Project ATC300ES

[MojaveRider]

Howdy! I don't have a ton of posts on here, but I'm starting a new project and I thought I'd share. I'm no where near as good as some of the great builders on this forum, but I'll give it a go. This post is kind of long, but there are pictures at the end :)

Almost 3 years ago I got a good deal on a pretty original '85 BR for only $400. It was my first Big Red and it's been a blast. It certainly isn't collector grade, but it is pretty clean and unmolested and runs fantastic. After seeing a thread about the 300SX running around in the UK, I thought it would be cool to do a 300 Big Red or 300SX. More power is always fun!

I began browsing Craigslist looking for a good deal on a TRX300 engine. I found a '95 TRX300 2WD less than a hour away. It was a complete bike, except the rear end was trashed and the wheels + tires had been taken for another project. The axle had actually seized in the rear end, and the previous owner beat the crap out of the axle and it wouldn't budge. We messed with it a bit and it started right up and didn't smoke, so I bought the whole bike for only $100. I was going to swap the 300 in the '85 BR, but I realized it was too nice of a runner to mess with. I didn't want to tear into a (mostly) original bike, just to swap it and go beat on it.

Over a year passes and I found a cheap '85 250SX. Owner said it ran, but it was rough. ROUGH. I ended up buying it in the dark and dragging it home. After pulling off the mismatched plastic, seat, and tank, it was clear this bike was done. The frame had been hacked on and the rear end had no oil and was instead packed with grease! It had a full size car battery mounted in an angle iron mount, random muffler welded on, and the wiring harness had been chewed up, with a full size car starter solenoid hacked in. I got it to run, but it smoked very badly and blew oil out the exhaust. I saved the engine, forks, and a few small parts, but ended up scraping the frame and rear end.

Nearly another year passes, and I decided I needed to do something with this 300 I've been sitting on, so back to Craigslist. I was hoping to find a non-running Big Red/250SX for cheap in decent shape, then swap in the 300, thus getting an old 3 wheeler back together and getting rid of this non-rolling mess I'd been holding onto. There doesn't seem to be too many Big Reds for sale around me (Central TX) right now, and most of the ones I found were running bikes. Then I found an '86. Listed as not-running, with some broken plastics. The kid selling it clearly didn't know much about it, just that it wouldn't run and he didn't know why.

I took a chance and drove 2 hours to go look at it. I made sure to bring tools to inspect the rear diff this time around. It was a bit nicer than I expected. Other than sun damage to the plastic tank, seat, and fenders, it wasn't bad at all. The rear end was full of oil, no water, and the brakes needed adjustment but were complete. The fishy part was the starter was off the bike, and clearly the entire air intact track was not installed right. The forks have zero oil in them, but aren't seized up and the boots are still intact. I bought it for $350.

Even though I wanted to swap it, I wanted to take a shot at getting it running first, to hopefully sell the 250. The starter had all of the teeth rounded off badly, and after pulling the starter gear cover the idler gear was just gone. The 250SX engine I had saved provided a good starter and gear. After connecting the neutral switch, cleaning the filthy carb, and replacing the fuel line that was gunked up solid, it fired right up and runs great. It doesn't smoke and shifts through all the gears. Success!!

After having some fun moving around the non-rolling TRX300 with my tractor, I got the engine out of the frame without too much trouble. My current plan is to take the output shaft off of the 250SX engine and swap it onto the 300, leaving the BR 250 intact. Once I get the 300 in the bike and running, I'm going to rebuild the front forks and get it ready for some trails.

I think it would be cool to do a big bore and/or stroker kit of some kind in the future, to build a 350cc or 425cc BR, but I'm not ready to dump that kind of cash (yet). Maybe after I get some miles on it...

On the trailer on the way home:
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Condition as brought home:
183280183287183291183288

After getting it running and removing the home-made add-ons, though the front basket is a Hondaline:
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Donor almost totally stripped:
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[6speedthumper]

Cool story! Both variants of that engine are bulletproof! I have 3 of those 300 fourtrax and love them all. You'll have an easy swap. Just be sure to swap the flywheel and stator from the ES over to the TRX engine so the electronics will work. Oh, by the way, does that TRX chassis have the mounts for the front diff? I could use a good frame if you'd ship.

[6speedthumper]

Oh, I would also be interested in that exhaust from the TRX as well.

[MojaveRider]

Cool story! Both variants of that engine are bulletproof! I have 3 of those 300 fourtrax and love them all. You'll have an easy swap. Just be sure to swap the flywheel and stator from the ES over to the TRX engine so the electronics will work. Oh, by the way, does that TRX chassis have the mounts for the front diff? I could use a good frame if you'd ship.

The frame is a 2WD frame, so no front diff mounts. It is pretty clean and straight though.

I'm not sure what I want to do with wiring. I have the complete TRX300 wiring harness and electronics. I wasn't sure if could use the 250 CDI with the 300 engine without swapping the stator/flywheel. Or I could use the 300 CDI. I need to get some wiring diagrams and compare connectors.


Oh, I would also be interested in that exhaust from the TRX as well.

The 300 exhaust is in pretty decent shape, except there is 1 small hole on the end of the muffler by the outlet. I can get pictures if you are interested.

[kb0nly]

If you have the wiring and electronics from the 300 and they were working i would stick with them instead of rewiring and swapping parts, just easier in the end!

Looks like a great project, will watch how this one turns out for sure!

[MojaveRider]

If you have the wiring and electronics from the 300 and they were working i would stick with them instead of rewiring and swapping parts, just easier in the end!

Looks like a great project, will watch how this one turns out for sure!

I need to spend some quality time with the wiring diagrams and see what's what, though the wiring on the BR is probably in better shape than the TRX.

[MojaveRider]

After studying the wiring diagrams and the wiring harness on the donor TRX300, I have a plan. My 250ES wiring is actually in really good shape and not molested at all, while the TRX wiring has seen better days. I plan to run the 250ES wiring and CDI. I purchased a bad '86 250 stator here on the forum (thanks redsox!) in order to steal the connectors to avoid mods to the 250 wiring harness. The 250ES has 2 grounds going to the stator while the TRX300 only has 1, but I don't expect that to be an issue. Hopefully I can simply put the proper stator connector on the 300 and it will plug right in.

If that doesn't work, my backup plan is to use the TRX300 CDI. As most people know the Big Red 85 and 86-87 CDI's are different. The 86-87 250ES and TRX300 have the same connectors to the CDI with the same wires, just in different locations. I'll re-pin the connector to work with the TRX300 CDI if the 250ES CDI won't run the TRX300, but at this point I'm hopeful that won't be required.

[MojaveRider]

Despite not posting here for over a month, I have been working on this. I pulled the 250 out of the bike:

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And temporarily dropped in the 300:

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It dropped right in as expected. I bought a bad 250ES stator to steal the connectors from. I connected it to the 300 engine, combining both grounds to the one ground on the 300. This allowed the 300 to plug right into the 250 harness. Sadly I did not have spark with the 250ES CDI.

As I said the '86-87 250ES shares the same connector with the TRX300 CDI but the pins are in different locations. I repinned the chassis connector to the 300 setup, and I still didn't have spark. I messed with this for a while and double checked my connections with no luck. Finally I admitted defeat and pulled the entire 250ES wiring off and replaced it with the TRX300 wiring. I'm using everything but the lights from the 300, including the dash panel and key. It lays out pretty well on the trike. I mounted the CDI and oil temp warning box behind the headlight where the 250ES fuses were located. I still need to get a headlight connector, and connect up the tail lights. Once the wiring is squared away, I'll start on the output shaft swap.

[JasonB]

cool project man, im in for some updates on this!

[MojaveRider]

cool project man, im in for some updates on this!

Thanks, it's no ATC500R, but should be fun. :)

[dougspcs]

Really interested in the outcome of this installation..also the overall performance change in the machine.

I'll be watching..

[King Trikester]

I've been following this thread since day one, I'm looking forward to updates and completion, I've always wanted to do this swap. Good luck

[MojaveRider]

I've finally finished (almost) all of the wiring work. In my last post I said I used the complete TRX300 wiring, and had mounted the CDI and oil temp box right behind the headlight (the TRX300 has the CDI up at the front by the steering shaft). After further investigation, I decided I didn't like that. A bunch of wires now had to move when the handle bars were turned, and that was making the entire harness shift around on the frame. I removed all the factory tape and moved the CDI and oil box back to the stock 250ES location. I had to extended and shorten a few wires, but it turned out pretty well.

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Everything is now in the original 250ES location. The wiring follows the original routing, and looks mostly stock. I had to make a sub-harness with a Y for the tail lights, and I still need to find a harness-side headlight connector to get the headlight hooked up. I used the donor TRX300 dash and key, which includes the oil temp light. I had to cut off the cable guide in the front to clear the headlight, but it came out OK.

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I'm going to run the TRX300 CV style carb. I did a test fit of the carb, which required swapping to the TRX300 throttle cable (different thread pitch on the carb side) and putting the proper fitting and plunger on the choke cable. Despite all the wiring shenanigans, she fired right up and settled into a nice idle. I verified the stator/rectifier are charging the battery and the tail lights work as they should.

The only part left to getting the TRX300 in the chassis permanently is swapping the output shaft. Hopefully I can get that done somewhat soon. There are a lot of details that need to get done before the final engine assembly: clean up and paint the exhaust, clean dirt/grime/grease off of the frame, maybe touch up the paint on the battery box, etc.

I need to get a move on so it is done before it's too hot to ride!

[Chopsaw]

Like this one , same as above wondering about the performance / power increase . I have a 85 completely torn down right now . Keep us updated .

[trike savior]

be careful swapping the output shaft. all the transmission gears are on it and if not careful there are shims that fall out of place really easy. I know from experience and had to split the cases on a good running motor. if I had to do it again I would pull the motor and set it on its side, so the gears would be stacked perfectly up and down. I guess it could be done in the bike but when you turn it on its side you would have to stack some wood under the front end to get it all level. GOOD LUCK!

other than that it looks real good. I have wondered about doing this swap for a long time and may need to attempt it sometime.

[MojaveRider]

be careful swapping the output shaft. all the transmission gears are on it and if not careful there are shims that fall out of place really easy. I know from experience and had to split the cases on a good running motor. if I had to do it again I would pull the motor and set it on its side, so the gears would be stacked perfectly up and down. I guess it could be done in the bike but when you turn it on its side you would have to stack some wood under the front end to get it all level. GOOD LUCK!

other than that it looks real good. I have wondered about doing this swap for a long time and may need to attempt it sometime.

I'm nervous about it for that very reason, and that's why I've saved it for last. I am going to pull the motor and do the swap on the bench, with the engine level. Currently it's not bolted in the bike, so it comes out in under 5 mins. I'm probably going to use the "insert a pipe as you pull out the shaft" method to ensure everything stays where it should.

[lokisbuddy]

cool project man that's one thing I have never tore into a BR. some day...

[djm0242]

If I understand correctly it's really the stroke that changes the cc on this engine right? Looks good I'll keep watching!

[dman10]

Where are in Central Texas? Cause I can agree about the heat. All you wanna do is either ride or lay around moaning about how hot it is.
Looking good by the way. Do you have to build custom mounts for the 300?

[MojaveRider]

If I understand correctly it's really the stroke that changes the cc on this engine right? Looks good I'll keep watching!

Yes, the bore is the same, but the stroke is longer. Also the gear ratios are slightly different. 1-4 is the same as the Big Red, while 5th gear is from the 250SX (which is a bit higher, so should give a slightly better top speed).


Where are in Central Texas? Cause I can agree about the heat. All you wanna do is either ride or lay around moaning about how hot it is.
Looking good by the way. Do you have to build custom mounts for the 300?

I'm in Bryan/College Station. No custom motor mounts are needed, everything lines up. The TRX300 output shaft is too long and has to be swapped with a 250 one, but other than that everything else matches up.

[Stonewall]

Very nice project, I can't wait to see how this turns out. I have a 250ES fame lying in wait at home for this very swap, just ain't found an engine yet (ain't looked real hard either lol....)

We should start an Aggieland 3 wheeler group...

[MojaveRider]

Very nice project, I can't wait to see how this turns out. I have a 250ES fame lying in wait at home for this very swap, just ain't found an engine yet (ain't looked real hard either lol....)

We should start an Aggieland 3 wheeler group...

I was lucky to find a whole bike for cheap, which made it easy to grab parts as needed.

I wonder how many 3 wheelers there are around these parts. A friend of mine desperately wants a liquid R but is currently 3 wheeler-less.

[Stonewall]

Not many judging by Craigslist, and most of em look like trailprotrailprotrailprotrailpro.

Found a real nice liquid R in Galveston for about $1000 too much. http://houston.craigslist.org/sno/4350791453.html

And there's a nice SX in Austin that I'd like to have but I don't have the money right now... http://austin.craigslist.org/mcy/4322161839.html

[MojaveRider]

That one in Galveston has been for sale for a while. He's actually lowered his price. There are some cheaper ones in DFW (along with a few expensive ones).

I got really close to buying that SX, I had exchanged several emails with the guy, but I ended up buying an '85 Odyssey instead.

[Stonewall]

Hey Mojave, do you still have the rear U-joint off the 300? If you do could you measure it and see how it compares to the Big Red joint? Im thinkin about buyin one of the all balls 300 HD U-joints but idk if it'll line up or not.

[MojaveRider]

Hey Mojave, do you still have the rear U-joint off the 300? If you do could you measure it and see how it compares to the Big Red joint? Im thinkin about buyin one of the all balls 300 HD U-joints but idk if it'll line up or not.

I think I do, I'll have to dig around in the garage and find it.

[Stonewall]

I'd appreciate it if you find time. That HD U-joint looks like it would work real good with more swingarm angle.

[MojaveRider]

Top is '95 TRX300, bottom is '86 ATC250ES:

188587

Just eyeballing them they look the same. Sadly I don't have any calipers to measure them.

[Stonewall]

Thanks! Looks like the transmission side is the same length on both of em, thats what i was concerned about because of pivot geometry. Driveshaft end looks longer on the 300 but thats no problem. Splines are the same right?

[MojaveRider]

Thanks! Looks like the transmission side is the same length on both of em, thats what i was concerned about because of pivot geometry. Driveshaft end looks longer on the 300 but thats no problem. Splines are the same right?

Engine side splines are definitely the same, the yoke to shaft splines look the same, but I'm unable to get the 300 shaft apart. I'll sell you the complete 300 yoke with u-joint and shaft for $20 if you want it.

[Stonewall]

Awesome, it'll work then. The trans and shaft splines are the same so it doesnt matter. Thanks for the offer, but all I'm upgrading to this joint (http://www.allballsracing.com/19-1013.html) so I know all i needed. Thanks for the help! I'll be bringin the BR down to CS after spring break if you want to take a look at it.

[MojaveRider]

I tried to swap the output shaft today, and I screwed it up :( It was all together and I ran it through the gears. Everything worked, except for 4th gear. More jacking with it and now it's totally screwed up. The cases will have to be split to fix it, and I'm probably not qualified to do that.

[Chopsaw]

You can do it . Lots of help here , Myself , I take pictures along the way so I can look back to see it before I touched it .
Maybe start a new thread on just fixin this , guys that know will jump in to help you .
Good luck .

[Stonewall]

Sounds like something (or several things) fell out of place when you took out the old output shaft. I'd be glad to help if you want to pull it apart and give it another try. I haven't been inside a 300 engine specifically, but I've rebuilt some similar ones. If I can find it, I also have a shop manual that I can bring down after spring break.

[MudBug]

Any updates on this? I am doing the same swap, just worried about swapping the output shaft

[MojaveRider]

Any updates on this? I am doing the same swap, just worried about swapping the output shaft

Nope, the engine is still sitting on my kitchen floor waiting for the cases to be split. Sadly I haven't really touched it since screwing up the output shaft swap.

[MudBug]

I'll let you know how it goes with mine. Might be enough motivation for you to get the ball rolling again lol thanks for all the wiring info though, I had no idea about that

[MojaveRider]

I'll let you know how it goes with mine. Might be enough motivation for you to get the ball rolling again lol thanks for all the wiring info though, I had no idea about that

I definitely need motivation, but lately all my time and money has go to another automotive project:

223861

I've always liked K5's, and I finally pulled the trigger on one, but as with all square bodies, there is so much little crap that needs attention.