Rough running problem

[78SilvAnniv]

I just checked the 2-3 recent threads of a similar nature and they don't seem to shed any light on what happened to me and the 78 today.

I had washed 78 and drove around the 'block' to blow most of the water off before a towel dry. Halfway around the block (about 1.5 miles) a truck came screaming up behind me as I made a turn so I stick my foot into it and she downshifted to second on her own and we maintained about 70mph for about 1.5 miles before I needed to make a sharp right hand turn.

As I slowed to nearly a stop for the sharp turn, I observed the engine running 'lumpily' and very rough as though it was about to die! I gave it a bit of gas and she smoothed out and we cruised the last 1/4 mile at 30-45 with no problems. UNTIL...we came to a stop sign and she wanted to die again! I goosed her around and we made it to our driveway where I shut her down.

My first thought was 'water in the distributor'...but there is no way for water to get there the way I wash the 78. My next thought was a vacuum leak, but everything looked okay to me.
:confused

It was several hours before Kenny could look at her and he took off the dist. cap/rotor and found that it had fouled, so we replaced it. We numbered the new cap to coordinate with the numbers on the old cap, and also numbered the spark plug wires before removing them. Die-electric grease where called for.

Fired up...but ran even WORSE! :eek
I also seem to hear a hissing noise when we goose the throttle...is that a vacuum leak? Kenny plans on changing the spark plugs tomorrow during daylight. I also thought I heard something knocking, but it could have been something moving and hitting something with the engine rocking the car so much.

What are we missing?
Heidi

GM crate 350 @ 250hp. Approx. 25,000 miles on engine w/ tuneups, rebuilt carb and properly timed oil changes. Carb and plugs/wires were done in the last year to year and a half.

 

[matchframe]

My gut feeling is it may be a burnt valve. With the hissing and knocking and lumpy running except at speed. The best way to determine this is with a compression test.

Hopefully it is just a vaccum leak!

Bill

 

[78SilvAnniv]

Okay, plan of attack...

Compression check
Check timing

The outside of the cap had a darker 'ring' circled around the center and the inside of the cap had red marks (looked like paint overspray) that was from arcing or crossfiring. The metal tab in the cap had dark, nearly black, 'stuff' all around it. Burnt die electric grease? Carbon? I've got no clue what it was.
Also, 4 of the 8 posts on the rotor were darker than normal.

Die electric grease came with the new cap and rotor with instructions on where to apply it and also to place it on the posts of the rotor before plugging in the plugs.

I don't know if we have vacuum advance and I'll have to ask Kenny.

I hope this is not a burned valve, I'm sure that involves a partial tear-down to re-do the head/s? If we do that Kenny is sure to want to replace the entire top end and I've been very happy with it as it is.

The problem appeared between when I stomped on it and then slowed for the turn. The stomp was milder than I have done while driving normally because the 78 was not fully warmed up, maybe 150*. D@mn chambered exhaust and the sweet sound it makes!

Thank you very much for your replies.
Heidi

 

[KOPBET]

Remember that by definition, dielectric implies opposition to electrical conductivity. The grease should be on the boot and not hinder metal to metal contact.

Dielectric grease is supposedly a necessity on HEI ignition.

 

[MoeJr]

If it ran worse after the cap and rotor change make sure you did not cross up any of the plug wires. Also because the car started to act up after your stomped on it I would look to see if you sucked up a piece of dirt in the carb that is holding the needle and seat open a bit. Also the float in the carb may have sunk a little at that time. And finally there is a power valve in the front of a QJet(if you have the original still on the car) if these get stuck as I have had happen to me before the car will run horrible. Hope this helps.

 

[Roy]

Rough Running

Don't worry. It is unlikely that you have damaged a valve.

Start with the simple stuff.

This time of year, most states begin a program of oxygenated fuel by means of adding methanol to the fuel mixture. The oxygenated fuel is a bit "hotter" and will clean out any residue in the fuel lines or carberator, often causing the fuel filter to plug and sometimes depositing junk in your float bowls.

If you haven't changed your fuel filter, now would be a good time to do so. Drain your float bowls if that doesn't work. Did you recently add fuel to your gas tank? If you, there is always the chance of getting fuel contaminated with water. I know you also said that you had washed the car and were carefull not to get water near any of air or fuel intakes.

The "hiss" is a vacuum leak. Happens all the time on C3 Vettes, especially if the original vacuum hoses have not been replaced. Look between your carberator and your distributor on your intake manifold and you will see a vacuum port with 2-3 hoses attached to it. This is the feed point for 90% of your vacuum operated devices so when you have a problem, always start here.

And "yes", your distributor should be vacuum operated. You can confirm this by finding a small rubber hose connected to a cone shaped diaphram on the outside base of your distributor. See if the hose is in good shape and connected to the diaphram. If you were working on the distributor, you may have disturbed this little hose - check the end of the hose where it connects to the diaphram to insure that the hose is tight and is not split. The distributor diaphram hose will connect to one of three places:

1. The main vacuum port.
2. A vacuum port at the base of the carberator.
3. A vacuum port on the carberator.

Once you have determined that the distributor vacuum hose is connected and in good shape, give a quick inspection to the other vacuum hoses feeding off the main vacuum port. Be sure to check the vacuum hose from the power brake booster to the vacuum port. Even if the distributor vacuum advance hose is connected properly, a vacuum lose through other hoses sharing the same engine vacuum will compromize the performance of the distributor vacuum advance.

O.K., so ... check/change fuel filter ... check vacuum connections. Based on what you have stated concerning the rough running and the hiss sound, I would say that these items should be the cause of your problems. Let me know what you find.

 

[78SilvAnniv]

From one thing right into another!

It appears I had 4 plugs not firing correctly. One boot (#6) was torn/burned nearly in two from the sparkplug heat. We are now timed correctly with the carburator adjusted accordingly, yet still having problems.
After my test drive the brakes did not want to work, so we need to chase down a vacuum leak. AND...Kenny wants to 'bench' the 78 until he can adjust the valves. The knocking I heard has decreased to a light clicking in time to the rpms on the driver's side. I can also feel a difference between the two exhaust sides, driver is not as smooth as passenger's side exhaust.
Heidi

Pic of sparkplug boot #6:

 

[Yoda]

Heidi,

Start up the car and remove the air cleaner then have Kenny choke the carb by covering it with his hands and see if it will die or if it is sucking air from a bad carb gasket OR you can spray wd40 around the base of the carb and see if it smooths out.

I've seen a few carbs suck a carb gasget and act very similar to what you are describing

Bud

 

[78SilvAnniv]

Thanks Bud,

Kenny didn't try the hands over the carb choke, but he did spray around the carb and it didn't make any difference to how it ran. Carb was rebuilt only last year and everything appears to be working fine on it. Original Quadrajet.

I think when we chase down the vacuum leak it will solve most of the remaining problems with the exception of the light tapping from the driver's side. It looks like he'll have to pull the valve covers and adjust the valves.

My idle in gear is now 600 and 850 in park. Normal?
It seems odd to me, but it could be because I'm not used to it...having had my car out of time / carb adjusted / advanced with all the systems for so long.

Kenny is pretty handy with a power tool or a wrench and I have confidence in him to fix my 78. I just hope I don't annoy him with questions like:
"Is it fixed yet?" :Silly
Heidi

 

[Stallion]

I idle at 600, no problem. But this is without the vacuum advance connected. It is usually around 850, but I keep it disconnected.

Good luck with everything, Heidi. I hate to hear of a Corvette, and mostly a '78 S/A, not doing so well.

TR

 

[Jack]

Re: Okay, plan of attack...

the inside of the cap had red marks (looked like paint overspray) that was from arcing or crossfiring. Heidi

Heidi/Kenny:
That red stuff inside cap is probably RUST thrown from the mechanical-centrifugal advance ... indicating it is badly in need of lube ... it'll hang up and cause erratic timing/rough running. Routinely happens if unit isn't lubed every so often. Even though you don't spray water on it ... distributor will get condensation inside it ... nature of the beast. Pull the cap & rotor ... clean, lube & inspect the mech-cent advance unit. Yes you also have vacuum advance ... and sounds like a vac leak somewhere ... others' suggestions about chasing down the leak should be considered. While you have the rotor out ... that's the perfect time to consider swapping the ignition module for a new one ... cheap at any parts store (though a GM is better) ... bad ig module can also cause some (some) of the ignition symptoms described. G'Luck!
JACK:gap

 

[Roy]

Rough Run

Your vacuum leak is related to your loss of power braking. The power brake vacuum hose is large and runs from the power brake booster to a main vacuum feed on the engine. There is usually no one-way vacuum valve in this line. If you blew the vacuum hose off, the resulting leak will cause your "hiss", reduced braking and poor performance.

Trace the hose from the brake booster and you will find your problem.

 

[cntrhub]

Since Kenny is going to do a valve adjment, have him take a compression test before the valve adj, and then do one after the adjustment. See if the is a difference?
A tight valve will make the engine run rough at idle for sure.

 

[78SilvAnniv]

Wow!

Thanks everyone for the GREAT advice!!!!!

Either we will purchase a compression tester or borrow one from our mechanic friend. The vacuum line sounds like tediousness by trying to trace lines and check for wear. I recently purchased vacuum fittings (little plastic pieces in dif. shapes/sizes) and I'm certain we have several sizes of hose somewhere in the garage.

As for the cap not being lubed...It must have been pretty dry because it was cooked and Kenny commented that it had probably needed to be lubed. I didn't know that was something I needed to do, so now it is on my list of things to check regularly.

I have a question about vacuum advance:
What is the difference in running with it, as compared to without it and why is it done?
Heidi

 

[78SilvAnniv]

Not worthless at all!

I don't think it was caused by the bath. It ran fine for the first half of the drive after the bath, but after I stomped on it and then braked for a turn is when I noticed the change.
Thank you for your advice, water in the distributor was my first thought.

After my husband and a mechanic friend were able to take a look at it Sunday (yesterday) we determined that it had several problems, some of which have been remedied and a vacuum leak that needs to be found pronto.
Heidi

 

[cntrhub]

By having the spark plug fire a couple of degrees sooner, you give the mixture more time to burn, thus burning more completely. So, at higher RPM, the more advance time you'll need to burn the fuel to give you the most peak efficiency.

 

[Roy]

Vacuum Advance

The vacuum advance in your distributor advances the timing of your ignition as vaccum in your engine intake system increases. The timing on the GM crate motor I have is 6 degrees BTDC (before top dead center) at idle. As the engine runs faster, the vacuum inside the intake manifold increases as the pistons are sucking in more air.

This increase in vacuum is transferred via that little hose between the carb/intake manifold to your distributor vacuum advance mechanism. The vacuum advance mechanism has a diaphram that flexs as vacuum increases/decreases with a small arm attached to the diaphram. As this diaphram flexes, the little arm rotates tha base plate inside your distributor that in turn advances/retards your ignition timing.

I do not have the specs in front of me but the timing can usually advances about 10-15 degrees ... it varies by motor and number of cylinders. So if an engine is set @ 6 degrees BTDC, at highway speeds it could be running at 16 degrees BTDC. The reason you want advanced timing is that at higher RPM, things are moving pretty fast inside the combustion chambers and the timing of your spark plugs firing has to be "advanced" to ignite the fuel/air mixture earlier in order to allow for complete and efficient burning.

The reason you don't need advanced timing at idle or start up is that too much advance at lower RPM makes it difficult for the engine to turn over or idle smooth. There is no need for advanced timing at low RPM. Too much advance at low RPMs and you get back fire through the carb.

Note: Some vehicles have centrifical advance units inside the distributor instead of vacuum units. The faster the engine spins, little weights force tiny springs to expand and the mechanism advances the timing.

I hope that this answers your questions. Tracking the vacuum leak is not difficult as everything you are looking for is right on top of the engine. The hose you need to look at comes off the power brake booster (that big bronze colored tank connected to your master cylinder) and runs into a plastic filter cannister, then exits at a 90 degree and the hose runs through a hole in a bracket and then the the vacuum port on top of your intake manifold.

 

[Bob Chadwick]

Re: From one thing right into another!

...a light clicking in time to the rpms on the driver's side.

Heidi,

This can also be an exhaust leak, which considering your recent change over to chambered should be considered as well.

Did you replace all of the plug wires? The boots look pretty bad and I'm assuming the wires have not been replaced in while.

Bob

 

[78SilvAnniv]

I'm pretty sure the clicking is not an exhaust leak, I know what those sound like due to my experience with headers on a Camaro many years ago.
I can also feel with my hands held just under the exhaust tips the passenger side goes: blat-blat-blat-blat, blat-blat-blat-blat while the driver's side goes: blat-blat-__-blat, blat-blat-__-blat.
Something is definitely sticking.
(the toasted wire boot was from the passenger's side, clicking is from the driver's side)

I have an entire box of new wires now, but the wires that were on it had been changed just last Fall. The vette received a tune up, chambered exhaust was installed and then the carb was rebuilt, all within 5 months of each other.
Heidi

 

[cntrhub]

Possible plug wires are deteriorating, or another wire failed due to heat like the other plug wire you posted? Sounds like the spark is traveling to the shortest path..... i.e. = misfire.
You can do one of two things. Change all plug wires to eliminate this variable. Or two..... wait till dark, look inside the engine bay without any light source, and watch for a spark shooting to ground from one of the spark plug leads? You an Kenny can stand on either side of the engine bay, and look at the spark plug bank. See if you can determine which spark plug is shorting to ground before it reaches the spark plug electrode like it should normally. There will be a spark jump from the plug wire right to the engine's head area or header. The darker area you work in, the eaisier to see the spark.