# Plastic camshaft on Briggs 17.5 hp



## labtek (Sep 26, 2015)

Hello to the group and good day. I have a single cylinder Briggs IC 17.5 hp, 500 cc engine powering my lawn tractor. Recently while operating it, it started backfiring and running poorly. The engine was manufactured in 2009.

The numbers are 31C707 0603 B2 080904ZA

I checked the flywheel key and it was intact.

On pulling the valve cover, I noticed that the pushrod and the rocker on the exhaust valve was very loose. When I applied pressure on the pushrod itself, it disappeared down into it's guide.

Obviously there is an issue with the cam or camshaft itself on at least the exhaust valve.

I have read several other posts and it was mentioned that Briggs had installed plastic camshafts and cams on some of their engines.

Might that be what I'm dealing with here? I was surprised to read that Briggs would install a plastic part on this critical engine component.

Obviously the engine will need to be torn down.

Thanks in advance for any thoughts?


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## AVB (Nov 28, 2014)

Actually it was manufactured on September 4th 2008.

Being a 2008 engine it probably more likely that it was needing the valves adjusted long time ago.

As for the camshaft (793880) itself, it is an all metal unit except for a very small bushing on the the decompressor assembly. On these cams I have seen several with shattered decompressor assemblies which makes it hard starting compression wise.

Since the push rod has come loose you will need to retrieve it and possibly the small valve cap button from the crankcase. Also these OHV can have valve guides that work loose but most times this usually cause bent push rods.

Also while you got the engine down you should do a leak-down test to make sure that head gasket isn't blown as this engine's design tends to blown these between the cylinder and the push rod galley. Most times this causes excess oil usage along a lost of power.


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## labtek (Sep 26, 2015)

Thanks very much for the post. The engine has been running very well over the time since was bought new. Never been hard to start or other issues. Whatever happened was a critical event.

Having said that though, my wife drives the machine most of the time and something significant may have occurred in the past that I did not get a memo about.

It will be interesting to discover what the innards of the engine show. My experience with Briggs engines has been that they were the ultimate in reliability and will run when others will not. I have re-tasked other Briggs engines and have kept them going many years past their projected lifespan.

Perhaps that is not now the case. I will post my findings in case it helps someone else.

Greetings from Nova Scotia Canada.


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## labtek (Sep 26, 2015)

Hello again.

I believe I have found the problem. On the end of each valve stem under the rocker arm there is a small round metal disk. The one under the exhaust valve had come off and was in the bottom of the valve pan. This meant that the exhaust valve was not opening far enough to allow all unburned fuel to exit the combustion chamber before the intake valve opened to allow new fuel into the cylinder.

This sometimes ignited the fuel mixture prematurely causing backfiring and poor performance including loss of power. The valves seem to be operating correctly now. It does not seem to be a camshaft problem after all.

I did not break open the crankcase so now will replace the cylinder head with a new gasket and test for operation.


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## AVB (Nov 28, 2014)

That small disc is what they call a valve stem cap. It only goes on one way as you should a depression for the valve stem to fit in to. These usually comes off when either valve clearance is too great or there is a sticky valve.

Make sure you set your valve clearances. IN .003" - .005" EX .005" - .007"

This done at 1/4" pass TDC of compression stroke or can use the alternate method of adjusting the valve when the other one is fully open; old school but works.


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## labtek (Sep 26, 2015)

I'm pretty sure that it was due to excessive clearance. The intake rocker seemed loose as well but not as much as the exhaust valve but then again, there's no way to know for sure because the valve cap was missing at that time. I did not need to open the crankcase- when I was fiddling with the push rod at first, it must have fallen into the oil channel right under it by gravity- that's why the push rod disappeared into the head when I pushed on it. It was just luck that the valve stem cap was still in the cover.

Interesting how clear things become when looking back.

I really did not know that the valve clearance needed to be examined from time to time to check for this issue. Now I know.

Your initial assessment of the problem seemed right on target- I'm grateful for the help and other information.


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## labtek (Sep 26, 2015)

I have re-installed the head with new gasket, adjusted the valves (including both valve end caps) and sparked the tractor up.

Runs smoothly. The problem was the missing exhaust valve stem end cap.


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## Basementgeek (Feb 7, 2005)

Good to hear

BG


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