Vintage Dirt Bike Q & A

Hey all. I posted earlier about a found MR250. I have gone over a lot of it and am putting it slowly back together. I have taken apart the carb (keihin) and I have cleaned it out and I had to get some replacement jets for it just to get it back to what I think is original. Well today I hooked it all up and turned on the gas petcock and after the bowl filled it just drained out the overflow. I fiddled around with the float height but I could not get it to stop draining. I followed the original manuals instructions on holding the carb vertical and setting the floats to 20mm. Well vertical threw me for a loop, I always thought you had the carb upside down to set the float correctly? Anyway I monkied around with it but could not get it to stop. So started to think that the main float and valve are worn out and do not top the flow. Am I on the right track or is there somewhere els I should be looking? I believe the main float valve and plunger are original. Any help would be appreciated. Thanks!!

The procedure on page 42 of the MR250 shop manual is correct. They recommend keeping the carb bore vertical as this allows the float to hang on its hinges and put no pressure on the float valve spring, making for an accurate float height reading.

The carb kit below should have the correct float valve, valve seat, and O-rings for your carb. The jets will be too big, and I don't know about the air screw or needle.

http://cgi.ebay.com/ebaymotors/Honda-CR ... ccessories

The OEM jetting for your carb is 110 main, 140 power jet, 48 pilot, 200 air, 2.5 throttle valve, and 271006 needle. Unless you live inside the arctic circle, this jetting will probably be too rich above half throttle with a new carb, and too rich everywhere if the needle jet is worn.

This is a very unusual carb for 1976 and will probably be somewhat difficult to rebuild and tune due to the power-jet design and limited parts availability, so keep that in mind. It can be made to work well if you're willing to make the effort, but replacement is also an option.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray Thanks for the reply. So let me see if I can explain the trouble I was having with the float. When I would hold the carb body vertical in my left hand the float will pivot away from the plunger towards the right. So either the float has been bent way out of whack over the last 30+ yrs or you need to push it back towards the plunger in order to get an accurate measurement. Is it possible to have the float barely touch the spring loaded plunger tip while freely hanging on its pivot? If so it feels like I will really have to bend those floats quite a bit. I did cob up a gauge for accurate measurement, but like I wrote last night I still could not stop the flow of fuel. I even rigged up the carb without the float bowl and tried to shut down the float plunger manually, still no luck. So am I correct in assuming that the float plunger and the valve seat are not creating a seal and letting fuel flow by? Thanks for your patience. Oh yeah this bike lives @ 5000-6000ft

Ray Thanks for the reply. So let me see if I can explain the trouble I was having with the float. When I would hold the carb body vertical in my left hand the float will pivot away from the plunger towards the right. So either the float has been bent way out of whack over the last 30+ yrs or you need to push it back towards the plunger in order to get an accurate measurement. Is it possible to have the float barely touch the spring loaded plunger tip while freely hanging on its pivot? If so it feels like I will really have to bend those floats quite a bit. I did cob up a gauge for accurate measurement, but like I wrote last night I still could not stop the flow of fuel. I even rigged up the carb without the float bowl and tried to shut down the float plunger manually, still no luck. So am I correct in assuming that the float plunger and the valve seat are not creating a seal and letting fuel flow by? Thanks for your patience. Oh yeah this bike lives @ 5000-6000ft

When setting the float height I put my mouth on the fuel inlet where the fuel line connects and blow gently. Hold the carb vertical and as you swing the float back and forth you can feel the valve open and close with your breath. Bend the float tang until the valve closes at the right spot. If the valve is leaking you won't feel your breath stop. (Blow gently though)
Also maybe put your floats in a dish of gas to be sure that they actually float.

Yes - this is right where you want the float to be when measuring float height (notice I added the word 'tang'). This will almost certainly NOT happen with the carb body exactly "vertical", which is only a rough guideline word provided by a Japanese shop manual writer with limited English proficiency.

If you can't stop the flow of fuel no matter what you do, there's a problem in the float valve area for sure - it could be a leaky valve, leaky O-ring, cracked carb body, loose hold-down clamp, damage to the O-ring groove or seating surface, etc. At this point, if you're sure you'll be keeping the carb I would go ahead and renew the entire float valve & O-ring, checking carefully for other the problems mentioned above while it's out of the carb. Once the new valve is determined to be leak-free and working well, then go ahead and set the float height.


The stock jetting will be rich for sure, then.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric