Vintage Dirt Bike Q & A

I need some help identifying a part. I am reassembling a Kehin Carb for an MR175 that was take apart earlier this summer. From it I have a small brass collar, about 2-3 mm tall by 2-3 mm in diameter. I cannot identify where it belongs.

Please help.

The air screw needle seat sounds similar to your collar. If you look into the empty air screw hole, you should see the threaded portion followed by a shoulder in the aluminum body, then a small brass collar way down inside. These are normally pressed in, but I've had one or two come loose during the cleaning process.

If the collar drops back in you might try a very small amount of epoxy around the outside to keep it in place.

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

Thanks Ray, I think that was it. Actually , I checked my other carbs, and none of them have the collar. I put it in one that wouldn't idle. Not it idles. I think I may have found the cause of all my mr's running rich on the low end.

Any idea where I can get 3 or 4 more collars?

A hobby or model-airplane shop should have small brass / aluminum tubing. If you can find something close on both I.D. and O.D. and cut / sand it flat on both ends - or at least the end towards the air screw - the spring should hold it in place and it will probably make a big improvement. You should be able to find something online that's an exact fit, which is probably what I'd do. There's no Honda P/N for this thing as it's supposed to stay in the carb body; you might ask an eBay seller that parts out carbs if they have any for sale, from MR175 / MT250 / FL250 / early CR125M (and possibly MT125) carbs.

I think these carbs will run super lean at idle without the collar, as tons of air will be sucked in from the idle air inlet in the carb mouth with nothing to stop it.

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

A hobby shop was the answer. I tried a bearing shop and they could not help. I had to use a hacksaw and jeweler's files.

It worked great on the magneto ignition bike. I now have to go back and reset timing, jets, needle position, etc. on the CDI ignition MR's.

Thanks again.

I think I was wrong on the location of that collar. I just pulled apart a '76 MR175 carb, and a small brass collar fell out of the idle speed screw hole. Both my previous carbs were missing this collar, and both had poor idle characteristics (either too low / stalling out, or too high / racing). Sometimes just wiggling the screw changed the idle, which seemed like an air leak to me. I think this collar minimizes air leakage into the slide tunnel through the screw threads, for a steadier idle speed.

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

I looked ans saw a sleeve in the idle screw of two carbs, but I also put the sleeves in the air screw. Funny thing is, it helped when I put it in the air screw. Could both use a sleeve? I don't see it on the diagrams for either.

I didn't see it in the diagrams either - you'd think the replacement screw set would show it. I don't think putting the speed-screw sleeve in the air screw hole would hurt anything, as it's quite a bit bigger than the air screw - it might change the air flow down near the tip of the screw though, and in your case that improved things so that's great. The sleeve fits the speed screw quite closely though, so it definitely has an effect there. It might be more of a support bushing than an air seal, although maybe it does both. If putting it back in evens out the idle I'll be real happy.

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