Back Fire through carb

...and yes you can check running engine timing advance at the dock. (Depending on what the true cause of your carb-backfiring it may or may not have to be under load to manifest.)
However, if the centrifugal advance mechanism was rusty and not advancing that would not be causing backfire through the carb. But a broken advance mechanism spring might allow excessive advance.

I'll throw in possibilities of burnt intake valve/seat, or bad int. valve spring in addition to everyone's suggestions above.

Did this issue just start after any other work was done? Is it just an occasional pop, or consistent backfire above a certain RPM or ..?

Just curious , does this blue '85 engine still have points, condenser and ballast resistor, or has the ignition been upgraded?
 
I have had backfiring problems many years ago but not through the carburetor. I found the capacitor in the distributor (older points type ignition) was my problem. Also if there is a resistor in series with the coil is somehow bypassed, the engine will run poorly.

By chance...... could you have mixed the spark plug wires?
 
I too suspect the distributor is the cause. It's not that difficult to pull the distributor and bring it to an auto or marine repair facility to check it out. They can replace any rusty or worn parts. Just mark the spot on the distributor the rotor point to before you remove it. I'm finishing the repair on one of our engines of a backfire through the throttle body. Turns out that, since the boat sat for two years without being used, one of the exhaust valves rusted in place. When I started the engine, the valve bent, pushrod twisted and snapped in two, and the engine missed. Until we found the cause of the backfire, I did a full tune-up, added Marvel Mystery Oil to the oil (possible collapsed lifter as the cause?), and switched fuel feed to second tank. Engines are Mercruiser 7.4 twin inboards, not very mechanic-fruiendly engines. Hopefully, repairs will be finished Tuesday and boat will be ready for launch by the end of the week. Good luck with your repair. Hy
 
Thanks for all the input. Turns out timing was at 8* BTDC and should have been 10* BTC. Reset the timing to 10* and all is well!
 
I find it hard to believe 2* of advance solved the problem. Either (1) You disturbed the real problem when you budged the distributor or (2) The problem will be back.
Hoping for the best but must honestly say I'm not optimistic.
 
+2

You report you just advanced the timing from too retarded . Retarded (late) timing should not cause backfire through the carb like too advanced (early) timing could.

But no sense in looking at a gift seahorse in the kisser. That's great it's running well again.
 
It could have been just at the limit needed to cause the backfire. But it's important to set the idle at the spec speed before, and during, timing. You need to use a remote gauge and not the one on the dash.

Test for a vacuum leak around the carb is easy if you spay some carb cleaner around the gasket. The engine will stumble if you find a leak. Also cover the intake with your hand to block the airflow. If the engine doesn't stumble, you have an internal leak in the carb.
 
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