XS750 to XS896 big bore

XS750AU

Coast to Coast
Hi
Recently I have finished rebuilding the engine in my XS750-2D. Did not have to do too much, but I did go to 1st oversize pistons using scheuerlein pistons. Ironic that it is easier to get German pistons to fit a jap bike! Plus is was skipping second gear, which is not unusual for this bike. It turned out that the PO had put the circlip with the wrong face against the gear and the circlip had been pushed out of its seat while remaining on the shaft. That bike is now on the road and has been rejetted for pods. The mains went from 130 to 140 and the pilot jet went from 42.5 to 45. All running well now. During this process I acquired another XS750SE that was a wreck and in pieces for spare parts. Fortunately the PO did not make much effort to get decent photos and I picked up for less than AU$100.

The engine moved for less than a revolution on the kick starter and then stopped rotating. The PO had started to pull the engine down but got cold feet at breaking the cam chain, and had left it for years with the plugs out and the cam cover off. When I bought it, it was encrusted in mm of dirt and crap, so I am hoping that there is not too much wrong inside. When I have inspected the cam journals, the bearing surfaces were better than my other bike and I don't think the generator cover had ever been removed before. As this bike was not complete and at some stage had had a side car fitted I felt it was a clean slate that I could modify to my hearts content!

On Ebay there was a 896 big bore kit for under $200 which is a third of the cost of new German 750 pistons, the catch being that 750 cylinders cannot be bored out from 68mm to 74.5mm, you must use 850 cylinders. I have searched for over a year globally to find a set of 850 cylinders, seems like there are a lot more 750's around than 850's! Finally I tracked down a set of 850 cylinders out of North Queensland - same day I ordered the 896 piston and head gasket kit.

The head on the XS750SE was the same as the head (including cams) on the XS850 so I do not have to change anything on the head. The carbies are Mikuni 34mm series II which I believe will be OK. The other benefit of fitting the big bore kit is that the compression ratio will immediately change from the 750's 8.5:1 to the 896 having 10:1, and that should be OK.

The drive line on the XS750 was totally over engineered and is the same as used on the XS1100 so it will be able to cope with the considerable increase in HP and torque.

I am hoping that there is someone else on the forum that has completed this conversion before me and confirm if I have missed anything before I start building?

I am going to use pod filters and have a triumph 955 header that I am going to use for a very free flowing 3 into 1 exhaust. If the stand main jet is 130, any idea as to what main I should start with, I am thinking it is going to be 150+? On the primary jet side it would go from the 42.5 to 47.5? Would the primary air vent also need increasing?

Thanks for any help.

Regards
Tim
 

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Ok looks like I need to work on attaching photos
The first photos are of the XS750-2D as advertised and now - nothing major but needs a final paint job.
The next photos are of the XS750SE as advertised (that why I was the only bidder) and at home, the final photo as of today, just getting parts together!
 

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Working on my own XS750 I'll be watching this thread ;)

Psy

Sent from my SM-G903W using Tapatalk
 
As i said to another guy on here - ditch the old rotten stock carbs and get yourself some triumph carbs. If youre going through all that work its nice to have some new, easily adjusted and maintained items on there. I did and im thankful! Also i built an engine with the 896cc cruzinimage pistons last winter, and it seems to be running fine.
 
I just picked up my 6th yamaha triple. this one has no papers and the motor is locked.....maybe it's time I start putting together a big bore kit....I think I may have a set of 850 cylinders knocking around.....hmmm
 
Cylinders and pistons have been dropped off at the machine shop. As you can see in the photo the pistons look OK, they are not in the same league as the Scheuerlein pistons out of Germany that went into the XS750-2D, but they should be fine. The cylinders look a little rough on the outside but all mating surfaces are in good condition.

Spent a couple of hours in the shed making up some tooling to form a flange on my headers. I have a triumph 1050 Street triple header which has a nice flat junction where all three header pipes meet, but the couplings onto the head are not compatible. Pipe diameter is also same as the XS at 38mm so I have just bought some 90 degree 38mm 304SS bends to solve the problem. In the photo you can see the first practice run. I will use the practice parts as the little clamp brackets that clamp the headers against the head. The split is along the tube seam, on the headers I will get my TIG welding mate to spot them on the inside before I flare them and see if that helps.
 

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The cylinders came back from being bored out to 74.5 that only leaves 1.8mm walls on the barrels! There was some variation on the piston diameters, only microns but it means the pistons need to go in the assigned cylinders.

A bit of distraction on the weekend as I finally sorted out the jets and air leaks on the 750's carbies. The following video is the first run with it running well. The road I was on was badly affected by a storm during the week with a lot of debris, so I could not go too hard.

https://www.youtube.com/watch?v=iXoosev1jwA

Cheers
Tim
 
XS750AU said:
The cylinders came back from being bored out to 74.5 that only leaves 1.8mm walls on the barrels! There was some variation on the piston diameters, only microns but it means the pistons need to go in the assigned cylinders.

A bit of distraction on the weekend as I finally sorted out the jets and air leaks on the 750's carbies. The following video is the first run with it running well. The road I was on was badly affected by a storm during the week with a lot of debris, so I could not go too hard.

https://www.youtube.com/watch?v=iXoosev1jwA

Cheers
Tim
Sorry for the technical hitch!
Video is now "public".

Sent from my GT-I9506 using Tapatalk
 
The cylinder is back after boring, and as in the photo the pistons had a few microns of variation. Not enough for any problems, 2 pistons are interchangeable and one, 0.008mm larger must stay in #1 position. I went back onto the Scheuerlein website as their pistons were close to perfect for the 750, to see if they sold big bore kits. And they do, BUT they cost Euro400 and are the same big bore kit that I bought on Ebay for AU$200 delivered. Have a look at the photos - they are out of the same tools!!! Scheuerlein state they are sourced out of Japan - but I cannot see how they justify the extra AU$380!!!

I have the gasket kit, so in theory I have all I need to rebuild the engine unless I find some surprises.http://www.dotheton.com/forum/Smileys/default/tongue.gif Made up an engine stand on the weekend to make it a bit easier. This is the second XS triple rebuild I have done , and the 3rd time to strip one down. On the 750, one of the journal studs striped out when I was taking it to final torquehttp://www.dotheton.com/forum/Smileys/default/angry.gif. One hellicoil and a day and it was fixed ;D.

Cheers
Tim
 

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datadavid said:
Did the same thing and with the right cams i think you will enjoy the power bump!
Hi David
Thanks for the reply. Can you give a bit more detail on the cams?
Thanks
Tim

Sent from my GT-I9506 using Tapatalk
 
XS750AU said:
Hi David
Thanks for the reply. Can you give a bit more detail on the cams?
Thanks
Tim

Sent from my GT-I9506 using Tapatalk
I dont really have great detail to give you but the earlier cams (77-78)are supposed to breathe a bit better. Web cams have a few profiles as well, to be welded on cores.
 
Hi Dave
What year model did you use as your base for the 896?
After your info on the cams I started to dig a bit deeper and had a Doh! moment. I had thought that all XS750's had the 8.5 compression ratio which when using the 896 kit would boost the compression to 10.5 which is not too radical. BUT the XS750E & SE which I am using had a compression ratio of 9.5 which means with the 896 kit it will be end up being 11.4, which is a bit higher than I would have liked. Anyone out there got an opinion on a 11.4 compression ratio on an air cooled 1978 engine???http://www.dotheton.com/forum/Smileys/default/huh.gif.

On the cams the XS750E appears to have the final design and the highest performance so that looks OK. Plus it keeps the valves open longer so I guess the actual compression will be a little less due to the swept volume with closed compression is slightly lower!!! The info I could find was per the attached image for the D and 2D and the following text for the E.

"Therefore the engine modifications were kept subtle and affect only the top end of the engine. For example the compression ratio has been upped from 8.5:1 to 9.5:1, which means it now requires premium fuel (at least 90.5 octane rating). The actual shape of the combustion chambers has also been altered and they now actually displace fewer cc’s than earlier XSs. Cam profile has been altered slightly, retaining the same lift but now having sportier intake timing. And intake breathing has been further increased by changing the venturi shape in the carbs (they’re still 34mm in size), modifying the main jets and adding on a, larger, better breathing air box.".

Guess if there are a couple of options on compression, think I will definitively use 2 cylinder base gaskets. If that is not enough there is always the option to swap heads with my other XS750-2D which has the 8.5 compression. That way I would boost the 750 to 9.5 and the 896 would end up at 10.5 (last option).
Cheers
Tim
 

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Since i built mine from scrap im not really sure what bike my head came from, but the starter wont turn the engine unless hot so either the starter needs a good cleaning or i got very high comp!
 
Hey guys!

sorry for hijacking your post but I shortly before buying two complete XS750 (stock bikes).
is there any way to check for the famous 2nd gear jumping without riding the bike?
both bikes won´t start cuz they were sitting for years,need carb cleaning et cetera...

if the second gear issue will arise is it hard to fix?i guess it means tearing the motor down to the gears....?

thanks in advance and sorry once again?
 
Ryan Stecken said:
Hey guys!

sorry for hijacking your post but I shortly before buying two complete XS750 (stock bikes).
is there any way to check for the famous 2nd gear jumping without riding the bike?
both bikes won´t start cuz they were sitting for years,need carb cleaning et cetera...

if the second gear issue will arise is it hard to fix?i guess it means tearing the motor down to the gears....?

thanks in advance and sorry once again?
Its not hard to fix, go to yamaha-triples.org for a few different methods. Only way to see if it slips is under load unfortunately.
 
Thanks datadavid!
Youre xs project is awesome...

I guess I will need to look far better carbs too,got the mikunis 2's on it...but i heard that these suck hard.
 
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