308 backfire while cruising! Help.

All,
Carb 308 backfires while cruising at freeway speed. Engine clears up putting a load on (climbing). More backfiring during deceleration. What to do?
Replaced spark plugs. Removed plugs look normal (brown/tan) without carbon deposits. Old plugs and new plugs gapped at .025" New plugs stopped most acceleration backfiring stopped but cruise and climb backfiring continues. What's next?

Backfiring during acceleration and cruise developed during a two day high altitude cruise back and forth over the Sierras (10k' peak/ 6K' cruise) now backfiring at sea level in the Bay Area. Suspected fouled plugs but plugs look normal with no carbon buildup.

Tach shows 1K RPM fast and fluctuates. Is that related or coincidence?

Is this an ignition or carburetion issue? Thx.

 

The tacho is hooked up to your ignition system, and since your plugs look good, I'd have a look at the ignition first. Do you still have the old points? These need their maintenance....

Hans

 

Is the car lacking power? Did you check the ignition timing? It sounds to me like an ignition issue. I second looking at the points.

 

Well, here's another for points. Hook up a dwell meter!

Timing? I doubt it. The three bolts holding the distributors on have to all loosen up. Or, the play-do grease for the advance mechanism wreaks havoc.

Also possible some other electrical factor could be causing it: coils, loose wire at the coil, etc. Seems like a hell of a long way to the Sierras and back for such acting up. I don't think you will be going much longer at this rate!

Seamus

 

Is it a "spit" type of sound and does it happen under light throttle? It couls be a plug wire or extender going south. Also could be a lean running carb. Please keep us posted.

 

All,
Apparently, cruising at altitude (four "hill climbs" over the Sierras) and backfiring are coincidental and unrelated.
update - 8 plugs are brown/tan wo/carbon build up.
inspected all extenders and found no evidence of carbon trace
8 new plugs reduced 95% of backfiring while under a load (climbing)
However, cruising at fwy speed exhaust still barks/backfires
Tach continues erratic fluctuations indicating "grounding" in one dizzy?

Next-dizzy inspection for dwell and possible internal grounding of point wires

all suggestions are welcome. Thx

 

Hi CCCVII,

I had the same problem a week ago and posted a thread "308 backfire".

I found that the point set that is in use when running at idle were not set to open at all on the front distributor.

So at idle only one bank of 4 was running. Above idle both banks would run as they were now both running on the 2nd set of points.

But when you back off the front bank had no spark. Pretty easy to check with a timing light.

See my thread for the rest of the story.

Brook308