72 LT-1 exhaust popping

[Gary S.]

My 72 LT-1 has moderate exhaust popping during deceleration, shifting gears and going downhill. This is on a 40500 mile car that is original down to AIR pump, coil, and all engine parts. All I have done over the years is drive it (not much due to work and injuries). Last year, I did a vacuum test and it was steady at 15". I intend to follow Duke and John's solid lifter cam adjustment - https://www.forums.ncrs.org/showthre...tings&uid=1362 - and then do new plugs, carb idle and mixture settings and of course, timing and dwell. Yesterday, I had my brother in the car behind me and he told me that the car stinks of unburned fuel. Ok, that is a good place to start.

A few years ago, I had the carb and distributor rebuilt to specs that would give me good driving. I will look up the guy (thinking it is Camaro Z28 guru that I believe John Hinckley mentioned a while back) Other than that and a tune-up about 7-8 years ago (again, I worked out of town and recovering from injuries) the car was not driven much, maybe 1200-1500 miles during that time. I had communicated here and remember testing the distributor and it was right on spec (I will have to check my emails to that effect) and I think 30 degrees total vacuum at 3000 rpm? The vacuum can is original and it was performing perfectly last time I checked it. Ok, found my repair slip from Jerry MacNeish of Camaro High Performance and my distributor was set to 30-32 dwell, 26 degrees centrifugal advance curve all in at 3000 rpm. Wow, this was 2004. I thought it was much sooner. Anyhow, about 1500 miles on it since the rebuild.

I will do some more searching here for spark plug information that is already out there and then will do the cam adjustment followed by a tune-up. Before the questions, I will tell that this car just "sings" when cruising along at 55-65 mph with plenty of pickup and a beautiful drive. A beautiful throaty song, btw. Its low speed partial throttle acceleration is a bit "jerky" and certainly not smooth like I recall my 1966 L79.

After reading here, the questions I have are:
1. it appears that a lean carb mixture can cause this popping yet I have a heavy gas smell at the exhaust. Can a rich condition cause this?
2. I was bringing my car home after have some yellow road paint removed from the lacquer on the car and drove for about 90 minutes. As I was nearing my house I hit several stop lights. Push in the clutch, and the engine slows from 800 rpm to maybe 600, then 500, and then dies. It starts right up but that continued unless I blipped the throttle. This one has me baffled. My vacuum test last fall showed good vacuum. Where else do I look for this?
3. I read that exhaust manifold or pipes leaking can create similar problems. I should be able to check that by running the engine and holding my hand over the exhaust outlet. Correct? The exhaust on the car is at least 30 years old and was rebuilt using GM parts.
4. I also read that a blown power valve can cause this but I can check that easy enough once I get through the basic tune-up stuff.

Anything else I should look at or do?

As always, thank you.

Gary

[James B.]

Re: 72 LT-1 exhaust popping

Bad AIR diverter valve?

My 72 LT-1 has moderate exhaust popping during deceleration, shifting gears and going downhill. This is on a 40500 mile car that is original down to AIR pump, coil, and all engine parts. All I have done over the years is drive it (not much due to work and injuries). Last year, I did a vacuum test and it was steady at 15". I intend to follow Duke and John's solid lifter cam adjustment - https://www.forums.ncrs.org/showthre...tings&uid=1362 - and then do new plugs, carb idle and mixture settings and of course, timing and dwell. Yesterday, I had my brother in the car behind me and he told me that the car stinks of unburned fuel. Ok, that is a good place to start.

A few years ago, I had the carb and distributor rebuilt to specs that would give me good driving. I will look up the guy (thinking it is Camaro Z28 guru that I believe John Hinckley mentioned a while back) Other than that and a tune-up about 7-8 years ago (again, I worked out of town and recovering from injuries) the car was not driven much, maybe 1200-1500 miles during that time. I had communicated here and remember testing the distributor and it was right on spec (I will have to check my emails to that effect) and I think 30 degrees total vacuum at 3000 rpm? The vacuum can is original and it was performing perfectly last time I checked it.

I will do some more searching here for spark plug information that is already out there and then will do the cam adjustment followed by a tune-up. Before the questions, I will tell that this car just "sings" when cruising along at 55-65 mph with plenty of pickup and a beautiful drive. A beautiful throaty song, btw. Its low speed partial throttle acceleration is a bit "jerky" and certainly not smooth like I recall my 1966 L79.

After reading here, the questions I have are:
1. it appears that a lean carb mixture can cause this popping yet I have a heavy gas smell at the exhaust. Can a rich condition cause this?
2. I was bringing my car home after have some yellow road paint removed from the lacquer on the car and drove for about 90 minutes. As I was nearing my house I hit several stop lights. Push in the clutch, and the engine slows from 800 rpm to maybe 600, then 500, and then dies. It starts right up but that continued unless I blipped the throttle. This one has me baffled. My vacuum test last fall showed good vacuum. Where else do I look for this?
3. I read that exhaust manifold or pipes leaking can create similar problems. I should be able to check that by running the engine and holding my hand over the exhaust outlet. Correct? The exhaust on the car is at least 30 years old and was rebuilt using GM parts.
4. I also read that a blown power valve can cause this but I can check that easy enough once I get through the basic tune-up stuff.

Anything else I should look at or do?

As always, thank you.

Gary

[Bill C.]

Re: 72 LT-1 exhaust popping

the diverter valve on my old 72 454 car made this happen.
replaced it and popping went away.

[Gary S.]

Re: 72 LT-1 exhaust popping

Hmm, now you are making me search here for diverter valve issues, maintenance, and repairs. I hadn't thought of that.

Seriously, thank you for the comments.

Gary

[Duke W.]

Re: 72 LT-1 exhaust popping

My prime suspect is also the AIR diverter valve. Use a vacuum pump and test that it fully closes and doesn't leak. It may take about 20" Hg vacuum to close it.

I also recommend you change to full time vacuum advance if you haven't already done so, and you will need to replace the OE VAC with a B28. Then go through the pre-emission idle speed/mixture procedure with about 900 as the target idle speed. Do not proceed beyond that with the "lean drop" method that is probably in the CSM.

Duke

[Les G.]

Re: 72 LT-1 exhaust popping

Check your intake manifold for leaks especially around where the fuel lines enter the carb, you can spray some oil see if it seeps in

[Gary S.]

Re: 72 LT-1 exhaust popping

Thanks for the replies. The very first thing I checked was to see if the small nipple on the back of the diverter valve would hold a vacuum. It did not. So, that is definitely part of the problem. I think I am going to find John Hinckley's bypass procedure and put it to work. To those have used the Hinckley bypass, do you notice and difference in the drive quality of the car? From memory (always dangerous anymore), I pull the AIR tubes out of the exhaust manifold and put some sort of a flat head screw in there, reinstall and then put something (a BB?) in the small vacuum tube to the diverter valve.

Thanks for the help. I did a LOT of reading about AIR systems, diverter valves and exhaust popping over the last 24 hours. Apparently this is a widespread problem and I should have read deeper before posting my question.

Les, I have periodically checked my intake manifold and it is tighter than a drum. This is a 40000 mile car and the engine has never been touched. That, in itself, means nothing of course. But it is a good engine.

Thanks to all,
Gary

edit:
Duke, I forgot to ask what going to full time vacuum gets me? This car has all of its emissions equipment in place to include the TCS. What would I do with that? Disable/Plug or ...

[Bill C.]

Re: 72 LT-1 exhaust popping

Gary -

this is what I did on a car to keep it cosmetically correct.

I purchased a used set of tubes for the exhaust manifolds. I then had a metal shop weld the tubes closed where they go into the manifold.
put it back together and looked great - nobody could ever tell.

I don't think it cost 25$ for the guy to do it for me.

this way gets you near 100% sure there will be no way to suck air back into the exhaust ports and cause a popping sound.

As for how the car runs, in opinion a little better. You don't get the leaned out exhaust and raised exhaust temperatures.
I have seen AIR systems make the exhaust ports glow cherry red after long runs etc..


Bill

[Gary S.]

Re: 72 LT-1 exhaust popping

Bill,

Good comments. I do think John Hinckley's method is slightly easier to do -

"To maintain the finishes on the air manifolds and keep hot exhaust gases out of the pump, rubber hoses, check valves, etc., remove each of the stainless extension tubes from the tube nut holes in the exhaust manifold and replace them with #10 x 1/2" stainless flat head machine screws. They will permanently plug the holes, and when the tube nuts are tightened, the same number of threads will show as with the extension tubes in place. Stick a BB in the rubber vacuum hose from the carb base to the diverter valve; if the valve diaphragm has failed (and most have), it will become a vacuum leak unless you plug the signal hose,"

I like John's method because of the lack of needing a welder.

Does anyone know how to pull the stainless extension tubes out without removing the manifold? My AIR has been on the car for at least 12-14 years (a Bill Hodel MAJOR expense) and I had actually forgotten that these were in there. So, pull the manifold or somehow fish the extension out?

Thanks,

Gary

[Michael W.]

Re: 72 LT-1 exhaust popping

Gary,

Simply remove the vacuum hose from the carb and put a cap on the port.

Done.

[Bill C.]

Re: 72 LT-1 exhaust popping

Gary

pretty sure you have to take them off and knock them out from behind.

this is why I elected to plug/weld the tubes. you don't have to screw with the manifolds and all the grief it takes to get them off.

bill

[Jim T.]

Re: 72 LT-1 exhaust popping

Gary your LT-1 has a Holley carb, start it up and remove the air cleaner lid, put a finger over the front vertical fuel bowl vent. If your LT-1 engine dies, your power valve has ruptured and needs replacement. Using Holley carbs on my 1968 and 1970 Corvettes since 1973-74 I have experienced ruptured power valves. I have used 5.5 to 6.5 replacement Holley power valves. You can buy one at most automotive parts stores.

[Duke W.]

Re: 72 LT-1 exhaust popping

Duke, I forgot to ask what going to full time vacuum gets me? This car has all of its emissions equipment in place to include the TCS. What would I do with that? Disable/Plug or ...

...less heat rejection to the cooling system at idle and low speed driving and better fuel economy. The TCS can likely be bypassed without removing it by plugging lines and/or insulating electrical connectors. Looking at the schematic should give you an idea.

The OE VAC does not meet the Two-Inch Rule. It will need a B28. Max centrifugal is 22 @ 2300, which is good, but initial timing needs to be advanced to about 16 for total WOT advance of 38 at 2300-up. These changes will yield noticeably better throttle response and torque/power throughout the rev range and still run detonation-free on 87 PON fuel.
'
Final step is to use the pre-emission idle speed/mixture adjustment procedure with a target idle speed of about 900 and total idle advance should be about 32 degrees.

It might not pass a PV with TCS disabled, but it can likely be done in a way to easily undo if/when necessary.

Duke