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Need help on the pw50!

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I have a 2000 model pw50 thats driving me crazy. It starts up first kick every time. It runs and pulls great until it gets to about 18-20 mph and sounds like it hits a rev limiter.you can back off the throttle a bit and it will clean up a little and gain a mile an hour or two  but eventually starts sputtering. No loss of speed when it sputters but no exceleration. Just like it hits a rev limiter.

 

Its had a new top end installed, new plug, cleaned carb at least 6 times. Cleaned exhaust and checked crankcase seals. I have ordered a new air filter, but it makes no difference in how it runs if one is present or not.

 

The adjustment screw is around 1 3/4 turns and it seems to run great at that setting.The needle in the throttle cable is set at the second notch from the top. The jetting is factory and no problem fouling plugs. The plug has around 8 hours on it. It is running on the injection system and it all looks fine. Smokes enough Im pretty sure its oiling.

 

I have had it on the stand running at full throttle and adjusted the screw and it made no difference in the running of the engine.

 

I havent checked the reeds but it starts and runs so well I dont think they are the issue.

 

No exhaust restrictor and the start/stop switch is unhooked.

 

Any help would be great. I have no clue where to turn. My son love this thing. He dreams about it every night. In the morning he says "I was doing Pop wheelies on my dirt bike daddy" Hes 4 and can clear at least 12' on his small ramp! (length, not high) And thats sputttering all the way!! He rides every day 10 times a day. I really want to straiten this out without a dealer keeping his bike for 2 weeks and charging me more than its worth.

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The pw should go around 25 mph. Its deffinately not right the way it is. And yes he probally is over riding it. This boy is a beast on a bike!

But still the pw's I have had experiance with do not run like mine. One that is right will smoke mine.

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That's unreal that he loves it so much!  We had a PW50 for years. My 2 daughters and son all rode it a lot.  2004 model.

 

Our PW50 revved to the moon and went like a rocket. I removed the rev limiter box and exhaust restrictor, and ensured the throttle opened fully.

 

But the biggest gain was opening up the muffler a little. Take out the baffle and do some drilling and bending.  Let the gases out. Noise increase aint much at all.

 

The problem is that Yamaha has a too big a main jet in there stock. Conservative to keep the temps down for mud riding, or whatever. Just too rich.  Open the exhaust to match the jetting and away it goes!

 

My son jumped it a lot. Before he got his KTM50 jnr.  He broke a few sets of PW50 steering bearings and stretched the frame a little. So keep an eye on that area. The forks eventually die too, but they are cheap to replace.

Edited by numroe

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Both the PW50 and PW80 are jetted far too rich from the factory.

Too get the most out of the stock bike, do the following:

1. Thinner air-filter foam

2. Drill holds in air-box

3. Drop main jet by 10 points

4. Lean out needle one notch

5. Colder plug by 2 points ie no6

6. Remove oil injection and run pre-mix at 32:1

7. Gut muffler

8. Grind out exhaust where it bolts onto the head.

After all of these your bike will rev out.

And if you are really keen then you can adjust the clutch for higher engagement.

Two thing you shouldn't do to the PW50 is raise the compression and raise the engine size ie. Big bore kit

keep me informed

Miller

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Both the PW50 and PW80 are jetted far too rich from the factory.

Too get the most out of the stock bike, do the following:

1. Thinner air-filter foam

2. Drill holds in air-box

3. Drop main jet by 10 points

4. Lean out needle one notch

5. Colder plug by 2 points ie no6

6. Remove oil injection and run pre-mix at 32:1

7. Gut muffler

8. Grind out exhaust where it bolts onto the head.

After all of these your bike will rev out.

And if you are really keen then you can adjust the clutch for higher engagement.

Two thing you shouldn't do to the PW50 is raise the compression and raise the engine size ie. Big bore kit

keep me informed

Miller

I do not agree with the "2 things you shouldnt do to the PW50", those things should be done AFTER you do everything else and you bike is running nice. In order to have it running ncie anyhow, you need to set your squish (Raise your compression)

 

Its all about FLOW...

Thanks,

Mark

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I do not agree with the "2 things you shouldnt do to the PW50", those things should be done AFTER you do everything else and you bike is running nice. In order to have it running ncie anyhow, you need to set your squish (Raise your compression)

 

Its all about FLOW...

Thanks,

Mark

 

A KTM 50SX has approx 12hp from a 50cc engine.

A PW50 has approx 2hp from a 50cc engine.

To get maximum performance and power from a two stroke 50cc engine you need to get it to rev high.

If you put a 60cc big bore kit in a PW50 and mill the head to raise the psi compression this is what will happen:

1. You will get a neat little boost in pick up off the line at low revs and you will gain a little mid-range power. Great for putting around the paddock.

2. But your peak HP will drop to around 1.8-1.9HP and your max RPMs will drop by at least 500-800rpms.

A PW50 isn’t going anywhere with a 12mm carb that sits on a restrictive manifold and reeds. The ports are set extremely low and the factory piston/rings will only handle a moderate increase in power.

It you want large increases in performance over and above what I listed earlier you need to do the following:

1. Wiseco Piston 50cc with 1mm rings (stock rings flutter at high RPM’s)

2. 15-16mm carb

3. Mod the manifold internal diameter including reed cage

4. Widen Exhaust port to 68% of piston cordial diameter, Raise the exhaust port high to achieve 180degrees of duration.

5. Widen Transfer ports and than raise height to around 125degrees of duration.

6. Blowdown should be around 26 degrees

7. Adjust the clutch to raise the engagement RPM to around 4000rpm

8. Adjust ignition timing

I highly suggest you invest in a digital REV meter with a MAX RPM recall feature before you do any of these.

Some hour meters have these functions.

After all this is complete then you can mill the head to raise the compression to between 140-150psi. You can go up to 0.5mm squish.

Your bike should be producing around 6 to 7HP at this point.

If you are brave enough then you can put a 60cc big bore kit in.

But chances are you will overheat the engine!!!!

Miller

Edited by miller234

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