KX 85 Cylinder still okay to use?

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10/26/2017 11:24 AM

Hi Guys,

I'm rebuilding my sons KX85 again after only 53 hours on Bottom end. The bottom end does not seem to be getting enough oil. Right now the rod has lots of up and down vertical play. It looks like it got hot on one side of Crank.
The bearings and seals are still good.

Couple of questions:

1. Does this cylinder look okay to reuse? It has very small blemishes, but check out pictures and let me know?

2. Will increasing the fuel jet size a couple of digits help get more oil down into the crank area? Or should i use 32:1 mix. I have been using Maxima and Yamalube at 36 to 40:1. We haven't fouled a plug in a real long time.

All of our 2 stroke bikes run great. With no fouled plugs, and no rebuilds in a long time. This KX has been completely rebuilt 3x in the past couple of years. All lower and upper ends plus all new seals each time.

Thanks for your time and responses.

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10/26/2017 12:22 PM

I would not re-use that cylinder. Send it in to get honed and re-plated. Crank needs a re-build too if you have any up and down play at all. If you have the time, I would just go through everything again. Otherwise, I wouldn't say its a great idea to use that jug again the way it sits. Increasing the jet size will increase fuel flow, your oil / fuel mixture will determine however much oil gets into the crankcase, as long as your carb is clean and functioning properly.

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Hope for the best, Prepare for the worst.

10/26/2017 1:03 PM

Cylinder - Not if you want it to perform well
Jetting - Well technically yes that isn;t your issue and the bike will run like crap.

What brand crank? It seems odd to have up and down play with thrust washers that look like new? I wouldn't use anything but OEM and they have the plastic inserts. If you are usuing Hot Rods/Wiseco cranks well, there is your problem. That looks like a Hot Rods crank to me.

What year KX?

This isn't a jetting issue. I would run 32:1 in a KX85 and keep an eye on your LH crank seal as it might be sucking air in.........

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10/26/2017 1:46 PM

Haha....welcome to owning the shittest 85 ever built. I owned 2 of them and couldn't keep 1 going. It doesn't matter what brand crank u buy the kx will eat it up and it will also eats pistons like crazy and all I ever used was oem but I did try hotrods also and nothing last. I ran 32:1 and did everything possible to try and make it last because we were racing every weekend. Piston change at 15-20hrs tops and change crank by 40 and you should be ok.

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10/26/2017 1:50 PM

Hard to tell from pic but I'm not seeing anything with the cylinder that would be a problem, take some scratch pad to it and give it a go

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10/26/2017 1:51 PM

Cylinder finish is key to ring seal. Ring seal is key to making power.

Will it run with the cylinder like that? I'm sure it will.

Will it last long or make good power? Probably not.

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Race Bike: 2018 KTM 350SXF

Other Bikes: 1985 CR80R, 1990 CR250R, 1998 PW80, Specialized Fuse Comp 29.

Sold: 2016 YZ250F, 2012 CRF250R

10/26/2017 1:55 PM

Thanks for the reply guys. Yes, it's a Hot Rods Crank/Complete bottom end kit. I just got another new hot rods crank kit this morning sad. Next time i will order it wrong the local dealer.

For the cylinder, do i take it to a car/hot rod engine shop and have them do it? Or look to my local MX shop here in town? Also, what will be the ball park costs to have it honed and re-plated?

Again thanks for the responses. I was going to put it back together and run it this weekend, now i'll wait and do it right.

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10/26/2017 2:01 PM

Might be able to get it honed, if it needs replated someone sponsors this site. Can't think of the name right now, anyways not hard to find.

You need to increase the premix, 28:1 or 32:1 depending on where you live and how hard your kid rides. This is an 85, not a 125. Note when you change the premix ratio you need to change jetting as well. More oil means you need to go richer on the jets to get the same amount of gas. No idea what jets you ought to be running though.

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10/26/2017 2:49 PM
Edited Date/Time: 10/26/2017 2:50 PM

Cost around $200 to get the cylinder replated. I sent mine to Millenium and it came back looking better than new.

We are at sea level and very close to stock jetting. 32:1 Motul 800 with a 75:25 pump/race gas.

I wouldn't run anything but an OEM crank.

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10/26/2017 4:38 PM

From those pictures I don't see anything wrong with the cylinders but the pictures aren't that good either as long as none of the coating is peeling around the ports or there's any grooves that you can stick your fingernails in. just cleaning up and reuse it. if you don't have a brush hone you can just use some Scotch-Brite to deglaze it, there's probably plenty of videos on YouTube on how to do it. As far as the rod are you absolutely sure there's up and down play and you're not just feeling the normal side to side play.

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10/26/2017 6:26 PM

RCF wrote:

From those pictures I don't see anything wrong with the cylinders but the pictures aren't that good either as long as none of the coating is peeling around the ports or there's any grooves that you can stick your fingernails in. just cleaning up and reuse it. if you don't have a brush hone you can just use some Scotch-Brite to deglaze it, there's probably plenty of videos on YouTube on how to do it. As far as the rod are you absolutely sure there's up and down play and you're not just feeling the normal side to side play.

100% positive, on the up and down movement, might be a 1/4" or more of play. It is bad enough that it made the bike stall at the track. My best guess is the rod is not getting enough oil, overheated in that area and stretched it. Everything else looks to be in good/great condition. Right now just deciding if i should put it back together after honing it with a drill and go riding with the family this weekend. Or waiting for the cylinder to be honed/plated etc. like a few guys suggested. In the mean time I was on ebay looking for a 2nd cylinder, so i can rotate them. Seems like getting a cylinder fixed will take at least a week with shipping etc.

It sucks when I always seem to have 1 bike broken out of 3 bikes and 3 people want to ride smile

Thanks for all the responses/suggestions!

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10/26/2017 6:44 PM

I can't imagine it running very long with that much up and down play. I would rebuild it before riding it again. If the crank or rod goes, then you are talking about replacing cases and possibly the cylinder and etc etc etc.

After rebuilding it, I would fatten that sucka up until you are seeing obvious signs of it being too rich (oil coming out of the silencer and plug fouling) and then lean it down slowly. I also would run at least 32:1 or richer.

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