Just bought a digital tach -- interesting discovery

[Deejays50]

Hey guys,

My agility 50 has a 70 or 80cc BBK (not sure exactly which... previous owner was a moron) with a CDI, exhaust, air filter straight off the carb and I believe a larger main jet. It has a Polini variator and malossi clutch (with heavier spring)

The scooter idles at about 1250RPM which sounds about right, not too high not too low.

I noticed that pinning the chinometer (a few clicks past the last number) the RPM was only 4500RPM, never more.

Don't I have a lot more RPM range to go?

Can I get it to go faster maybe?

Thank you in advance.

[de-dee]

  what speed are you at 4500 rpm. , I think those scoots go to 8,000 rpm.  does it rev higher on the center stand,.

[Deejays50]

It's past 50mph on the chinometer but its way off to begin with. Maybe 43? 45?


Edit: will check center stand RPM (hate revving it on center stand, but Ill do it)

[ophelia]

A small engine like that might idle at 2000 rpm. My 250 cc bike idles at 1500 rpm, so maybe your displayed revs are half what they ought to be?

[Deejays50]

A small engine like that might idle at 2000 rpm. My 250 cc bike idles at 1500 rpm, so maybe your displayed revs are half what they ought to be?

Well i think displaying half would be right, because I was pinning the speedometer trying to see if the RPM's would go any higher, and they did... and i blew the engine. must have been revving to 9k if I had to guess?

It got hot, very hot. I don't know what happened.

I think that's it for me and scooters.... im really bummed now.

[Vivo]

That's why they have the rev limiter...

[ophelia]

Yeah, a CDI with a rev limiter might have saved you. Sustained 9000+ rpm is brutal on the motor. But just throw on another 70 cc jug and piston. Add a bit of 2T oil in the gas to keep the bigger cylinder well lubricated.

[Deejays50]

I wish the CDI had a limiter. It must not.

I rebuilt the top end on a 139qmb before but I'm worried the bottom end could have damage.

How do I know if the crank and bearings are okay?

[ophelia]

Oops, I thought the Agility was 2T. It is 4T.

[Deejays50]

So now I'm left with two questions:

1) how do I know if the crank and bearings are OK? Best way to test?

2) Do they sell CDI's that have an improved speed but include the rev-limiter?

Thank you!

[baddi]

So now I'm left with two questions:

1) how do I know if the crank and bearings are OK? Best way to test?

2) Do they sell CDI's that have an improved speed but include the rev-limiter?

Thank you!

You check the bearings by testing if they can move exessively from side to side and up/down.
The connecting rod has the service limits of 0.55mm side clearance and 0.05mm radial clearance.

I can't seem to find any specifications on the main bearings, but they must also not move too much. Also, no moving part can have any "hard spots". If you feel any hard spots, change it all.

Anyway, you should start finding out what caused the engine to malfunction.

I have had my agility (50cc with fuel injection) up at 12500 rpm without it complaining, but it also has double valve springs, which some of the older agilityes does not. Too high rpm can cause problems with the valves not closing properly in time.

Another problem that can occour is that the fuel film between the piston rings and the cylinder can break. It breaks easylier if the oil temperature is very low or very high, but a general rule is that it tends to break if the piston speed is over 25 m/s. At 11500 rpm, the piston speed exeeds this for a short amount of time, but the mean piston speed wont be near 25 m/s before 33000 rpm.

If your cylinder or valves has broken, it is kinda likely that metal pieces can be found in the system, so to prevent further damaging, do not move any parts before it has been inspected and remember to flush everything with engine oil before fitting the replacement parts.

About the CDI's: i have not seen anyware that it can be bought, but i have seen a picture of one on This website. Anyway, if you cut the original CDI, it should be restricted to somewere around 9000 rpm instead of around 7500-7800 that it normally is restricted to.

[Deejays50]

Thanks for all of this info.

I think the lower end is OK. the connecting rod moves freely with very little play side to side.

I've already dumped the oil out but will need to add more oil to flush it a second time.
What is the best way to do this? Just add oil to the case and let it dump out?

Cylinder and valves didn't break, the piston lost a chunk of itself as well as the piston ring and started having a field day in the cylinder.

I'm pretty sure it ran crazy lean causing the destruction. I need to focus on carb tuning now.

I bought another 80cc kit + head from scrappy. I may put my stock airbox back on and see what happens.


Thank you for your reply!

[baddi]

Thanks for all of this info.

I think the lower end is OK. the connecting rod moves freely with very little play side to side.

I've already dumped the oil out but will need to add more oil to flush it a second time.
What is the best way to do this? Just add oil to the case and let it dump out?

Cylinder and valves didn't break, the piston lost a chunk of itself as well as the piston ring and started having a field day in the cylinder.

I'm pretty sure it ran crazy lean causing the destruction. I need to focus on carb tuning now.

I bought another 80cc kit + head from scrappy. I may put my stock airbox back on and see what happens.


Thank you for your reply!

Oh, it is you in the other topic also, great.

To get the metal pieces out, you can do a couple of things, depending on how much work you want to do. In most cases, i would just fill the crankcase with oil, stir around and drain it, then fill it to the normal level and drain it again. If you think there might be alot of pieces in the oil i would first let it stay in the same position for as long as possible to get the oil to drain to the bottom of the crankcase, then drain it and flush after with gasoline because it flows easylier. But remember that gasoline is not good for the oil, so you will have to flush it with engine oil some more after this and then drive the engine very slowly to get the oil into the bearings and then changing the oil after a short time of use because it is contaminated with gasoline.

So it depends on how much metal is in the crankcase, how much time you want to use on it and how much oil you can afford to waste.
If it is only metal dust, i would just change the oil, stir the oil up with a short time in idle and then change it again, but if there are real pieces of metal, i don't think that is enough.

[Deejays50]

I've already drained the oil since pulling it and believe it or not the oil was quite clean. I didn't see anything shiny. The oil filter was also spotless. No large pieces or anything. I think most of whatever happened at the top end stayed there.

I will do the add oil drain oil and add again method. Don't want to go the gasoline route

Thanks for your help.

[Deejays50]

Hey guys just an update.

My tach actually was setup correctly. So yes, it definitely blew up at around 6k.... lol

I do remember hearing the exhaust note change significantly. It was loud and somehow got quieter and faded in volume a bit.

Either way, new top end is in and just doing some carb tuning as shown in my other post here: http://www.kymcoforum.com/index.php?topic=10279.0 in case you're interested.