MudEye

Over the past year or so I've noticed after a 5 mile cruise into town....and a resting period of an hour or 2....On starting the 330 it backfires and black smoke shoots out of one tailpipe.......
Valve guides? It's also noted that one cylinder has less compression on the side of the tailpipe that's backfiring with the black smoke following it.

1: If it was valve guides...suggestions? Just new valve guides?
2: Go ahead and have heads redone......Motor is still strong and is an everyday driver......I have no idea the condition of bottom end. Original motor with 115,000 miles. Again motor is strong..

don71

I would think more on valves and valve seats, sealing properly than valve seals themselves. Backfires, means raw fuel is in the exhaust being ignited after the combustion chamber process. Its not oil, its fuel.

You mention lower compression on one cylinder, that is the likely culprit.

My guess is the exhaust valve is not sealing properly. Yes, some head work is likely in your future.

classicmuscle442

Does it backfire when first started for the day? May just be carbon build up if it only does when warm.

droldsmorland

Before you jump into ripping into a potentially good engine why not follow some troubleshooting basics before tossing money and parts at it???

1. Read plugs. How do they look? Keep track of which cylinder the plugs came from.
2. Is the choke working properly?
3. Is this just a hot start problem due to a heat soaked carb?
4. How far off is the one "low" cylinder? How did you perform this compression test?
5. A leak down test will show whether its a valve problem or a ring problem(wet & dry test)... if there is a problem...
6. How does it accelerate with your foot into the gas? Smooth or does it buck n fart back?
7. How is the ignition system, points, plugs, cap, rotor etc...

Hopefully, you run ethanol free gas in this car???

Sounds like a hot start problem to me.

Could be as simple as a carb rebuild, adding a heat isolator gasket.

MudEye

Doesn't backfire in morning. Only after a drive. And a restart.

MudEye

Plugs are new. Carb has been rebuilt , although he did put it on the lean side . not sure what a heat isolated gasket is... Used a pressure test kit from auto zone and the one cylinder was down about 10-15 psi than all others. No backfiring when I lay my foot heavy...it roars and takes off smoothly. Just installed new hei distributor but it backfired before then. Use only ethanol free gas and add lead additive. Never done a leak down test before. But it sounds like that would isolate a few factors.

droldsmorland

10 to 15 PIS difference is well within a general spec. Compression isn't your problem. Google wet and dry leak down testing.
I had this problem with my 68 400 engine. On hot summer days, I go to restart after it sat for 30 mins and it would pop out the exhaust and out the carb. So I had a cloud of smoke from under the hood and out the pipes. Embarrassing. I put the insulating/isolation gasket under the carb. That helped but didn't cure it. Next step was to remove the intake and weld in block off plugs in the heat riser crossover passage. This is a passageway under the carb in the intake mainfold via the heads. The driver's side exhaust manifold has a flap in it which is activated by a bimetal spring which opens this flap when it's hot. When it's cold it closes and diverts hot exhaust gas through the heads and under the carb to enhance warm up and pull the choke off. But with today's gas this excessive heat doesn't play nice.

Heres what an insulator gasket is: Jegs, Edelbrock, Summit carry them.


https://www.summitracing.com/parts/e...le.com/search?

There are two or three ways to block off the crossover passages. One is the weld in cast metal plus(mine came from Mondello many tears ago). Some guys install it into the heads some into the intake like me. I didn't want to rune the heads. I'd rather alter my intake as it's easier to reverse. There is a thin tin flat block off provided with some of the intake gasket kits. Some guys will do only one side some do both. The tin flat block offs can rot through and rattle. Same rattle for the cast plugs if not welded in and just dropped into the heads.

The driver's side exhaust manifold needs to be removed. Then remove the heat stove flap and weld up the shaft holes. You will need to tweak the divorced choke so it pulls off much quicker or convert the divorced choke to electric (simple cheap kit).

The car will now be cold blooded. You will need to let it warm up for several minutes if temps are below 45-50*f. But your hot start problem will be solved.

Try the insulator gasket first. Next try lowering the float level 1 or 2/32" of an inch. Tune your carb and timing with a vacuum gauge to obtain the highest vacuum signal by adjusting the timing and Air/fuel mix screws. Then if you dont solve it do the above modifications.