big block XS250 (400) Turbocharged


XS400 Addict
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Perth, Western Australia
Gday guys, I hail from Western Australia. In my state we have a stupid law that when you get a motorcycle licence you can only obtain one for a 250 for the first 12 months then go for another test to get your big bike licence.

I have other bikes (70 Commando, 71 Benelli Tornado and XS650 project) that I cant legally ride.
I recently found the penalty for riding out of class is loss of licence and Ill loose my new job if I cant drive so Ive finally got a 250 licence, put the other bikes under tarps and now need to wait............

So I brought an XS250.............. but then that wasnt going to do so I brought a 400 barrel from Ebay in the US
Then I found out the 400 head was different so I tracked down a complete 400 top end (which are rare in Australia, XS250s are a dime a dozen)

Im now putting it all together but I also brought this in a moment of madness :)




Ive started this post to pool info on installing the turbo and would appreciate any suggestions or ideas you guys may have

This is only supposed to be a side project so Im doing it as cheaply as I can, also my new job is working for comission and I havnt been paid sufficiently for 12 months so buying fancy stuff off the shelf is out, thats why after a fair bit of research I brought this particular turbo (I think it was $140 landed here in Australia)

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I brought the turbo a couple of years ago when I first brought an XS250 but then moved away and was making more money so I went on to other projects, now Ive come full circle.

The turbo is off a 600cc ForTwo Smart Car.
It has an internal wastegate but the exhaust housing is part of the cars exhaust manifold so Ill have to do a little cut and shut with it.
Ill post some more photos of the turbo later

From what I read the turbos dont like draw through and many 4cyl bike guys are blowing through CV carbs and raving of the useability so thats how Ill go Carburetor R1.html

Ive hit a wall now with what carbs to use?

I need to get the bike registered first (as a 250 ;-) ) then can play around with the turbo but dont want to buy another set of carbs.

I have a stripped set of 250 carbs, complete set of 250s, set of 34s of a 650 and 38s off a 650.
The 34mm, 650 sets are spaced too wide and I thought I could use parts off the 250 carbs to shrink the spacing but after stripping them the throttle shafts I have wont work.
The wreckers want $160 for unknown condition carbs
Will the 250 carbs flow enough? I wouldnt imagine It would be ideal and take full advantage of going to 400
Also There appears to be a lot of passages I will have to block and re route to boost up the carbswards after so possibly I should start with another carb to begin with......... and If I do that why not the R! carb the guys have explained the DIY on in that link????

Any Ideas/ thoughts/ experience??
G'day Icgtr. The laws are a bit different over here in nsw. The fuzz would have a field day with an L-plater on a turbo bike but no issue with an xs400.
I am in the middle on a jugs transplant on an xs250. Most difficult part so far was lowering the compression ratio. Jets may need a bit of fiddling too.
I would be interested to see how a vacum carb deals with forced induction.

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Ive been reading up as much as I can and VM carbs seem to be the wat too go from all accounts and the standard carbs should work well as all the vents are in the inlet tract so I dont need to plumb anything up either except I may block the vents for the float bowl and do the Pitot tube mod.
Ill also modify the fuel delivery passage to get more fuel to flow faster into the carbs as I dont want to use a fuel pump if I can help it, especially since you need to set the reg at 1 PSI, barley seems worth it.

Ill see if I can con a mate into machining the shaft on the XS650 carb so I can use them
my turbo


its a Garrett GT Turbocharger GT1238S

Turbo Model: GT1238S
Turbo OE No.: 724961-5002S
Application:Smart 600 Benzina 0.6L M160
Certified by ISO/TS16949:2002

Turbo Model: GT1238S
Turbo OE No.: 724961-5002S
Vehicle OE No.: A1600960199/299/399/499/599/699
Fitted To Model: Smart
Fitted To Engine: M160
Fitted To Make: MCC part

VSR Balance Calibration
<0.65G Max 0 - 90,000rpm / <2.00G Max 90,000 - 168,000rpm

Replaced by
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Ive also been looking into Replacement pistons but havnt come up with out of the box to use in an overbore. I think its the 16mm pin that is the problem

The best bet so far is either aftermarket big bore pistons from a LT230 or a GS850 pictured below


LT230 wiseco VS GS850 piston

The other makes Ive found with similar bore sizes and 16mm pins are:


Unconfirmed but around 70mm in bore:


Famous Yoshimure big bore kit once featured on Ebay Germany


Head that came with that kit with modified Combustion chambers


I heard ther were GSXR750 pistons and I managed to get hold of one out of an early 90s bike
Below is one pictured next to a NOS XS400 0.50 piston




Idealy I would like to use this type of modern piston design but it has a 18mm pin and will require machining
just done a search.

crazyPJ said;

I have fitted 1mm OS GS500 liners and pistons (75mm) into XS400 for a friend.
You can use sohc or dohc XS400 as pistons are the same (2 valves/cyl)
I'm pretty sure the stock liners are too thin to bore more than 2.5mm OS, you need to fit new ones.

this is on a dohc though

I would think it would cause some issues with the head working right as well as cooling that much more displacement:shrug: More power, get bigger bike:laugh: I don't think the stock clutch would hold a lot more power anyhow.
its only 74cc extra.

bigger carbs, gas flowed head should be ok!

clutch can be upgraded, stronger springs etc
For the time and money or when I see someone with a 70hp xs400 I would just get a bigger bike:laugh:
that's what the majority would do, then there's the dreamers.....

Right on Drewpy!!!
Some of us get off on making the most power from any given motor. It's the ingenuity and improvement more than the final hp/torque number that makes it fun and worthwhile.

And in many locales, it beats out the insurance premium due to small displacement!