2 stroke scooter engine suddenly lacks power when going up a hill

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  #1  
Old 10-16-08, 09:56 PM
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2 stroke scooter engine suddenly lacks power when going up a hill

I own a Yamaha BWS 50 scooter (2 stroke). It runs great but recently, the engine will "bog down" when going up a long hill at full throttle. Naturally, with a 50 cc engine, the scooter engine will slow down going up hills, but this is different. The engine suddenly lacks power, almost like it is running out of fuel. When this happens, I release the throttle once or twice and its fine after that.

There is no fuel filter on this bike. It never happens on a straight road, even a full throttle. This is not a transmission problem, it's definitely the engine lacking power for whatever reason. Could the carb become flooded when going up a long hill at full throttle ?
 
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  #2  
Old 10-17-08, 07:35 AM
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No, Paul, it would a lack of fuel about 90 per cent of the time.

In that context I would say you have a fuel flow restriction which could either be in the tank, lines, or carb itself.

If it "fixes itself" when you close the throttle and reopen it, I would lean toward something in the float bowl.

What year is your scooter - I'm working off a '99 diagram showing a standard float carb (versus an all position carb). With that carb the above would get you in the right direction.

If you have the float bowl carb with the slide throttle, I would first disconnect the line from the carburetor, empty the tank with the fuel line into a clear container. Watch the flow of fuel from the line and then check the container for residues. If the flow is restricted to any level you would likely have an obstruction in tank. Most tanks have a screen to filter the fuel. That could be plugged. Usually when you take out the petcock, you'll find the screen attached.

If you determine that fuel path is clear, the carb would need to be taken apart and cleaned.

Post back with the year and we'll look at it a little more.
 
  #3  
Old 10-17-08, 05:44 PM
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Thanks a million Marbobj. The scooter is an 02. I have a service manual I downloaded off the 'net. I can e-mail it to you if you give me your e-mail address. The bike has a float bowl carb.

I'll drain the tank and see what I get.

Paul
 
  #4  
Old 10-17-08, 06:37 PM
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No on the manual. I shouldn't need it. An 02 Yammer Hammer, huh? The carb is the same one I looked at. See if you get something when you work on the tank, then post back if we need to take it to the carburetor.

It should be something pretty simple if it normally runs well.
 
  #5  
Old 10-19-08, 09:30 AM
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Also, check the fuel cap or tank vent for restriction, this was a problem for me I almost never found.
 
  #6  
Old 10-26-08, 05:23 PM
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I have the same problem on a 2 stroke pocket bike
 
  #7  
Old 12-02-08, 01:03 PM
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I finally got to removing the fuel hose from the carb. I expected the fuel tank to drain through the hose but nothing came out. There is no petcock valve on this bike. I sucked on the fuel hose and could not suck any fuel out of the tank. Yet once I reconnect the fuel hose to the carb, the bike starts up fine ! There is a device at the bottom of the fuel tank. It has a bowl on it. I removed it and it was clean. The only way I could drain the tank was to start the engine and the fuel drained out of this device (with the bowl removed). The fuel was clean.

So I am assuming I have some pluggage in the carb jet ? Any advice on taking this apart would be appreciated.
 
  #8  
Old 12-03-08, 04:05 AM
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These little 50's have a governor built in the carb. or so I have been told and it could be this that is malfunctioning. Have you ever noticed how they do not go much faster down hill as on level ground? to control the speed they richen up the carb. if they wern't designed this way they could easily reach 45mph. my old yamahopper would barely reach 30 mph. before the governor kicked in. I suspect something in the carb. for sure, did it always do this or did it just start?
 
  #9  
Old 12-03-08, 08:29 AM
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This just started. It ran fine going hills in the past.
 
  #10  
Old 12-05-08, 11:16 PM
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My problem is getting worse. What started as an intermittant problem now happens on every hill. As I start climbing a hill, the scooter slows down (normal) and then the engine bogs down and looses almost all power. This happens at WOT. If I go up a hill at 1/2 trottle I am OK albeit I go up the hill very slowly. Once back on level ground, I can accelerate slowly and once up to say 50 km/hour (30 mph) I can go to WOT and the engine revs up and runs fine. Until I hit the next hill.

In a nutshell, my BWS engine bogs anytime I am under load (going up a hill) and at more than 1/2 throttle.

My next step is to take the carb apart and clean it.

Could my muffler by clogged ? Are the symptoms I describe consistent with a clogged muffler ? The scooter has 10000 km on it. It never used to do this.
 
  #11  
Old 01-15-09, 08:11 PM
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I finally got around to cleaning the carb and it fixed nothing. The scooter is still bogs down at full throttle going up a hill or accelerating from a dead stop (again at full throttle). Note that slowly go to full throttle on flat surface, then no problem. Anyone have any suggestions ?
 
  #12  
Old 01-16-09, 03:38 PM
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Did I understand you to be saying that if you remove the hose from the carb and suck on it you can't get any fuel flow? That sounds to me like what KattSilver said a few replies back, take off the fuel tank cap and see if you get any fuel flow then. If you do then clean the vent in the cap, or if that isn't possible replace it.
 
  #13  
Old 01-20-09, 08:40 PM
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I can't suck any fuel out even with the cap removed. Yet when I connect the fuel line to the carb the bike starts up fine and runs fine (other than at WOT). So evidently, fuel is flowing to the carb. I stumped.
 
  #14  
Old 01-22-09, 10:39 PM
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older two stroke scooters used to clog mufflers. It was in the repair manual to burn them out with a torch. The oily exhaust used to build up and clog the muffler. What tuning have you done? Do you have a repair manual? That is a reed valve engine. A cracked reed can effect performance from idle to wot. Have you checked/read your spark plug?
 
  #15  
Old 04-12-09, 03:31 PM
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Resolved?

I think my Yamaha Riva 1996 is having the same problem. Did you get this resolved?
Originally Posted by Paul Doyon View Post
I own a Yamaha BWS 50 scooter (2 stroke). It runs great but recently, the engine will "bog down" when going up a long hill at full throttle. Naturally, with a 50 cc engine, the scooter engine will slow down going up hills, but this is different. The engine suddenly lacks power, almost like it is running out of fuel. When this happens, I release the throttle once or twice and its fine after that.
 
  #16  
Old 04-20-09, 09:29 PM
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Yes it is resolved

I replaced the torque spring on the transmission rear pulley. My original spring must have become too soft (not stiff enough) over time thus causing the scoot to stay in too high a gear as it was slowing down. This was flooding the carb and causing it to eventually stall. I put in a new stock spring and it now works well.
 
  #17  
Old 01-10-11, 11:07 AM
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Cool 2 stroke bog

check that the screen in mufler is not clogged.that was my prob.it runs like new now.same prob you have.best of luck.
 
  #18  
Old 03-21-14, 05:40 PM
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more info please

I know this thread is really old but could you be more specific on what you mean by replaced "the torque spring on the transmission rear pulley"? Do you have any pics or diagrams?
I'm having the exact same problem on a Yamaha Jog 100cc scooter, sudden loss of power and everything fine if I release the throttle and pull again, took it to a "mechanic" but he claims nothing is wrong and only replaced the air filter. Now it has gotten worse, it losses power and completely stalls and stops and I have to push the kickstarter once or twice before the electric start will work again, and then everything fine again for a couple of miles till I have to repeat.
Nothing goes wrong if I stay at less than say... 3/4 or 1/2 of throttle.
 
  #19  
Old 03-22-14, 04:40 AM
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What do you mean by "release the throttle and pull again"?

Be more specific by what you with "stalls". Does the engine quit running? When does the engine quit running?
 
  #20  
Old 04-05-14, 05:35 PM
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is the fuel tank under where your feet go? Does it have a vacumn operated fuel pump? Or does it have a carb that has the fuel pump within the carb?
 
  #21  
Old 08-04-14, 08:57 PM
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Hi, thanks for your answers, for some reason I wasn't receiving notifications on updates to this post.

Release the throttle: let go off it when driving the scooter, let it go to its original position as I would if I want to stop the scooter (no acceleration). After this I accelerate again and this prevents the engine from "stalling".

Stalling: Yes, the engine quits running if I keep going and don't release the throttle. It will vibrate for a couple of seconds and stop. Originally happened at top/high speeds but now it has worsen and will happen almost continuosly, obviously can no longer use my scooter until I fix this.
 
  #22  
Old 08-04-14, 09:00 PM
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Hi, no, the fuel tank is under the seat, but it does have a vacuum operated pump which has been replaced, and the problem persists.

Carb has been cleaned and adjusted, electric system checked, a fuel filter was installed. All of this will temporarily solve the problem but it will just come back after a few days of use.
 
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