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Hi. I have a B&S 18.5 engine (31N707 0301 B1) that will start but backfires real bad. So I took carb off and cleaned it real good then put back on, no change. Adjusted the valves and no change. Cleaned coil contacts, no change. Took Flywheel off and checked the Key. No damage to key. I then took off the carb and put a new carb on, still no change. Backfires and blows mist out of air filter intake. This engine is on a Poulan riding mower. Would appreciate any help. Thanks

There is a few things that can do it.

• Bad exhaust cam lobe
• Intake air leak
• and a new one I ran into this month.

So for one thing try adding choke with running to see if the problem clears up. Here I just had that 3/4 choke solved the backfiring before I reworked the carburetor.

StarTech said:

There is a few things that can do it.

• Bad exhaust cam lobe
• Intake air leak
• and a new one I ran into this month.

So for one thing try adding choke with running to see if the problem clears up. Here I just had that 3/4 choke solved the backfiring before I reworked the carburetor.

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Thanks I will check into those.

Bat57 said:

Thanks I will check into those.

Click to expand...

I took the block apart and pulled out the camshaft and found the exhaust lobe was almost completely rounded off. So I am putting in a new camshaft and that should take care of the backfire issues. Thanks