Removing and replacing a cylinder head on a Yamaha TTR250

DISCLAIMER: The modifications described in the following text are for educational purposes only. In no way do I recommend that you apply these modifications to your own motorcycle. If you do choose to go ahead and modify your motorcycle based on the information in this document you will accept all responsibility for your own actions. The author(s) of this document, and host(s) providing it for you, accept no responsibility whatsoever. If you are unqualified to make any of the changes described herein but are bent on doing the modification, seek out a knowledgeable friend or professional mechanic for assistance.

Repairing a broken float pin post on a TTR250 carburettor

It is an unfortunate fact of TTR250 life that a float pin support post can be broken off when removing a stubborn pin no matter how careful you are.

This step-by-step guide takes you through my recommended repair from start to finish. I hope you find it useful! Thanks to Ron on the TTR250 forum for the idea.

Tools needed: 4mm tap, 2mm and 3mm drill bits, centre punch, hammer, small round file and, ideally, a Dremel with cutting disc and the use of a pillar drill.

Part needed to use as a replacement post: M4 x 10mm steel winged thumbscrew - available from eBay and elsewhere costing just over 2 for a pack of ten.

This was my first attempt and I have been putting off doing it for ages as there is potential to trash the carb body! But be reassured, it was far easier than I thought and quite a quick job.

I was probably being over cautious and did some test drilling, etc., before tackling the actual job.

Firstly I test drilled a 3mm hole in some scrap aluminium and tapped it out to 4mm (a) to gauge the depth I would have to drill the float post and (b) to make sure that I drilled a hole that would take the tap. Ideally you should use a 3.3mm drill for a 4mm thread but I doubt many non-professionals would have that size one in their toolkit.


I then checked the thumbscrew fitted and checked the depth I needed to drill the post.


Put some tape on both the 2mm and 4mm drill bits at the depth you require.


Centre punch the pin post stub on the carburettor body.


Because of the need for accuracy in drilling, I used a pillar drill and a 2mm drill to make a pilot hole.


If the pilot hole is OK, follow up with the 3mm drill. You should end up with something like this - the hole ready to run the tap down.


Run the tap down trying to keep it at right angles to the carb body. Do the usual i.e. do a part turn in and then remove the tap to clear the threads before going back in again.


Test fit the thumbscrew and estimate how much it needs to be shortened by. You really want the hole you later drill for the pin as near central to the "ear" of the thumbscrew as possible.


The thumbscrew is awkward to hold firmly to do work on. I lock nutted it so that I could hold it in the vice to shorten it with a cutting disc on the Dremel. The added advantage of having the nuts on it is that the help clean the cut thread when you remove them. Nevertheless, you will probably still have to dress the end of the threads so that it easily screws into the post.


Fit the thumbscrew and use the float pin to work out where to drill the hole and then centre punch it to make the drilling more accurate.


My float pin was 2.27mm in diameter and I only had 2mm and 2.5mm drills so I drilled some test holes with them to see which was best to use. The 2.5mm hole was too sloppy so I used the 2mm drill knowing that I would have to enlarge the hole somehow,


Holding the thumbscrew for drilling took a bit of trial and error. This is what I ended up doing using a pillar drill clamp.


I found a burr to fit the Dremel to do some initial enlargement of the hole and finished off with a small round file. It is important that you don't go too far as you need the float pin to be a snug fit and not loose in the hole.


Test fit the thumbscrew and pin.


After successfully test fitting the pin, I ground the sides and top off the thumbscrew's "ears" to within about 2mm of the hole so that it will clear the float bowl and the float itself can fully pivot.



Float fitted and all looking good!

Don't stress if your pin is not 100% level as this is quite difficult to achieve using this method - unless of course you are a master machinist with access to tools not found in your ordinary garage! The copper float hinge has quite a bit of movement on the pin so the float will find its own level.

Not such a difficult job after all - thank goodness.


Job done!

Brian Sussex

Compiled by Brian Sussex, Devon, UK - all you ever wanted to know about TTR250s - the forum for TTR250 owners - everything you need (possibly!) for your TTR250 -
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