Gradual reduction in top speed

Mez · Trackday Trickster Joined: 13 Mar 2004 Karma :

Ok, this is a really really dumb, obvious question but as one or two of you must know by now, I really don't know much about the technical/mechanical aspect of bikes.... anyway...

I bought a brand new 125 last summer, and in the initial few months I could hit 50mph fairly easily, 55mph if I pushed it, but as time has passed, this has dropped to little over 40mph.

What I'm asking is, why does this happen? And can anything be done to bump it back up again? The bike hasn't been particularly well maintained, is it a simple maintenance absence in a certain area? FYI I've just reached 1500km on the clock.

While I'm here... under front end braking, it sort of shudders to a halt... not shudders as such, but kinda start-stops. It's really not as smooth as it was.

When I took it to the garage for it's 2nd service (1st one I did myself with help) I mentioned that the brake fluid level was low as I couldn't see it at all in the little window thing. Having collected it later on and noticing the level had not changed, I asked the guy and he said it was just below the mark and that he would top it up next time I drop in.

Was he wrong? Would a top-up fix the start-stop braking?

Thanks in advance, guys. Mr. Green

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Itchy · Super Spammer Joined: 07 Apr 2005 Karma :

Has the chain been adjusted properly? in that poorly adjusted chains rob you of power.
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Mez · Trackday Trickster Joined: 13 Mar 2004 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Reduction from ~55mph to around 40mph means a fairly hefty loss of power.

Could be simple things such as a dirty air filter, valve clearances out, etc. Could also be more severe things, such as worn piston / bore.

As to the issue with braking, that will not be down to low fluid. Lightly to be a warped disk, or possibly a contaminated disk (ie, oil in one place on it) or possibly something left on the disk (eg, a buildup on a cast iron disk where the pad has been when parked up after being out in the rain).

All the best

Keith
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Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

Yep, sounds like a warped front disk. Also if your brakes are rubbing/binding, it'll soon kill MPH!! Wink

What sort of 125 is it??

2-strokes also lose power if the exhaust becomes coked up. What 2-stroke oil do you use and how often do you cane the bike? Does it spend much of its life pootling round town sat at lights?

If it is a sports 125 (not a scoot) then your power valve servo might be faulty ... (assuming it has a power valve)
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Mez · Trackday Trickster Joined: 13 Mar 2004 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Hold the front of the bike in the air and spin the front wheel. It might be obvious. To make it a bit more obvious hold a screw driver against the fork leg with the tip just off the surface of the disk while you do this (the tiny gap makes any tiny warping of the disk more obvious). Or do it properly with a dial gauge.

However to knock that much speed off I would expect the brake would need to be dragging quite hard, and it would likely get very hot when you were riding the bike. Quite likely that would be very obvious (at least from the smell).

All the best

Keith
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Blue_SV650S · World Chat Champion Joined: 27 Nov 2008 Karma :

Mez · Trackday Trickster Joined: 13 Mar 2004 Karma :

Thanks for the replies guys.

The Shaggy D.A. · Super Spammer Joined: 12 Sep 2008 Karma :

Is there any warranty left on it?
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Mez · Trackday Trickster Joined: 13 Mar 2004 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Speak to the dealers first. Fiddling too much yourself would give them an excuse to get out of the warrenty issue.

What is probably needed is to pull the caliper off and clean the pistons up thoroughly.

All the best

Keith
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rockers · Scooby Slapper Joined: 29 May 2008 Karma :

sorry to hyjack, does the chain tension ajustment make a massive difference in speed ? if the chain is slightly to tight i could see there being a small small decrease but a slightly looser chain would involve less friction ? or am i wrong ?

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Within reason it will not make a difference to top speed. Stretched like a bow string it might (but is also wrecking the sprocket carrier bearing and the output shaft bearing), and massively loose it might manage to slip (but virtually dragging on the ground loose for that).

All the best

Keith
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