tr 3 backfiring into air cleaner

Now that I cured my miss by changing the spark plugs, i was working on tuning the 3 over the weekend. I had a hard time getting it tuned. Unlike using my Colourtune in the past, I had a hard time finding the spot where the mixture burning in the cylinders changed color. When I finally thought I had it about right, I tried blipping the throttle to see if there was a stumble. I had a stumble and I also got a backfire into the air cleaner.
That was the first time I have had that happen and it made me jump about a foot.
In the back of my head I know that I have been told what the cause of an air cleaner backfire is, but I can't remember it.
When I got done tuning, the lifting pin in the rear carb would cause the engine to stumble and then quit and the lift pin on the front card had no effect at all. I know that isn't right, so I will have to work on the tune some more.
Who knows what is causing that backfire into the air filter?

Dick

A leaking intake valve or late timing might be leading candidates.
A compression test could identify a valve problem.
Late timing could be the distributor setting or a bad distributor cap. When I say distributor cap that could be dirt or a crack that a spark might follow to ignite a cylinder with an open intake valve.

Before you do anything else go out there and be 100% certain that the plugs are wired 1-3-4-2. It is possible to get them switched in a way where it will still run, you can still get some semblance of tune, and if you romp on the throttle it will backfire into the carbs. Heck... you can even drive it around for a week wondering where all your power went.

Not that I would know this from personal experience of course /ubbthreads/images/graemlins/blush.gif

If the firing order is OK... think back to the adjustments you made with the color tune. Did you have to make one carb extra rich? Lean running (like one carb having a stuck float) can easily cause backfires through the carb.