boring carburettors

[trailietrash]

I can recall a few hop up magazine articles back in the mid to late 70's suggesting boring out a carburettor as an alternative to purchasing a new carb.

As my TS125 24mm carb has a flange mount I was wondering if it would be cheaper to bore the carb rather than have a aluminium adapter plate made to mount a 26mm spigot carb.
Looking at giving the TS a very mild port & polish, so looking for options to increase a bit of flow through the motor.

Anyone know of any old shops who still do this? Or was it found to be more of a money pit than a service and the better option is to use a bigger carb?

[MX?]

From what I've been told for my XL, it can be as simple as taking to it with a file, wet & dry or Dremel tool which surprised me at the time, but apparently the results are very good.

[trailietrash]

From what I've been told for my XL, it can be as simple as taking to it with a file, wet & dry or Dremel tool which surprised me at the time, but apparently the results are very good.

I would be happy to try to file out a carb if I had a useless spare to practice on but, unfortunately, I don't have that luxury. Early Honda XL carbs are oval in shape and probably would, with the touch of a file, increase the airflow, by rounding out the intake but again, this is something that I have not tried. Not even on my Postie  but I did remove the choke to reduce its restricting airflow 

I'm more interested in knowing if sticking a boring bar down a carb makes it worse than just mounting a bigger carb ??

[DT_Mania]

That title had me wondering if Mikunis bore you and Bings excite you or maybe the other way around.

[JohnnyO]

It is possible to bore out most round slide carby's about 2mm. It has to bored offset upwards so as not to alter the flat area where the slide bottoms out. Just make sure the slide will still seal on either side and you don't make the intake spigot too thin.

[tony27]

It's always possible, I'm sure quite a few mikuni carbs are done like that. The 36mm & 38mm have the same size bodies as do a number of others.
I have an oko flatslide carb for my trials bike that measures 28mm across the bore but 30mm high, from the factory they offset the bore towards the top after its round. Hopefully that makes sense.
Any decent turner should be able to do it for you

[suzuki27]

Johnnie is right from what I know. I had a 32mm Mikuni bored out, at an automotive head & cylinder place, to 34 and a bit. Make sure you clean out all the debri from the boring really well etc. You may need to supply more fuel/air to get the best out of it- not just slightly larger jets but,on the suzuki, a bigger fuel tap and line and some enlarging of the fuel delivery apertures in the carby.

[DR]

I would be fitting a TM125 cylinder. This allows you 24mm up to 28mm with normal spigot mounting. Stock TM cylinders aren't wildly ported, might make for a good combination. Just looking at the TS cylinder the stud spacing is tiny as is the inlet tract Always used TM bits on my TS/TM's and never really looked at the stock setup

[Lozza]

There are 20 things to do before this, unless you intend to rev the engine a lot harder the actual output (measured on a dyno) will normaly drop and usualy makes the engine peakier, as the bigger bore looses airspeed and the amount of fuel it pulls from the needle jet.
BTW nobody 'polishes' ports any more

[suzuki27]

On the RM125 I had the headed shaved to raise compression, opened up the airbox a bit and ran good fuel- this with the bored out carby did improve performance.

[whitey 43]

If you have the carb bored, dont forget to machine/grind the top of the slide or it will hang down into the bore...

[bazza]

Easy  1.5 to 2mm fine

[maicomc490t]

TT

Boring the carb is reasonably easy so long as you don't overbore, and yep, you bore up from the base.

Diameter is limited by slide width and height, not too mention the amount of meat in the carb body itself.

I can do the job but if it were me I would be looking for alternatives first - machining time could well exceed the cost of a more suitable larger bore carby (ie love to take your $$$ but I prefer to be honest and steer guys on the cheaper path if it can be done)

Dave Mac 

[DR]

Trailie, what sort of riding will it be doing? mainly street? light trail? just puttering around? If you are not retaining the lighting system then a simple way to make the engine 'more responsive' would be fit the TM125 inner rotor ignition. This will not increase your HP but it feel like it has a few variables to think of but the main ones for mine are the rider weight and it's intended use. You could make it go like TM but I don't think I'd like something as peaky in stop start traffic. Another easy way to get a little more response in remove flywheel weight by having the stock flywheel lightened. Probably not as expensive or time consuming as the carb boring as it's simply a matter of it machined then bolting it back on, no re-jetting or rooting around required I have a spare if you'd like to try that first.

[Nathan S]

DT2/DT3 Yamahas have a 26mm flange mount carby, FWIW.
Dunno about bolt spacing, but they're relatively easy to get hold of.