KX85 engine vs KX100 engine

[griffbones]

What is the main characteristic differences between the KX85 engine and the KX100 engine? Would the 100 have more low end? Does one rev out quicker or further than the other?
I ask because I just purchased a 2007 KX100 engine to put in my son's 2005 KX85. I am hoping I will not be disappointed with this conversion, but I can always put the 85 top end on this 100 bottom end if I have to. I was just curious what the power characteristics will be like and any pros or cons to this set-up.
BTW he is not racing, but he is riding some pretty steep and knarly trails.

 

[gwcrim]

I put a 100 top end on my son's 85 and it's doing well. It pulls my 175+ lbs. around and he rides it in hare scrambles. The 85 would rev higher and have more of a hit to the powerband. The bigger bore of the 100 mellows it out just a tad.

 

[griffbones]

Boy I hope he isn't too disaapointed, the KX85 already has a pretty weak hit compared to the CR's and YZ's. Does the 100 have anymore low end grunt or is it just like a mellowed out 85?

 

[gwcrim]

Nothing mellow about it. Just beefier and not quite as pipey.

 

[griffbones]

Well that sounds good! Do you think it will make it better for tight woods riding with lots of steep uphill trails?
BTW thanks so much for your input gwcrim! :cool:

 

[gwcrim]

That's basically what we ride. My boy is a big 9 yr old and he does great on it.

 

[gwcrim]

That little bike actually scares me more than my 500.

 

[griffbones]

I know this sounds like 100 questions. :blah:
Have you done any special mods or did you just bolt on a stock KX100 top end and a stock KX100 pipe or?

I am getting the engine, electrics, cdi, ignition coil, carb and stock KX100 pipe all from a 2007 KX100. So I guess my set-up will be 100% stock KX100.

 

[gwcrim]

All I did was buy a 100cc jug, piston, and head. The KIPS valve needed shaved a little to clear the piston, but that was basically it.

 

[griffbones]

Thanks again for all the info. :cool:

 

[motometal]

some folks like to add a flywheel weight for woods. I've never tried it.

Personally I thought the KX100 had a good motor but stock had a pretty narrow powerband. Decent low and mid but once it hit it wouldn't rev much above that. Did "team green" porting and careful jetting, with stock pipe and it ran significantly better.

Other mods were a higher pressure radiator cap (stock cooling system is marginal for a heavier rider and/or tight trails and/or wide open running). Carbon fiber reeds (the new ones may come with these). Also heavy springs, especially fork springs...the stockers are way soft if you are jumping at all, of course this depends on rider weight. I think I changed to a heavier fork oil as well, and ran it as high as I could get away with, but for woods this may not be the best setup.

Also depending on the conditions I geared it down quite a bit, one tooth down on the front and may have had a larger rear as well.

Main weak spot other than forks was the powervalve system. I kept breaking the cross pin for the flap valve. No catastrophic failure (no damage to piston and cylinder), but it would just make the engine run poorly because the powervalve would stop working. I'm not sure if this has been fixed/updated.

hope this helps. My current mini is now an RM85L.

 

[lefty20]

Nothing mellow about it. Just beefier and not quite as pipey.

yeah it got more power then my 91 rm 80 if that helps,lol its decent mid good low but overall speed isnt great my little bro lost to a ttr 230 in a long run :O

hers a list of what is is on the kx100,which isnt mine anymore its my bros:
v force stage 2 carbon reeds
pro circut platnum header
ported and polished
acebecs plastics,gripepr seat
excel rims
fmf shorty
powercoated frame,
jetted to not sure what,
silencer,new spokes,renthal chain and swingarm,
protaper bars,protech front suspension,

and thats all i can think of now,its not exactly piped out but its better then stock

 

[griffbones]

You guys are really starting to scare me! My sons KX85 would easily out run his sister’s TTR230. I am starting to get the feeling we are about to go way backwards compared to the 85 engine. Sounds like the 100 is going to be an obsolute dog. The KX100 engine hasn't arrived yet so I am still in suspense.
I guess I could put our 85 top end on this new 100 engine, but I will probably have to buy a new CDI box. In 2006 the stator assemblies changed on both the 85 and the 100, and so did the CDI boxes. The Kawi parts fiche shows the 85 and the 100 as using different CDI boxes so I won't have the correct 85 CDI box to mate up to the new style stator assembly. :yell:

 

[gwcrim]

Give it a try as a 100. I'm not sure why you think it's going to be a dog.

 

[griffbones]

Give it a try as a 100. I'm not sure why you think it's going to be a dog.

I hope you are right, it just seems like everyone is leading me to think that the 100 doesn't have much of a hit and no over rev? I like the idea of good low end and mid power, but my son definetely liked the over rev of his 85. I spent a pretty good sum of money for this engine and related parts so I guess I am just getting nervous about it.

 

[motometal]

overall, it's a great motor. Over-rev is nice but shifting and clutching works too (he will be doing that regardless with this type of bike). Regarding over rev though, if you think about what really makes this motor tick, it is the 85 lower end with a larger bore. I'm pretty sure the carb, exhaust, and even the basic size of the ports is the same (as the 85), so since we are trying to pump a greater volume of gasses throught the same size holes, naturally we can only expect so much out of the over-rev.

 

[Cody305]

We looked at building a supermini and using the 100cc topend to do so.
At least 3 different engine builders said the same thing. Buy a 85cc jug and build that.
The 85cc jug just flat out makes more(read better) usable power than the detuned 100cc jug. I'm not saying that the 100cc motor can't be made fast, just that the 85 is what is being used on 105cc-112cc builds.

Don't take my word for it, call around and find out what works best.

 

[griffbones]

We looked at building a supermini and using the 100cc topend to do so.
At least 3 different engine builders said the same thing. Buy a 85cc jug and build that.
The 85cc jug just flat out makes more(read better) usable power than the detuned 100cc jug. I'm not saying that the 100cc motor can't be made fast, just that the 85 is what is being used on 105cc-112cc builds.

Don't take my word for it, call around and find out what works best.

I completely understand what you are saying, and in fact I am being told that a stock KX85 will out run a stock KX100 too! I still have our 2005 KX85 top end but I will have to replace the CDI as the 100 and the 85 have different advance curves and a different amount of total advance. I can't use the 2005 KX85 CDI box that I have because this (2007) engine has a different stator assembly that will NOT accept the older CDI boxes, it must be a 2006 or 2007 CDI unit to work.
If this stock 100 engine runs as strong as the stock 85 engine did, my son will be happy, but anything less will not please him at all! I would feel the same way if it were my bike.

 

[motometal]

it depends what you want. If you want less peaky, more meaty low end and midrange then the 100 will be good. I'm not sure I would call it "detuned", this is no enduro bike folks...but it is a bigger piston trying to breathe through some of the same holes.

Actually while the KX has a good motor, the RM probably has more potential. It has been updated more recently (as well as the suspension, etc.) I've been pretty impressed! 6th gear wheelies no problem, if I had a 7th gear it would wheelie in 7th as well.

The KX had softer plusher suspension for the woods and handled great, but the RM85L flat rocks the house when the riding gets more agressive. A bit bigger too, for folks like me that aren't kids anymore. :)

 

[lefty20]

You guys are really starting to scare me! My sons KX85 would easily out run his sister’s TTR230. I am starting to get the feeling we are about to go way backwards compared to the 85 engine. Sounds like the 100 is going to be an obsolute dog. The KX100 engine hasn't arrived yet so I am still in suspense.
I guess I could put our 85 top end on this new 100 engine, but I will probably have to buy a new CDI box. In 2006 the stator assemblies changed on both the 85 and the 100, and so did the CDI boxes. The Kawi parts fiche shows the 85 and the 100 as using different CDI boxes so I won't have the correct 85 CDI box to mate up to the new style stator assembly. :yell:

maybe on a shorter distanve or if ur sons a better rider but i tryed my freinds ttr230 and he tryed my kx100 and i was demolishing him untill he slowly (it took like 10 min) got right up side by side...i still beat him but not by very much

 

[griffbones]

Ok I got the new 100 engine installed Friday night and did the first warm up cycle on it, and then last night I did the second warm up and ten minute ride. Today I am going out to do the 20 minute ride, cool down and then the 30 minute ride, cool down and change the oil & spark plug. Then I will get to see first hand how this thing is going to feel. I will give a ride report good or bad after I find out. Wish me luck guys, and BTW thanks for all of the opinions and input on this topic. :cool:

 

[snb73]

Porting will liven the motor up if you need more power.

 

[griffbones]

Ok here is the ride report.
1. The bike now has way more low end power.
2. The bike is smooth yet still has a pretty good 2 stroke hit. It's kind of hard to explain, but it is smooth and yes it still has a nice hit, just not quite as hard.
3. The thing hits the power band way quicker but we did loose a little bit of overall rpm, but it does pull hard up top.
4. We ran 13/50 gearing with the 85 engine, I left that same gearing on for this engine. With the 85 if I rolled on the throttle quick in first gear off of a slow corner it slowly built up to the powerband. Now if I roll on the throttle moderately in first gear off of a slow corner it hits the powerband imeadiately and I have to shift real quick! 2nd gear seems to be a better choice in slow corners now. I really think that I probably need to go back up to the stock 85 gearing of 14/50 or possibly some where in between the 13/50 & the 14/50 gearing. Basically the bike feels like I just dropped a couple more teeth off of the front sprocket of the 85, yet smoother.

In summary I feel that this is going to make an excellent torquey woods bike for my son. BTW I don't know if it is due to Kawasaki adding the vent to the power valve chamber, the updated ignition or what, but this thing still has some decent over rev too, way more than I expected it to have! :cool:

I rode my buddy's kid's CR85 with a Eric Gore 99cc kit and low to mid range port job (bike is used for Harescrambles racing), I thought the two bikes seemed to have some very simillar characteristics. The big bore Honda was a little more potent, but they both were very smooth and turned less rpm's than the stock 85's. My friend rode our bike today too, and he seemed to like it, he also thinks this will make a good woods bike for my son. :cool:

PS. You are right Motometal, this ain't no endure bike, it is definetly a MX bike, just a little smoother than some. :nod:
And Gwcrim, you are also right, there ain't nothing mellow about it! :nod:

 

[motometal]

glad you like it, if it has decent over-rev then that's a good indication that the main jet isn't too rich.

another thing I like about these little bikes is that the tires don't wear out very fast, and when they do they are really cheap, even for the good Dunlops!

The 420 chain was a weak spot on the little KXs I had, put a 428 Renthal kit on and it still wore out fast (at least the chain is cheap), do the new ones come with a 428 yet?

 

[griffbones]

Idle question

The plug seemed to be a nice tan color after my son spent the afternoon riding it. This wasn't doing throttle chops, I just checked the plug at the end of the day.

The only issue I have is that it won't always maintain a good idle. When we got home instead of trying to fix this new carb I just swapped it out for the carb from our 85 engine (that had been working perfectly) and changed out the main jet and needle for the ones for the 100 engine. But it still seemed to have the same identicle problem. I did noticed that with either carb if I turn the air screw all of the way in the engine will not imeadiately die, but after a while will slowly start getting rougher. I started with both carbs set to factory specs, stock jetting and air screw at 1 3/8 turns out. It seems like maybe it will idle better at only 1 turn out but I have not run it long enough at one turn out to be able to tell yet. It really bothers me that the engine will not imeadiately die when I seat the air screw, using either carb. I know for a fact that this current carb is spotless clean and in perfect working condition! If I use the correction factor chart supplied by Kawasaki, it shows that the stock 45 slow jet should be fine at the 45 - 50 degree temps we are currently having. Is it possible that this engine will require a bigger slow jet than what is recommended? I did check for air leaks by spraying brake cleaner all around the cylinder base, carb, carb boot, reed cage etc. and found nothing. Before someone asks, yes I tried setting the curb idle screw on both carbs, it will only raise the idle for a couple seconds and then it falls down again. For some reason the throttle still seems very crisp and responsive, even coming off of idle. So other than this air screw/ low idle issue the bike runs great. But for my son to ride this bike in the woods I need for it to have a decent idle. So does anyone have any answers or ideas on this issue?