Vintage Dirt Bike Q & A

Good morning, I finally got my 1974 MT125 Elsinore started. It has probably been 30 years since it has been started and has less than 1200 miles on the bike. When I did get it going oil or something drips out of the exhaust pipe right at the head. It sprays and drips and the oil is black. I changed the oil in the transmission but only filled the 2 stroke oil reservoir.

I noticed on the parts diagram there is a gasket that goes there, part number 18352-361-000 but it is not available anymore. I haven't taken mine apart to check the gasket but that's what I am assuming is wrong.

Does anyone one know a good way to make a gasket or where to buy an aftermarket gasket or could it be something more serious I need to worry about?

Thanks for the help,

Keith

Keith,

First off, your leakage may be partially due to a clogged pipe. The MT125 pipe is super-restrictive even when new, and clogs up real quick (I'm using a gutted one on my MR175, I've seen it inside). If it were me, I'd connect a shop vac to the input flange and see what happens; you may get lucky and it will flow sort of OK afterwards. If you have any doubts, pop off the cleaning cover on the bottom of the pipe just ahead of the stinger - if the motor runs stronger (it will definitely be way louder), the spark arrestor's probably clogged. If the pipe's plugged up, the following repairs will only allow the oil to stay in the pipe instead of draining out.

The MT125 and MR175 share the same exhaust manifold, so I've had to become familiar with that too. By today's standards it's horrible, but it was actually fairly good for the 70's.

The 18352-361-000 gasket is a semi-soft (asbestos?) ring seal that fits into a recess on the forward edge of the exhaust manifold, and seats against the pipe/flange joint about 1.5" into the pipe end. As this area usually has weld bead and other crap, sealing is generally poor unless the joint area is ground down smooth, which is what I do.

There's also a piston-ring type secondary seal on the manifold, which works poorly at first but eventually clogs up with carbon enough to seal OK, usually just about the time the pipe needs to be removed. There's also the copper manifold/cylinder gasket, which works fine if a new gasket and hi-temp sealant are used.

Look inside the pipe end and see if the ring gasket is still inside; usually it is, as it carbon-welds to the pipe pretty quickly. If it looks good, I'd just leave it for now. If you clean everything up well - pipe flange, manifold, piston ring, etc. - you can try reassembling everything with hi-temp sealant and strong springs (the OEM ones are OK when new but pretty expensive; the 83mm Helix stainless exhaust springs 53-0633 are a lot stronger, although one of mine came apart).

If this leaks, there's a Kawasaki O-ring (P/N 671E2535) that replaces the piston ring and will seal very well. It's a VERY tight fit, and you'll have to clean, lube, and heat everything to get the pipe on.

Probably too much info - sorry.

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,
You can never give me too much information, thank you for the help. I will take a look and let you know what's going on.

Keith

I finally after all other attempts to clear my clogged mt 125 pipe , cut the back open of the pipe . Wow what a design honda had ! I had a mouse nest on top of a lot of carbon in the spark arrester .the pipe has three sections from the front is the chamber area , with a baffle with tube separator . Second area is muffler it has muffler packing held on with heavy screen , separated with a baffle with tube , tube is spark arrester it has a turbine type baffle in it. That is where mine was clogged . It is the tube you see when you remove cover ! I have pics if interested Dave .

Yes, it's a real marvel of engineering (not). At least it's quiet, I guess.

My advice: cut it, gut it, hold on tight. The hollowed-out MT125 pipe on my MR makes huge power everywhere, and should do the same on the MT125 motor.

The top section in the photo is from another pipe, showing what used to be inside.

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