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bad cam?

Posted by buddycraigg (My Page) on
Mon, May 13, 13 at 3:17

briggs and stratton 18.5 intek

on a craftsman tractor
model 917.273638

This was given to my stepfather because it kept breaking starter gears.

I did a quick internet search and found the valves may need to be adjusted.
I replaced the gear.

with exhaust valve open, set intake at 0.004
with intake open sent exhaust to 0.004

It started right up and ran for about 10-15 minutes of mowing.
Suddenly lost RPM and stalled.

Now it will not start.

More searching on the internet I find they have weak cams.

The exhaust valve moves 0.066
while the intake moves 0.175

Did the exhaust lobe give up?

Follow-Up Postings:

RE: bad cam?

I hope that you mean that there is an "open space" between the valve stem and the rocker arm when you say VALVE OPEN.
Because the valves actually must be in the CLOSED position when adjustment is done.
But even that is not the entire story when it comes down to properly adjusting valves on Briggs engines.
It is possible that the exhaust valve SEAT loosened in the head and jumped out of its pocket, and is now holding the valve from closing all the way when it should.
This would have resulted in zero compression and hence the stalling.
Does the engine now have any compression? And does it seem like the gap or clearance between the exhaust valve stem and the rocker arm has increased to an exaggerated amount after the stall? Maybe the gap is now .060" to .100" or so? if you see a great increase in the exhaust valve clearance now versus what you set it at, that supports the hunch of a valve seat coming out of place.

RE: bad cam?

Early 31 series B&S engines were notorious for having faulty camshafts on which the cam lobes wore off prematurely.

I can send you detailed instructions on how to set the valves CORRECTLY and it will also include a test which may show if the camshaft is bad. Both Intake and Exhaust should move the same amount.

Address below, put in proper format and include model number and what you want.

Walt Conner
wconner5 at frontier dot com

RE: bad cam?

I had the OPPOSITE valve open when I set the clearance.
There is not excessive clearance between the rocker arm and the button on the stem of the valve.

Compression is at 25PSI.
I don't know what a small engine should run at normally.
And with the decompression feature on the intake valve, I have no clue if this would be a normal reading for this engine.

You say the valves should move the same amount.
The exhaust valve moves 0.066
while the intake moves 0.175.
That's 0.110 difference.

I would suspect that is a problem, but I cannot find any hard numbers for the cam specs.

RE: bad cam?

Well, Briggs does not really give any hard compression specs as they espouse using a cylinder leakage test (with compressed air) to pin point the cause of a true loss of compression due to a defective part.
But a cylinder leakage test does nothing to reveal a worn cam.
But usually a compression reading on one of these engines will show around 60 to 70 PSI if all is good.
And worn cam does not cause an engine to just suddenly stall out and not have compression all at once.

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