Briggs - Gas Blowing Out of Carb

[GAVolunteer]

Key in blade adapter ok??...it might be a key or key-way built in to adaptor

I believe I found the problem. The crankshaft is "a little" bent. I removed the blade and adapters to check the blade adapter key and pulley key. They were fine. After reinstalling the blade and pulley, I turned the blade several times and the blade bolt seemed to have a slight oval pattern. I attached a wooden skewer stick to the underside of the mower with the point of the stick centered on the bolt. When I turned the blade, I could see a small oval pattern of the bolt as it turned.

What is perplexing is that the flywheel key was not sheared yet the crankshaft appears to be bent. When the engine is running, there does not appear to be any vibration.

Would a bent crank cause the timing to be slightly off and cause a fine spray of gas to blow out of the carb?

This is a 1995 engine. Before I junk the engine for parts, I want to be sure that my diagnosis passes the "smell test". Any other tests or troubleshooting I need to do?

Thanks,

Robert

 

[Michael72]

I believe I found the problem. The crankshaft is "a little" bent. I removed the blade and adapters to check the blade adapter key and pulley key. They were fine. After reinstalling the blade and pulley, I turned the blade several times and the blade bolt seemed to have a slight oval pattern. I attached a wooden skewer stick to the underside of the mower with the point of the stick centered on the bolt. When I turned the blade, I could see a small oval pattern of the bolt as it turned.

What is perplexing is that the flywheel key was not sheared yet the crankshaft appears to be bent. When the engine is running, there does not appear to be any vibration.

Would a bent crank cause the timing to be slightly off and cause a fine spray of gas to blow out of the carb?

This is a 1995 engine. Before I junk the engine for parts, I want to be sure that my diagnosis passes the "smell test". Any other tests or troubleshooting I need to do?

Thanks,

Robert

The bolt might give a different reading to the shaft itself in terms of it being bent... as in the bolt is slightly bent)....Its why I mentioned the blade adapter earlier on....Was the blade fitted when you felt no vibration?.. is it burning oil?...compression rising up past piston rings wouldnt help matters....see my post.. "something to look at"... in this case the timing was out at camshaft and scored piston and cylinder...

 

[KrashnKraka]

Don't know why but all 4 briggs 12hp i have found did this shortly after acquiring them. The last one i did about 6yrs ago, knock on wood.

Still what leaves me scratching my head is i dont see how hitting tall grass will stretch a valve but that is what the symptoms point to.

I am with you on this one, cylinder pressure leaking back past an inlet valve.
Makes sense.
I am wondering if in the circumstance you describe there isn't maybe a valve
spring issue?
Like, busted partially, lost tension or even the wrong spring (temperature coefficient).
And in the OP's case he reads as very switched on so maybe he is finding an existing
symptom post the blade 'damage'.... you think?

KK

 

[GAVolunteer]

The bolt might give a different reading to the shaft itself in terms of it being bent... as in the bolt is slightly bent)....Its why I mentioned the blade adapter earlier on....Was the blade fitted when you felt no vibration?.. is it burning oil?...compression rising up past piston rings wouldnt help matters....see my post.. "something to look at"... in this case the timing was out at camshaft and scored piston and cylinder...

I rechecked the crankshaft with the blade and adapter removed. The amount of "wobble" was much less about 1/16th". I then checked put the blade and adapter back on and checked the each end of the blade at a fixed point. One end was 1/16" shorter. It could be the blade was shorter one one side. But it was consistent with the crankshaft measurement.

Which makes me wonder if that small amount of variance in the crankshaft would cause the timing and carb issues.

When I lapped the valves, I checked the piston and cylinder and there was no scoring.

Was the blade fitted when you felt no vibration?.. is it burning oil?

I have not felt any vibration - before or after I removed and reinstalled the blade. IT does not appear to be burn in oil.

Remove the air filter and hold a sheet of paper over the air intake and it is blowing(spraying gas) the paper up instead of suck down is not normal.

That was a neat test. When the paper is about 1" from the carb opening, the paper gets wet with gas quickly. That clearly showed the amount of gas blowing out of the carb. If the paper is put closer, the carb sucks the paper with quite a bit of pressure.

Also, I had already ground down the intake valve stem to .006 clearance and did a leak down test both cold and hot. See previous post. I believe the valve is closing completely.

From all of the tests and posts, I believe I have a timing issue. Looks like it is time to look inside the engine.

 

[Tinkerer200]

Note that most B&S engines relieve compression via the Intake Valve so you are going to naturally get some blow back when starting and at idle. Running great then badly then great again does not sound like timing to me and the amount of gas you are describing sounds like flooding to me. By the way, when you lapped the valves,did you use Bluing or anything to be sure you had a good seat?

Walt Conner

 

[GAVolunteer]

Thanks for the feedback.

By the way, when you lapped the valves,did you use Bluing or anything to be sure you had a good seat?

Walt Conner

I did not use any bluing. Afterwards, I did a leak down test with engine cold and hot. Only had 10% leakage which I assumed indicated a good seal between the valve and seat on both the intake and exhaust valves. I could hear a little air coming from the oil dipstick tube. But not an excessive amount. Did not hear any air from the muffler or carb.

the amount of gas you are describing sounds like flooding to me.

I agree with the flooding idea. There are no adjustments on the carb and I've tried two different carbs both with the same result. I assume the only cause of flooding would be a bad carb needle or seat. Any ideas on how to troubleshoot a flooding issue?

 

[Deeruser]

What often happens is some pertain fact is accidentally overlooked, omitted.

Walt Conner

English please

 

[slomo]

 

[slomo]

Key in blade adapter ok??...it might be a key or key-way built in to adaptor

Or the flywheel key and its groove.

 

[slomo]

Is the spark plug black?

Take an old plug. Gap to 1/4" wide. Check for a good blue spark.

Wonder if someone put a steel key in the flywheel groove causing it to not shear??

If the crank is bent, it's done. Or find a mower shop that has a crank straightener.