Calling all people with knowledge on XT250/SRX250 engines!

[JeremyW]

I currently have an SRX400 which I want to do up really nicely. EXCEPT I think that putting effort into doing up a bike that doesn't really have a following may be a waste of effort.

Therefore I am thinking about performing a 250cc transplant and making a sweet learner bike for someone.

BUT I know very little about Yamaha single cyl engines. So.. I think the best and easiest way would be a head, rod and piston change and keep the bottom end and gearbox the same. Does anyone know the similarities between an XT250 / SRX250 and XT400 / XT350 / SRX400 engines?

I know the SRX600 and SRX400 engines are completely different so I think things may not be that simple.

Any advice or pointers would be great.

[Ixion]

As far as I am aware (I've had SRX250 mand XT600), it's the other way round.

The SRX/Xt 400 is just a small bore XT/SRX 600 (check with Mr Motu he's the oracle, but I'm sur eon that)

The SRX250/XT250 is a completely different engine. DOHC instead of SOHC for a start. The 'big brother' of the SRX250/XT250 was the XT350. There was nver an SRX version of the 350.

Note that the XT250 we are talking of here was a rare beast, only made for a couple of years. DOHC , NOT the earlier SOHC, and NOT the later 225cc which was called an XT250.

If you could find the bits you could do a top end transplant from an XT350 onto the SRX 250, but it doesn't give you a lot mor epower (some extra torque, though), because the XT was lower tune than the SRX.

[MacD]

As far as I am aware (I've had SRX250 mand XT600), it's the other way round.

The SRX/Xt 400 is just a small bore XT/SRX 600 (check with Mr Motu he's the oracle, but I'm sur eon that)

Yes, the 400 is more or less a small bore 600, but you can't just swap the 600 cylinder/piston onto the 400 bottom end unfortunately (actually check Motu's thread linked below - he discusses this). You can supposedly fit an XT600E engine in with a bit of work.

Motu has an interesting thread about his XT400 rebuild here. I wonder how it's going?

Your profile picture shows one of the later monoshock 400s by the look of it. These a really cool little bikes and do have quite a following overseas, although not so much in NZ. IMHO it would be a shame to put a 250 in it. The SRX250 is a completely different bike. Maybe look at upgrading it to a 600?

Great SRX600/400 Site - http://www.srx600.net/

Smaller SRX400 site - http://www.srx400.co.uk/

[Motu]

Motu has an interesting thread about his I wonder how it's going?

Going great as a 621cc,a transplant engine.The 400 has a smaller bore and shorter stroke.In retrospect the 495cc engine would of worked well,for awhile anyway.The main concern was the weaker rod.Still have the 400 to rebuild,but it's well down the list.

Difference between SRX and XT is sprocket fitting (circlip on SRX,nut on XT) and aligment,and gear ratios.Clutch cover is different on XT to clear brake pedal,the SRX has rearsets.Pity,as the SRX cover is polished.The SRX has a small cult following,but would expect more to be into them as they are a great bike and better at what they do than Honda's GB....but the GB has the classic retro look I guess.

[FROSTY:Me]

An interesting conversion would be the srx400 roller with a SR250 engine

[MacD]

Going great as a 621cc,a transplant engine.

I occasionally look at my SRX400 and wonder about fitting a 600 into it. The 400 is a great bike around town, but could do with a few more HP for open road travel (well compared to the SRX600 I used to own). Maybe I should just look at a DR650 and keep the SRX standard.

[Motu]

In retrospect,after using 4 different engine sizes in one bike - the 400 is a sweet little unit.The 600 shakes,the 400 has practically no vibration in comparison,and the bigger capacities effect the bike more - more surge on the throttle,more lunge forward with shut throttle...gear changes and corner entries require some thought and skill.I could ride the 400 so much harder,just row it up and down through the gears,it was hard to upset the bike and I could just concentrate on nailing all the aspects of cornering.Now I have to choose the correct gear,and make the change at the right time using appropriate clutch and throttle application.Point to point,particularly on gravel,I don't think there would be much between the bikes on time.But hills,passing and head winds - the 400 sucks.

[JeremyW]

Well I have decided to keep the 400 as it is a good motor (only done 18000K!) and with the proposed rule changes it could be a lot of work for nothing.

I have decided to do a single carb conversion and make a free flowing exhaust.

Parts:
Manifold off a yfm660 grizzly (fits perfectly!)
Carb off a GB400 which I inherited

They match together perfectly which was a nice surprise.

Decided to take the airbox out as I wont be needing it and it was in the way. WHAT A MISSION! Starter motor off.. no joy..oil line and side cover off... no joy...lever, hit, push, pull... no joy...drop the engine...joy!! got the ugly weird shaped restrictive piece of crap out!

Now for reassembly and get the exhaust made. Getting excited!!