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Yamaha SR250 carb rebuild, maintenance, tuning

May 17, 2018

This post is meant to complement a video we clipped together explaining the function, details, pros and cons, service, maintenance and repair of the SR250 carburettor, which is a Mikuni BS34 CV carb.  Some issues were a little difficult to cover in the video so I hope to capture the remaining ones here.

 

Manual scans.

I figured it would be a good resource to all to have access to the chapter in the workshop manual about the carb.  So here you go! (open in a new tab or download).

 

 

 

 

 

Stubborn screws and jets.

If there is one thing you should do before working on a carb, it is to get the right tools.  More accurately put, get the right screwdriver or bit for your driver that matches the screw (or jet) head perfectly. Do not use some shitty old crappy tool or one that is too small or too large, you will fuck the screws or the jet heads and make the whole job a nightmare.  Mentioned in a couple of the Jadus youtube videos is the use of JIS (Japanese Industry Standard) screws on Japanese motorcycles.  These are slightly different to standard Philips screws and it really helps if you order yourself a JIS screw driver set or bit set.  Check out the difference below:

 

 

The clear difference between JIS and Philips drivers

 

The next important thing to mention is how much force is required sometimes to remove these bastards! Not just rotational force either, rather pressure force into the screw and jet head as well.  This does not help remove the item, rather it prevents the tool from torquing out of the detail and damaging it. Brass and aluminum are soft metals after all. Last, patience, if something is stuck and you feel it is not budging, take a breath and step back, then do the following…

 

Apply both heat (preferably with a heat gun) and penetrating oil (5-56 or WD40 works as well). This will hopefully break up or at least loosen any corrosive bond the two subjects have formed. If this doesn’t work, try an impact driver. If the screw or jet head is stuffed, try getting a hold of it with a pair of big ass poligrips, this works a lot of the time.  If none of that works, try modifying the screw/jet head with a dremel style tool - to recreate a larger and/or deeper slot for a flat head screwdriver. 

 

Good old polygrips on the screw head!

 

A lot of the time, you will inherited a carb that has been worked on by a ham handed mechanic, the type with no feeling for the art whatsoever.  Anywho, sometimes the screw heads and jets will already be right and proper fucked. In this case, the aforementioned trick with poligrips or head modification with a dremel might work. 

 

Rotary power tool with disc cutter bit.

 

Make a nice big slot in the place of the stuffed head detail - now it is a flat head screw!

 

One trick to remove a stuffed pilot jet is to drill a small hole into it, then find a torx head driver that is just slightly larger than the drill bit you just used.  Then pound in that screw driver into the head of the pilot jet. Once in there nice and snug, apply a lot of heat (preferably locally) to the carb and spray with oil. Do this a couple of times - hopefully a couple of heat expansion cycles will break the corrosive bond.  Then try turning out the pilot jet. Obviously you destroy the jet, but this is a cheap sacrifice for saving the carb.

 

Bash that torx driver into the pilot jet, apply heat and penetrating oil, turn out.

 

 

The soft brass takes on the form of the star shape torx drive from the force of the blow, then you have a nice grip on it to turn it out (with help of heat and oil).

 

Float height.

You will see the method below suggested in the manual.  This is a really easy and clean method for checking actually, but it only works if the carb is on the bike or the float bowl full with fuel! We have measured the responding height the float bowl should be, according to the more conventional way of measuring - see below.  It should be 23mm from the gaskets surface to the highest point of the float when the float needle valve is seated and the internal compression spring is extended (know what I mean?).

 

 

 Ruler to gasket surface.

23mm to top of float.

 

Diaphragm issues.

Proper operation of the diaphragm is paramount to clean and smooth engine running.  The smallest of pinholes can mess everything up and have you wasting your time with jetting in the carb.  Make sure to inspect with a close eye and the diaphragm either held up to the sun or a backing light and inspect the entire surface area of it.  Sometimes a hole will be very obvious, other times it really will be a pin hole. You can try and repair pin holes with silicone or something, I am sure some people have succeeded with this for a while, however I have never done it myself and think it is a less than optimal solution.  Why be a cheap bastard with something so critical and crucial as the operation of the carb? Entire slide assemblies are still available from Yamaha and this is a great option. However they are a little pricey. We decided to test a few different aftermarket solutions and found the best option is to trim out the old rubber part of the slide and replace it with a new one - which is made entirely of rubber therefore not requiring any of the old part.  These work just as good as the stock ones if not better. We did back to back tests on the dyno with stock and aftermarket and got exactly the same results. We now offer these replacement diaphragms here and I will make a replacement video for this part in due time.

 

Obvious diaphragm problems.

 

Not so obvious diaphragm problems.

 

Jetting.

The stock jetting for the SR250 to our knowledge is the same the world over (please correct us if we are wrong) which really says something about how adaptable and versatile this CV carb really is - see pros and cons in the video.  We created a table below to illustrate this and other suggested jetting settings for various setups. (right click and download or open in new tab).

 

SR250 Carb Jetting Settings Guide.

 

Finally, if we have missed anything or you want clarification on something, feel free to send us an email or post up a comment on the youtube video and we’ll try help you out!  Cheers.

 

Carb parts we sell:  Diaphragms, Carb refurbish kit


 

 

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