I buy and collect MTD minibikes. 11/03/20. Email: firstname.lastname@example.org
Overall MTD History and Info.
The MTD "Trail Bike" models 361-100 (2.5hp), 361-150 (3hp), and 363 and 364.
Probably MTD's most conventional minibike design.
The 361-100 is their weakest design with no supension and a 2.5 hp Tecumseh motor. Note
in different catalogs the model number changed (see below).
From the 1971 catalog, the model numbers seemed to have changed from 363 to 361.
The MTD "Trail Bike" or "Trail Flite" model 540-100 or 361-200 (3.5hp) and 361-250 (4hp).
I believe that MTD used this "trail flite" monkier for a lot of their bikes, as they evolved.
A really bad picture of a 1970 MTD catalog.
It shows the three models MTD offered. From what I can tell from this
incredibly crappy photo, is the model 540-520 (left) had full suspension
and a 4hp Tecumseh motor. The 540-250 (middle) had a 3.5hp Tecumseh motor and
just front suspension. And the model 540-100 (right) had a 2.5hp Tecumseh motor
and no suspension
MTD (also marketed as Columbia) made basically three different models of mini bikes. Their catalogs indicate six (or more) models, but really it boils down to three models. The bikes with the leading link front suspension, the ones with (kind of) suspension, and the one with no suspension! I can hear your question already... "what is a leading link front suspension???" We'll get to that in a minute. Note nearly all the models with suspension had the nickname "Trail Flite", so really that doesn't help differentiate the models.
MTD page from the Western Auto catalog. Judging from the rounded engine blower housings
and the removable handlebars,
this is probably from 1971. Also notice the silver engine color on the $264 model.
The MTD/Columbia mini bikes models. My guess is this ad is from 1975. Notice the
squared black engine blower housings on the 603 and 703 models. Also notice the different
gas tank styles.
The MTD 'Trail Bike' minibike.
This model was entry level with a 2.5hp Tecumseh motor and no suspension. There's not much to it, the basic of basic minibikes. It used a motor-mounted gas tank, unlike all the other models that had a separate discrete black plastic gas tank. MTD used several different model number for this bike including 540-100, 363-100, 361-100 (2.5hp) and 361-150 (3hp).
A lot of people call these MTD models "ugly". I guess it sort of is, but it does have its charm. Comes with 6" wheels and a separate black plastic gas tank. The front suspension is really next to nothing (as far as front suspensions go.) But it does have a nice 20 degree angled gray Tecumseh HS40 or H35 engine. Also uses a V-plex symmetrical torque converter (like a Comet series 20) with a jackshaft and a nifty "thin" TC cover. The Tecumseh has an angled intake manifold to accomodate the float bowl carb. The rear brake is an interesting item. Imagine a drum brake housing, but instead of an internal drum brake, it uses an external band brake (like a clutch brake.) It works well but not a common thing, as far as mini bikes go. The exhaust is a nice Taylor muffler (a standard mini bike muffler), but with an added two foot chrome extension (which bolts over the edge of the Taylor), to give it a nice custom look! Also these models uses a center stand style kick-stand, which again is a very nice touch.
Note after 1970 MTD changed from the V-plex torque converter to a Fairbank Morse torque converter. This demanded a changed in the torque converter cover to the thicker (deeper) version (4.5" deep, versus 3" deep.) The thinner cover design doesn't fit anything but the V-plex torque converter because of the flat shape of the driver. The thin cover can be used on the Fairbank Morse torque converter, but the mounting brackets must be modified.
The 1971 MTD "ugly" models
Note at some point MTD changed the prefix on their models from 361 to 368. I don't know when/why this happened, but it did.
A step up on the MTD line, model 361-300 (3.5hp).
Note rear suspension (kind of) and a rear chrome hold bar and chrome fenders. Also this
bike has the "thick" clutch cover and a nicer seat.
A refined MTD "ugly" model 361-520 (1971 catalog model) with 4hp Tecumseh, and sold as the Wester Flyer SS400,
with springer front suspension. Has the thicker (deeper) clutch cover.
MTD "ugly" model torque convert set up. It's a series20 symmetrical style (inward moving driven.)
Leading Link MTD minibike models 6xx and 7xx.
The MTD leading link ("Earls") suspension on a model 753.
Another change that happened was an increase in length to the front fork
U-bracket. This was probably done to allow the usage of a 12" front wheel
on the leading link front suspension models. But interestingly, even with
the short U-bracket, a 12" front wheel fits fine. Bottom U-bracket is the
newer, longer style in the picture below.
There's a bunch of other nice features about the 6xx and 7xx bikes that make them quite nice riding mini bikes. The wheels are spoked 10" diameter with trial tires. They have full rear suspension with a swing arm and a jackshaft. And again the leading link full front suspension. They also use an assymetrical torque converter (made by Fairbank Morse, but is the equivalent to a Comet 30 series.) This is an outward moving assymetrical driven system (unlike the inward symmetrial driven used on the "ugly" models.) Also like the lower model, a center stand kick-stand is used along with a great custom chrome exhaust. The Tecumseh engine is 20 degree mounted with a slanted intake manifold and float bowl carb. The seat is a snap variety (actually riveted), like Rupp was doing too. The torque converter cover was the same style as the later entry level "ugly" models (the deep version). The plastic gas tank was the same too as the later "ugly" models with rounded edges (though the Columbia models seemed to have a different red gas tank, though I have never actually seen that tank used on any MTD mini bike).
So what is the difference between a 6xx and 7xx MTD bikes? It's not a huge difference, but basically the handle bars and motor, and later, the front wheel. The 6xx and 361-6x0 series uses non-adjustable chrome handle bars with a more street bend, with a knob on each side of the triple tree (which allows easy handlebar removal.) It also had a Tecumseh HS40 four horsepower engine. The 7xx models use motorcross style handlebars (with a cross member) which are fully adjustable. And the 1972 and later 7xx models had a Tecumseh HS50 five horsepower engine. That's the main difference between the two models. But when the leading link model was introduced about 1971, all flavors had 10" wheels and a silver 4hp Tecumseh HS40 engine. In 1972 when Tecumseh introduced the HS50 engine (5hp), this became an option for these MTD models, and these had a 12" front wheel and were usually yellow bikes. Also the motor color (usually) changed from silver to black with the move from 4hp to 5hp.
Another restored MTD/Columbia model 753 with 10" front wheel. Custom exhaust,
and the carb is a slide variety (for better performance.)
Around 1974-ish MTD came out with a copy of the Fox "Desert Fox" model with red plastic flarings (opposed to Fox's yellow flaring). This was model 368-850c and it used a two cycle Tecumseh AH817 motor and a torque converter. The motor mount was an aluminum adaptor, because the frame's motor plate was drilled for a Tecumseh footprint. Used a 16" front wheel and a 14" rear wheel.
Restored model 368-850c, but the torque convert and flaring decals are not quite correct.
Bottom line, all the leading link MTD minibikes are really quite nice to ride. Particularly the 703-2 and 753-2 MTD mikebike models. They look a bit funky with the leading link front end, but they ride great (the motorcross style handlebars help with handling.) And with the Tecumseh HS40 or HS50 motor and torque converter, they really take off quite well! To be frank, the MTD 7xx series is one of the best vintage 1970s mini bikes made. There closest rivals were Rupp and Fox, but the MTD just rode a bit nicer with the leading link front end, and had more motor (often a 5hp, compared to 4hp on the Rupp.)
MTD Repair Issues.
There is however one big problem with these MTD gas tanks, and that's where the metal L gas line fitting mounts to the tank. The metal L fitting goes through a rubber tank bushing that is 33/64" in size (MTD #735-0149 or Oregon #07-392). The outside of the rubber bushing should measure 1.050" and the outside diameter of the inner section is .550". Remember NEVER use gasoline with ethanol in these tanks! Then ethanol will react with that rubber bushing, and it will most likely leak. And/or the ethanol will dry out the rubber bushing (and again, gas leaks).
Over time the rubber bushing will get hard and gas leaks will occur. Fortunately it is replaceable (MTD 735-0149 or Oregon #07-392). First pull the metal L fitting out of the bushing using channel locks. If the rubber bushing is really hard, you may have to cut the L fitting (but try not to!) Once the metal fitting is removed, the rubber bushing comes right out. A new bushing can be purchased and pushed into the bottom hole in the tank. Then the L fitting pushed through the bushing. Now your gas tank leaks should be gone. Just remember, use "recreational" gasoline with NO ethanol if you don't want to replace this bushing again!
Why do I say this? First the original Fairbank Morse system is probably worn out, being 50 years old. But the main thing is parts are hard to get. Even the belt (200397a) is not that easy to get (obsolete). Also the springs on both the drive and the driven are pretty light duty. And if you have a decent motor, they just won't do the trick. What happens is the original Fairbank Morse system switches gears too soon, and don't allow a decent motor to get into it's proper RPM band.
If you have to replace the torque converter jackshaft, you will need a 5/8" diameter keyed jackshaft about 7 3/8" long. Also it should be drilled with an H drill bit and tapped for a 5/16-24 bolt on each end. Note some people will use a 5/8-18 die on the end of the jackshaft and a nylon 5/8-18 nut. If you take that approach a longer jackshaft will be needed.
Another part that is often missing from MTD mini bikes with torque converters are the mounting brackets for the original MTD cover. There are two of these used to support the TC cover, and they mount to the Tecumseh engine side PTO cover mount holes with three 5/16-24 bolts. Because they are often missing, the bracket is available in a DXF file: MTD cover bracket DXF. With this file you can have it laser cut. I use the website sendcutsend.com to cut the DXF parts in 1/8" thickness (.125") and in stainless steel or mild steel.
The MTD torque converter bracket (two needed) that mounts to the original Tecumseh engine.