Yamaha PW50 does not want to rev up

[kingscourt]

The PW50 is basically a 49cc scooter dressed up as a little dirt bike, so I'm hoping the expertise on these forums can help me resolve my issue.  Here is the history.  I bought a used PW50 which was running well, but not perfect.  There was a hesitation halfway up the rev range, where it would bog/gargle, and then if you held the throttle open, it would get past that, and rev to the upper range nicely.  In my quest to fix that, here are the things I tried.

-clean carb
-took to the dealer
-baked the exhaust
-replaced carb (new never performed better than old)
-replaced with yet another carb, with same results as previous new carb
-replaced the top end
-replace the reed valve
-tried by-passing the oil injector and using premix instead
-removed spark arrestor from silencer
-ordered a selection of jets (62.5, 65, and 67.5) to complement the 70 stock
-lastly I tried opening up the airbox by drilling four 1/8 inch holes in the airbox and tried running with a dry foam filter element

Along the way I replaced the spark plug several times.  Nothing has resolved my problem.  Right now it starts first kick, idles nicely, and revs nicely until 1/2 to 3/4 of the way up the rev range, where it becomes a turd, and bogs, gargles, and simply does not want to rev. Once it does that gargling, it just seems like it's too rich, which is what lead me to put the holes in the box.  When you remove the airbox/filter, it revs great.

[niz76]

Weird. It seems it would be running lean by putting smaller than stock jets in. Then drilling holes/running open air filter would lean it out even more... But you say it revs fine without the airbox?? Seems strange. Have you tried any bigger jets? Is the needle adjustable? Seems upjetting would be necessary after those airbox mods, but the reed valves could be a factor too... I'm sure another knowledgable member will be along shortly to chime in!
Welcome to the forum BTW- I'm fairly new here but jumped in with both feet and love it!

[kingscourt]

Weird is how I've felt for quite some time. You do what you think should be done, but it just doesn't give the expected results. It seems like a perpetually rich situation that I can't seem to resolve. When I went from 70 to 67.5 it improved a little. Then when I went down to 65 it didn't run as well as the 67.5. Then when I went down to the 62.5 it started making a popping sound that made me think it was too lean, so I went back up to the 67.5. I have not tried anything above 70, because since it sounds like it's drowning in gas in the upper revs, it seems counter productive. But since things just don't make sense with this bike, I actually thought about going higher on the main jet, but I don't have any jets higher than 70.

I appreciate your response. If anything it made me realize that my message is reaching people on this forum. I could tell that there is a lot of knowledge, expertise, and experience on this forum, and hopefully the summation of the symptoms and characteristics will help someone give me the direction and advice that I need. I'm absolutely worn out from trying everything I could think of. The most recent thing I've thought of was to replace the entire exhaust, but I've already unsuccessfully dumped so much money, that I'm reluctant. If I knew it would resolve my issue, I would spend the money in a heartbeat. But I'm all too familiar with spending the money to replace something, and it doesn't accomplish anything. Besides, it runs great without the air-box, so that to me means that the exhaust is fine.

Some other information: I've read that the PW is supposed to be very sensitive to air-screw (pilot jet) changes, but from my experience, it's very unresponsive. I was thinking of experimenting by changing it to a different size, and seeing if it helped. But supposedly the pilot has more to do with low rpm and idling than upper rpm's.

I've had two very mechanically inclined friends take a shot at it, and both gave up. I will say that although both are very good mechanically, neither is a 2-stroke specialist. I know there have got to be some 2-stroke specialists here, and hopefully the right person will read my message.

[2TDave]

Check that you don't have a float that's sticking or has fuel in it, also a stuck enricher maybe a possibility.

[scootertech]

Since the carb has been replaced twice without results check the wire to the enricher to make sure it's getting power. If it doesn't you will be rich all the time.

[kingscourt]

I'm not very mechanically inclined, but I've learned a few things along the way. I know what a float is, but I do not know what an enricher is. Is enricher another term for the choke? Or is it something else.

[2TDave]

It is. On our scoots it allows more fuel in rather than limiting air, thus the term enricher.

[kingscourt]

The "enricher" or choke suggestion made a lot of sense, so I was hoping that was going to be the root of my problem. On the PW there is a cable that runs from just in front of the handle bars, down to the carb, where it enters the top of the carb, alongside the throttle cable. At the bars the choke handle was moving to half and full choke position nicely and freely, and you could see the movement of the cable end. Now the question was if it was moving at the carb end. I unscrewed the screw holding the cover of the carb, and pulled out the throttle and choke assemblies. I once again engaged to half and full choke, and you could see the little piston that slides in the carb move a little for half, and even more for full. At full it was almost entirely compressing the spring. Now the question was if it the little choke piston was fully sitting down at the bottom of the cylinder that it slides into. I measured how much it extends, and how deep the cylinder is, and it's pretty close ... so it should be extending all the way down, and seating where it's supposed to. The measurement was close, so maybe, just maybe it's not extending all the way down to fully disengage the choke. I looked into adjusting the cable slightly, to allow the choke piston to extend just a little further, but I couldn't see how. Some other details related to the choke:   When it's running and you rev it with choke disengaged, it slowly comes back down to a nice low idle. But if you half-choke it when you let go of the throttle and the revs are still elevated, the revs drop quickly but it doesn't turn of the engine. Full choke doesn't turn off the engine, either. I was under the impression that if you choke a running engine, that it would turn it off. I don't know if that little detail means anything, but I threw it out there just in case. I thought it had the 67.5 jet and four 1/8" holes in the box, but it was actually eight 1/8" holes and the 70 jet. It was running pretty good under load, except a little sluggish, and it would bog/gargle, about 3/4 way up. A few times it cleared the bog, and revved pretty decently. Maybe the extra holes in the box are helping. I wanted to drop the jet to 67.5, and see if it helped even more, but I ran out of time. Even though I feel the 8 holes have helped, they should not be necessary when you are running the stock jet size, stock engine, stock carb, and stock exhaust. This makes me think that there is some underlying problem that we are trying to compensate for, by trying to give it more and more air. Tomorrow I will try the 67.5 main jet to see if it improves more. I might also have to look into the pilot jet as far as the idling.

[kingscourt]

Here is the report from yesterday. I put the 67.5 main jet in, and adjusted the airscrew to the manual recommended 1 and 1/2 turns out. When I screwed the airscrew down, it was initially more than 2 full turns out. With those settings, it was just missing one thing --- top end revs. It started first kick every time. Revs wonderfully up to a point, felt pretty responsive, idles perfectly, and after revving, came back down to idle quickly. It felt great, but did not want to really rev out. When I took it for a test ride, it would take off great, rev up nicely, but it just wouldn't scream on the top end. It would either stop before it really wound out on the top end, or it would gargle, and just didn't want to clear out past that.

Then I thought to duct tape the 8 holes in the airbox to see if there was a noticeable difference. To my surprise there really wasn't a noticeable difference. Still ran pretty good, except for the very top end, where it just didn't want to rev.

[niz76]

I would try a bigger main jet. Maybe 72 or 75 and see what happens.

Sent from my XT603 using Tapatalk 2

[kingscourt]

Even though the bigger jet sounds counter intuitive, considering it sounds rich to me, maybe I'm misinterpreting what I'm hearing. Since the problem is in the higher revs which is supposedly linked to the main jet, and going lower on jet size isn't helping, I guess there is no reason to not try a bigger jet. I'm going to order the next 2 sizes above 70, and hope for the best. As always, thanks for the input.

Also, would you recommend I order a selection of pilot jets while I'm at it?  Jets-R-Us offers flat shipping, so if I'm gonna need the alter the pilot jet, too, this would be a perfect time to order.

[niz76]

Doubt you'll need to alter the pilot jet size. Sounds like you have that in tune already. They're cheap though, so depending on your budget...