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Yamaha BWS 100 2-Stroke Scooter Problem

08/30/2010 3:19 AM

Hi all. I have a Yamaha 100cc 2-stroke scooter. It is self lubbed. I can get about 90km/h out of it, but it is very lazy. Takes ages to get there. The scooter also sort of drop revs and then picks revs up slowly when the accelerator is opened midrange to full.The scooter was serviced. Airfilter and carburator cleaned and new sparkplug. Exhaust was checked as well. Runs on leadfree petrol. When I accelerate from 0km/h with a wide open throttle it slowly picks up revs. Before the first service I had it would rev high on the getaway. I now think they did something wrong when they serviced it. I took it back, but they assured me they did nothing wrong. I changed the sparkplug again, but the result was the same. Any ideas why it does not rev high on the takeoff?

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Guru
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#1

Re: Yamaha BWS 100 2-stroke scooter problem

08/30/2010 3:52 AM

Does it rev freely & smoothly off load?

80% of problems are carburation, the other 80% are ignition.

With only a 100cc to play with timing and fuel mixture are critical.
Check the timing. If you want it done properly, learn how to do it yourself.
Check you have a big fat BLUE spark. If you want it done properly, learn how to do it yourself.
Check the air filter.If you want it done properly, learn how to do it yourself.
... and fuel line filter. If you want it done properly, learn how to do it yourself.
Check the tank for sediment. If you want it done properly, learn how to do it yourself.
Strip and clean the carb...heck, you can guess what I'm goint to say next.
None of it is rocket science, get someone to work though it with you first time, be slow tidy and methodical, use the right tools and if you get stuck ask.

Check the ancilliary stuff too, like all oil levels, and that your brakes aren't binding. Remove any anvils or cows from the vehicle too.
Del

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Commentator

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#2
In reply to #1

Re: Yamaha BWS 100 2-stroke scooter problem

08/30/2010 4:13 AM

Del, I grew up with 2 strokes. When I was still at school I stripped the carbs and engines cleaned the airfilter and burnt the carbon from the exhaust. I even changed the rings on the piston after seizures. So I can do all that stuff. I am older now and dont have the time to do all that stuff. I thought maybe there was someone who had a similar problem. Quick fix you know?

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Guru
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#3
In reply to #2

Re: Yamaha BWS 100 2-stroke scooter problem

08/30/2010 5:00 AM

Ah yes the work life ballance... welcome to the 21st century where the problem will be too much leisure time and free energy.
That's what they told us back in the late 50s.
Good luck...
If I had to go for one thing, I'd say timing.
Those guys who sell sparkplugs must make a fortune, I've only ever known one plug to actually fail. Plenty needed cleaning or re-gapping.
Del

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#13
In reply to #2

Re: Yamaha BWS 100 2-stroke scooter problem

11/26/2010 4:39 PM

Do you know where I can get a manual for the BWS 100?

Thanks

S

shayla@tampabay.rr.com

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Guru
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#4

Re: Yamaha BWS 100 2-stroke scooter problem

08/30/2010 5:19 AM

I'd agree with Del, probably timing. I had similar problems many moons ago when I had 2-strokes. Once the timing was too retarded (a bit like the rider!), or was not advancing properly, the other time the ignition coil was on it's way out, but that showed other syptoms too, so more likely just the timing.

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#5

Re: Yamaha BWS 100 2-Stroke Scooter Problem

08/30/2010 9:56 AM

Wrong spark plug.

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Commentator

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#8
In reply to #5

Re: Yamaha BWS 100 2-Stroke Scooter Problem

08/31/2010 2:30 AM

Nope. It is according to the spec.

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Guru

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#6

Re: Yamaha BWS 100 2-Stroke Scooter Problem

08/30/2010 12:09 PM
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#7

Re: Yamaha BWS 100 2-Stroke Scooter Problem

08/30/2010 11:02 PM

I'd say carburetion. Sounds similar to a racing kart I raced several years ago. 100cc single cyl 2 stroke fixed drive, no clutch.

Went out onto the track one session and when I used anything over 1/2 throttle it actually ran slower. Turned out to be a diaphram/gasket in the carb. part of it operates like a reed valve over an orifice inside. It was faulty. It was making the mixture way too lean, too early and hence the motor did not want to rev up. Put in a new part (diaphragm/gasket) and ran like a bought one, as they say.

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Anonymous Poster
#9

Re: Yamaha BWS 100 2-Stroke Scooter Problem

08/31/2010 8:34 AM

The guy with the diaphram is likley correct. I have fixed hundreds of chainsaws and countless outboards. The first thing I do is clean the carb and replace any diaphrams even if they look good. It seems they fatigue over time is the problem especialy if the carb heats up any after you shut the engine off. It also seems that ethanol harms these diaphrams too by making them too flexible. Make her happy with a new diaphram.

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Commentator

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#10
In reply to #9

Re: Yamaha BWS 100 2-Stroke Scooter Problem

09/01/2010 2:22 AM

I will try this and come back to your guys. Thanks

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Commentator

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#11
In reply to #10

Re: Yamaha BWS 100 2-Stroke Scooter Problem

09/08/2010 2:54 AM

Yesterday I opened up the carb. I noticed the clip on the needle was in a groove near the top. It spelled a lean fuel setting to me. I dropped it one groove to lift the needle somewhat. Problem solved. The hesitation when I opened the throttle was gone. Thanks guys.

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Guru
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#12
In reply to #11

Re: Yamaha BWS 100 2-Stroke Scooter Problem

09/08/2010 4:09 AM

Well done. And thanks for letting us know. It is always good to hear of the eventual solution, and how it comes about. It makes our discussions more meaningful, and we can all learn from the experience. Cheers.

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#14
In reply to #11

Re: Yamaha BWS 100 2-Stroke Scooter Problem

04/23/2014 11:51 AM

Hey Carel,

Great to hear u fixed the problem using needle seat positions, if you have the problem again one thing I have found with these bikes is they are very fussy as to how much air they get...too much and they run like a wet noodle, very lazy etc. But as long as they have the stock air filter on them with also the stock restrictor in the air intake its all good. You are essentially doing the same thing as u did when correcting the needle clip positions (ie richening the air/fuel mix) but from the air intake side of things . I had this problem and it was solved by making sure my air box was properly setup with stock restrictor in place, went like a gun immediately after I corrected it.

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