Diagnose this sound

Can anyone tell me what is going on by listening to this? My timing is set to 10 after (to the right) dead center. Fuel mixture screws are about 1.5 turns

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For what it's worth, it does it really bad after coming off the throttle. Also, hear some knocking when coming off throttle.(I know that ain't good)

Hi, how long has it been running like crap? Timing is set before TDC, but I think you mean BTDC, not after TDC.
I would go over the complete tune up. If your cap and rotor, or wires are really old, replace them. Run your engine in the dark and see if there is arcing from the wires. If you have points and condenser, change them or update to a Pertronix ignition and coil. Check the firing order.
Check the simple things first. Good luck

B RON CO said:

Hi, how long has it been running like crap? Timing is set before TDC, but I think you mean BTDC, not after TDC.
I would go over the complete tune up. If your cap and rotor, or wires are really old, replace them. Run your engine in the dark and see if there is arcing from the wires. If you have points and condenser, change them or update to a Pertronix ignition and coil. Check the firing order.
Check the simple things first. Good luck

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Well, believe it or not, it's running much better now. I rebuilt the carb and I think that helped. I also got the timing set last week.(I knew I had ATDC wrong). But I haven't driven it any in the past 4 months cause I have been working on it. Just got it back on the road today. I'll check to see if there is any arcing and then move to the other things.

I assume the fuel/mixture screws have nothing to do with it?

Also, just so I understand the general problem. Is there unburnt fuel that is getting into the exhaust? Is that what causes the popping coming off throttle?

The screws can actually have a lot to do with the sound. At least at idle. They have minimal effect after the throttle is opened.

Don't worry about the number of turns (other than as a general reference) and don't try to keep the two sides matched. The screw tips are not always identical and carburetors and fuel distribution in the intake is not always consistent. So just turn them in or out as needed.
You typically don't want to see less than 1 turn, or more than 3.5 turns, but even that is subject to individual carburetor whims.

As you turn the screws in or out you should feel the characteristics of the idle change. You generally like it to go higher and smoother, but there is a tipping point where it might just be smoother when turning the screw back in from it's max. The idle might come down and that's ok too.
Whatever the idle ends up at from adjusting the mixture screws, you re-set it to the desired speed using the idle speed screw.
This speed also changes when you advance or retard the timing.

So a tune-up is really a dance, where you swap between adjusting this, that or the other thing until it's balanced just right.
And don't be afraid of timing. Although 10° BTDC is actually a very good setting, every engine is different and yours might like more or less.
And that's just assuming that the timing marks are correct!

Good luck. Play with the screws until you find the sweet spot. Remember that the number of turns listed in the rebuild manual are just starting reference points. Everything can change once the engine is running.

Paul

Sounds like some misfiring. Too lean or bad ignition.

I know this sounds crazy, but check for crossed plug wires. Several years ago I bought a 79 CJ7 with a 304 from a guy whose son had been working on it. Had put several new parts on it after it had sat for several years. Before the guy sold it to me he said that he wanted to get it running first and he shot me a price. Couple weeks later he told me that he had it running but there was a miss in the engine. He then lowered the price if I still wanted it.

I ended up buying it and trailered it home. I had run the engine off and on over a week or two just moving it around. When I started to take the valve cover off I noticed the paint on two of the brand header tubes was not burnt off. I pulled ALL of the brand new plug wires COMPLETELY off and reran them. Ran like a top. Matter of fact it still does, with nothing ever done to the engine. I bought it in '97

Pull the plugs and keep them in order. Compare each one and see if one looks odd. Any looking odd man out check the wires and plug in that hole. I once had a plug that had a bad center core and when I pulled the wire the core came with it. When I pulled the plug the center electrode was still in the plug. The spark had to jump the center core and then the spark gap. After it burned back the soft center core it started missing.

Put a vacuum gauge on the engine manifold vacuum. The needle should be very steady on a good properly tuned engine.