Back firing through the carb - smell fuel

I have a 21.0 HP ELS646 (B&S) motor. The coil (Armature-Magneto) on the right side (looking from the front of the motor) went out and started back-firing through the exhaust. I check the head pressure on both sides and have 125 PSI. I replaced the coil and I have blue spark on both sides. Now its back firing through the carburator and you can smell excess fuel while its running. Any advise would be great. I also changed the spark plugs, air filters, and oil/ oil filter.
Thanks..........

Take off the valve cover and plugs, and turn the flywheel by hand, you will see something more than likely. Make sure the valves all travel and sit all at the same heights when at rest. You should find something amiss.

If its back firing through the carburetor check the inlet valve, which is a little odd because you say you are getting good psi in cylinder(s)

EngineMan said:

If its back firing through the carburetor check the inlet valve, which is a little odd because you say you are getting good psi in cylinder(s)

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It could be from a little flare from an exhaust valve not opening {pushrod}, or not opening enough {cam lobe}, and the fuel smell is from the compression going the wrong way. The greatest thing about getting them to pull the valve covers, is that it is free and easy to do, and they quickly see how the thing works and they no longer fear tearing into it a bit.

Another free and easy test that I promote is starting and running with 1 plug wire off, then trying it again but on the other side. It will tell you which side is giving you the problem.

The Classic Briggs and Stratton valve guide sliding out of place. You will find that the guide migrated out of its place and the pushrods are all bent up not allowing the valves to open at all.

Flyin2low said:

The Classic Briggs and Stratton valve guide sliding out of place. You will find that the guide migrated out of its place and the pushrods are all bent up not allowing the valves to open at all.

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Flyin2low is on to something. That B&S twin had a rash of problems with Bad pushrods also. They used aluminum tubing with billet ends to save a buck. I'v done 3 in the past couple years, all the same,Bent exhaust pushrods that popped outta the rocker arm cups. Deceptive, cuz you's still get full compression readings. With the exhaust valve not opening, You'll definately get backfiring thru the carb. I've even seen the air filter tops blowing off under that situation. They even ran [poorly,on just one Cylinder firing. Easy to check, just pop off the valve cover tops.The local dealer didn't have any T.S.B's, but 3 of the same motor is more than a coincidence.The sad part is that the replacements are still aluminum w/ the billet ends. I think the only way to avod a repeat [if thats the problem] is annual valve lash adjustment, as I believe that too much clearance is what allows it to hop ot & get mashed up while running. If that's it, I'd love a reply,outta curiosity. Double check, but if memory serves, the correct lash is .004 to .005 thousands clearance, Also, has any of you all ever encountered that ? Good luck, keith / Sgtrudz. Semper Fi.

And before you rip the engine apart slip the top cover off and remove the flywheel bolt / nut and washers.
Have a close look at the slots for the flywheel key.
If they do not come together to form a perfect square, pull the flywheel & replace the key.
Sticky valves can cause backfiring through the exhaust and through the carb.
Slipping flywheels can do exactly the same thing.
In any case if you have never adjusted the valave lash it probably needs to tbe done.

You can also have both problems as the backfires can cause enough of a jerk to split the key.

bertsmobile1 said:

And before you rip the engine apart slip the top cover off and remove the flywheel bolt / nut and washers.
Have a close look at the slots for the flywheel key.
If they do not come together to form a perfect square, pull the flywheel & replace the key.
Sticky valves can cause backfiring through the exhaust and through the carb.
Slipping flywheels can do exactly the same thing.
In any case if you have never adjusted the valave lash it probably needs to tbe done.

You can also have both problems as the backfires can cause enough of a jerk to split the key.

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Valid point, Bert.1st, many of us [ as I'm guessing you ], Have been doing this since we took apart our 1s toaster as kid's ! On walk behinds, checking the flywheel key would be automatic. It's been my experience, folks are embarraased to admit they hit something]. On Tractors, though, The "shock train" is much longer. Yes, advanced timing could cause that, but unless the timing was changed, how could it ? Look, We're all here to help, so please don't take my interjection as a personal affront. Many idea's make a solution. Again, I'd still love to know the outcome, for future use. keith R. / sgtrudz

This thread is 3 years old.

BlazNT said:

This thread is 3 years old.

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Thans. Good info is still worthwhile.