Hot Rod HS40-Powered Full Suspension Cat 250

Got this beat up 1969 Cat 250 with some more desirable frames earlier in the year. I had to cut off a LOT of extra, rusty metal, including the front end which was in real bad shape.

I had this idea of making a "Cat" suspended front end with the Cat tree I had laying around, so that was what I did first.

I tried using a solid rear like original, and it looked so terribly UGLY that I am not posting the photo for it! :laugh: So I started investigating using "mini cycle" swing arms, because the "mini bike" swing arm design was well known, and would have just been a copy. I ended up using an old XR 75 arm I got off of Ebay. The white set is for measurement purposes, as I ended up using a different set.

See those shock absorber mounts above? They do NOT work on bikes with slanted rear frames, unless you want to place torque on the upper mounts, or are able to use horizontal upper mounts. I was unable to do either, so had to remove and replace the lower mounts at an angle.

I had to relocate the brake anchor arm mount to the opposite side.

First Mock Up

And follow on work getting the JS mounts fabricated and welded in and swing arm components squared away.

Here is a comparison with another Cat 250 I have.

More work.

The bike and I went through several iterations, but it will probably end up being just like this, without the fenders. The rear wheel and brake assembly are modified versions of the stuff the ebay guy from Kansas sells. Front wheel is his version of a precision CNC hub to work on Cat wheels.

I had several thoughts about how to power it, but ended up going with a BB HS engine, that more closely represents what was available in 1969.

In addition to the ball bearing PTO, the engine sports a Dyno 245 cam, a billet rod, Animal valve springs, and Briggs FH keepers and retainers.

Rebuilt the magneto section with new parts, and timed it to stock HS 40 spec. even with the cam, as there is no compression release. Note: I have terrible luck getting the original Tecumseh wire mag. retainers to work with the Chinese repops, so the mag is epoxied in place. It works well like this.

I will post updates as they occur. The next steps are to finish out the engine with a custom exhaust, intake manifold from OND with a Mikuni 22 on the end I got from Chipper, and I'll have to stick a fuel pump somewhere.
Note, there are several of us doing HS builds right now using these components. Thanks to Delray and Ole4 among others for paving the way.

Dyno 245 cam for the OHH engines.
ARC 6282 Billet Rod for the 5.5 OHH engines.
Dyno AN-001 18 LB valve springs for the Briggs Animal.
Briggs DCSH103 valve keepers for the Flathead.
Briggs DCSH102 lower valve spring retainers for the Flathead.
Ok ….I'm digging this build ! I'm loving this rear swing arm Idea … :thumbsup: Keep it coming [MENTION=31239]Havasu Dave[/MENTION]
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Well-Known Member
nice dave,that bike will make a good test bed for the hs40

what was the plain for transmission. looks like your going with a clutch setup?


Well-Known Member
that's a interesting piece on your engine. I don't even have a mounting hole for that piece of tin. what year motor is that again?
I've had a few with that piece. Markus mentioned which years used them in a post IIRC. There is a boss in the aluminum for the mounting hole. And the thing is, since I will be using a fuel pump and a Mikuni, I wont have to shove the fuel line between there like we always have to when building them like stock.

Engine block is HS40-55513E. Most of the guts are earlier Rupp era 1970. The piece in question is part number 34212. I "think" this engine was 1979.


Well-Known Member
yes dave that sounds about right. my engine was right around 68-69 before tecumseh must of decide to do this change over and add this piece of tin.


Well-Known Member
Just noticed your Engine Mounting Bolts. Thought of a question......Why don't more people just Tap the Block and run Bolts up from underneath?
I know Tecumseh actually did that on some of the electronic ignition models, for example I have one off a snowblower the motor mount holes are tap. the only problem with that sometimes is the bolt gets bigger and some factory motor mount slots on minibikes are not big enough for the next size bolt to go through the slots.
This is an original design that has been floating around in my head for a few years, and I definitely did not glean inspiration from anyone, except maybe Honda. :no:

The engine mounting bolts are temporary, while I fabricate fuel and exhaust system. Actual mount bolts will be Grade 5 5/16-24 flanged, with serrated flange nuts.
Great looking project Dave. That little extra stretch gives it an awesome looking stance and looks like it will fit an adult more comfortably. I always admire your builds.
Great looking project Dave. That little extra stretch gives it an awesome looking stance and looks like it will fit an adult more comfortably. I always admire your builds.
Thanks Pat, and I always admire your builds as well. I especially missed you while I was fabricating those handle bars. Aesthetically, I wanted to keep all of the parts "Cat," and the bars are just half of a Cat front fork set with the top plate cut out, then aligned, and welded to the front plate again, only 2 inches taller to go with the over all theme. The front plate is some 16 ga. I had laying around.

What I really needed was your bending skills, so I could push the bars forward enough with a bend right at the plate to get the bars perpendicular to the frame. I fear their rearward slope is going to push the rider off the back of the seat.

I think all I've accomplished is taking a bike built for an 8 year old, and making it more suitable for a 12 year old. LOL!
Keeping my build thread updated, here is the throttle method I used to get the Mikuni VM22 to work on the HS without knocking the carburetor way out to the side. Thanks to OND for the spiffy manifold. As always, this is in mock up phase, so take no note of fasteners, ugliness, or anything, anything at all, that my be wrong. What I lack in technical expertise, I make up for in posting verbosity.

Elbow from EC that I tried out on a Tilly for a kart. The end beneath the rubber grommet is threaded for the lock nut that holds it in place. I didn't use that, just shoved it in the hole, it fit snug, and I can pull vacuum on the seat there at the throttle cable. Spring holds it all together.