Rough running problem

[78SilvAnniv]

Thank you cntrhub,
Kenny and I did do that Sunday morning in our totally enclosed and darkened garage and we couldn't see any sparks...not even from the boot that was torn to shreds.
We only replaced the torn boot/wire since the others looked just fine, but perhaps we should replace the others too. Maybe tonight.
Heidi

 

[cntrhub]

When you replace the plug leads, grab hold of each spark plug end and see if one is loose? Not loose in the plug hole, but a broken plug? It is so important to torque plugs. If there is just the slightest angle of the wrench when installing the plugs, you could almost hear a "tink!" (breaking the plug's shaft) if you were paying attention. A broken plug will surly cause a miss in the engine.

 

[78SilvAnniv]

Just replaced the plugs yesterday and I'll have Kenny give them a check as you suggested, but I trust Kenny and our mechanic friend's abilities.
Heidi

 

[JohnZ]

Vacuum advance is absolutely essential to a good-running street engine; the centrifugal advance in the distributor is only rpm-sensitive - nothing else affects it. Vacuum advance is engine load-sensitive, and works together with the centrifugal advance system to provide proper spark advance based both on rpm and engine load conditions. Vacuum advance has no effect whatsoever on wide-open-throttle performance, as vacuum drops essentially to zero at wide-open throttle and the vacuum advance drops out. Most people (including many mechanics) don't know anything about vacuum advance - why it's there, what it does, and what it DOESN'T do, and and unfortunately listen to people who know even less about it than they do and disconnect it "because race cars don't have it". Race cars run all the time at wide-open throttle, full load, and high rpm, so they don't need it; street-driven cars do.

See my technical articles in the October/November and December issues of "Corvette Enthusiast" magazine for a complete description of ignition timing, centrifugal and vacuum advance and how to optimize it, written in terms hobbyists can understand. This is the least-understood subject in the Corvette hobby, and 99% of what you read on the Internet about it is wrong, especially from those who live, eat, and breathe the Summit and Jeg's catalogs.

 

[78SilvAnniv]

Thank you John, that was clear and concise. Easily understood, too.
Heidi

 

[Stallion]

Vacuum advance is absolutely essential to a good-running street engine; the centrifugal advance in the distributor is only rpm-sensitive - nothing else affects it. Vacuum advance is engine load-sensitive, and works together with the centrifugal advance system to provide proper spark advance based both on rpm and engine load conditions. Vacuum advance has no effect whatsoever on wide-open-throttle performance, as vacuum drops essentially to zero at wide-open throttle and the vacuum advance drops out. Most people (including many mechanics) don't know anything about vacuum advance - why it's there, what it does, and what it DOESN'T do, and and unfortunately listen to people who know even less about it than they do and disconnect it "because race cars don't have it". Race cars run all the time at wide-open throttle, full load, and high rpm, so they don't need it; street-driven cars do.

See my technical articles in the October/November and December issues of "Corvette Enthusiast" magazine for a complete description of ignition timing, centrifugal and vacuum advance and how to optimize it, written in terms hobbyists can understand. This is the least-understood subject in the Corvette hobby, and 99% of what you read on the Internet about it is wrong, especially from those who live, eat, and breathe the Summit and Jeg's catalogs.

John, unfortunately, my Vette was subject to this "Bubba" approach. I guess my previous owner took it upon himself that the vacuum advance was lacking, so he disconnected it. I just never re-connected it because I never 100% understood its use, and why the Vette was running fine without the vaccuum advance.

 

[JrdnsEdu]

78SilvAnniv-
Did you ever figure out the clicking noise? I'm really curious because my car has been doing that too and I cannot find it. I even started to get used to it after a while.
Let me know if you ever find it.
Thx- Eric

 

[78SilvAnniv]

I cornered Kenny today about when he is going to take a look at the 78 and he said he will do it this weekend.

The clicking sounds like a vacuum leak, but it is definitely from the engine. It increases and decreases with rpm's.

I am hoping (and I'm sure Kenny is, too) that it is an easy fix like tightening the valves or something. The click is coming from the driver's side and I can tell from the sound and feel of the exhaust air blowing out of the duals that the problem is on the driver's side.

I believe we will do a compression check, check sparkplugs, wires and such on the drivers side, check/tighten the valves and if that doesn't fix it...I don't know WHAT I'm gonna do!
wringing hands, pulling hair and chewing fingernails

Kenny said we may need to rebuild the carb. I don't understand this because we had it rebuilt last summer by a mechanic friend and it worked wonderfully. Kenny commented that the 'backfiring' when I let off the gas pedal at hwy speeds may have damaged the carb.
Can that really happen?

I also seem to be running fuel rich. Don't know if this is the case or if the funky engine isn't firing one on the drivers side and making the extra-fuel smell out the pipe.
Heidi

 

[cntrhub]

The carb is fine. If a backfire happens through the carb, it goes directly out the venturi opening. It may pull fuel out the jet because as you know, "for every action, there is an opposite and equal reaction."
Sounds like either a tight intake valve, or a burnt valve? Remember, a valve sitting on the seat will dissipate the heat from the combustion chamber. If that valve sits off the valve seat, there is where you start to burn valves. The heat will travel up the valve stem and through the guide, but that's just not enough. That valve face needs full contact to cool off.
I would do that comp. test before and after. This way, you will tell you have a possible burnt valve problem? The real test is of course, a leak down test. But the comp. test should give you a reading that will match, if you have a burnt valve no matter how many valve adjustments you do. Meaning..... the reading will still stay the same.
Your fuel will not burn because there is not enough compression. This is one variable. Usually a ticking valve means the valves are set too tight. This may be why you smell raw fuel out the drivers side exhaust?

 

[Macgyver]

Ok heidi, I read this post all the way through a second time and now some buzzers are going off. How you describe your engine when it wanted to die is exactly what happened to mine, except it did die. What they told me at the shop was that the gas over flowing from the carb had gottne the spark plugs wet and flooded the engine. I also had a very bad spark plug wwire that was missing a boot, spliced and shot plugs that they changed. I couldnt reach the front four for the life of me. He showed me the wire and it looked like yours except a little worse. It runs a little better after replacing the wires and the front plugs.

But I still here the exhaust doing the exact same thing you described yours doing but on the passenger side pipe. I have true duels no cats so its quite apparent. I had exited the highway and slowed to about 7mph to take a turn off the access road which was at a nice incline adn she puttered out. I bet yours might have died if you had been on an incline as well becuase these problems seem to be so similar.

The interesting thing is is that I tried to start it for 10-15mins and at the very end it almost cranked but then wouldnt becuase it wasnt getting any fuel which seems the opposite of what the floats are doing. Ten minutes after I left they said it cranked right up. Ill let ya know how the carb rebuild goes this weekend.

Steve'O

 

[78SilvAnniv]

What is a leak down test and how do you do it?
If I do have a burnt valve, what is entailed...do I have to tear into the heads?

Steve'O, I was on a slight incline for the last turn for home, but I feathered the gas to keep it from dying. If I had not quickly done so I am certain she would have died.
Heidi

 

[cntrhub]

Heidi, A leakdown tester has to be used along with an air compressor. It will need constant air pressure which the gauge needs to show you a reading. Most Snap-On trucks have these units and they have the spark plug adaptors for most plug sizes.
Basically, the leakdown is showing you 4 basic air leaks that are present in the engine. It is critical that the crankshaft is positioned on the compression stroke when checking leakdown. The test will show if there is a piston ring seal problem (hole in piston even!). This will be detected by listening to the crankcase ventilation (any hole in valve cover... oil fill cap for example). Compressed air will escape past the rings, and you will hear the air, and also the guage on the unit will give you a high reading. Next problem area will be the valves. Problems can be anything from a burnt, bent, or out of adjustment (held open) valve. Simply listen to the air leaking through the carb for an intake, and the exhaust pipe for a leak at the exhaust valve. Finally, a head gasket leak will show you both cylinders having 100% leakage, because when one cylinder is on compression, the cylinder next to it will be in a valve cycle that is open. Ideal leakdown will show a 2% leakdown. 100% or less leakdown, will mean one of the above mentioned is the cause of excessive air escaping.
Head removal will be the mode of repair and inspection. No way around it.

 

[78SilvAnniv]

Whew!

Kenny took the valve covers off and we found two loose rockers that were so loose they were not operating correctly. One on each side.
Engine still sounds 'off', I think it is because our friend adjusted the carb and I don't like the way it sounds now. I think he set it to what he likes to hear, not how I like it to run. If we can't get the carb and timing set...I'm afraid I may be getting a Holley 650 double-pumper for Christmas.
Heidi (I like my quadrajet!)

 

[MoeJr]

Keep the QJet. Find someone that knows how to set them up and you'll be happy with it again.

 

[78SilvAnniv]

Keep the QJet. Find someone that knows how to set them up and you'll be happy with it again.

Our Qjet was just rebuilt last year and was running fabulously until our mechanic friend started fiddling with it. I think I will call around and find someone to adjust the carb and just not tell Kenny what I'm doing.
Heidi

 

[Bob Chadwick]

That new shop up there with The Last Detail has a guy that is supposed to be good with carbs. Call up there and ask for Jerry, the shop manager. Tell em Bob sent you.

 

[MoeJr]

Keep us posted Heidi on how you are making out. If ya were up here near NH I would adjust the carb free of charge. Can you say VACATION!! lol

 

[vmrod]

a vaccuum idea...

Is it possible that the vaccuum line that connects to the modulator (on the transmission) came off. You should have a vaccuum line that runs from the engine area to the rear of the transmission. (on the passenger side) If it came off down there, you would not even see it unless you looked under the car near the back of the transmission.

-Vic

 

[Scooter76]

It may have mentioned in the above responses, but your engine wanting to die sounds like a stuck float/ needle & seat. It will cause raw gas to dump into the motor, thus creating an extremely rich mixture. Happened to me the other day. Might be worth checking out.

 

[78SilvAnniv]

It may have mentioned in the above responses, but your engine wanting to die sounds like a stuck float/ needle & seat. It will cause raw gas to dump into the motor, thus creating an extremely rich mixture. Happened to me the other day. Might be worth checking out.

Thanks Scooter,
It wanted to die when I was slowing nearly to a stop, but when I gave it gas it evened out and appeared to run just fine. I don't think it was anything stuck...if it was I would expect it to try to die at all times, not just at low rpms.

I still think it is running "off", and I plan on calling Jerry at The Last Detail tomorrow. Kenny also says he wants to time the engine first, to see if that fixes anything. Maybe we should do another vacuum check to be sure everything is where it is supposed to be.

Right now, at idle the engine shakes the whole car and seems too "lopey" when I feel it should be running smoothly.
Heidi