Decompression Valve?

[Iplayfarmer]

My neighboor asked me to help go through his mower and tune it up. It's a toro 17-44 HXL.

It acts like the battery is low... the engine will turn just a bit and then quit. But, I've charged the battery, and I've put in another battery that works in another mower. I'm sending off the starter with a friend to have it checked and re-built if necessary, but in the mean time I got to thinking (Dangerous, I know)

Don't some of these B&S engines have a decompression valve on them or something that releases the compression pressure so that the engine can get to spinning easier? What would be the symptoms if that went bad? What is that valve called? How do I check if this engine has it? How do I check if it's good or bad?

 

[shot_gun]

B&S had a problem with this a couple years ago. They were not being set right from the factory. I remember you rotate the engien clockwise up on compression stroke and get to top dead center. Then take a 1/4" drive extension(or suitable replacement) and make a mark on the extension at the spark plug hole opening. Then measure a 1/4 inch form that mark further up the extension. (spark plug out of course for the whole process) Then turn the motor in a clockwise direction until the second mark reaches the spark plug hole. Then adjust your valves to .004 for the intake and .006 for the exhaust and your good to go! Hope that helps

 

[Iplayfarmer]

Well, it's definately not the starter. The motor shop put it through every test they could come up with and it passed all of them. I'm going to check electrical ground connections next, and then I'll check the decompression valve.

 

[dbdartman]

Replace the battery cables. BTDT, got the T-shirt...

 

[Iplayfarmer]

It's not the battery or cables. I jumped the starter straight from my diesel tractor...Negative grounded to the starter and the positive touched directly to the power lead on the starter. Same result. Engine turns about 3/4 revolution and then stops.

I pulled the spark plug and the engine spun fine just with the onboard battery.

I pulled the valve cover and stuck a screwdriver between the exhaust valve pusher and valve stem. The engine spun great and as soon as I took out the screwdriver, the engine fired right up.

I'm conlcuding from all this that it is definately some kind of decompression valve I still need to go through the process that Shot Gun suggested. Are those clearances universal for all B&S engines or is there someplace that I need to look up the clearances for this specific engine?

 

[shot_gun]

They the same for all new style briggs engines. ie Intek, ELS, IC ect. Chances are you have an Intek. Good luck

 

[Iplayfarmer]

They the same for all new style briggs engines. ie Intek, ELS, IC ect. Chances are you have an Intek. Good luck

I've going to have to get to it on Monday.

How do these decompression systems work anyway? I thought it was a whole separate valve for some reason.

 

[TedLaRue]

I think shot_gun is dead-on in his diagnosis that you should try adjusting the valves first, but I think you should make sure you're using the right clearances. If your exhaust valve clearance is too small you could cause the valves to burn.

I've attached a snapshot of the Brigg's manual showing the proper clearance ranges for various models and series of engines. Hopefully, yours is included.

To summarize the attachment:

for models 120000 or 150000 use
.005 for intake (.004 to .006)
.010 for exhaust (.009 to .011)

or for models 200000 or 210000 use
.005 for intake (.004 to .006)
.005 for exhaust (.004 to .006)

If your engine isn't one of these models, then go to the Briggs & Stratton website and download the manual for your engine. You can also find parts lists here.

Be sure to follow shot_gun's instructions about turning the crankshaft beyond TDC until the piston is 1/4" below where it was at TDC (top dead center).

The compression release is a device that holds either the intake or exhaust valve open at low RPM, and then moves out of the way for normal running speeds. It's possible that this device isn't working correctly, but you should definitely try adjusting the valves first. If that doesn't work, then you'll have to break down the engine and make sure the device can move smoothly. On some engines there's a spring involved (which could be broken) and sometimes burrs keep the device from moving smoothly.

Best of luck...

 

[Iplayfarmer]

I looked up this engine on B&S's website and all I came up with were three different owner's manuals. I did notice that the valve clearances listed in the posted pic were the same and corroborated Shot Gun's advice.

Where are the marks typically on these verticals for TDC?

 

[shot_gun]

I do it by feel. With both spark plugs removed get the cyl on compression stoke and put the 6" 1/4 " extension(or equivelent) in the spark plug hole and watch it come out as far as possible and stop there. Make a 1/4 inch further up the extension and turn it till you reach that mark set your valves and done.