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Old 12-12-2012, 01:14 AM   #11
pommie john
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Joined: Apr 2008
Location: Brisvegas, Australia
Oddometer: 1,410
Quote:
Originally Posted by Tosh Togo View Post
Yuppers... I've seen that one before, and wonder if the info's got a typo for the 750's exhaust timing.

-Here's why-

Using those cam timing numbers, we get:

750: IVO 38 BTDC, IVC 58 ABDC LC=100
EXO 56 BBDC, EXC 36 ATDC LC=100
Overlap=74


And these are the timing specs for the 900 and 650, from the Yamaha manuals:

900: IVO 38 BTDC, IVC 58 ABDC LC=100
EXO 66 BBDC, EXC 26 ATDC LC=110
Overlap=64
 
650: IVO 34 BTDC, IVC 58 ABDC LC=102
EXO 66 BBDC, EXC 26 ATDC LC=110
Overlap= 60

All three engines have a the same intake closing point, and the two with known timing figures have the same (or nearly the same) lobe centers for their respective camshafts, so I'm questioning the XJ750 timing we've both found that indicates a 100 LC on the exhaust cam, and a big change in overlap.

This is getting interesting, and even if the real XJ750 valve timing doesn't surface in time, I have no problem checking the cams before tearing it down, so I've got a known place to start from after we begin to put it back together.

It won't be the first time I've had to do that, and this puppy has so much low-end and mid-range that I'd hate to lose any of it.

Hmm. I see your problem. The cam with 100/110 centres might be described as 105 lobe centres running 5 degrees advanced which is a way of shifting the power down the range. Compared to the other cams the spec on the website does look suspect. Mind you, maybe they were looking to make it a bit more racy. Sorry I can't help more.
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