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Author Topic: XS750 to XS896 big bore  (Read 22936 times)

Offline XS750AU

  • Posts: 310
Re: XS750 to XS896 big bore
« Reply #40 on: Aug 13, 2016, 03:21:08 »
Reassembled the head today. New valve stem seals were the only new parts. All straight forward stuff, final timing of cams when the cam chain goes in later on.
Then I just continued to clean grim and muck from all the bolt heads and other crevices. In some ways the oil and dirt appear to have preserved the painted covers, as it is cleaning up pretty well.
“Engineering is the art of being approximately right rather than exactly wrong.”

Yamaha XS750-2D
Yamaha XS896
Husaberg FE550
Yamaha TT250
Yamaha IT200N

Offline Tune-A-Fish©

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Re: XS750 to XS896 big bore
« Reply #41 on: Aug 13, 2016, 08:38:03 »
Just wondering if you did a valve clearance feel before buttoning things up?
"I didn't come here and I ain't leavin"  Willie Nelson

"love hard, live fast, die fun" Kacey Musgraves

"Like a Wreckin Ball!" Eric Church

Offline XS750AU

  • Posts: 310
Re: XS750 to XS896 big bore
« Reply #42 on: Aug 14, 2016, 00:26:22 »
Hi Tune-A-Fish
After torquing the journals down the clearances are 0.16mm on the inlet and 0.22mm on the exhaust. So there is no need to play with shims at this point.
Thanks for reminding me.
Regards
Tim
“Engineering is the art of being approximately right rather than exactly wrong.”

Yamaha XS750-2D
Yamaha XS896
Husaberg FE550
Yamaha TT250
Yamaha IT200N

Offline Tune-A-Fish©

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Re: XS750 to XS896 big bore
« Reply #43 on: Aug 14, 2016, 08:55:44 »
Hmmm... Seems fat for the 896 bore, if you took notes for each valve, you might need to readjust if it's noisy or boggy off idle, that big bore will be hungry for air and tighter multiplies not just open more but longer.

I can tell you know all this crap, just offering some friendly thought  :o
"I didn't come here and I ain't leavin"  Willie Nelson

"love hard, live fast, die fun" Kacey Musgraves

"Like a Wreckin Ball!" Eric Church

Offline XS750AU

  • Posts: 310
Re: XS750 to XS896 big bore
« Reply #44 on: Aug 14, 2016, 19:29:49 »
Hmmm... Seems fat for the 896 bore, if you took notes for each valve, you might need to readjust if it's noisy or boggy off idle, that big bore will be hungry for air and tighter multiplies not just open more but longer.

I can tell you know all this crap, just offering some friendly thought  :o
Thanks for the info. The factory limit for the inlet is 0.16mm which is where we are now. The valves are going to grow as they heat up in operation and close the gap. So  how far can you close it up with the shims before you have gone too far?
The exhaust valves will get hotter, hense their min clearance is 0.21 and we are currently at 0.22. The 896 is going to run hotter, higher compresion and the basic fact that more power = more heat.
Really interested to get you experience on the true minimum clearances. I will admit to being conservative.
Thanks
Tim

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“Engineering is the art of being approximately right rather than exactly wrong.”

Yamaha XS750-2D
Yamaha XS896
Husaberg FE550
Yamaha TT250
Yamaha IT200N

Offline Tune-A-Fish©

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Re: XS750 to XS896 big bore
« Reply #45 on: Aug 14, 2016, 19:54:54 »
Unfortunately, I don't know what the pistons are and what relief if any is cut in them. I may well be wrong, just seems loose to me.
"I didn't come here and I ain't leavin"  Willie Nelson

"love hard, live fast, die fun" Kacey Musgraves

"Like a Wreckin Ball!" Eric Church

Online datadavid

  • Posts: 1120
Re: XS750 to XS896 big bore
« Reply #46 on: Aug 15, 2016, 01:46:05 »
My valves, new in a head with new guides and 3 axis seat grind,  clearances actually grew slightly the first few hundred miles. Only reason i can come up with is carbon from ring seating collecting on the valve seats?

Online datadavid

  • Posts: 1120
Re: XS750 to XS896 big bore
« Reply #47 on: Aug 15, 2016, 01:48:06 »
Point is, run it for a while then the routine stuff, retorqueing head, camchain tension, valve clearance. After the first check they seldom change.

Offline XS750AU

  • Posts: 310
Re: XS750 to XS896 big bore
« Reply #48 on: Aug 15, 2016, 08:16:18 »
Most round slide (VM series carbs use Large Hex jets.  Large hex jets are numbered to represent flow rates at 30cm of pressure IIRC.  Large round jets are measured in mm of diameter, so a 150 is 1.5mm diameter.

In simple terms double the size of a hex jet from say 90 to 180 and it doubles the flow, but double the size of a round jet and flow increases by a factor of 4, so a 180 round flows 4 times as much as a 90 in a large round.  There are tables on teh web that show a pretty close correlation for equivalent sized jets.

Thanks for the heads up. I found that comparison chart and as you said a huge difference in flows (refer attached). The 130 AMAL design that were in it are actually the equivalent to about a 105 round mikuni (reverse). So the PO actually leaned out the carbi significantly. The 160 AMAL that I bought are equivalent to a 120 round mikuni!!!!!! It is never a mistake, if you learn without doing damage - Thank you Teazer.
Cheers
Tim
« Last Edit: Aug 15, 2016, 08:31:54 by XS750AU »
“Engineering is the art of being approximately right rather than exactly wrong.”

Yamaha XS750-2D
Yamaha XS896
Husaberg FE550
Yamaha TT250
Yamaha IT200N

Offline XS750AU

  • Posts: 310
Re: XS750 to XS896 big bore
« Reply #49 on: Aug 15, 2016, 08:27:45 »
Point is, run it for a while then the routine stuff, retorqueing head, camchain tension, valve clearance. After the first check they seldom change.
I have not done as much work on the head as you Dave, just a bit of basic maintenance. Totally agree with you though, run it a see how it goes. Should be OK at lower RPM and run out of puff towards redline.
The exhaust valves had a heap of carbon build up on the seats and valve crown, the piston gudgeon pins were distinctly blue which indicates to me that it has been running bloody hot. With the jetting being totally lean (per previous post) it looks like it was running bloody hot with no power - no wonder it did not see much speedway action!!
Cheers
Tim
“Engineering is the art of being approximately right rather than exactly wrong.”

Yamaha XS750-2D
Yamaha XS896
Husaberg FE550
Yamaha TT250
Yamaha IT200N