ignition or fuel problem?
08-10-2013, 12:31 PM
#1
ignition or fuel problem?
1974, manual transmission, smallblock w/pertronix unit in the distributor replacing points. Origional Quadrajet 7044207. Rebuilt again maybe a year ago. Quite warped air horn/float body, caused by some knucklehead (that'd be me) overtightening the front bolts.
Had the car running perfect, using an air/fuel ratio gauge to set it up. Settled on a 74/41 jet/rod combination. Showing about 14-15 to 1 on cruise, and about 13 to 1 on WOT. Ran great for months.
All of a sudden, a week ago, began running ridiculously rich (9-10 to 1) on WOT. Backfires through the carb, randomly while at cruise, and worse at heavy throttle.
Changed the coil, distributor cap, fuel filter and frame to fuel pump hose (which was kinked a bit).
Pulled the top off the carb, fully expecting to find the tips broken off the primary rods, but they were fine. Swapped in a 73/39 jet/rod combo. No improvement. Tried isolating the vapor canister, thinking maybe it was dumping a bunch of gas back into the carb.
Still runs like @#$%.
Now what?
Had the car running perfect, using an air/fuel ratio gauge to set it up. Settled on a 74/41 jet/rod combination. Showing about 14-15 to 1 on cruise, and about 13 to 1 on WOT. Ran great for months.
All of a sudden, a week ago, began running ridiculously rich (9-10 to 1) on WOT. Backfires through the carb, randomly while at cruise, and worse at heavy throttle.
Changed the coil, distributor cap, fuel filter and frame to fuel pump hose (which was kinked a bit).
Pulled the top off the carb, fully expecting to find the tips broken off the primary rods, but they were fine. Swapped in a 73/39 jet/rod combo. No improvement. Tried isolating the vapor canister, thinking maybe it was dumping a bunch of gas back into the carb.
Still runs like @#$%.
Now what?
Last edited by gcusmano74; 08-10-2013 at 01:19 PM.
08-10-2013, 01:00 PM
#2
Team Owner
Is the advance can on the dizzy still working properly? If you have an auto tranny, also check the vacuum line/hose to the vacuum modulator and also that modulator. Any major vacuum leak will give you the symptoms you have.
08-10-2013, 01:22 PM
#3
Manual shift car. Just changed the notation.
Will check the vacuum advance canister and the choke pulloff this weekend.
Yes, but wouldn't a vacuum leak make it run lean?
Will check the vacuum advance canister and the choke pulloff this weekend.
Yes, but wouldn't a vacuum leak make it run lean?
08-10-2013, 04:34 PM
#4
Team Owner
Yes it would...at idle. But if the vacuum can were ruptured, that would also throw timing way off at idle.
08-11-2013, 09:50 PM
#5
Advance canister and choke pull off are both good. Finally thought I'd check the plugs. Found the #7 was at least a full turn loose, and looked richer than the rest. Tightened it up, and it seemed to run fine.
So here's my theory. Most likely the rich condition was caused by unburned gas coming through the #7. And since it was so loose, it had a lousy ground, barely sufficient for idling and part throttle. When I opened the throttle, the ignition requirements were higher than the plug could deliver, so it didn't fire, and the voltage dissipated itself through an adjacent plug wire, causing the backfire.
Sound plausible?
So here's my theory. Most likely the rich condition was caused by unburned gas coming through the #7. And since it was so loose, it had a lousy ground, barely sufficient for idling and part throttle. When I opened the throttle, the ignition requirements were higher than the plug could deliver, so it didn't fire, and the voltage dissipated itself through an adjacent plug wire, causing the backfire.
Sound plausible?