Guys;

I went on a Poker Run tonight and during the run, she started what sounded like a backfire through the carburetor or somewhere under the hood, not the pipes (sidepipes).  It happened only with the xmission in gear and at 45mph, 55, 65, 73, and 80 and over 2K on the tach.  Whether in second or third, it happened, and also whether on a flat or on a hill.

When I got back in I found one of the little rubber block off nipples laying on the manifold.  I replaced it however, due to losing the power steering belt while revving the engine trying to duplicate under load, I wasn't able to go back out to see if this would fix it.

She began missing on a run last weekend, but only at 65mph or over and then only a little bit.  I thought I had a burnt plug wire but found all wires tight when we stopped.  I put her up on the ramps before the run today and found a loose plug, not fouled.  I replaced it and went on the run today, with the above results.

The question is, could the rubber vacuum nipple falling off cause a backfire through the carb under load?

If not, any suggestions?

Also, I tried the stick thread locker today for the first time.  I HIGHLY RECOMMEND IT!!!

Any help would be appreciated.

Dennis

OK, it is now Sunday nite and I changed the fuel filter (in carb block), added fuel drier and 104+ octane boost in case I had bad fuel.  I had never used the station before and it was a full tank prior to yesterdays run.  It only seems to have made the problem worse.  I took her to the store and barely made it the 10 mile round trip.  It is spitting and missing constantly.

I checked for any other vacuum issues but found none.

I really could use some help on this.

Thanks,

Dennis

Chances are the constant backfiring thru the carb blew the vac. line/plug off.
It sounds like you may have a timing problem, or possibly an HEI coil taking a dump, but...I would be suspicious of the ign. module, inside the distributor. The best bet would be to replace the coil, module, cap, and rotor.

It is also possible that it has jumped time, due to a worn timing chain/gear, or your carb could be in need of repair/rebuild.

Joel;

Thanks.  I thought about jumped time but it starts with barely a turn of the key every time (even cold) and idles smoothly around 650rpm.  It has a new rotor bug, cap and wires from less than 500mi ago however, I did not change the coil or module.

I have a typical leak at the lower timing chain cover and front/rear main seals.  That is next on the list.  While I am in there, I guess a timing chain swap or at least check for slack is in order.

If it were the carb, wouldn't it do it even if it weren't under the transmission load?

Dennis

A carb problem can show up in different ways, depending on where/what is the source. It can idle fine, but not have the proper "transition" from low speed circuits to high speed circuits. In other words, it may have a restriction in one of the circuits due to contamination("dirty"), or it could have an internal leak of some kind.

Thanks Joel.

Is a carb rebuild something for the first timer or is it better to just swap at our local carb shop?  This is not a numbers match car.

Dennis

Well, that I could not say. If you have good mechanical aptitude/skills, it would be something you can do yourself. If you get frustrated easily, then I would suggest letting someone else handle the carb rebuild. Check out Ken's Carb Rebuild article here(Click on "Ken's Projects"), and see if it is something you think you could do yourself.

Hey All;

It now appears that the coil is dying a slow horrible death.  It didn't help that I put the new fuel filter (in carb) in backwards...

On the good side, it does appear that the temp problem has been fixed.  After rodding and coring the radiator, changing to a 180 deg thermostat, changing all hoses, fresh fluid and 2 bottles of water wetter, I found a 7 X 12 hole in the bottom radiator shroud.  71 vette and I fabricated a piece out of 18 ga steel, riveted it in and now she runs below 200 again  Thanks Tim!

I will pick up a coil tomorrow and give it a try.  The symptoms include not having the cutting out/spitting problem until after the thermostat started to move off 100, indicating heat buildup in the distributor that is xferring into the coil.  There is still the possibility that it will be or will also be the module however, I'll try the coil first.

Along the way I have found out that the motor in the car is not a 1980 (knew previously that the #'s didn't match) and there are several parts that don't fit.  I'll have to find a way to decode the engine number to see if I can find out what it is.

Again, Joel & Ron,  thanks for the input!

Dennis

Darn it.

71 Shark, not 71 vette. 

Sorry, Tim.