Tonio

Vital signs: 1972 LT-1, bone stock, healthy engine (I think?), it develops 20" of vacuum measured at the carb base (PCV port) and at the intake manifold. With the A.I.R. hoses connected up to the exhaust manifold valves, I get an obvious exhaust popping when I decelerate. With the hoses disconnected, the pop goes away.

What's the likely cause? Excessively rich fuel mixture? I don't have a tool to test AFR.....

Thanks in advance.

cooper9811

Popping on deceleration usually indicates a lean mixture. I have been told other things can also cause this, but the times I have experienced this, I found it was running a little lean. Has any work resulting in a less restrictive exhaust been done? That can cause a carb to run a bit lean sometimes.

I would try a small adjustments to the carb mixture screws first (1/4 turn at a time), since that's an easy thing to start with. If that doesn't make a difference, may need to go up a jet size or two.

Tonio

I haven't touched the exhaust, it's all original style. OK, will play with the screws a little. Thanks!

CanadaGrant

On your smog system the diverter valve closes the flow of air from the pump at high vacuum like idle. If there is a problem with the valve and you have constant fresh air flow through the lines into the exhaust at idle or any high vacuum condition you will get a popping as the spent gas is ignited again. Check for diverter valve failure.

stingr69

I agree with CanadaGrant, the diverter valve diaphram has failed and is not holding vacuum anymore. They get old and die.

Due to the diverter valve failure, The air keeps flowing out of the smog pump and goes into the exhaust manifold at all times.

When the carb snaps closed quickly on decelleration, it causes an immediate high vacuum lean condition as air is sucked around the edges throttle blades but only a little fuel is metered into the mix. ....the mixture is too lean to ignite and the extra air pumped in to the exhaust by the smog pump allows the unburnt mixture to ignite in the hot exhaust manifold causing the "BANG" or "afterfiring".

Disable the smog pump system and the issue goes with it. Several ways to do it without trashing the parts. I like to keep the systems in place but make sure they do not function anymore.

cooper9811

I guess that would do it - I learned something today.

CanadaGrant

Those diverter valves are also a bit hard to find and can be expensive. Below is an extreme example. If you find that's the problem there might be someone on here who knows where you can get yours rebuilt. Whatever you do, don't throw the original away.

http://www.ebay.ca/itm/like/37182698...728&rmvSB=true

lvmyvt76

BOY, that is extreme!

Tonio

Great info! Thanks guys. My pump is definitely pumping air to the exhaust manifold trees at all operating conditions, even at idle. So it sounds like my valve isn't doing what its supposed to be doing. Question: where does the diverter valve get it's vacuum signal from?

Perhaps the manifold vacuum source to my valve is not connected, so the valve is always open. That would do it, yes?

CanadaGrant

Yes, that would do it. On my 69 the vacuum line from the diverter valve is connected to manifold vacuum on the right side of the carb between the choke linkage. Not sure on yours but any vacuum source is ok as long as it is manifold vacuum and NOT ported.

Tonio

Perfect! Thanks, CG!