Increasing bore on XV125

[The Bellandaine]

Hi folks,

I’m looking for some advice on increasing an XV125 to a 250cc

My plan is to swap the cylinders and pistons to 49mm bore set. The rest of the bike is pretty much the same as the 250 model so I don’t foresee any big issues. 
 

Question: Will I also need to change the con rods? Or will the existing 125 con rods work?

Are there any other factors I should be aware of?

Any advice or guidance is greatly appreciated.

Thanks,

Philip

 

[The Bellandaine]

Any takers?

All advice gladly accepted 

Philip

[neversaydie]

If successful, thatll give a good gain in HP, but its a fair bit of work to do, and theres always the risk of failure. Youd need to notify insurance, DVLA.  Id have alook at the CMSNL website and compare the parts numbers for the two models before starting, just to check compatability

[BrianOfDenmark]

I am planning to do this operation on my daughter's xv125.

Cylinders, heads and another main jet. That's what I am expecting.

And perhaps the tci.

 

[tomXV]

Hi first post.

Been giving this some thought and I think yes and it will give 180cc.

The con rods are 1cm different in length, the 125 is 1cm longer. But the cranks have different placings with the 250cc about 1cm further out giving a longer stroke.  Essentially by the looks of it you would get proper compression of fuel when fitting a 250cc head to a 125cc but with a shorter stroke so reducing the cc you get. 

Based on the kits that used to be sold the piston shape and 49mm bore looks identical to the 250cc cylinder and piston and those kits where 180cc. 180cc because you have a shorter stroke with the 125 crank set up.

Here is a 250cc cylinder kit from China that looks identical to the old 180cc big bore kits you used to be able to buy. So much cheaper than the old kits, but quality unknown. 

https://www.aliexpress.com/item/32762639325.html?spm=a2g0o.productlist.0.0.525713c42Rj4gz&algo_pvid=bf04f88d-4f15-4ffc-a08a-daafbdbcdc63&algo_exp_id=bf04f88d-4f15-4ffc-a08a-daafbdbcdc63-11

You would of course have to rejet the carb.  I think the 125cc has 12.5 pilot jet and 77.5 main jet.  The 250cc I think has 17.5 pilot jet and 110 main jet. So go for somewhere in the middle of those sizes as a guide. The actual carbs are the same on both 125 and 250 but with jet size differences.  There is an air box mod you can do that would help get a bit more power too.

Also check your CDI (actually a TCI) and see what model it is. You want the TDNF45 as it is unrestricted 66mph. The TDNF44 is 50mph and the TDNF43 is 57mph.

I have just got my hands on a XV125 1997 (TDNF45) with 6,700 miles on the clock and might go this direction myself. Garaged for 20 years now needs new intake and chain for MOT.  But with a taped up intake (cracked) and a service it is sounding lovely. Has the original Cheng Shin tyres.

[tomXV]

I really want to go for this 180cc conversion but have a little concern over the piston from a 250cc. The piston for the 125cc is 12.7mm from top of pinion to top of piston, the 250cc is 12.4mm from top of pinion to the top of the piston. So I  have been doing some searching on the net for companies that might still have big bore kits available for XV125.  There are not kits as such anymore but there are still companies that can swap out your cylinder sleeves and provide pistons and gaskets etc. Downside, apart from costs, you would have to dismantle your engine and send off the cylinders to be altered. Perhaps something to be done during winter.

http://www.kit-moteurs.com - These guys will do a 180cc conversion, I have emailed them for a quote as to cost.

www.up-power.eu - These guys will do a 170cc conversion and costs 459 euros (about £400). Up-Power also have a carb tuning kit that according to their data will up the HP to about 14 from 10, but you would have to follow the instructioins provided with the kit and alter your airbox. Cost of carb tuning kit is 69.99 euros (about £60) and might mean enough of a  power boost that you do not have to convert the cylinders.

www.piecemotoquad.fr - These guys still stock the BIHR pistons from the old 180cc kits.  They are 49mm so should fit a Chinese XV250cc cylinder. At only 25.22 euros each (about £22) they might be my easy solution when matched with a chinese cylinder set.

The big advantage to having your existing cylinders converted with a new sleeves is that the external markings will still say 125cc not 250cc.  Some MOT stations and all insurers would not be overly happy with 250cc written on the cylinders.