Project 2-4: Getting a 2-Stroke Motor in a 4-Stroke Frame, Part 4

Apr. 04, 2011 By Rick Sieman
Here was our task: we had the amount the Mikuni carb to the engine and fabricate some sort of airbox that would work.

So far, the work on putting our two-stroke RM250 powerplant in the four-stroke YZ 426 chassis has been straight-forward work. A lot of it, but no real mystery. Next came the hard part, or at least one of them. We would we have to figure out how to get a carburetor and a functioning airbox on the bike.

Project 2-4 Previous Stories:

Part 3

Part 2

Part 1

Naturally, we wanted to use as much of the stock parts as possible. It didn't make a lot of sense to fabricate a brand-new airbox if we could make the Yamaha box work.

The back of the carb had to clear the shock spring in order for it to work properly.

First, we had to mount the carburetor on the engine and make sure that the air intake side cleared the shock. This is real pain in the neck on modern single-shock bikes and makes you long for the days of the old twin-shock setup.

Here's the stock Yamaha YZ 426 airbox complete.

We were able to find regular old radiator hoses at our auto parts store that would fit the carburetor. It never hurts to become friends with these guys, and if you're nice, they'll let you go in the back where all the hoses are hanging and see which ones will fit your carburetor.

A piece of radiator hose of the right diameter got carved up with a razor blade to meet our needs.

With a bit of trimming, we were able to get the carb mounted with sufficient clearance from the shock on the other end.

This is the final result of our little modification.

Then came the fun stuff. The existing inlet off of the YZ 426 airbox was cut off with a razor blade and we decide to make a new front mounting setup for it. At this point, a bit of brain fade set in, as we made the mounting piece out of ABS plastic. Big, big mistake. Even though the stuff was very easy to cut and make to the shape we wanted, when we tried to get some sort of adhesive to stick to the ABS, we realized it was indeed hopeless.

As you can see, the carb slipped nicely in place.

That was when we decided to go with a nice simple sheet of metal. This was cut out to the desired shape, and regular black RTV sealant was used to hold the new hose from the carburetor to the airbox in place. After it dried for a few days properly, the connection was very strong and we were able to mount the airbox properly in place and hook everything up.

Everything cleared. The air inlet hose fit around the shock and the carburetor still went into the engine inlet at a good angle.

We double checked to see if the inlet side of the carburetor cleared the shock spring. It did and we were ready for the next step.

Another trip to the auto parts store and more rooting around in the radiator hose section got us the right piece of hose for our needs.

In this case, we wanted a large cross-section on the inlet side of the carburetor to connect another piece of hose to the airbox.

All of the front of the airbox was simply cut off and discarded.

We had a nice thick piece of ABS plastic laying around that we decide to use on the new airbox front. Big mistake on our part.

Even though the plastic was easy to work with, it gave us huge problems later on.

The plastic section was bolted home to the front of the airbox.

A piece of hose was carefully cut to go from the carb to the airbox inlet.

Here's how the hose lined up.

This is when we discovered how difficult it was to work with the ABS plastic. Nothing would hold properly to attach the hose to the air inlet on the box.

The plastic was scrapped and we went to a regular piece of thin sheet metal instead.

After the hole was cut into the metal, a small portion was bent upward for increased strength in the area.

Regular old black RTV sealant made a strong and flexible connection that's holding up quite well.

Here's the airbox with a hose attached. Simple, yet effective.

Success! The carb is now attached in the airbox is firmly in place. Newsletter
Join our Weekly Newsletter to get the latest off-road news, reviews, events, and alerts!