swapping to a 2-smoker kx65 motor

[oscarmayer]

well as some of you know my son has been racing ATVs for the last 4 years now. he competed in the 70cc class at the TQRA Pro challenge last month. I actually dropped in the baddest engine we have build for his class yet. (114cc allowed) we were making upwards of 15hp. went from a 42T rear to a 34T rear and front from a 13t to a 14T. we should have been smoking it! well nope... on the back straights he had it pinned in 4th gear and them dam CVT 2-smokers passed him by like he was walking. So after asking around we decided to get a kx65 and swap the motor into it. So that is the next project. I'll post photos and stuff as we go. But ummm yea... int he small CC class a 4-stroker just cannot compete.. Let alone with a hopped up 70cc 2-stroker. that is pretty much like running a tweaked out 200cc 4-stroke air engine.

[86T3]

At all the races i was at this year it seemed like there were a couple kids per class that were running bike motors with manual clutches. I also noticed that these kids were always near the back. Id bite the bullet and run one of those cvt motors

[oscarmayer]

we re going to use a recluse semi-auto clutch conversion for the engine so no manual clutch. rules do not allow for manual clutch. usually the ones in the back are learning to ride. my son has been racing a while and knows how to shift gears. the problem is an engine that can compete now. :( I had thought about that, but the cost of swapping to a DRR or apex then trying to make it competitive is so high, it's crazy. I know guys with over $6k into an engine assembly. not me.... he's not a grad champ racer. he's ok and decent but now grand national one. so for me to fork over more $ in an engine than I spend all season is beyond my ability right now.

[86T3]

Ouch, i can see why youre going with the 65. I didnt know they make a rekluse for those, thats pretty awesome. I dont know what your max cc is but eric gorr has a kx65 big bore that puts it up to 72cc. Here s the link http://www.eric-gorr.com/bbmasterlist.php I havent bought anything from him so i cant give a review but i am planning on buying a big bore for a kx100 this winter from him.

[oscarmayer]

cool man, thanks!

we are limited to 114cc 4-stroke or 70cc 2-stroke. no manual clutches allowed. stupid rule on the clutch...

[oscarmayer]

got the kx65 and will soon be working on mounting it to the atv frame. i am planning on running a yfz450 radiator as it is larger and should work easier for atv use.

[oscarmayer]

updates:
I talked to the racing guys, they said we would only be able to run the mini-open. the good thing is that allows for up to a 104cc 2-smoker and does let you run manual clutching. so we so not need the recluse clutch now! $$ saved!
I got the old engine out, cut the old engine mounts out and even removed and patched the frame where it had been factory notched for the 4stroke. I put in a full tube there and welded it up so no more notch or dip. much stronger!
now, I mounted the kx65 motor, welded in now engine mounts and even put in pipe mounts too. I installed the rear swinger and put the chain on and spun it over by hand looking for chain bind. none to be found! fits like a glove!!! I have not had time to get pics up, but I will when I'm done. this is turning into a nice project! I think it is really going to pay off too!

[86T3]

Get a kx100 motor if you're allowed that much more displacement. I have a yfz radiator but i dont know when ill be able to get to it. If you can wait a little bit maybe we can work something out.

[oscarmayer]

thanks man. I made some mods to the bike frame. I am now able to stuff a 7" tall radiator behind the bumper where before I could not. width is not a factor it was height between the bumper supports where it mounts to the frame.

the problem was last year's rules let us run in the 70class but this yr they made some changes that forced us into a different class. sucks, cause I already purchased the bike and tore the engine out and sold the rest of the bike when i found out. so i am sorta stuck. So in order to help, we will drop in an Athena 82cc BB kit. That will probably be the best bet we can do at this point. 82cc with 12.5:1 compression is still a decent setup. plus this is a 6speed. unlike the CVT bikes that eat up performance when converting power through the CVT setup. so in the end, he will actually come out pretty close to the CVT rear wheel power, but with 1 advantage. His engine will have a faster top end in 6th gear.

[86T3]

Sounds like it going to be a potent combo, can't wait to see some pics. Are you on facebook? There is a guy in the northeast selling a Nacs kx80 race quad from the 80s, its posted in the tecate group. Its pretty neat, dont know if youd be interested but it may give you some ideas.

[oscarmayer]

ok here you guys go, pics!!!!!

this is a 155Z that was built to a 180cc. I actually have a typhoon I picked up for me and I will drop this into it so I can chase little man around torturing him on the track to make him go faster. :)
186360

This is the old 125cc that made close to 17hp. fully built stroked, and all custom work done. has 15:1 compression
186361

here you can see I chopped away all the old mounts and installed the new kx65 with new mounts into the chassis.
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another engine photo in chassis. (close up of mounts)
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assembled in quad. fits nice!
186364

here you see everything fitted perfectly! fit like it came that way!
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another angle of fitment.
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[oscarmayer]

engine updates:
today I torn into it and rebuilt it. I installed all new seals and bearings. I installed a new hotrods crank so it is stronger and better balanced than factory for better RPMs. I upgraded the old stocker to a tusk performance clutch.
the engine will be sent off next week to a pale that will punch it to a 85cc and machine and custom mod the ports. it should run as fast as a 100cc when completed!
I still have to buy a pwk 28mm carb and some other odds and ends, but it is getting there.
I will post photos as I go.

[SELF27]

What kind of quad is that? Looks good.

[86T3]

Your son is a very lucky kid, good job. I would love to take that for a spin, meet at trikefest? Lol

[LT80]

"went from a 42T rear to a 34T rear and front from a 13t to a 14T. we should have been smoking it! well nope"
Did you ever figure out why this gearing combo was slow?

[oscarmayer]

LT80:
The internal gearing was not up to par (trans gearing). Apparently any of the 125 transmissions no mater what you do to them really will only hit around 40-43 mph even at 10k rpm (which we can run to 11,500) the 2 smokers run 14-16k and can hit upwards of 55mph. That was why he would get passed on the straits like he was hitting the brakes. We had the 16-17hp same as the CVTs', but not the rpm's and in the upper R's we loose the torque while the 2-smokers' gain it.

self27:
It is a prototype design biased off a typhoon xtreme style but highly modified.

Thoughts,
If I had ran a 155 gearing It would have hit the same as the CVTs. with this new motor we can hit 60+ and keep going till it blows when done.
it has 6 gears and we will have a 13T front with the same 34t rear, so it will be setup really nice! with 6 gears he should have no problems with the hills. If we need to I can change the front out bigger, or even the rear. (rear currently requires complete rear disassemble due to where current ring gear sits inside swinger, but having a custom swinger made and it will run a raptor 125/250 rear end which will solve that issue)

86T3, not decided if I am going to TF yet, but if I do, this bike could not go. the suspension was specifically designed and build for his weight range, if you say in it, you would destroy the shocks on the first bump. I am building a 180cc one for myself, and that one I will bring to TF for fun. it will hot around mid 50's and let you slide sideways the whole time doing it. :)

[Toaster556]

Oscar, did you try the XL/CL/SL70 transmission? It runs a 0.96 fourth gear instead of 1.043 found in the chinese transmissions. I did the swap last year on my Zongshen 125, and the difference is noticable.
However, the KX65 with the 82 kit makes a fantastic torquey powerband. It will help your kid for sure. What will you be doing for a pipe?

[oscarmayer]

gonna run the stocker, I found a site that tested every pipe (not custom made) for the kx65 engine available and the stocker appeared to be the best power band of them all on where we needed it. most may smooth it out, but sacrifice lower end, and some lots top end, we needed lower mid through higher end, WOT is not as critical for over all as 2/3-4/3 throttle is where most of the time he rides, so I am setting it up for his riding style. the stock pipe hit hard at that point and dropped flat at the 3/4 mark exactly where I needed it. so it works out and saves me some $$$.

[oscarmayer]

here are the latest photos of the progress.

New radiator was installed, finished engine mounts and their supports, installed harness, installed new XFR nerf bars, finished motor with the Athena 82cc BB kit installed now.

[oscarmayer]

finished it last sat and touched it up Sat. night before Sunday's race.
Let me know what ya think. it runs fast! now the rider just needs to learn to operate the bike. so we will probably spend most of the season this year int he back of the pack, but he will get the hang of it and do better soon.

[SELF27]

looks good. great job.

[oscarmayer]

Thanks, took lots of planning and thinking on how i wanted to make it work all together.

[oscarmayer]

well bike would not run for the race this last weekend, torn into the motor and found the piston had cracked and the ring broke. time to replace piston.

[86T3]

Dang, that sucks. Has your boy been getting used to it more? Any videos?