mefreema

I have a 79 vette I am getting ready to sell. I pulled my built up motor and put the original moter back into the car. Now it is backfireing like crazy, popping back through the carb.
To find TDC I put my finger over the # 1 plug hole and turned motor until I felt compression then rotated to motor until the balancer mark light up with timing tab on zero.
I checked and rechecked and rechecked the plug order tried different sets of plugs, plug wires, caps, rotors, and coils. Nothing seems to be fixing it.
The car starts up fine, two pumps on the accelerator and varoom starts up every time but frequently pops from out of the carb. Also, the # 8 cylinder doesnt seem to be fireing at all. I can put my had right on the headder pipe after running it for 15 sec with no problem. Two other cylinders (2 & 3 i think) get very warm and the rest will leave your skin on the headder pipe they are defently fireing.
The vacuum gague is my big concern. It is bouncing like crazy probably close to 4 or 5 inches of vacuum and also sways up and down averaging between 8-18 inches of vacuum. It is a stock cammed car so I would guess it should be at a steady 20 at least. The gauge is plugged into the base of the carburator.
I would like to think I have limited all ignition and timing problems by maybe not since #8 is cold. Also, I am concerned about my intake gasket maybe having a leak because of the crazyness on the gauge. Could I have a really bad vacuum leak on the intake on runner #8 causing the backfireing and also causing the cylinder not to fire?

DaGoat

Do a compression test on the #8 cylinder. I almost guarantee you don't have any compression on that cylinder. Same thing happened to my #1 cylinder.

If you don't, check your valve lash.

mefreema

Already checked it. 150 PSI. Also my oil looks ok, not milky on the dipstick.

DaGoat

Damn, nothing's easy today.

Use a spray bottle of water on a light mist to help find any vacuum leaks around the carb and intake.

F-BIRD'88

Could be a sticky valve from sitting so long.

mefreema

I did spray starting fluid along each side of the intake manifold but did not notice a rise in the rmps but it is running so crappy now I'm not sure if I caught it. I was low on eather too so I didnt have much to spray to check. I could try that again or you say use water?

Also, what's the best way to check for a sticky valve? When I bought this car like 4 years ago I did get to original motor running ok from what I recall. So I think the motor was good before I stored it but cant be 100% sure.

This is killing me I want to get this motor running right so I can put the car up for sale. I just bought a 78 camaro with the 4 speed manual and 3.73:1 posi rear. It is a good solid car but the interior is trashed. I have one kid and another on the way so the vette was not pratical for me. I KNOW I am going to regret getting rid of it down the road but oh well

Any more suggestions? I am leaning towards vacuum leak. I may try to tighten the intake bolt a little to snug them up. Then disconnect and plug every vacuum line from the motor that absolutly does not need to be there. Then fire it up and check with the starting fluid/ water trick. I didnt get a chance to work on it tonight, maybe tomorrow but definently this week.

Anyone interested in a 79 vette in the Michigan area?

carbed82_Z28

Check your timming. I had the same problem, then we found out the car was out of time.

RB83L69

It's not timing if some cyls are working right. Leave that alone.

I've never in my life seen a "sticking" valve. I've seen broken springs, and real weak springs; but that's about it. Given those odds, I'd say the chance of 3 "sticking" valves in the same engine, is pretty damn remote.

Vacuum leaks tend to affect all cyls more or less equally; unless they're completely heinous (like the PB fitting that's on only one runner left off. Probably not that.

Sit back and calmly analye the problem, without trying to invent some far-fetched probability or one-of-a-kind new defects. Work through the obvious first.

Sounds to me like the plug wires are on the dist in the wrong order.

"The simplest explanation that fits all the facts is usually the right one."
—Occam, ancient Greek philosopher, from when plug wires hooked onto the plugs with thumb screws

DaGoat

While checking the order of the plug wires get a voltmeter and do an ohm test on each of the wires. Could be that one of the wires is bad. While you've got the #8 plug wire off pull the plug, hook it to the wire, ground the electrode and see if it's firing at all.

TMX

My thoughts exactly. This is a very common occurance on motors that have been sitting for an extended period of time, particularly those with a few miles on them . A compression test will often be misleading under these circumstances - a cylinder leakdown test will show you what's really going on.

A quick test is to remove the plug wire from the suspect cylinder and ground it, and then start the engine - are the backfires gone, or at least drastically diminished?

That said, the fix is usually very simple - remove the rockers from the affected cylinder and give the end of the valve stems a quick pop straight down with a dead blow hammer. You don't want to wail on it, just a few light-but-sharp hits to help reseat the valve.

-Bob

mefreema

Fbird88 and TMX you guys are what make this board so AWSOME :hail:

I pulled the #8 plug and the severe popping from the carb stopped and there is just very very mild backfiring. The motor motor will now idle down and hold a steady rpm. The vacuum is now bouncing between 15 & 17 which is much steadier than before.

Just so I understand, the intake valve is sticking open slightly and when the #8 cylinder fires it pops back through the intake and carb? That is interesting that it would still hold that much cranking compression but I guess that is what the leakdown test of design for.

Now to fix it I need to pull the valve cover and take off the rockers for the #8 cylinder. Pop the intake and exhaust valve stems with a (will a rubber mallot work?) dead blow hammer a couple time but not wailing on it. My one concern is knocking the keys loose and the valve dropping into the cylinder. Should that be a concern? Do I just make sure that cylinder is TDC or could I accidently tap the valve into the top of the piston? Or would that be concidered wailing on it

Once that is done is there a way to tell if it is now seated ? A particular sound to listen for or do I just reset the lash test and repeat the process if necessary?

Thanks for all the help

TMX

Hang on, we're not there yet.....

A couple of things before we get into this, just to be sure we're on the same page and that all the obvious bases have been covered up to this point.

RB83L69 hit on some valid points regarding possible firing order and/or valve spring problems. The only thing that lead me (and, I assume, F-BIRD'88) to lean towards a sticking valve was the fact that the engine was out of service for an extended period. If you're pretty certain that this problem didn't exist before you originally pulled this motor, then I'd say you're still on the right track. Anything is possible, you just need to start troubleshooting in the order of what is most likely given the circumstances (and given the absence of a proper leakdown test).

That said, are you absolutely certain that the firing order is correct? Two crossed wires can deliver symptoms similar to what you're describing

If you are, there's one more thing I can suggest before you pull the valve cover.

Assuming that we are dealing with a sticky valve, it may be possible to free it up without pulling the valve cover. leave the #8 plug wire off and grounded, start the car, and run some carb cleaner through the carburetor. Use up a good amount (say 1/4 can) while working the throtle by hand to keep the engine running, and then finally flood it with cleaner until it stalls. Let it soak for a half-hour or so, and then fire it up and see what you've got. It also might help to run a little Marvel Mystery Oil in with your motor oil for ten minutes or so before trying this (you can use something like Motor Flush in the oil instead of the MMO, but I've seen it do some bad things to older engines).



You can have a small amount of leakage past the valve and still spin up a decent compression reading. This is something that a leakdown test would show pretty readily. Also, you'd have the added bonus of being able to hear (in most cases) the compressed air escaping through either the exhaust system or the carburetor.

A rubber hammer won't do it, you need something capable of delivering a solid hit to the valve. A deadblow will generally have a relatively soft face, and won't damage the tip of the valve stem.

Nah. just be sure you're squarely hitting the stem and not the retainer.

The cylinder you're doing this on should be at botton center to minimize the risk of doing any damage. However, you're not going to be hitting the valve that hard anyway. You're not trying to compress the valve spring or get a lot of travel out of the valve, you're just trying to bounce it past any gunk that may be causing it to stick - 1/8" to 1/4" or thereabout should be plenty. I used the term "sharp-but-light" when describing how to hit the valve in my previous post, and I'm afraid I don't know of a better way to describe it..... think of a "fonzie hit", where you hit it solidly but with little follow through. I hope that makes sense.

Test and repeat if necessary. I've personally never had one that didn't free up after a half-dozen hits or less. At the risk of sounding like a broken record, this is another place where a leakdown tester would be the thing to have - as soon as the valve seated, the cylinder pressure would shoot right up to normal levels, letting you know you were done.

Also, do check the condition of the valve springs while you're in there. Any broken spring or one weak enough to cause this problem at idle will make itself obvious.

Anyway, I hope this helps. let us know how you make out.

-Bob

mefreema

I have checked and rechecked the fireing order 1-8-4-3-6-5-7-2 for 74-82 vehicals. I have a diagram from my chiltons manual and am fairly certin I have that right and those #'s are stamped on the intake as well. I have checked the timing. I have been through all the wires with a multimeter checking ohms.

I am going to try out your suggestions and see what I come up with. I will also see if I can get a leak down tester from work.

mefreema

It is fixed and I am a jackass :lala:

How many threads end with a somewhat similar statement

I decided to dig straight into the valve cover to have a look at the valve train. I found that both the intake and exhaust rockers were just loose as a goose. I thought that was strange as hell so I just started to set the valve lash. When I was spinning the pushrod to feel for the resistance I noticed a wobble. Then it hit me like a Mac Truck!

I remembered that I pilaged that motor for a pushrod on a previous rebuild and put the bent one in its place. I had intentions on replacing it and resetting the lash before I dropped it back in but that was years ago. Needless to so I completely forgot about it. So this could have all been avoided if I had not killed soooo many brain cells in my youth.

Thanks to EVERYONE for all your help. Sorry my bone headed move had all of you racking your brains for me.