Need some advice on two stroke bikes - Wrist Twisters
 
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post #1 of 12 Old 04-15-2010, 08:16 PM Thread Starter
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Need some advice on two stroke bikes

I have three sons. They are 17, 14 and 10. The 14 and 10 year old have decided they want to race motocross. The 14 year old wants to make it a career. The 10 year old is in it for the fun.

Here's the deal... we currently own four stroke Honda CRF's. We had three and are down to two... an 80 and a 100. We are selling to buy two stroke bikes. Looks like we are looking for a Kawi KX 65 and either a Yamaha YZ 85 or Honda CR 85.

I know nothing about two stroke bikes. I am an engineer and do most all of the current maintenance on our bikes, which isn't much. They just don't need it. I see adds for the two strokes with rebuilt engines...pistons, rods, etc... and then with just new reed valves.

Are they difficult to work on? Do they need constant adjustment/work or whatever? How long do reed valves last? Easy to replace? At this point I'm not even sure what to ask... I don't know what I don't know.

Any ideas what I should look for when I go look at these bikes? Any problem areas that I can focus on?

Any and all advice is appreciated.

Doug

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post #2 of 12 Old 04-15-2010, 08:24 PM
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get a CRF150R for the 14 year old... 22hp outta the box... 28hp easily made with mods in the size of a CR85 (it uses the CR85 frame) kx65 seems bout right for the 10 yr old.

if you wanna go all 2 smoke just make sure theres still parts availability for em all the two strokes u listed are pretty popular so i dont think you would have a problem. 2 strokes like you said are pretty simple... reed valves, and piston n rings every now n then is really all thats needed.

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post #3 of 12 Old 04-15-2010, 08:46 PM
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That's not much of anything that needs adjustment on a 2 stroke. no valves to shim or anything. a top end rebuild consist of replacing the rings and running a bore hone down the cylinder. Stock up on spark plugs 2 strokes foul them alot.

The 2strokes in my family don't go through reed valves often unless they don;t get ridden it seems.

Common sense. So rare it's a god damn super power.
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post #4 of 12 Old 04-15-2010, 08:47 PM
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I don't have a whole lot of experience with them but have had some. They are dirt simple. The concensus on bigger bikes is they are actually less maintenance than current MX four strokes (note not your CRF's those things are bombproof trail bikes that will live forever) but 80's may be a different story.

I imagine with today's coated bore's you just slip in a ring every so often, ring + piston at some greater interval & ring + piston + cylinder at some large interval.

Make sure the low end is good if you can, that is where it gets expensive & more time consuming. Pop off the magneto cover, grab the flywheel & try to move it up & down. Any up & down movement & it needs a low end. A little side to side is OK but not much.

I managed to turn this (exactly as I bought it & brought it home):





Into this:






Learned alot, started right up, wasted WAAAYYY to much money (over $260 on missing engine parts alone). Sold it for about $280 less than I had in it

PS, not to burst any bubbles but 14 is a little old to decide to make it a career. Kids make their debuts as pros around 17 unless they recently put in an age limit. They usually start around 4. Doesn't mean can't have fun though.

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post #5 of 12 Old 04-15-2010, 09:29 PM
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I don't know much about the smaller crf bikes but the 450 is highmaintanace but then again neither of your children have any business on a 450 dirt bike. I couldn't imagine them being high maintanace like the 450. Most of the 450 MX motors out there are high maintanence do to the level the y are designed to compete at.

Common sense. So rare it's a god damn super power.
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post #6 of 12 Old 04-15-2010, 10:04 PM
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there will be valve adjustments on the crf150.

I would recomend a 125 2 stroke for the 14 year old. my neighbors kid is 13 and has an 85 and it's not enough for him and he's only started mx racing. Coarse he has been tryin to keep up with our built 450 based quads so he has a steep learning curve.

Common sense. So rare it's a god damn super power.
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post #7 of 12 Old 04-16-2010, 06:59 AM Thread Starter
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Thanks folks for the advice. No bubble bursting here. He knows it's a huge learning curve. We went to watch some amateur AMA racing around here a couple of weeks ago. After watching the first 85 class he turned to me and said, "I think I'm gonna need a lot more practice."

I work in the NASCAR industry and am hoping that a little good old American know who will help. Maybe he can start by sweeping the shop somewhere to get his foot in the door... or at least sit down with a current pro and ask questions.

If he doesn't make it, at least we are going to have some fun. There is lots of AMA sanctioned and outlaw racing within an hour or two of our house. I think I'm looking forward to it as much as the boys are.

Doug

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post #8 of 12 Old 04-16-2010, 07:13 AM
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Like others have noted, two-strokes are quite simple to work on. Why have you decided to go with two-strokes? I thought that things were moving pretty quickly over to the four-strokes - even for racing. I know that for quad racing it is quite rare to see a two-stroke at this point.

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post #9 of 12 Old 04-16-2010, 10:52 AM Thread Starter
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The way the AMA has classes set up, the smaller four strokes don't compete. I would love to have a CRF150R but there is no class that it runs in. Once you get to bigger bikes... 250cc's and up, then the four strokes make sense and you see two and four strokes running in the same classes. It's the 65 and 85 classes that seem to be mostly two stroke.

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post #10 of 12 Old 04-16-2010, 12:54 PM
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Two strokes are the simplest & cheapest engines to maintain and or rebuild. Reed valves are a breeze to replace. You can find alot of useful info on them at Boyesen Engineering
If you google the word "two stroke" and click on images you will see just how simple they really are. The Maxima oils website has a very cool oil migration diagram as well as alot of great info on premix oils.
The four stroke CRF150R that nd4spdbh mentioned is a great little bike, but it's been know to have cam breakage and valve train issues. When you blow one up (notice I didn't say "if"), it's going to cost you at least four times more than it would to rebuild a two stroke. It looks like you already figured out that not all tracks have a class for it.

If two strokes are jetted correctly for the premix oil / ratio you're running, your air filter is clean (this is vital with two strokes) and you're not lugging it around letting load up, then modern two strokes don't really foul plugs. I haven't fould a sparkplug on a two stroke since the early 80's. (I'm currently racing a 2009 YZ250 two stroke)

Don't let what Bryce said about age discourage your Son from becoming a Pro (although for the most part he's right). John Dowd didn't start racing Motocross until he was 20 years old and at 14, Jeremy McGrath was still racing BMX. If your son has desire and he can block out alot of teenage distractions and dedicate himself, 14 is not to old to try and make a career of it.

Best of luck to your boys!

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post #11 of 12 Old 04-16-2010, 02:14 PM
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I would listen to that man above me. He's rolled around in a lot of dirt.

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post #12 of 12 Old 04-16-2010, 08:46 PM
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Only problems you'll have working on 2strokes are the bottom end. You'd do well to leave that to a shop as special tools are required. They had better get started, especially the one wanting to make a career out of it. He's about 9-10 years older than most mx racers when they started out.

XX motor in a 900 frame...Come get some.
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