lawnmower engine trouble

[96blasteryfs200]

Hey guys I know you are all two stroke guys but I figured you could put your 2 cents into this. I have a briggs and stratton 3.5 hp model 92502 4 stroke motor. Me and my buddies are using it to make a motorized bicycle. The motor was running great before, but then it died as if you simply shut it off.

I then discovered the spark plug was loose, so i tightened it and tried starting it, but no luck. We tried spraying engine starter in it, but it wouldn't even make a pop. We had plenty of spark, but i checked the magneto gap just to make sure it was in spec, which it was. The plug was wet with fuel so i figured we flooded it, so i left the plug out and let it sit for a while. I tried starting it and still no luck.
I then took a compression test and it read 62psi. 85psi is recommend for the motor, and anything below 65 should be adressed. So there is low compression, but that doesn't explain why it would run great then die suddenly.
So a Re-cap: There is fuel, spark, and compression although it's on the low side and everything is in spec. The only thing i can think of is that the plug is fouled and won't spark under compression. The only thing we haven't tried is a new plug. What do you guys think?

 

[harley1-5]

is it ohv brigs motor or flathead?

 

[96blasteryfs200]

side valve (flathead)

 

[harley1-5]

could be leaky valves if I was you I would rebuild easy and inexpensive to do

 

[96blasteryfs200]

perhaps but could the valves suddenly go out of spec and cause it to stop running? We actually had the head off and from what i could see they were seating nicely.

 

[Blaaster]

The only thing we haven't tried is a new plug. What do you guys think?

Try a new plug is the first thing that I would do.[

 

[harley1-5]

I would try a plug but I have had valve issues with briggs before

 

[dirtysquared]

check the oil. Possibly has a low oil shut off.

 

[harley1-5]

most side valve briggs motors don't have low oil shut off but they are without a doubt the easiest motor in the world to rebuild

 

[Blaaster]

Valves do not usually fail just like that, unless a foreign object or carbon holds them open.

A motor needs only 3 things Fuel, which you have,
spark at the correct time, which I feel you have but you fear the plug could be suspect.
Compression which you say is below the recommended pressure.

A new plug would a good start.

 

[harley1-5]

only saying that about valves because of what blaaster said about carbon I have seen carbon build up around valve seats on briggs engines but yes is is not the norm

 

[kevinsranger]

check to make sure the valves are moving. briggs at some point went to a plastic cam and or plastic gears. also was there rust or corrosion on the pick up coil when you measured the gap? buildup in that area will lead to weak or no spark, usually some emery cloth will fix the issue

 

[96blasteryfs200]

We had to whole engine apart to find anything that was wrong. I made sure the valves were working (which they were). We cleaned every nook and cranny including the engine, gas tank, and carb. I'm thinking that the plug is bad. We are going to work on it this friday i think, so i will get a new plug and try it then.