Vintage Dirt Bike Q & A

I'm dialing in my Keihin carb and want to know any tricks or advice that might help. I've read the general instructions about screwing out the idle from stop to 1 1/2 or so and then the air screw in then out and find the middle. I have the u clip on the second from the top ring currently.

I've got a little oil in my gas since I wanted to make sure the oil pump was working. It's working and most likely fouling my plug currently and making this adjustment difficult, but I am glad to oil the internal components heavily before using straight high octane gas and trusting the pump.

I still feel the air flow is restricted due to a heavily carbonized exhuast system which I have already cleaned along with the spark arrestor as best I can. I see some smoke near the exhaust manifold most likely because I need a new ring where the exhause connects to the exhaust manifold. Unless I can use some liquid gasket that can handle the heat there?

I read in Clymer that I can take off the spark arrestor and run high rpms to blow the carbon out "momentarily". I tried that and it seems relatively open other than one back fire and an occasional hard kickstart indicating some pressure issue.

Motor compression is 150 or so.

Air filter is new and clean, gear old fresh gas and tank cleaned and all electronics are up and running after cleaning and protecting with electronics grease.

Here's my advice:

1) Remove the tailpipe insert, clean out the openings around the spark arrestor (at the front of the tailpipe) with pipe cleaners, and drill out the center of the spark arrestor cone at least 5/8", or more if possible - unless you're OK with an exhaust outlet on a 250cc motor of less than 1/2 sq. in. (when clean), in which case leave it alone; the motor will be running as much on it's own exhaust as what it receives from the carb, but it will run. Leaving the insert out won't help at all; the restriction is in the spark arrestor.

2) Adjust the oil pump cable; it's a 5 minute procedure, you basically just align the mark on the oil pump cable arm with the mark on the pump, using the cable adjuster.

3) Start the motor, warm it up, let it idle, then turn off the petcock and wait, with your hand on the kill switch. On my buddy's bike, the motor will idle fine for a while, then takes off to over 7,000 rpm right before it dies (bad float valve), but hopefully your bike will just idle to a stop, as it should.

If the float level is off, the float valve is leaking, or the exhaust pipe is plugged, there's no point in trying to jet the carb until these problems are addressed.

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,
If I drill out the spark arrestor, is there somthing I can add to make it forest service legal?

Well, you can certainly maintain the original functioning of the spark arrestor; just weld a 'nose cone' onto the front of the tailpipe insert, on the 3-bladed center section of the tailpiece. This will direct any sparks from the newly-drilled hole in the same direction as the original spark arrestor outlets, i.e., away from the tailpipe inlet and down the length of the tailpipe, to be trapped at the far end where the tailpiece bolts are. As far as "legal"... hopefully you'll never have a ranger disassemble your exhaust to check the spark arrestor, as I'm pretty sure you'll be screwed no matter what he finds. ;>)

A better solution would be to cut off the entire tailpipe and spark arrestor and weld on a 1.25" O.D. stub for an FMF Turbine Core II universal silencer, but if you're going to that much trouble you might want to investigate pipe upgrade options as well, and get some more performance out of the motor while you're at 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

Thanks for the input Ray. My friend says he was checked by a Ranger sticking some type of rod in to his bikes exhaust pipe. Never seen that done myself.

Thanks Ray! I will drill it out. I wondered if there was supposed to be a hole in the center with the little slots around it. It does not let much of anything into the hole now. How deep can I drill without a problem?

As far as rangers inspecting my hole they would most likely be pleased because I'm guessing it gets full of carbon again real soon.

On a legal note I would ask the ranger to reference the law that allows him to remove bolts from my vehicle and perform this inspection without a search warrant or any probable cause I am violating a law. I would then tell him I will not give him consent to perfom such an invasive procedure, and I would like to leave unless I am being stopped and lawfully detained for some violation. If so I will cooperate, but will give no consent for any searches and therefore "write me a ticket or let me go." If he procedes with an illegal search the ticket will be thrown out of any court and you can sue him for violating your consitutional rights preventing illegal search and seizure. He will be afraid of this and proceed with great caution when he hears you exercising your rights. Many people are nice and think they are supposed to let police do anything they want. Police like that because it makes their job easy and once they have your verbal consent you are not protected unless you tell them to stop.

Educate yourself on search and seizure laws and protect yourself, because this is not nazi germany or stalin's russia . But it could become that way if we allow government officials to break the law they swore to uphold.

I've gotten to know these old Honda/Krizman 2-stroke spark arrestors pretty well by now (unfortunately) and the MT125/MT250/MR175 units are all basically identical; a center cone aurrounded by curved passages resembling a turbine engine. The cone is just thin sheet metal, you can drill right through it leaving a nice clean hole. I started with a 3/8" bit on my MR175's pipe, and stopped at 5/8" as it just made the pipe louder with no additional high-rpm performance gain. I imagine the MT250 pipe could be drilled to 7/8" or maybe more if the cone area still had some metal left between the new hole and the 'turbine' outlets.

Apparently the ranger will simply poke a rod in to make sure your silencer is not a straight-through type, which is a pretty good field test for a spark arrestor, actually. With this kind of test, as long as the rod hits some kind of solid barrier, a decent ranger should let you ride - of course, there are all kinds of rangers, as you know.

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