YB100 down on power

[russeleoin]

Hi All. I am hoping to benefit from you combined vast experience. I bought a YB100 3 years ago off ebay and I have to say I love it. I use it every day for the 28mile to and from work run and it does me just fine. However I have never owned a 2stroke before and I am noticing a very slight loss of power. My measure is Reigate Hill, I could do 35mph up the hill but now unless I get a run at it I am down to 30( ha bloody ha I hear you say) I can only just overtake these nippers on scooters now and that is bad. Anyway, I have a couple of considerations:

1) With the warmer weather I would expect a drop in power, but would it be noticeable?
2) If I put in a new piston et el, do I have to change the carb settings or is the difference so small it does not matter.

The bike starts first kick and will idle smoothly(for a 2stroke)?

Thanks Russ
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[stinkwheel]

Decoke the exhaust. Check the ignition timing and points gap. Replace the sparkplug. Clean the air filter.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[russeleoin]

Hi Stinkwheel, thanks for that. The exhaust is new( for MOT in Feb10) and the plug and filter were replaced last year (done about 1000 miles) but I will replace them this weekend just in case.

May be all this volcanic ash is detrimental to YB's as well!!

Russ
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[stinkwheel]

Could well be the points then. My girlfriends one was down on power but started well. The ignition timing was retarded due to the points being worn out (easy starting and lack of top end are symptoms of a retarded ignition. The plug would usually be darker than normal too). I replaced them and it made a world of difference.

It's really basic. There is no adjustment for timing as such, you just adjust the gap so they open at the correct timing point. If the gap lands up too big or small when it's timed up, you need to replace the points.

You'll need a flywheel puller to get at the points. A honda one fits them. I ordered part number: 079330010000 from www.davidsilverspares.co.uk . Came in at just over a tenner.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[shleppy]

my rxs100 starts to behave poorly just before the carb messes up and starts flooding. clean the carb and away i go. probs not this but worth a try if uve tried everything else.

[Genital Giant]

Have you tried losing a few stone?
____________________
Harley-Davidson: The most efficient way to convert petrol to noise without the adverse side effect of horsepower.

[27cows]

Yeah, most likely the points to blame. Failing that, probably bore/piston/ring wear. You normally have to put a pretty high mileage on a YB before the power drop off is noticeable, though.
____________________
The RXS100: vehicle of choice for Chuck Norris

[russeleoin]

Ja ja, loose a few pounds, easier said than done. I keep trying to convince her in doors that I am a Chinese Racing Snake in disguise!

I have bought a new plug, fuel and air filter, will fit them this weekend and whilst I am at it I will have the carb off and give it a good scrub. The milage on the bike is 37k, but I have no history on it so I have no idea if the piston has ever been replaced.

I also want to finish up on the GPZ this weekend so with all this nice weather it will be a bike weekend!.
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[EnergyInducti...]

Actually, You dont need a flywheel puller to adjust the ignition on a YB100, Trust me =P I did it myself! Just fit a screwdriver down the slots of the magneto! Easy to adjust =)
____________________
Riding: Yamaha RXS100
Aiming for : RD350 + Test
Dream bike: Suzi RG500 Gamma

[russeleoin]

Thinking about it, the points may well need to be adjusted as I fitted new ones about 5 months ago but didnt start using the bike again until March. I have a puller, oh and being a Rhodesian, I made a plan!
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[27cows]

If it's got 37K on the original piston/bore, that's probably the reason for the decline in power. They need reboring after about 35-40K.
____________________
The RXS100: vehicle of choice for Chuck Norris

[russeleoin]

Hi All. So I replaced air filter, and plug and cleaned out exhaust although it was not dirty at all and I have seen no improvement. Last night I took the carb off, cleaned the bowl and jets with compressed air, and still no improvement.

So, onto the next thought. I can only think now that it is the piston and bore. When I put my thumb over the plug hole, and turn the kick-start by hand, my thumb can remain in place, but does that relate to speed of the engine turn over( am I not pushing down on the kick-start fast enough?) Does anyone have idea's on new pistons? I will take the head off tonight and check on the size of the piston, but is there a rule of thumb for measuring the gap between the piston and cylinder? If I buy a new piston and rings, does that manufacturer matter( I have seen NewOldStock advertised and other stuff on Wemoto, and Yambits). I would rather get the best I can.
Whilst I have have the head off, is there any other maintenance type work I can do that will be good for the bike?

Thanks, Russ
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[stinkwheel]

You would normally measure the piston ring end-gap.

Gently remove a piston ring (they break easily) and insert it in the bore. Use the piston to push it into the barrel so it is sitting totally square in the "working" part of the bore. You can now measure the distance between the ends of the ring using a feeler guage. There will be a value for this in the workshop manual.

If you are going to replace the piston, it would be best to just have it rebored. If the rings are worn out, there is probably significant wear on the bore too. This is a job for a specialist engineer. You'd usually get the next size up piston to the one you have (the size is often marked on the piston crown somewhere if you scrape the crud off). The engineer will machine out the barrel to the correct size.

Of course, you might even find it has a broken piston ring. Not an unusual finding. As such, make sure you stuff a rag into the crankcase opening BEFORE you pull the barrel all the way off the piston. So lift it up maybe 1/2" and shove the rag in. Broken piston ring segments invariably make a bee line for the bottom of your crankcase, then stay there sulking.

Changing the small end bearing while the top end is off is a good idea.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[russeleoin]

Thanks Stink, I have the manual and will check the end gap.

The rest of you ??? what no idea's. I get to pick the collective brain on "1", HA,
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[Noxious89123]

[russeleoin]

I took the head off yesterday, de-coked and all. The piston has an A on it and no size, I presume that means that it is standard. I also measured the ring gap, tolerance from the manual is 0.15 - 0.35mm, I have 0.75mm. I guess new piston and rings is on order, but I dont think that was cause of my power loss. About a month ago, it was okay, the power loss has been quite quick. What I did find was that the araltite repair I had on the pinholes in the floats had come off. I have soldered them closed now. Could the powerloss be due to float position? Possibly too much fuel?
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[27cows]

A leaky float definitely won't have helped the good running of your motor.

Bore wear and the power drop off is usually a gradual process that you don't even notice until you're struggling. The carb looks far and away the most likely culprit.
____________________
The RXS100: vehicle of choice for Chuck Norris

[russeleoin]

Right I will look to the carb again, however I have been thinking!!!
The piston looked in okay condition, the bore was in good nik too. Would it make sense to fit standard rings again? While I dont mind fitting a new piston and all, I would have to send the cylinder off for work and money is tight.

Any thoughts?

Thanks
Russ
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[stinkwheel]

[Vincent]

You need a new ring to measure the end gap with, sometimes just new rings will suffice without having a rebore. Are you sure the "A" on the piston crown isn't an arrow?

[russeleoin]

Hi Vincent, yes definitly an A, the arrow is there too, pointing toward the exhaust port. I have decided to buy the rings off ebay, probably a tenner delivered so I can put up with that. I am sure someone on here would have them off me if the tolerance is out.
I took the old girl out for a spin today, she is definitly a bit better, so I am sure that the original problem was the carb. I will have it off again tomorrow and work on the float height as I think it is just short of flooding. The plug is very slightly black, but not too bad really.

This coming weekend I will have the cylinder off again( hopefully rings will be here) and I want to paint it black. I bought a tin of BBQ black and will use that to make it look good.

Russ
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[Vincent]

Hi Russ,

you need to be a bit cautious about the quality of pattern pistons and rings, best to stick to OEM wherever possible or at least Japanese made parts. I'm not sure if it's the same with the YB but my RD does not get on with pattern head gaskets either.

[russeleoin]

Hi Vincent, thanks you just reminded me about the gaskets!. I just bought these: https://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=360257095886 It is newoldstock. I had though of pattern parts but decided I would not chance it. The saving of a few quid was not worth the worry. One thing I did notice was in the pictures(not only this one) there is a third ring. Looks like some kind of spring, or spacer, which I definitly do not have on the piston now. When I took the piston off, I cleaned up the ring space with a wire brush and there was no third bit!. Is this normal?

Russ
____________________
I love classic merc's but bikes take up less space.
Current ride is a Yamaha YB 100 and I love it! Oooow and now a running GPZ400R!
Next stop, Direct Access hopefully by the end of summer.

[stevo as b4]

If you do decide to fit a set of new rings, then be careful and check the piston ringlands for wear first. There should be a maximum tolerance that is with the ring fitted, there should be a very small gap measurable with a feeler guage between the side of the ring and the ringland groove.

I have had problems before when fitting brand new rings to a worn piston, and for this reason i would reccomend them to be changed together. OEM YB100 engine parts are not expensive and there is only one of everything, so even if the bore is not worn out of the service limits, i would still change the piston at the same time as fitting new rings. As said a good hone will be needed before you put new rings or piston in.

I reckon that you would see a fair bit more sparkling performance from the bike if you convert to electronic ignition if you have points currently. A nice expansion pipe and another 5-8degrees opening time on the disc valve would probably make a bit of difference too and make it a bit more snappy!

[Vincent]