OK, Gearheads!

[LubeDude]

Do you think that a 15# difference in one cylinder on a 327 would be enough of a leak to cause a backfire under guick acceleration? This cylinder is 135#s, the rest are 150#s. I dont know yet if its exhaust or intake. Im thinking intake.<br /><br />Under acceleration from a start, it will backfire every time, I have done everything I can imagine as far as carb and ignition. Car runs great after you get it going.

 

[QC]

Re: OK, Gearheads!

Lubedude,<br /><br />No comment on this thread, but I had a question for you over on the Wal-Mart Supertech oil thread . . .

 

[muskyone]

Re: OK, Gearheads!

sounds like a valves sticking

 

[craze1cars]

Re: OK, Gearheads!

Just one pop? Or does it backfire multiple times? 15 lbs is usually not enough to cause a significant problem, but anything's possible. I'd guess maybe you have an exhaust valve that's not seating properly and allowing raw gas to seep into the exhaust manifold. Or a lazy intake valve that's allowing it to fire back through the carb. If it's backfiring through the carb, you should see some evidence of this inside the air cleaner.<br /><br />Having said all that, my first guess would actually be carburation...dumping too much gas into the intake manifold at idle. But if your comment about doing "everything" with carb & ignition is fully accurate, then I'd say you likely have a valve issue on that one cylinder. I'm sure others here will have their own ideas to share...good luck!

 

[QC]

Re: OK, Gearheads!

LubeDude,<br /><br />Sorry, it is in the Mercruiser section. Down a little now. I think last comment sometime late last night.

 

[LubeDude]

Re: OK, Gearheads!

Thanks Craze1cars for your reply. I have been a small block chevy guy for more years than I hate to admit, (Like from when they were born). well almost. This one has me totally stumped. It is deffinatly backfiring through the carb. The engine in question is a 1965 Corvette, 350 HP 327, single four barrel. The car was purchased two years ago, sense then I have done a bunch of repairs on the car, but it hasnt been driven much so I cant really say if it always did it or not. The car sat for a long period of time, like several years at a time. Im thinking maybe it has a weak valve spring. It will do it when you are driving along at say 45 MPH and you let off the gas quick and then step on it also.

 

[LubeDude]

Re: OK, Gearheads!

Originally posted by Quietcat:<br /> Lubedude,<br /><br />No comment on this thread, but I had a question for you over on the Wal-Mart Supertech oil thread . . .

Someone "PLEASE" help me, I cant find it.

 

[magster65]

Re: OK, Gearheads!

It sounds to me like the float level is low or the accelerator pump isn't giving it a full shot. That would cause a very brief lean condition. Also, I think that engine has a solid cam, check for tight clearance(s) on a valve(s). Nice car

 

[LubeDude]

Re: OK, Gearheads!

Carb has been all gone through, great accelerator pump shot, float level is right at the site hole.<br /><br />Hydraulic cam.<br /><br />Thanks for the input though.

 

[kenimpzoom]

Re: OK, Gearheads!

How bout the choke.<br /><br />I know this is pretty much unrelated, but my 2 bbl carb always backfired on my 283 cid Impala SS until I wired the choke to stay about half closed. That was just a temp fix till I got my 4 bbls carb.<br /><br />You could try to disconnect the spark on that cylinder and see if it still backfires. That would narrow it down to if you should still be looking there.<br /><br />Ken

 

[kenimpzoom]

Re: OK, Gearheads!

Cam lobe may be worn out, or the tappets may not be working on that cylinder.<br /><br />Did ya check for bent pushrod or broken spring and check the rocker arms really good. Did the rocker arms studs pull out.<br /><br />Also check for vacuum leak from under the intake.<br /><br />Have you run it with a vacuum gauge?<br /><br />Ken

 

[kenimpzoom]

Re: OK, Gearheads!

Is the distributor cap new, I once had a bad backfire and found out the cap had micro cracks in it (you could barely see them).<br /><br />Ken

 

[LubeDude]

Re: OK, Gearheads!

Keep em coming.

 

[dolluper]

Re: OK, Gearheads!

Centrifical advance, you running points in that if so condensor Vaccum advance,metering rods, bushings binding on distributer shaft won't advance right, Coil crapping out,chain worn, distributer off a tooth,cross firing,push rod,cracked rocker,weak fuel pump<br />What have you fixed is it Quad carb

 

[kenimpzoom]

Re: OK, Gearheads!

Check to make sure vacuum advance is plugged into the correct vacuum port on the carb.<br /><br />Ken

 

[Skinnywater]

Re: OK, Gearheads!

I think a cylinder leak-down test is your next diagnostic step. That difference in compression is likely a intake valve that isn't seating properly. Carbon deposits, wear or burning on the valve face or seat.<br /><br />Imagine with pre-emission motors where the valve/ignition timing allowed more mixture to burn before TDC. The carbon deposits that develop inside the combustion chamber develop more heat.<br />The air/fuel mixture that passes through the open intake valve spontaneously ignites before the spark plug fires.<br /><br />This is possible without an intake valve sealing problem.<br />The symptom would only be exacerbated with even a slight intake valve seating problem.<br /><br />And here I'm only assuming your discribing a pop through the intake on a quick acceloration.<br />If it a rythmic pop through the intake, roughness at idle and mechanical noises..... bent pushrod...sticking valve....

 

[LubeDude]

Re: OK, Gearheads!

Originally posted by dolluper:<br /> Centrifical advance, you running points in that if so condensor Vaccum advance,metering rods, bushings binding on distributer shaft won't advance right, Coil crapping out,chain worn, distributer off a tooth,cross firing,push rod,cracked rocker,weak fuel pump<br />What have you fixed is it Quad carb

Thanks for all your help, Im getting ideas as to several things to check.<br /><br /> Vacume advance, Pertronix ignition, I installed a new vacume canister, it holds vacume fine, I lubed all the workings of the distributor when I had it apart. Timing is perfect, Not enough miles on the engine for the chain to be worn much, (So we were told), New Taylor wires, I have a new fuel pump on the shelf Im installing today, I have the valve covers off, and doing what I can to inspect what I can, and I will run the valves today also even though they are hydralic, there could still be a tight one. Its a holley carb, I went through it again just a couple days ago, Holley parts, even jetted it one step richer just for the heck of it, but it didnt help, Its getting a great accelorator pump shot. I dont think its a weak valve spring, it runs strong all the way to 6,000. Only pops at initial acceleration.

 

[kenimpzoom]

Re: OK, Gearheads!

Check of pushrods pushing through the rocker arms.<br /><br />I suggest pulling the rocker arms completely off on that cylinder. Look at everything closely.<br /><br />Then use a box end wrench to make a poor mans valve spring compresser. With rocker arm off, put box end wrench on the stud, next a big washer, then follow with the rocker nut. Pry down with wrench on the spring. Watch out for the keepers. Put 100 psi on the cylinder with air hose and you can pull the valve spring off. Dont drop the keepers! This is a fast way to check out the valve springs and guides and maybe see how easily the valve moves in the guide.<br /><br />(I am typing all this, not knowing if you already know it, sorry if I insult your knowldge).<br /><br />Ken

 

[dolluper]

Re: OK, Gearheads!

Humm Pertronix eh! does it have the whole bundle including the flame thrower coil 40k one or you running a stock coil Trying to remember we had a problem similar to yours and it ended up what? it was part of the pertonix thinking it had to do with the grounding Off for a root cannal back later I'LL think about it if my head don't explode from the freezing

 

[LubeDude]

Re: OK, Gearheads!

Originally posted by KenImpZoom:<br /> (I am typing all this, not knowing if you already know it, sorry if I insult your knowldge).<br /><br />Ken

No, not at all, many heads are better than one on a situation such as this. By no means do I think Im any smarter than the rest of you folks.<br /><br />We have all had those moments of,<br /><br /> "Why didnt I think of that?"