How to install a 125cc Pitbike engine in a TRX90

[Pappy]

This could not be a more simplier swap for the average person to do." Why" may be a more complex answer but atleast you will know going in that with average hand tools you can accomplish the project.


Most TRX90 owners run into issues when looking for more power for thier son or daughter. While there are alot of hop up parts available, not too mention enough qualified engine builders that will be all to happy to get more power from the trusty 90, sorting it all out can be difficult. I was not worried with building our quad to fit any rule or sanctioning structure so the pit bike engine fit the bill for us.

With that said, when we went shopping for a more powerful engine, we could not over look the fact that 125cc pitbike engines sell new for $550 or less and offer a manual clutch to boot! That is exactly what we did, scavanged an engine from a Xtreme 125cc pitbike. If you can get your hands on a complete bike your money ahead seeing that it contains almost every part you will need!

We borrowed the following from the pitbike( You will need these parts and equipment to do the swap)

Engine(Most 125cc pitbikes come with a 24mm carb so make sure you use it!)
Oil Cooler
Clutch Perch and cable
Airfilter
Electronics
Kill Switch
Welder to fabricate a small mount for the oil cooler
1/4 inch fuel line to lengthen the oil cooler lines

You will need to keep the following off of your stock TRX90 engine

Shifter lever
Intake

[Pappy]

Remove the fenders, both footpegs and exhaust. You can take off all the wiring from the stock TRX90 and set it aside.


You will need the intake off the TRX90 engine as it is not angled to the side like the pitbikes intake.

Once you have the engines out, swap over the carb and check for clearences. Our donor 125 needed to have a mount removed on the cylinder to make room for the carb.


(I will edit back in the proper pictures, I seem to have lost the file they were in )

[Pappy]

The one item you will need if will be a longer throttle cable for the 20/24mm carbs. Robbs Racing as well as other mini quad specialists can get you what you need.

I used a petcock off of a 400EX sometime back to make use of the larger carb and trimmed down the fuel knob to clear the carb

[Pappy]

Install the 125 engine in the frame and then you can move onto the wiring. It is a straight forward deal, and the cdi from the pitbike is used so the only wiring you really need to do is a few grounds and thats it!


No matter what exhaust you run, there will be 2 areas that need attention. One is the dipstick clearence, and with most pipes its tight to get it out to check the oil level but it is not so bad that you need worry too aweful much with it. When we switched to a Robbs Racing pipe, its bend and 2 piece design gave us ample room.

Second on the exhaust is the rear most mount. You will need to either slot the mount or drill a new hole. The 125 engine is just a tad longer in the cylinder area and this causes the hole for the bolt to be about 1/4 inch off.

[Pappy]

Our engine was a kickstart, we wanted every bit of weight savings we could get. The kic start lever WILL NOT clear the frame as it comes off the pitbike.

I cut the lever just above the area that slides onto the shaft, added a small piece of metal to bring the kick start lever out far enough to clear the frame and TIG welded everything back together.

If you decided to use the electric start 125's, you will need to do a bit more work. A battery tray will be needed(will fit in the rear cargo area of the rear fender but add some support and a holddown, some of which can be used from a donor pit bike)

Foot clearence is tight, but with a few different trail and error positions and the rider getting acustomed to kicking over the easily started engine, starting will be simple. You can trim the fender a small amount to add some room for a larger riders boot.

Once you have the kick start in order, your almost finished

[Pappy]

The oil cooler is small, and finding a good spot for it is not that hard, but can pose problems if you do not have access to a welder and a few tools to fabricate a mount and weld it to the frame.

When we first installed this engine to work out the bugs, we simply zip tied it to a frame crossover and it never moved.

On the finished quad, seeing we had switched to hydraulic brakes, making a mount was easy and installing it was simple.

[Pappy]

After you have the engine in, exhaust on and are begining reassembly, you can install the clutch perch and lever from the pitbike. The bracket that comes with the pitbike (or bought seperatley) bolts right on and the cable fit perfectly saving me from searching for the correct cable!


The clutch perch and lever work well for smaller hands

[Pappy]

Thats pretty much it, it is not very involved depending on how you go about getting the donor engine.

If you buy one, they run around $500 but check with the seller to see if it includes the carb. I found it easy enough to just buy the pit bike and sell back the parts I did not use to the seller then pick out the individual pieces needed to complete the swap.

I did this swap initially at the track in an hour with simply hand tools and a grinder if that shows how easy it is.

Performance wise, the manual clutch will make the mini quad a rocket ship in short order! The 125cc put out around 10hp stock, come with usually a 11.5 ~12-1 piston and run very well. There are a ton of hop up parts to get even more power from them if you so desire. The biggest thing we have liked is that the power robbing auto clutch is gone, and our rider can now step up and develop his skills before learning the clutch on a larger machine.

[redrunner]

Wow! very nice install, bettin' Cody is luvin' that.

[Doibugu2]

Wow, swapped at the track???


I see a Monster Garage episode with you in it in the future.

It will be the first episode that they will make money because you will buy the whole car and start selling the parts on ebay!