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Old 12-11-2012, 11:49 PM   #10
Tosh Togo OP
Beastly Adventurer
 
Joined: Nov 2010
Oddometer: 1,594
Quote:
Originally Posted by pommie john View Post
Hey Tosh, look what I found

"Beyond the YICS feature, the 750's cylinder head is conventional, with two valves per cylinder and dohc driven from the crank by a roller chain. The valve sizes are the same as on the Maxim 650, 33mm intake and 28mm exhaust. But valve timing is different, to match the larger displacement, the 750's intake valves opening 38" BTDC and closing 58 ' ABDC for a duration of 276; the 750's exhaust valves opening 56 BBDC and closing 36 ATDC, for a duration of 272. The 650's intake valves are timed at 34-58 for 272 duration and the 650's exhaust valves are timed 66-26, again for 272 duration."

That works out at 100 degree lobe centres for both inlet and exhaust.

It came from here if you want to check ..

http://www.motorcyclespecs.co.za/mod..._sega%2081.htm
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.

Tosh Togo screwed with this post 12-11-2012 at 11:54 PM
Tosh Togo is offline   Reply With Quote