air/fuel mixture is perfect first.

The EGR can easily be tested. Start the car and get it hot. Give it a burst upto about 2000 rpm with your finger on the underside of the EGR valve you should feel it move up and down.

Remove the EGR and clean the port with some carb cleaner. If you want start the car. It should sound like you have an exhaust leak with the EGR off and air should be coming out of it. Keep your face away if you do this after using carb cleaner. It will shoot out. If nothing happens the port is clogged.

FWIW my same problem was the nylon timing chain gear. Easy to test. Crank the motor until you get as close to "0" on the timing mark. Your rotor should be pointing at 1 or 6 in the distributor. If you have a long torch wrench put it on the bolt in the center of the harmonic balancer and move it back and forth. The rotor should move instantly and in sync. I had about an inch play in mine. Keep in mind that my car idles at 600 so that play is real notcible at idle. At 900 though I noticed nothing. Which brings me back to fuel/air mixture and EGR on yours. But you never know.

I also assume you have your spark plug wires on right. Some will mix up the 5 and 7. The car will run but poorly.

Jim

impalaman

Well I know it's been a while since I responded to this thread. Life happens.

Since then I've pulled the distributor and installed the cam gear shims to take out the massive amount of play that was in it.... about 1/8 inch.

I also replace the timing set. The old one was quite loose. I used a Melling double roller.

Also replaced the rear main seal, front seal, and oil pan gasket while I was at it.
(Had help though. It was a big job.)

The car runs great. BUT...............!!!!!!!!!!!!!!

It still runs like complete xxxx at idle when the AC is on.

I checked all the vacuum lines again. I plugged everything at the carb. No changed.

I am assuming at this point that the distributor either needs to be rebuilt or replaced.

jcloving

Tim and Larry above are right.

I had a jumpy timing mark too. I rebuit the dizzy and shimmed the shaft to specs. It cut out 2/3 the jumping around. I think the timing chain may have stretched and that accounts for the rest. I have 118k miles on my motor.

You said you checked each plug wire and each is firing. Make sure each boot is firmly down on the plugs.

Good Luck

impalaman

Well I thought I'd finish up this thread. I know it's been a while, but better late than never. I replaced the timing set a few weeks ago with a Mellings double roller. This improved things a good bit, but my idle was still crappy when the A/C was on.

So I did a search to distributor re-curve service and found the following site:

http://paramountd.com/

This business is owned and operated by Mark Saunders in Michigan. I pulled my stock HEI distributor and sent it across the country to Mark.

What I got back was a completely restored and re-curved distributor. My idle is perfect with the AC on or off. The engine vacuum is also in the green and steady. (unlike before)

Also since my timing is all-in by 3100 after the re-curve, performance is MUCH improved. I am very please with the job Mark did and I highly recommend him if you want your HEI distributor serviced or restored. He even glass beed blasted the housing.

Take it easy.

impalaman

Here are photos of what my distributor looked like when I got it back from Mark Saunders at Paramount Distributor Company.

http://www.flickr.com/photos/impalam...in/photostream

http://www.flickr.com/photos/impalam...in/photostream

TimAT

Always nice when someone comes back and posts what the final fix was.

Glad you got thru it. And you learned some things in the process. That's the real big PLUS!!

7t3shark

Looks better than new!