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B&S Losing power again! Ideas?

14 years ago

Hi to all,

I have a 3 year old riding mower with a 17 hp briggs & stratton engine, model 31H777.

The engine began to get harder to crank (it always was)and after I found this forum Walt sent me his instructions for adjusting the valves.

After adjusting the mower ran great! really smooth and strong.

About a week or so later I got the mower out for its realy first workout and it ran great for about 30 minutes, then it began to get week, then it started sputtering and coughing and even spit out a good bit of bluish smoke for a few seconds. It barely had power to get back to the shop

and a lot of oil had came out the top of the engine around the crankcaase reed breather gasket.

After some research I decided to replace the head gasket which did appear blown. After I replaced the head gasket it ran OK for a few minutes then lost power again. But no oil blow back or leak. A compression check came out about 65psi. It has always been at least 100psi.

I pulled the head back off and although the gasket seperated into 2 pieces it did look like oil was moving between the cylinder bore and rod gallery.

After using a straight edge I found the head was pretty warped and I ground it down flat and replaced it with a new head gasket.

Started right up and ran good for about 15 minutes then begin losing power, not as bad as before just enough to make a 1 hour mowing a 3 hour mowing, but may get even worse with time.

A compression test is a solid 100 psi + and there is no oil leaking around the engine anywhere. I believe the new head gasket is holding just fine.

It starts easy and idles smoth but at high speed it hunts a little. Once the blades are engaged it slows and feels week.

One thing that does not seem normal is that there is alot of air blowing back through to the carburator "intake". I know some of this is normal but it feel like hurricane force winds blowing against you hand if you hold it over the intake, you have to really push down to get the intake to start to suck against your palm...

I know this is a long post but want to be thourough.

This seems like a valve problem to me but I have adjusted and readjusted the valves to spec (unless I am missing soemthing).

I cleaned the fuel filter even though there seems to be good flow (I can see fuel moisture in the intake).

I took the carb off and completely cleaned it (looked like new inside) and verified the fuel flows good when I lower the float.

Flywheel and Key are positioned correctly.

Replaced the spark plug just a couple of weeks ago. The electrode of the spark plug looks dry and kind of brownish white.

I feel like there is not enough fuel getting pulled into the combustion chamber because of the back pressure but then again Im just guessing.

It obveously runs better cold.

Fuel lid brather is clear.

Im starting to fear something is wrong with the cranshaft or something in that area?

Any ideas would be appriciated, I really cant afford it but Im about ready to shoot this thing and put it out of my misery ;)



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