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Old 12-09-2012, 10:48 PM   #1
Tosh Togo OP
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Need '81 750 Seca cam timing

Will be freshening up the Olde Beastie's top end soon, and have been unable to find the opening and closing specs for the camshafts.

If someone has a factory manual in their collection, would you mind finding and posting that info?.
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Old 12-10-2012, 12:25 AM   #2
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Quote:
Originally Posted by BOETJE View Post
Even the factory manual doesn't mention opening and
closing angles. I have made valve lift curves in the past for standard and special cam : If you set them on the standard timing marks, you'll be fine.

I once had a Yamaha manual for my XJ750, and it listed the opening and closing numbers for both camshafts, just like it does in my XJ650 manual. Unfortunately the 750 manual was loaned to a friend, and never came back.

The head that's going on has been milled .020", and the top of the cylinder block may get the same treatment, so "setting them on the standard timing marks" will only guarantee that the valve events will all be late.

Over the years I've degreed in about a dozen cams, and have yet to find ANY that were close enough (<2 degrees from spec) to suit me. I've seen two DOHC engines (both a were XJ Yamahas) that had one cam within one degree, but the other was worse.

It's my engine, and "close enough" isn't going to work.

Opening/closing numbers or lobe centers are welcome.
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Old 12-10-2012, 04:34 PM   #3
pommie john
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Quote:
Originally Posted by BOETJE View Post
Go and sit somewhere quiet and think very carefully about what you just wrote...


What he wrote makes sense to me. If you mill the head, the cam chain tensioner will take up the slack at the back, pulling the sprocket backwards, retarding the cam timing... won't it? Am I missing something?
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Old 12-10-2012, 06:00 PM   #4
Tosh Togo OP
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Quote:
Originally Posted by BOETJE View Post
Go and sit somewhere quiet and think very carefully about what you just wrote...
I did, and I'm still right, while you are apparently confused.

Quote:
Originally Posted by pommie john View Post
What he wrote makes sense to me. If you mill the head, the cam chain tensioner will take up the slack at the back, pulling the sprocket backwards, retarding the cam timing... won't it? Am I missing something?
That's the gist of it.

Quote:
Originally Posted by BOETJE View Post
Yes you do .
If the timing marks line up , like he said, then it doesn't matter how much is milled of the head. Thing is that if you mill a lot off, you might have to slot the cam sprockets in order to make them line up.
If, if, if.... there are a lot of assembly tolerances in the cam-driving system: the crank sprocket, the cam sprockets, and the mounting holes to mate each cam to its' respective sprocket. cam. There's also the assembled height of the cams' axis versus the crank sprocket.

This potential stack-up of tolerances is the biggest single reason that otherwise identical stock bikes will often have very different personalities... two degrees will make a measurable change in the torque curve.

The Japanese factories do a good job of it, but there's still inherent variation in the results: there are many little opportunities for the valve timing to end up to not be what you think it is, and the only way to be certain is to degree in each cam. Your presumption that the standard timing marks is optimal is not shared by very many other engine builders, so please stop with your enthusiastic-but-misguided ways to "help" me.

I need actual timing numbers, not someone's opinion that the lazy way of engine building is OK.

You do your cams your way, and I'll do mine the way I was taught.
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Old 12-10-2012, 06:04 PM   #5
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Quote:
Originally Posted by BOETJE View Post
Yes you do .
If the timing marks line up , like he said, then it doesn't matter how much is milled of the head. Thing is that if you mill a lot off, you might have to slot the cam sprockets in order to make them line up.

The timing marks only get it lined up to the right chain tooth. Once you add a variable of machining the head, you need to degree the camshaft in.
The timing marks are on the sprocket so slotting it won't help. The marks might line up, but the cam could be anywhere. To get it right you need the timing spec of the cam.

For what it's worth, Megacycle make a couple of cams for that bike and they have 108 degree lobe centres. That might be a good starting point.
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Old 12-10-2012, 06:25 PM   #6
Tosh Togo OP
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Quote:
Originally Posted by pommie john View Post
The timing marks only get it lined up to the right chain tooth. Once you add a variable of machining the head, you need to degree the camshaft in.
The timing marks are on the sprocket so slotting it won't help. The marks might line up, but the cam could be anywhere. To get it right you need the timing spec of the cam.

For what it's worth, Megacycle make a couple of cams for that bike and they have 108 degree lobe centres. That might be a good starting point.

Thanks!..
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Old 12-10-2012, 06:53 PM   #7
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I have a factory service manual for the XJ750 RH Seca (which is really just a small addendum added to the XJ650 service manual) but it does not list the specs you desire. Maybe you had a factory design (or other) manual. You might also ask this question at the http://xjbikes.com/ forum as this is a huge repository of XJ-specific info and expertise.
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Old 12-10-2012, 08:30 PM   #8
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Quote:
Originally Posted by BOETJE View Post
No they are not : they are on the cam itself.

I wish you all good luck , and a long and happy life.

My mistake regarding the marks. They are on the sprockets on most bikes I've seen. The principle is the same though, messing with deck heights means the cam timing needs to be properly checked.

Have a good life yourself.
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Old 12-11-2012, 10:47 PM   #9
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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
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pommie john screwed with this post 12-11-2012 at 10:48 PM Reason: spelling
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Old 12-11-2012, 11:49 PM   #10
Tosh Togo OP
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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
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Old 12-12-2012, 01:14 AM   #11
pommie john
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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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