How can I detune a KX60?

Joined
May 1, 2000
Messages
156
Likes
0
#1
I got a KX60 for my son, and it's too much to handle. I'll have to get him a PW80 or XR70 or something if I can't mellow this thing out. Any suggestions?
I don't want to buy a pipe, which probably won't make enough difference. It seems that the port timing is the problem, and I can't change that. How about the ignition? If I retard it, will that help?
I don't care if it loses some power.
-Toby
 
Joined
Apr 9, 2003
Messages
819
Likes
0
#2
You could put a flywheel weight on it or try a larger rear sprocket. It will slow it down a bit and mellow out the hit.
 
Joined
Oct 12, 2003
Messages
66
Likes
0
#3
When we bought our first KX60 in 1998 it was secondhand and the guy we bought it from gave me an aluminium plate, around 1/16" or 3/32" thick with an oblong hole cut in the centre. He said it cut back the horsepower considerably. The plate needs to be the same size as the inlet stub and it fits between the stub/reed block and the carb. I can't remember the dimensions now but essentially you are restricting the airflow/fuel mix into the engine. I'd guess you need to make the aperture around 75% of the original opening. You can use the reed block to mark the shape of the plate and the position of the four holes that locate the finished item on the studs. As long as the plate isn't too thick there is enough thread on the studs without having to change them for longer ones.
Don't gear it down with a larger rear sprocket - it only makes them more responsive and easier to spin the rear wheel. Depending how old your rider is, I found it only takes a couple of weeks for them to learn to moderate throttle control anyway. Both my lads were eight when we got them KX60s - They now ride 85s so they lived through whatever the KX could sling at them!
Good luck.
 

gwcrim

Subscriber
Joined
Oct 3, 2002
Messages
1,881
Likes
0
#4
There might be some way to put a throttle stop on it. Or... PW80s come with a washer in the exhaust port that restricts flow.

I do agree that your boy will probably learn to handle it pretty quickly if he gets to ride much.
 
Joined
May 1, 2000
Messages
156
Likes
0
#5
Has anyone tried moose torque reeds? Do they soften the hit? Their website says so, but I'm wondering if the improvement is marginal, and the hit is more controled by the pipe.
-Toby
 
Joined
Jan 16, 2003
Messages
282
Likes
0
#6
I would agree with Martin Cole. That sounds like a good plan. As far as the reeds go, the first ride will probably be noticeable if he has riden it enough to feel the bike. But I don't think the reeds would do that much, correct me if I'm wrong.

-Red Rider
 
Joined
May 1, 2000
Messages
156
Likes
0
#7
I tried the Martin's suggestion. It does seem to help, a little bit with the hit, but it keeps it from running away after the hit. I figured it would be impossible to nail the hit exactly with the restricted flow. I think this is probably a better fix than a simple throttle stop. I put an oblong opening in about the bottom 2/3 of the inlet hole that goes through the rubber manifold. I used stiff /060" gasket material. If it works, and it seems like it will be there a while, I'll substitute aluminum.

The reed change still sounds interesting. I felt these ones, and they're pretty stiff. It could be that the hit comes when the air flow gets fast enough to open those reeds better. That hit seems pretty hard, like something mechanical is happening (e.g. reeds).

I wouldn't mind getting a pipe off ebay and tweaking it at some point. Maybe a fatter or shorter stinger, or increase the header volume. Any ideas on this?

-Toby
 
Joined
Oct 12, 2003
Messages
66
Likes
0
#8
I'm pleased the restrictor plate idea had some effect but I suppose it's only natural you don't get a good run-on. Another tip could be to try a couple of extra cylinder base gaskets and/or another head gasket. This would slightly alter the port timing and reduce the compression ratio. One or both ideas are inexpensive and you have'nt made any mechanical changes that can't be rectified quickly.
Here in the UK most people tended to stick with the stock KX60 pipe as aftermarket units produced little benefit for the cost. It is a different story with the KX65 which does seem to respond better when fitted with Doma or DEP pipes. I've seen some with Pro-Circuit and FMF but personally I can't give you any advice here as we don't race in that class any more. However, most of the quicker kids all seem to be equipped with Doma - but then the pipe is only part of the story - you'll get good results with well set-up machine that really handles and inspires confidence from the rider - and better reliability. A Technoflex rear shock, extended swinging arm and better brakes are all the KX60 needs to put them on equal terms with the KX65s and KTMs - but if you are spending that much you might as well buy a KX65 anyway.
 
Joined
Mar 23, 2002
Messages
37
Likes
0
#9
i remeber people having problems when adding base gaskets to the 60's, but someone in the UK did make thicker head gaskets that reduced the compression when the KX60 was run in the auto class with the centrifugal clutch. Can't remember who did the auto conversions, but if you can find out I would contact them....
 
Joined
May 1, 2000
Messages
156
Likes
0
#10
I thought of adding gaskets. Adding base gaskets would make the transfer port opening more advanced, I think. It would be something you would do to make it more radical, producing power at higher RPM's.

Adding a head gasket or getting a thicker head gasket might help. That would lower the compression. The flywheel is a good idea, but I'm not sure I want to spend the money. I might just sell it and get a mellower bike.

Does anyone think that leaky crank seals might cause the problem? It seems that that would affect it across the rpm range. This bike does have a lot of oil in the exhaust. I'm still not convinced it's running like it should.

-Toby
 
Joined
Oct 12, 2003
Messages
66
Likes
0
#11
Hi again, I don't think you can find anything more mellow in that size range. You'd probably be looking at a PW80 and the gap is really too great. Can your son change gear easily yet? If he can, get him to short shift through to third or fourth and that will mellow it out a lot. When my kids started I had them on a tight track with hardly any straights and plenty of small jumps. I would push start them in second gear and let them get a feel for it. After about 10-15 laps I pulled them in for a rest and then push started them in third. They found that great as the acceleration was more controllable and by then they were getting used to keeping it rolling. As they grew in confidence I got them doing clutchless shifts and they were not long picking it up. It was better than riding a flat grass field where, if left alone, they just go in ever decreasing circles. A couple of weeks of that and they soon start jumping instead of rolling and quickly learn to use the power band to get a better take-off. Personally, I'd persevere instead of changing the bike.
 

Jasle

Sponsoring Member
Joined
Nov 27, 2001
Messages
1,358
Likes
0
#12
the auto clutch is made for the kx by gene berg. basically you just leave it in 3rd and run around the track like it was a cobra or ktm 50.
Personally I'm for just letting the kid learn the bike. Since I am a Cobra dealer we bought my son the Cobra DC65. its way mean. nasty power band. way faster than the kx65. 4hp more.. So anyway my kid is a newby coming off a 50. At first he cannot ride the DC65 for crap and is 30 seconds a lap faster on his 50 than his 65. We just gave him time. he is just now coming in to his own on the 65. Lap times are pretty close and I'd suspect in the next month or so he'll be faster on the 65.

One trick reccomended by his coach was to use the large flat parking area of the track. He just had Will blast from one end of the parking lot to the other rowing through the gears and feelling the power band. He'd ride next to him telling him when to shift. Helped him get over the feeling that the bike had too much power fairly quickly

We also start in 2nd gear. 1st has just too much power.
 
Joined
May 1, 2000
Messages
156
Likes
0
#13
I got the flywheel weight, and I got some Boyesen 2 stage power reeds. The reeds were only $21 shipped from Boyesen, so I thought they were worth a try. I'll let him go from there.
-Toby
 

Jman271

Subscriber
Joined
Oct 18, 2001
Messages
317
Likes
0
#14
To tame down a pipey powerband, you can turn down the cylinder base to lower port timing, raising it by adding gaskets will only decrease effective stroke, increase trans duration, and make it worse. You have to check squish before you take material off the base to see how much you can get away w/ taking off. It will be kinda hard to get an accurate reading b/c it has a dome piston but you could realistically be around -1mm, but make sure you run good fuel. Also, I did a auto clutch, it was the best mod yet. It drags out the hit from roll on, so you get this longer climb to WOT, it also keeps the kids worrying about riding, and not about shifting. Most of the kids I see riding them leave it in 2nd gear and ring the p*ss out of it anyway. If none of these options sound like where you want to be, you could make a throttle gov out of surgical tubing and insert it inside the cable in the cap. Just a thought, see ya-
 

fuzzy

Subscriber
Joined
Jul 26, 2002
Messages
447
Likes
0
#15
Most "hit" on a 2-stroke can be easily cured w/ jetting. Most MX bikes(even more so on smaller bores) are jetted from the factory to produce a big "hit." Most MX riders want this surge "on the pipe" when exiting corners and such....Especially on a 60 as the little thing has to accelerate fast enough to clear the same jumps as a big bike(and the damn kid riding it usually has balls of steel).

On a stock bike, the jetting is usually rich accross the board so I'd try a leaner pilot, and main. The biggest thing that will change your "hit" is the needle(also the most neglected w/ jetting)--get one w/ a less-radical taper along with being richer.

I can change my WR from a 500-style wheelie monster to electric-smooth via needle changes....Both would dyno the same net HP on top.