Thread: Tricky Timing Question
09-28-2012, 01:58 PM #1
Tricky Timing Question
Hi Everyone,... been working on this SLT 780 with exploded top ring landing damage on the PTO. (I discovered that whoever had this machine before me split the head gasket (maybe it was compromised by lack of proper clearance with the head)
Anyway, I checked the crank index 5 ways and discovered it to be off by two degrees short (retarded) ATDC from the front two cylinders every time.
Is two (2) degrees out of index a problem? Does it need to be rebuilt?
Although the shop manual says +/- 2 degrees is within tolerance, I wonder if I should compensate and adjust the timing for this "limiting" crank index factor. And, if so in which direction should I adjust the stator and by how much?
Any suggestions would be greatly appreciated.
p.s . I used this proceedure (below) to determine crank index:
Disconnectbattery ground cable and all spark plug high
tensionleads; ground high tension leads to engine. Disconnect lanyard from engine stopswitch before proceeding with the following steps.
1 .Securely fasten a degree wheel on the flywheel or
outputcoupler/driveshaft. Use a large degree wheel
for moreaccuracy, and make sure it is mounted
concentricallywith the crankshaft center line. Cut a
slot in apaper or cardboard degree wheel and slide it
over thedriveshaft if engine and pump shaft is
installedin craft. Again, be sure to secure the degree
2 . Sharpena coat hanger or section of welding rod and
anchor itto a convenient spot. Point the sharpened
end at theouter perimeter of the flywheel or drive
couplermounted degree wheel.
3 . Installa dial indicator into the magneto end cylinder
spark plughole (front) (#1). (The ignition timing is
referencedby the magneto end.)
4. Rotatethe engine to bring the piston to top dead
center(TDC) on the cylinder with the indicator
5 . LocateTDC as accurately as possible by finding the
center ofthe point where there is no piston
movement."Zero" the dial indicator at this point.
Continue torotate the crankshaft in the normal
directionof rotation (counter clockwise as viewed
from therear of the craft) until the dial indicator reads
.100"(2.54mm) after top dead center (ATDC).
IMPORTANT Do not allow the crankshaft to move
6 . Bendthe pointer or move the degree wheel until the
pointeraligns with the 180 or 120° mark on the
7 . Withthe pointer aligned, make sure the degree wheel
and pointerare secured and will not move out of
position.Re-check accuracy of this location by
repeatingsteps 4 and 5. The pointer should align
with the180 or 120° mark when the dial indicator
reads.100" (2.54mm) ATDC.
IMPORTANT Do not move the degree wheel or pointer
after theinitial setting on the mag end cylinder - simply
read thewheel and dial indicator.
8 . Removethe dial indicator and install in cylinder #2
(center).Repeat steps 4 and 5 for cylinder # 2. Note
the degreewheel indication when the dial indicator
reads.100" ATDC. It should be 180 or 120° (± 2°)
fromcylinder #1. Proceed to PTO cylinder (#3).
Cylinder #3should also be 120°<± 2°) from cylinder
Symptoms ofan out of index crankshaft can include:
•Difficulty calibrating carburetor (repetitive plug
fouling onone cylinder with no other cause);
•Unexplained piston failure on one cylinder (i.e.
severedetonation, broken ring lands, piston holing);
• Excessivevibration of engine, backfiring, etc.;
• Rough idle, poor top speed.
Last edited by polaris780slt; 09-28-2012 at 02:19 PM.
09-28-2012, 02:08 PM #2
I would leave it as is. 2 degrees is within spec and you should notice no ill effects as a result.
Maybe you misspoke, but the reading should NOT be from TDC, but past TDC.
09-28-2012, 02:18 PM #3
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