Scooter Problem

T

Tosser

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My Dad got himself one of these cheap little Chinese scooters as a runabout, called a Kazuma I think.

Anyway, info on these things are very thin, and he has been experiencing a strange problem which he has been unable to diagnose. I should add that he is no amatuer when it comes to mechanics, so it's unusual for him to be stumped like this.

The problem exists only when the bike is hot. When cold everthing is fine, but after riding for 3km's, it feels as if the bike is "pulling back", with a loss of power.
When the bike is hot it won't start either.

Have checked the following:
Spark plug - has some oil on it but otherwise seems fine, bike isin't smoking excessively so this doesn't seem to be the problem?
2 stroke mix - the scooter has separate tanks & mixes these automatically. Also tested this by manually forcing the scooter to use only 1 tank ( manually mixed & blocked off the 2 stoke oil tank)
Rings checked & seem to be OK.

Dunno what else to check actually, but I'm wondering if incorrect valve clearances could cause this.
Any suggestions?
 
Have you checked the rings when they're hot?

Only thing I can think of is that the rings and/or cylinder walls expand because of the heat which would definitly lead to a loss of power when the gas start leaking round the rings
 
Check to see if its the right type of plug, maybe you are using a cold spark plug or something, I had a problem with the wrong spark plug once and the lack of power was noticeable.
 
My money is on rings. Have the dealer replace them through the warranty, if it's still covered. Is the fuel tank vented? Next time it loses power, pop up the seat and take off the gas tank cap. Another thing that it could be is vapourisation. If you have a fuel line running near the engine for instance, the heat evaporates the fuel in the line and starves the engine of fuel. Stupid question but it happens to me often: is the fuel tap open?
 
Tux said:
Have you checked the rings when they're hot?

Only thing I can think of is that the rings and/or cylinder walls expand because of the heat which would definitly lead to a loss of power when the gas start leaking round the rings
Click to expand...
How would you check piston ring gaps when hot?
Piston ring gaps are checked cold in the bore and a standard is used which accounts for the bore diameter and type of ring to determine the gap when measured this way cold.

metalcore said:
Check to see if its the right type of plug, maybe you are using a cold spark plug or something, I had a problem with the wrong spark plug once and the lack of power was noticeable.
Click to expand...
These small 2-stroke engines are very sensitive to heat range of plugs - this would be my first check.
 
wabbit16 said:
Stupid question but it happens to me often: is the fuel tap open?
Click to expand...

+1

Number 1 cause of my bikes problems. Also check the fuel pipe as mine had cracked and actually burst (thank goodness it was parked then). It could be fuel starvation (check the filter too).

I cannot comment on the actual engine problems, but having a bike is a never ending learning experience.
 
Tosser said:
The problem exists only when the bike is hot. When cold everthing is fine, but after riding for 3km's, it feels as if the bike is "pulling back", with a loss of power.
When the bike is hot it won't start either.
Click to expand...

not familiar with the bike, so here are some things to think about
1) the choke is still on.
2) perhaps the choke is off, you think, but it is still on (lever/cable broke)
3) nah werner, the choke is automatic....in which case, it is still on and the mechanisn is broke.
4) werner u knob, i have checked everything to do with the choke, and it is ok. in which case, screw off the top of the carb (the accelerator cable end)...inside you will find a brass needle, about 2 or 3 inches long. This has a clip at the top, and various notches...maybe 3 or 5. Normally it is in the middle notch. Moving the clip UP means the needle sits lower...and this leans out the mixture over the rev range. Moving the clip DOWN a notch increases the fuel in the mixture. See which one applies.

P.S. I'm going down this route because as you say, when cold all is ok. If it was lean, you would say "problem starting when cold etc"..
It could also be a partial seizure as mentioned by previous posters, but as you say this has been checked and you even ran it on premix leads me to belive we arent correct to go down that path.

If your old man knows his stuff then checking the choke and needle position is a piece of piss. Sometimes the clip even goes missing and the needle just sits there floating around depending on air pressure, making for interesting time. (clip=circlip...very small)
 
stoke said:
werner, that is an awesome response, and thanks for the tips on how to change the mixture.
Click to expand...
If you haven't moved to a considerably different altitude, then the fuel mixture shouldn't need changing, unless there's been tampering or somethings come adrift.
 
hold on a sec stoke...the proper way to change the mixture is via the airscrew...should be on the side of the carb.
there will normall be two screws on a two stroke carb...one sets the idle speed, the other is the airscrew. pls dont mess with the airscrew unless you are competent. it can really mess things up.

the needle position...well, think of it like "+10%" or "-10%" across the whole range (up to 3/4 throttle).....hmmm...i can see i am going to have trouble explaining this, as it is similar to the airscrew (which controls the first bit of the throttle)

aw shucks:-) the needle sits in the jet, and has a very specific taper...as you turn the throttle, it raises the slide (air flow) and raises the needle (petrol flow)...the taper means as it raises the needle, the needle gets thinner (more pterol)..
moving the circlip sets the initial needle position, thereby massively changing the mixture across the range.
picture of typical needle, showing circlip snad grroves
http://crfsonly.com/howto/450x/jetting/needle-clip-1.jpg

(this bit nicked from somewhere else because i am lazy to type)
Idle-1/8th Throttle is called the Pilot Circuit. It's controlled by the size of the Pilot Jet and Fuel Screw setting. (*ahem* airscrew..)
1/8-3/4 Throttle is controlled by the needle and slide. The further down you lower the clip (towards the point) the richer the fuel mixture is.
3/4-Wide Open Throttle is controlled by the size of the Main Jet


the OP problem seems to be choke/mixture/needle related, but with the rattling going on in a normal 2 stroke, the circlip often jumps off. Or wasnt set right in the first place, so the whole mixture settings go out the window.

ok, here is my rough guide, (very rough)
start bike, let it warm up. (go for a ride).
now, set idling speed using idle screw.
so, now the bike is idling, call it 1000rpm. or 1500 or wherever, you will hear when it sounds comfortable. turn the mixture screw out till it almost dies (this is richer)...then turn it back till the idling starts rising (it is getting leaner)...count the turns you went from one extreme to the next, and set the airscrew in the middle position.

all should be ok now, with the needle clip in the middle position (call it factory default). now, you should be able to ride it around, but at some stage get it checked by a competent mechanic.
 
Last edited:
werner said:
hold on a sec stoke...the proper way to change the mixture is via the airscrew...should be on the side of the carb.
there will normall be two screws on a two stroke carb...one sets the idle speed, the other is the airscrew. pls dont mess with the airscrew unless you are competent. it can really mess things up.

the needle position...well, think of it like "+10%" or "-10%" across the whole range (up to 3/4 throttle).....hmmm...i can see i am going to have trouble explaining this, as it is similar to the airscrew (which controls the first bit of the throttle)

aw shucks:-) the needle sits in the jet, and has a very specific taper...as you turn the throttle, it raises the slide (air flow) and raises the needle (petrol flow)...the taper means as it raises the needle, the needle gets thinner (more pterol)..
moving the circlip sets the initial needle position, thereby massively changing the mixture across the range.
picture of typical needle, showing circlip snad grroves
http://crfsonly.com/howto/450x/jetting/needle-clip-1.jpg

(this bit nicked from somewhere else because i am lazy to type)
Idle-1/8th Throttle is called the Pilot Circuit. It's controlled by the size of the Pilot Jet and Fuel Screw setting. (*ahem* airscrew..)
1/8-3/4 Throttle is controlled by the needle and slide. The further down you lower the clip (towards the point) the richer the fuel mixture is.
3/4-Wide Open Throttle is controlled by the size of the Main Jet


the OP problem seems to be choke/mixture/needle related, but with the rattling going on in a normal 2 stroke, the circlip often jumps off. Or wasnt set right in the first place, so the whole mixture settings go out the window.

ok, here is my rough guide, (very rough)
start bike, let it warm up. (go for a ride).
now, set idling speed using idle screw.
so, now the bike is idling, call it 1000rpm. or 1500 or wherever, you will hear when it sounds comfortable. turn the mixture screw out till it almost dies (this is richer)...then turn it back till the idling starts rising (it is getting leaner)...count the turns you went from one extreme to the next, and set the airscrew in the middle position.

all should be ok now, with the needle clip in the middle position (call it factory default). now, you should be able to ride it around, but at some stage get it checked by a competent mechanic.
Click to expand...
To summarise, you have described setting the idle/low-end fuel mixture which is unlikely to be the OP's problem - what he is describing is a loss of power at the high end.
 
i described all three options, idle, low speed-3/4 as well as WOT.

the OP doesnt say the problems exists at WOT, and we can assume the main jet didnt magicaly change size on a fairly new bike, so lets wait for OP to reply.

P.S. what was your solution?
 
werner said:
i described all three options, idle, low speed-3/4 as well as WOT.

the OP doesnt say the problems exists at WOT, and we can assume the main jet didnt magicaly change size on a fairly new bike, so lets wait for OP to reply.

P.S. what was your solution?
Click to expand...
The fuel mixture doesn't usually/magically change.

I'd first suspect a wrong heat range spark plug.
 
werner said:
i described all three options, idle, low speed-3/4 as well as WOT.

the OP doesnt say the problems exists at WOT, and we can assume the main jet didnt magicaly change size on a fairly new bike, so lets wait for OP to reply.

P.S. what was your solution?
Click to expand...

I'd imagine that idle & WOT are the only 2 feasible settings on these little scooters :-)
Just given the great suggestions to my Dad a few minutes ago, he's gonna start with the fuel & plug solutions 1st.
Will feeedback tomorrow once I've gotten an update.
 
Just spoken to my Dad. Looks like the problem is hopefully solved. It's seems it may be as simple as a 'crimped' fuel line i.e. insufficient fuel getting through at high RPM's due to the fuel pipe being squeezed.
He's sorted this out & all seems to be fine for now.
A big thank you to all those who offered great advice, specially Werner - dankie Okes!
 
Whahaha, so my guess at fuel starvation was rite! Muwahahaha lol jk but glad to see you're up and running again.
 
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