Three things can cause popping through the intake. An experienced ear can
usually tell what the source of the problem is.
1. too lean of a mixture during the time of the backfire (twisting the idle
mix screw won't tell you anything)
2. Firing of the spark plug while the intake valve is open (for any reason,
timing, or misfire)
3. Mechanical problem that allows passage of the combusted mixture back
through the intake valve.
So it's got to be either fuel related, electrical related, or mechanical!
That sure narrows it down!! :-)
Very difficult to diagnose over the internet. Could be many things.
Suggested Strategy:
1. Verify electrical first by oscilloscope and experienced technician
(easiest to do).
2. Investigate fuel and egr system (tougher to do, but more probable than
mechanical).
3. Investigate mechanical - last resort
Good luck
Les Myer
At 01:15 PM 4/16/99 -0400, you wrote:
>OK. Next is the shotgun fix. I'm thinking 10 gauge birdshot, or maybe just
>a few lead slugs.
>
>Quick symptons:
>
>1. Intake backfiring when accelerating and when coasting in gear. Once or
>twice when decelerating. Most noticable and annoying when beginning to
>accelerate out of coasting (RPM of 3k or so). Car bucks a lot.
>
>Car: '78 Midget 1500
>
>Things tried:
>
>1. Recheck for intake manifold and vacuum leak - nothing - still think I'm
>doing it wrong
> at idle, spray carb cleaner (gunk?) all around carb, intake
>and exhaust manifold
> Spray near EGR valve and distributor vacuum line hookup.
>Listen for increase
> of engine RPM. Right or Wrong? How much of an increase?
>Not more than 100RPM
>2. Adjusted valves - nothing
>3. Advanced timing - now worse
>4. Retarded timing - ran worse overall
>5. Enrichened carb (single ZS CD140) - more gas smell in exhaust - no other
>change
>6. Checked plugs - good shape - slightly white 1st time - darker after
>richer carb
>7. Distributor cap - new as well as the wires (8mm Accel)
>8. Oh yeah, the plugs are splitfires - less than 1K miles on them
>9. Checked distributor vacuum advance with MityVac. Moved about 1/4 inch at
>the edge
> (counterclockwise, I think)
>10. Topped up Carb dashpot with Dextron III Tranny fluid - no change
>11. Checked Compression - all within 10 PSI around 130 or so.
>12. Voodoo incantations - no change - neighbors think I'm nuts -
> engine red from chicken blood
>13. Excorcism - no change - Catholic Church thinks I'm nuts
>
>Anyone have any other ideas?
>
>Michael
>
>Oh yeah: When towing, should i disconnect the driveshaft or leave it
>connected? ;-)
>
>
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