XS750 to XS896 big bore

When I was playing with the headers I checked if there was much mismatch between the exhaust port and the header ID - it is considerable. The red circle is the 34.5mm ID of the header pipe, as you can see there is a lot of metal inside that circle for the dye grinder to remove. The exhaust valve OD is 31mm and it has a stem which also eats into the flow diameter, so there is no need to take it out all the way to 34mm. I am thinking the area down the bottom of the port where is has flattened out is in for particular attention. Any body who has experience in these matters I am all ears!
Cheers
Tim
 

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"D" shaped exhaust ports are quite common on a lot of high end machines. Are you porting this head just based on guesswork? There are a ton of factors to consider before you go making the ports bigger. I would clean up the casting marks and have the head bench flowed to see what cfm it's doing. It is not uncommon on these old Japanese bikes to have valves and ports too big, so you may be close to the sweet spot right now. By making the ports bigger you may actually reduce velocity and kill horsepower. Same thing goes for the header ID. Going bigger than the exhaust port is only going to reduce velocity as well. How you plan on riding this thing should be the main thing to consider here. Is it a bike you are going to race or scoot around town? You obviously got some chops so I apologize if this post is redundant. I have been slowly researching for a kz750 twin hot rod build so I am watching with interest. I may build this DIY flow bench which uses a laptop and a shop vacuum
http://www.flowperformance.com/system.html
 
+1

Touch that floor especially if u take it back to the gasket and u will kill exhaust velocity and power.

Taller higher ports flow better.

In fact, the overhang might even prevent some exhaust gas reversion.
 
Thanks Doc & jcw.
Greatly appreciate your input and help to avoid disaster. Once the material is removed there is no going back.
This bike is not going to race. I prefer to avoid built up areas and find a nice twisty country road. So the bottom line looks like leave the ports as they are and only touch any obvious casting defects (which there are none).
Thanks again for for you great help.
Cheers
Tim

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there's no saying what is optimal unless you do a flow check and do some calculations based on your cc size, cams, carbs...etc I'm just guessing that if you do porting it won't be much. You can polish the exhaust ports and clean up the intakes but leave them rough. Finding a header tube size that matches your exhaust port will help a lot as well
 
I am nowhere near an engineer so i just gave my in and x ports a light sanding. They were a bit coarse.
 
took the plunge and bought an ultrasonic cleaner after reading a NASA research paper.
They are bloody great - petcocks before and then after 10 minutes in the ultrasonic with a little bit of dish washing detergent.
 

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Wow really nice!
is there a reason you stick to the stock petcocks, i heard that the membranes tend to break pretty easily?
I thought about swapping out for "normal" petcocks with on and off function on my project.
 
I just blank off the membrane part on my taps, and have Up as the off position. One less thing to worry about
 
The theory wad that the leaking petcocks diluted the engine oil and contributed to the big end failures. I have been using the standard petcocks on the 750 for 5 years without problems. The current rebuild kits appear to be robust. Plus it nice to have original spec. Means one more kick for cold starts though!


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No progress this weekend as I was down in the country.
I did find some inspiration though. After coming across the images of the bike below, I have decided that with my leading axle forks, it does look more dirt bike than cafe, so a scrambler it will be. Hopefully it will look half as good as this one by Bunker Brothers in Turkey.
I did put new carbi boots on an old TT250 I have down the farm for my son to ride. The new boots have transformed it from an mongrel to an absolute gem to start. Just shows how critical air leaks are on the induction side. Plus of course I had some real fun on the Husaberg - love that bike.
Cheers
Tim
 

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Not that productive this weekend.
Painted the barrels of the 896 black, as a contrast to the rest of the motor. Also retrieved all the calipers to strip down and clean. Went to strip out the swing arm from the frame. but I need to get a 28mm socket to undo the pivot nuts.
On the XS750 I made up a manometer to balance the carbies. Thought I had stuffed them up, but it turned out that even though there was fuel in my temporary tank, the fuel was not feeding through under a certain level!!!! Doh!!!!!!!
All good now. The manometer was made using $6 of aquarium air hose and one T piece plus some old sump oil (for colour!!). Very very sensitive to minor adjustments. As it is a closed loop system it is really just measuring the vacuum differential between each carbi at idle. 1/8th of a turn on the balance screw makes approximately a 200mm difference. As it is in the video that was the best I could get, the slightest movement of one balance screw gave a 100mm movement.
https://www.youtube.com/watch?v=n72dNNkNX0E
Cheers
Tim
 

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XS750AU said:
No progress this weekend as I was down in the country.
I did find some inspiration though. After coming across the images of the bike below, I have decided that with my leading axle forks, it does look more dirt bike than cafe, so a scrambler it will be. Hopefully it will look half as good as this one by Bunker Brothers in Turkey.
I did put new carbi boots on an old TT250 I have down the farm for my son to ride. The new boots have transformed it from an mongrel to an absolute gem to start. Just shows how critical air leaks are on the induction side. Plus of course I had some real fun on the Husaberg - love that bike.
Cheers
Tim
The leading edge forks gave me the exact same feeling. So I raised the rear end 1 1/4" with longer shocks to make the tank / frame line parallel with the ground and improve ground clearence. Mine will not be a show bike... it will see gravel roads ;)

Psy

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Psy
That was my thinking also. Do you have any photos to share?
Tim

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I have a build thread here in the Projects/trackers section but for convience here is one as she sits now... have some electrical issues to sort ( no tail light but brake light works!?!?! and the blinkers are not blinking) leaking petcock, and I should change the fork oil but I havent figured out how to remove these funky fork caps. Tank will see some black highlights of some sort and the front fender needs chopped and painted. I havent gone full resto or mod because this will see action I left the plastic section of the rear mud guard for example because it will see rain and mud, battery box is staying for now. havent decided on which knobbies to order for it yet.. there is not a lot of room in the back thanks to the narrow swing arm.


Psy
9d85fa77b0a994c3a738a10bd2d490db.jpg


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Thanks for the photo.
To remove the fork cap, you have to push the adjuster cap down 1 to 2 mm. There is a 1mm wire clip sitting in a groove that you then have to dig out with a small blade screw driver. Once the clip is out, the adjuster cap and spring will all come out the top. If you want to remove the damper rod, there is a cap screw down the bottom of the fork that you need to undo (good idea to undo before releasing spring tension).
They are pretty easy to work on. They do suck dirt and crap in, so it is worth the effort to pull them appart and clean.
Cheers
Tim

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Thanks for the info.. thats what i figured.. mine are missing the caps and are pretty corroded so i cant see the retaining clip... i just need to take some time and go at them... other stuff to do for the time bieng... ill tackle them when the new front tire comes in... gotta install my gators too ;)

Psy

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Hi Psy
One of your earlier photos really shows the change in stance with the 365mm shocks. Almost looks like a dirt bike. One point about the Thruxton shocks, even though they are longer they only have 60mm of travel. That damper tube is bloody long! Think I will go for a set of 365mm long with 90mm of travel.
Forks arrived from straightening and chroming. It costs AU $400 to get done, what does it cost where you guys are?
Cheers
Tim

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