timing marks/TDC

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stian buksa
Cucciolo - the Lil Pup
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Joined: Mon Oct 12, 2009 11:50 am

timing marks/TDC

Post by stian buksa » Thu Jul 15, 2010 2:36 am

I find it really hard to find TDC on the rear cylinder to set the ignition. Tried the "feel the piston" method, but that gives me +/- 5 degrees or so accuracy.
Shouldn't it be possible to use the timing marks on the beveldrive for this ? I know they don't align at TDC, but does anybody know exactly when they align ? Then I guess I can set the degree wheel after that....

Also when I put my engine together, I followed the manual to every detail, but couldn't get the ignition right afterwards.
Turned out the cams were 180* off. The timing marks of the bottom end line up every 3 turns of the crank, but all the marks (including those of the bevels) line up only every 6 turns.
Did I miss something putting the gears together, or is it a hit or miss operation ?

wdietz186
Cagiva Alazzura
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Post by wdietz186 » Thu Jul 15, 2010 7:11 pm

The most accurate way to find TDC is using a degree wheel and a positive stop for the piston to contact.With the stop contacting the piston some degrees before TDC note the position on the degree wheel and turn the engine backwards until the piston hits the stop again. Note the position again and try to adjust the wheel so it indicates the same number of degrees when turned forward and back. Halfway between the two is TDC. Back the engine up to the timing number before TDC and set the points to open at that point.Remember the engine turns clockwise/backwards [as viewed from the left side] unlike most other engines.As for the timing marks they should all line up as you assemble the engine,if you turn it over a turn or so it will just confuse things.

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DesmoDog
750 GT
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Re: timing marks/TDC

Post by DesmoDog » Sun Jul 25, 2010 7:56 pm

stian buksa wrote:The timing marks of the bottom end line up every 3 turns of the crank, but all the marks (including those of the bevels) line up only every 6 turns.
Yep. They do that. :lol:

If you figure out the gear ratios of the bevel gears it all makes sense. They do that to keep the same pairs of teeth from engaging each revolution, which would make the gears wear out faster.

The bet way to do this is with a degree wheel and a piston stop as already described. You can check/change our cam timing then too.

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