Howdy! Been looking and gleaning for about a year, but finally my first post. I have a stock original 84 Z28 with the L69 and auto tranny. I bought this car new and it has been a fun car to drive. About six years ago it started to start hard and idle rough with the typical excessive black tail pipe smoke.

Last week I put on my fourth rebulit carb on it since the original problem (yes, I did a stupid and gave the factory carb in as a core on the first rebuilt). The first three rebuilts were from auto part stores. I never have been able to get it to run right and I'd get frustrated and put it in storage for awhile until I would get a wild hair and try messing with it again.

This last rebuilt was bought from a guy off of Ebay that runs a one man shop. It started right up, and after adjusting the idle (a little fast) it starts and runs the best it ever has! I'm elated!!

I now notice that it does an occasional subdude and uneven popping sound from the tail pipes at idle. This starts only after it warms up a bit. It also does what I call the exhaust rap sound on deceleration at times. It had never done this before. It does not affect the performance or starting in any way when it does this. The car runs great!

Keep in mind I have kept it in perfect ignition tune and fluid service, even while in storage. I would take it out and drive it a little (crappy as it was) every month while in storage. It was stored in a heated enclosed garage.

Any ideas on this popping, or no worry? There does not appear to be any exhaust leaks (new exhaust, including the cat) and it is not heard back through the carb when it does it.

Thanks! Sorry for the long post.

 

Welcome. What's your timing set at? Should be 6 degrees advanced with the EST disconnected. Have you put a dwell meter on it to check MCS cycling?

 

The last time I checked the timing was about a year ago and it was right on then. I have not done anything to disturb it, but I will verify it again. Have not checked the MCS, yet. I will do that. I know just enough about auto mechanics to get myself in trouble, so I wanted to ask first before I started. I will not be able to check these until Monday. I will post what I find.

Thanks!

 

Did it start with the new exhaust install?

 

I used to get some backfiring but that was the A.I.R. system on cold start up which was normal.

Auggie

 

Isn't that for TPI motors?

 

naf said: Did it start with the new exhaust install?

No. The new exhaust was put on about 6 months ago. Seems to have started after this last carb change, however the prior carb idled way too rich to be able to tell anyway. It did not do the decelaration rap with the carb I just changed out.
I also was seriously thinking an exhaust leak, but I've done the ol' stethescope listening trick (mainly concentrating around the manifolds) and I sure cannot find a leak.
I had a Toyota pickup that had an exhaust leak at the manifold gasket and this thing would bang like a huge backfire until I fixed it.

Thanks.

 

Problem fixed! Thanks! Timing was still right on, but in checking the MCS dwell, it was running too rich in closed loop. Small amounts at a time, I ended up backing the IAB out a complete turn. Now, hovering around 30 degrees, give or take 5 degrees. Idle pop and deceleration rapping - history!