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[T3] Backfiring at 2000 RPM and higher


I'm trying to resolve a problem where my '68 Notch starts backfiring as the engine speed increases. I think it's backfiring through the exhaust, and not the carbs, because the sound is accompanied by a puff of blackish smoke from the exhaust. It sounds like it's running great at idle.

Timing and dwell appear to be set correctly. Also, I've checked the timing advance with a strobe light, and it appears to be advancing. I would hazard a guess of 15% BTDC when the idle increases to 2000 RPM, but that's a just a guess. My distro number ends with 009, and there are no vacuum lines connected to it.

Before this happened, I did two things:

1) Repaced the points. The guy at the parts store wasn't sure which points I needed, so he had me bring in the old ones and he compared them, and they appear to match.

2) Checked the compression. This might be relevent because I may have disturbed the electrical connection to my left side carb's cut-off jet, when I was trying to get my tester connected to #3. I searched through the mailing list archive and lack of power to the cut-off jets has been listed as possible backfiring culprit before. Compression was 1-90, 2-95, 3-105, and 4-95, which from what I've read is within the acceptable range.

When I get home from work today, I will check the cutoff valve conn to ensure it has power. If it looks ok, I'll put my old points back in.

Can any of you offer other suggestions on what I can check if these two avenues fail to identify the problem?

Thank you,


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