Backfiring at start up when cold

[Donnie B]

All of our cars have a start up cycle they go through in order to heat up the catalytic converters. It's a fast-idle for about 20 seconds before dropping down to normal idle speed.....now my problem.

After letting my car sit for a few days, it backfires like heck and the fast idle fluctuates anywhere from .6K to 2.1K or so. After the fast idle cycle ends, everything is hunky dory and not a problem in the world when driving. But at start up for that 20 seconds, the intermittent backfiring wakes up the entire neighborhood.

Another point is that it only backfires from the left bank. My only modification is a Kenne Bell supercharger and yes, I have fresh 91 octane gas. I'm also at 9,000 elevation and the only tune I am, running was from Kenne Bell at the time of supercharger installation over 10 years ago.

Any suggestions would be greatly appreciated.

 

[Juice]

Hard to say you didnt give many specifics.
If it was fine for the last 10 years, and now its acting up, time to do basic diags. Time to see what the spark plugs look like. Scan for codes.
Just to clarify, high idle is not to warm the converters. The pcm retards the timing to warm the cats.

 

[beefcake]

Id check the basics first check hoses spark plugs some simple diag. Or if you have a tuning device see iof you can send a datalog over to KB