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Cam timing on old 91 surpass

Old 02-02-2015, 10:16 AM
  #1  
Jennifer Curtis
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Default Cam timing on old 91 surpass

I have a 91 surpass that I bought in 1991.
I recently replaced the bearings and when I
was installing the camshaft, I could not get the
dimple to line up with the pushrods. It would
go straight down or slightly forward of the
pushrods. (Inline with the pushrods was
about half a tooth from straight down.)

So I put the dimple straight down thinking
that it is at least a direction that could be
described easily, and so might have a
better chance of being right.

The engine runs OK, I guess. I mean the
rpm peaks at about the same level.

Should I leave it this way or mess with it some more?

Jenny
Old 02-02-2015, 12:57 PM
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Propworn
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The cam will make a half rotation (180 degrees) for every full rotation (360 degree) rotation of the crankshaft. If the mark was to be up or in line with the lifters at top dead center (compression stroke) you timed it at top dead center (exhaust stroke) no difference because it’s a single cylinder, if you were to rotate the crank another 360 degrees the cam mark should line up where its supposed to be with the piston at top dead center. If the timing were wrong it would not run as well as it does.

Dennis
Old 02-03-2015, 04:06 AM
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Jennifer Curtis
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If I turn the crankshaft 360 degrees the dimple
is not in line with the pushrods. It is straight up.

Since you think running ok is good enough evidence
that it is right, I'll leave it that way.

Thanks

Jenny
Old 02-03-2015, 06:49 AM
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Bax
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Here's the proper cam setting with the engine at Top-Dead-Center. Note that the dot is not at 90-degrees to the crank, but lined up with the pushrods.

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Old 02-03-2015, 07:08 AM
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Jennifer Curtis
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OK.

Now I'm confused.

The dot won't go in line with the pushrods at TDC.
It will go straight up or farther back towards the
piston. It is a half a tooth from lining up with the
pushrods. It won't go into that position because
the gear teeth don't line up that way. That is
what this thread is about.

Maybe it is because the engine is very old and
was made using a different alignment technique
back then. Maybe they just put the dot in the
wrong place on the cam gear.

So the question is should I leave it in what
appears to be the wrong setting it is in now,
or change it to the other possible wrong setting?

Jenny
Old 02-04-2015, 06:46 AM
  #6  
Bax
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If the dot on the cam doesn't line up, then the crankshaft may not be at Top Dead Center. We usually set the crankshaft manually, by observing the position of the crankpin in the crankcase. There's almost always a bit of play on the drive washer that can fool you into thinking the crank's at TDC when it's really not. That being said, if the engine will turn in the high 9,000's when leaned to just rich of peak RPM on the ground, your engine has the cam set correctly.
Old 02-04-2015, 07:08 AM
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Jennifer Curtis
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I set it in its current wrong looking position with the
back off and the crankpin straight up. The drive
plate has a mark that also points straight up so
I'm not being fooled.

With 5% nitro and an APC 14x6 propeller in a
3 inch spinner, I can get 10,100 rpm peak, which
I need to back off to 9,800 so it doesn't lean out
during flight. This is well below the 11,000
specified in the instructions. When it was new
it got about 10,400 peak, so it is not running
like it originally did, but from what you say,
I guess the cam is in the right position.

What RPM would I get if it was off one tooth?

Jenny
Old 02-05-2015, 06:52 AM
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We don't know, but it would be lower.
Old 02-06-2015, 05:16 PM
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Propworn
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I have a book that speaks directly with pictures for setting the cam timing on an OS four stroke.

The camshafts punch mark must line up directly under the pushrods with the crankshaft at TDC picture (649)

I have never used tape but they recommend rotating the crank until the piston is at TDC and the marks on the crankshaft and the block line up picture (645) Tape the crankshaft in position picture (646)

The cam will rotate slightly as you push it in to fully engage the gears so start it one tooth early like in picture (648). When push the cam in until its fully engaged the cam will rotate until it lines up like picture (649)
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