Engine Knock, Briggs & Stratton 18 HP

plumduckyJuly 25, 2007

I did some repairs on a ten year old Murray Lawn Tractor with an opposing twin cylinder 18 HP B&S engine (serial no. 42A707 Type 1238).

This engine was apparently switched at some point - don't know how old the engine is. The tractor body says 17 HP Twin II (Murray, 46" cut, Automatic Drive).

Wasn't running when he dropped it off to me. Is running now and it has a loud metallic knock of some kind and in sync with the engine cylinder firing, which I only notice when first starting it up and idling.

I had assume it was a crankshaft rod knocking from worn bearings (only have automotive engine experience), but someone pointed out that it doesn't have rod bearings (but no chance for more discussion on the phone with that person).

I'm wondering if anyone is familiar with my description of the problem, and could tell me what my options are. A B&S service rep told me that Briggs doesn't offer a short block for this serial no. anymore - something to do with emissions non-compliance.

Thx very much for any help,

PlumDucky

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fisher40037

Take off the heads and scrape off the carbon and see if the
noise goes away.

Fish

    Bookmark   July 25, 2007 at 8:43PM
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fisher40037

Take off the heads and scrape off the carbon and see if the
noise goes away.

Fish

    Bookmark   July 25, 2007 at 9:04PM
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jimc_vt(z3Vt)

Do what Fish says first,the connecting rod has no bearing,matal against metal with a thin film of oil seperating the two pieces of metal(welcome to small engines)...Jim

    Bookmark   July 25, 2007 at 9:10PM
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walt2002

"which I only notice when first starting it up and idling."

Well you could run some SeaFoam in a few tanks of fuel to clean out the carbon however, a heavy clunking at idle is not too unusual for an old opposed twin. Usually caused by top main bearing/bushing play. Doesn't hurt anything but the top bushing is replaceable.

"but someone pointed out that it doesn't have rod bearings"

Well it doesn't have replaceable inserts, the rod casting serves as the bearing the cure for this is new connecting rods but doubt that is what you are hearing.

Walt Conner

    Bookmark   July 25, 2007 at 9:16PM
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larryf

I have owned an opposed twin on an old Gilson for about 20 years and it has a knocking sound when idling with no load. The knock disappears when the blades are engaged at idle. The knock cannot be detected at high speed - load or no load. I believe this is a characteristic of the vertical opposed twins with splash lubrication. They seem to run forever.

Opposed twins with full pressure lubrication are as smooth and quiet as any engine out there - no knock.

the question is - Are you describing the characteristic knock - or do you have something more serious. As mentioned, cleaning the head carbon is a good idea and it is easy to clean. Change the oil to SAE 30. Make sure the oil level is maintained in the operating range. Make sure the dip stick is screwed down and not simply rested on the dip stick housing.

If your engine does have a serious problem, there are many good engines remaining on expired equipment. Cooling fins must be kept clean to reduce the possibility of a valve seat failure. A Murray mower is not worth the expense of a new short block - even if you can find one.

    Bookmark   July 25, 2007 at 9:40PM
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plumducky

Thanks Gentlemen, for your replies. I'm encouraged to take my first small engine apart, if the owner of this tractor doesn't claim it soon. Will post again if I do.

Can't thank you enough for your help,
PlumDucky

    Bookmark   July 26, 2007 at 10:44AM
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