On of the twins is a 1970 Sport Custom Camper Special with 360, C6 and (disabled) air conditioning.

I was driving it home tonight through Oakland, climbing the hill towards the Caldecott tunnel when the truck started backfiring.

I ended up having to move into the slow lane and dropping down into 2.

Got through the tunnel and was headed downhill again, and it seemed to run fine in D again.

Is this a timing issue? I don't know what would change the timing if the engine load changes.

The last time I had an issue like this, it was on an old Dodge Van... turned out to be a bad condenser.

This truck has had a recent tune-up and I checked the timing after I painted the harmonic balancer marks.

Carb seems to be ok, but I haven't set the high idle up very high (it doesn't get that cold around here), so I'm not sure the automatic pull-off is set like it should be. That probably doesn't matter, since the engine was operating at normal operating temperature when this happened.

I'm wondering if it could be the vacuum advance.

Any suggestions?

 

If the engine runs well under light load and you say you just had a tune up I would check the distributor for wear in the bushing. You can just take the cap off and feel how much side movement it has or you can connect either a dwell meter to read dwell changes as you rev the engine.

If there is bushing wear the dwell will change with engine rpm and the timing will retard causing backfire.

Hope this helps.

 

2x... with the above.

... but yer still running points? C'mon man, this is one area where modern tech makes total sense!! Be gone with points and condensers!!!!

 

Ok, I'll look into that. Thanks.

 

Yeah, both the twins still have mechanical points.

The guy downtown at the auto parts store sez I'm probably the only guy in town that's still doing that. This town's about 26,000 people or so, but there's lots of other towns & cities nearby. Seems like there ought to be other vintage cars & trucks around that still use points.

There was one other bumpside running around town, but the girl that owned it rolled it one morning when she was late for work She said that the tow truck got it upright again and then she drove it to the junkyard. Made me sad.