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2021/05/19 11:26:13
paulbaker1954

Tecumseh engine rebuild - won’t run

Just rebuilt my engine after it seized and now won’t run. It was running fine before.

There is fuel and spark but am now wondering if it is possible to get the cam timing 180 deg out?

I aligned the marks on the crank gear and cam gear but have been pondering if it is possible to get them wrong.

Where should the magnet on the flywheel be when the engine is on the compression stroke I am wondering.

I guess I can check if it’s right by watching for the inlet valve to close and then next TDC should be firing point
8 comments Leave a comment
Roy
If the cam is 180 degrees out rotate the crankshaft one full turn and it will be correct. The cam turns at half crankshaft speed. To check the timing rotate the crankshaft until the intake valve is opening as the exhaust closes, this is called valve overlap. The piston should be at tdc.
2021/05/19 12:13:08
paulbaker1954
Roy
If the cam is 180 degrees out rotate the crankshaft one full turn and it will be correct. The cam turns at half crankshaft speed. To check the timing rotate the crankshaft until the intake valve is opening as the exhaust closes, this is called valve overlap. The piston should be at tdc.


Sorry I may sound thick here. Do you mean rotate the crank 360 and then realign the timing marks on crank and cam gears.

Still not clear how to check if it is 180 out. If I watch the valves then once the intake valve has closed and then rotate the engine until TDC again should the magnets on flywheel and coil pretty much line up.

The Tecumseh service manual just says line up the marks on cam gear and crank gear but says nothing else
2021/05/19 12:29:02
GBertolet
Check the compression also. The piston rings could be damaged, or cylinder scored. Also, you could be off a tooth on the timing, or you misinterpreted a mark or indentation, on either gear, as a timing mark. Just trying to cover all the bases here.
2021/05/19 12:57:49
Roy
The cam timing cannot be 180 degrees out. It rotates 1/2 turn for every revolution of the crankshaft, if the marks are not aligned rotate the crank one revolution so they meet.
2021/05/19 15:40:22
paulbaker1954
Roy
The cam timing cannot be 180 degrees out. It rotates 1/2 turn for every revolution of the crankshaft, if the marks are not aligned rotate the crank one revolution so they meet.


Got it now just twigged that these engines aren”wasted spark” in that they fire on both the compression and exhaust stroke so as long as the crank and cam marks are correctly aligned it is not possible to get the timing 180 out.

Need to go back and check all the basics (alignment, valve clearance, etc .... again which sadly means pulling the motor again.
2021/05/20 02:50:54
paulbaker1954
Just had a thought......

Crank I used in the build was a different part no to the original as the original is no longer available. I checked the shafts and journals and stroke and all exactly the same but......

One thing I didn’t check was the flywheel key way position on the crank. If this is different then obviously the spark timing will be off so I will check it but cannot till next week.

So in the meantime anyone know if the flywheel key way position changes on different Tecumseh cranks. I guess it will depending on where the mag coil is mounted.
2021/05/21 03:21:51
Scrubcadet2010
If you can, look at the position of the piston, on the compression stroke, coming up to TDC, relative to where the flywheel magnets are to the ignition coil.
2021/05/21 15:33:02
paulbaker1954
Scrubcadet2010
If you can, look at the position of the piston, on the compression stroke, coming up to TDC, relative to where the flywheel magnets are to the ignition coil.


I checked the key position on the old and replacement cranks and they are the same.

I believe that for correct timing the magnet should that the magnet is just about to exit the area of the ignition module with piston at TDC as in attached image
2021/05/22 02:42:37

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