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John Deere model d lh exh valve setting

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I have a john deere model d . I over ran above 900 rpm on startup . now I checked the valve setting and the lh exhaust valve open setting occurs about 20 degrees after cover mark . could the cam have shifted that far? also if I set the spec lh exh to cover mark it appears that the impulse for the #2 cylinder will occur about 20degrees after tdc??? and same for the #1 cylinder ?? seems like it could be 20 degrees before or tdc but not after tdc?? the impulse marks on the flywheel and lh exh open mark are about 20 degrees off parallel which makes the firing 20 degrees after tdc also ?? what am I missing ??
 
Perhaps its as simple as not knowing where to line it all up at? 3 O'Clock position of flywheel there will be mark sometimes on a transmission cover. This is where the flywheel timing marks are supposed to be lined up to. I doubt your engine can run well 20 degrees out of time which seems to be saying to me that it is timed just the way it's supposed to be and you are just using the wrong line up reference point. Lh exhaust mark is the point at which the #1 cylinder's exhaust valve is supposed to just start lifting off it's seat. Magneto is to trip it's impulse spring on the IMPULSE mark at 3 O'Clock.
 
The IT manual sez with spark fully retarded for starting it should fire (or snap if magnito) at "TC". Running timing is 25 before TDC with the centrifugal (or manual) advance all in.
 
The mark on the tractor is clear and lined up. Lh exh lined up , the piston is not at bdc ? the bdc is at 20 degree past the mark .. and that's where the exh valve starts to open . the impulse clicks exactly on the mark and opposite side 180 degree also .. it seems to me that with the marks on the flywheel , the impulse mark is not 180 degrees apart but about 150 degrees . using this spec setup
, it appears to me that the #2 cylinder will be 20 degrees past it's tdc when the impulse fires ?? I'm missing something ..
 
Yes . the normal marks on flywheel are 150 degrees apart . Impulse is 150 degrees ahead on flywheel . same as all jd's I guess . but with this alignment then it appear that when Lh exh align at mark , then the impulse will spark when it reaches the opposite 180 degree position . which is when the #2 piston would be on the power stroke and piston would be 20 degrees past the tdc ??? .. ( #1 exhaust , #2 on power ) I know that this cannot occur and most likely 20 degrees before power tdc
 
Think I figured it out .. the lh exhaust valve opens 20 degrees prior to bdc using last part of power cycle to start the upper motion of piston .. the bdc occurs 20 degrees after exhaust valve opens .. the impulse is correct .. I was thinking that the bdc would start the exhaust cycle -- wrong ...
 
i think what you are missing is the mag lag angle which is probably 30 to 35 degrees. don't forget the cam timing can be adjusted without affecting the ignition timing.
is it possible that you have bent the lh ex push rod effectively retarding that valve event.
barry
 
I figured out now that the exhaust valve is supposed to open 20degrees prior to bdc . mine currently open at bdc . so I've either got to rotate cam so exh valve opens at mark or can I adjust the tappets ? the exh tappet is loose until exh opens , so can I tighten ahead or do I have to rotate cam?
 
hi.
set tappets by tightening adjusters till no clearance then back out 1/2 a turn.
turn engine forward till l/h ex valve just starts to open at this point marks on flywheel and spline cover should be in line or within one inch. if not adjustment can be made by removing l/h cam bearing and removing bolts from r/h bearing, it is now possible to push end of cam down to separate drive gears turn crank to line up marks lift cam and replace bearings. may have to try a a few times.
hope this helps if you feel something changed after overspeeding i would check for a bent push rod.
barry
 
Barry - I think you are on the right path . to turn the camshaft , I guess I hold the crank ( flywheel) on the mark and then rotate the cam until the cam lobe just makes contact , then reassemble the bearing holding the cam .. do you think it will work .. I already removed the head and had valve seats machined and push rods were okay ( one was bent ever so lightly , but this is resolved during tappet adjustment ..
 
Barry -what remains is a mystery in how did the tractor run before shutting down and how the cam could be rotated without more damage . cannot believe that bearing would move ? maybe it was how I reassembled the head . might I have not positioned the valve rods correctly ( not in the sockets? ) should this snap into place?
 
a bent push rod can and will spring under load so should definitely be straightened as much as possible. it is possible for one to catch on reassembly so i would recheck l/h ex push rod can be removed without removing rocker. if you do need to retime cam easiest way is to position cam pull gears apart and turn flywheel till marks line up then re engage gears and refit bearings. i would think its almost impossible that overspeeding altered cam timing unless gears are very very worn/ broken teeth can be seen if you remove the crank cover and drain gear compartment. can't remember which way your timing is off if late is it possible that it has been this way for a long time and just a result of worn cam and gears these old engines will still run with massive amounts of wear.
 
Barry -Normally , the exhaust valve should open about 20 degrees before it reaches bdc . however The lh exh was opening at bdc instead of on cover mark . so I adjusted the lh exhaust valve to start opening on the mark . Then checking the exh valve again it just stayed open the entire cycle ?? so I adjusted the lh exh back to opening at bdc ( which is 20 degrees after it should )..after adjusting it back , the exhaust valve open and closes fine , but just at the wrong time again .. just fine .. except it is opening later as before ... so Barry , I guess I cannot change the valve timing by adjusting the exh valve opening .. this can only be done by rotating . So to adjust the cam ( exh valve opening on the mark ), I guess that means pulling the bearings and rotating the cam while holding the crank in position ??? hope I can hold the crank steady .. boy what a night mare .. thanks for your help
 
Barry - So I turn the flywheel until the #1 cylinder exh open mark on flywheel lines up with the cover mark .. I then adjust all of the tappets just prior to this point to correct clearance - that is to tighten all then back off all 1/2 turn at this point .. .. how will I know if this is not correct ? must the exhaust valve open prior to reaching bdc for each valve?? if so how much ? should I measure all valves tappet threads so equal ?how will I know if I should rotate the cam ??
 
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