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  #11  
Old 09-15-2007, 04:40 PM
TommyC TommyC is offline
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For RestroRob

RestroRob -

Thanks for weighing in on my thread - I hoped you would. I appreciated the drawings you provided! I transferred them to PPT and will study them. Wish I had an actual cam in my hand to study this "new" feature.

Per your question, I've rebuilt several motorcycle & VW Type I engines, as well as several B&S. The last B&S was my old 3.75HP (Model 98902; Type 011001; Code 93083151), which hails from 1993. To be sure, this engine has an "ordinary" cam. At the risk (again) of seeming ignorant, naive, or otherwise moronic, I never heard of a compression release on a mower engine, and in any case would never think of such a device as being anything but a HP thief. 2-cycle flat track race motorcycles of yore used to use them to effect engine braking on turns. It also reminds me of the mechanical advance distributors I preferred to use on VW Type I engines.

My difficulty with Mr. Kapaun derived from his terse responses. I appreciate his attempt to help, as I do for anyone here. When I mentioned my surprise observation of the slight exhaust valve opening on the compression stroke, he focused on the valve clearances. It was only in context that I got a clue about compression release for post-1995 (or later) engines, and later learned about the effect of higher RPMs in canceling the effect. I wish he'd made that clear from the outset, and I'd erased that as a factor for my engined not starting. I'm not big on acronyms either. I was not trying to "2nd guess" him, I was just trying to keep it real.

Having layed out the serious details of my engine starting failure, the 1st thing to address would NOT be the valve clearances (imho). Furthermore, when he made the emphatic point about 1/4" past TDC, he didn't mention which (of the 2) TDC in the 720 degrees he meant. Many people would think of the main TDC at 0 degrees- the start of the 4-cycle loop. I posted this:

"... when the piston is ~1/4" past TDC, the valves are "on the cam". The only place where the valves clearly come off the cam is at the top of the compression stroke (as strange as that may sound)."

which got no response. In further tinkering, etc. I now believe it's the "2nd" TCD (top of compression stroke) that's the reference position for setting valves. Folks should be clear on this difference: very important.

OK - I'm, past that (just wanted to help explain my view). Regarding coil degradation, I may have misquoted when I mentioned that you said they "wear out" - my bad. In reviewing posts covering 100 pages on this forum (and I noted many good ones by you), I did pick up on your claim that they break down under heat, which I agree with.

Which is a good point to end this thread, because as fate would have it, I finally got around to removing the flywheel and voila! SHEARED TIMING KEY! The mower was trying to run w/ spark advanced ~240 degrees. How it ran over the past several weeks is beyond me. This should explain not only recent failure to start, but also intermittent backfires thru the carb. Happily, it probably ain't the coil after all.

I have a new key and expect to have the engine running in a couple hours.

Thanks again (Bill,too) for help - especially your nice diagrams of the "MCR" cam. A picture IS worth a 1000 words, and more detail is always better than less.

TommyC

p.s.
I enjoyed reading many posts here in the archive.
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  #12  
Old 09-15-2007, 07:32 PM
Restrorob's Avatar
Restrorob Restrorob is offline
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Glad to hear you isolated the problem Tommy,

This is one of the problems with on-line (Internet) diagnostics. We can't see/touch or hear the problematic equipment. All we have to work with is what the poster explains (which you explained well) but on larger post as your's some details tend to get over looked.

The fact that you said you had rebuilt many 4 cycle engines lead me/us to believe you had basic knowledge of the functions/operations and knew that most lawn and garden equipment small engine valves are adjusted on compression stroke, Our bad....

Also, There are some that do not know of the 1/4" past TDC on some engines to get past the compression release to obtain a proper valve clearance adjustment.

A mechanical compression release is nothing "new" to lawn and garden equipment engines, Kohler introduced this fetcher back in the late 40's with the introduction of the K-Series engine line.

http://www.kohlerengines.com/about/history.html

The first cam picture I posted is from a Kohler K301, I knew that one showed in greater detail the compression release mechanism and I could blow it up without much distortion.

Enough of that.....

There is a process of elimination one should follow when doing diagnostics. That's starting with the easiest probable causes first. Myself had rather remove 3 or 4 screws and check valve clearance first (since spit-back/backfire through valves in 'most' cases is found to be valve related) than remove the whole top of the engine to check the key.

So, It looks like the valve assumption was invalid in your
case but very well could have been reversed, Pull the flywheel and find nothing wrong then check valve Clarence.

In any event; Glad you found the problem and don't be a stranger, We all really try to give each other a hand the best we can......

BTW Tommy,

No pun intended,

But....At times....I would give anything to get my hands on the engineers that design some of this equipment.

Take Care
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