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2004 Scorpa SY250 help please


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Hi all, just got my first trials bike . Need a little help/advice . This has probably already been asked so sorry , what fuel mix should I be using ?   Also been out for a little play today and the bike will only start with choke even when hot, After its warmed up turning the choke off causes the bike the rev very high then cut out . If I leave choke on after about 15mins it wont tick over and bogs down a bit. 

any advice is appreciated

cheers R  

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I think you have described extreme lean fuel/air mixture with throttle slide at idle position. The "choke" is not a choke. It is a fuel circuit that provides a rich fuel/air mixture to help starting a cold motor. When the throttle slide is above the idle position, the starting circuit does not flow. That's why you can ride it around with the "choke" on but with the motor warmed up, it is too fuel/air rich when the throttle is in the idle position.

There's a couple of things it could be.

Most commonly due to blocked or restricted pilot circuit in the carby. Usually caused by the bike being stored with fuel left in the carby.

Possibly due to air being sucked into the motor that is not going through the carby. Commonly due to leaking flywheel side crankshaft seal or at the rubber boot between the carby and the reed cage. Sometimes (very rarely) leaking cylinder base gasket.

Edited by feetupfun
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Years ago a kind member wrote down the steps to tear down the bike & carb, how to clean it using carb cleaner & electrical wire rather than compressed air then how to reassemble everything, have a search under Scorpa & I’ll see if I can also find it

takes a little time & once done leave fuel tap on for 10 minutes before kicking over

in my experience with the Scorpa will need doing on a regular basis so you will become quite proficient at it

good luck, great bikes once set up & running properly

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Here you go, was back in May 2016, good luck! Take pictures as you go to help with the rebuild

You can get the carb out if you need to at a meet, without doing these steps, but here's the easiest way get the carb out and in if you're working in a garage.

1. Put the bike on a stand so the rear wheel is off the ground.

2. Remove the rear fender.

3. Turn off the gas.  Disconnect the fuel hose at the pet cock.  Remove the fuel tank (two screws in the front). Leave the fuel hose connected to the carb.

4. Remove the aluminum sub frame.  You'll need to disconnect the coil from the aluminum sub frame.  Watch how it's connected in the rubber holder so you can get it back that way.  Be careful with the airfilter spring connectors as it's easy to break them.

5. Put a block under the rear wheel to support it cause you're going to remove the top shock bolt and don't want the wheel to hang on the brake hose.

6. Remove the air box.  Disconnect the rubber boot at the air box and leave it connected to the carb.  Makes it easier to put back together.

7. Unbolt the top shock bolt and pivot the shock back (REMEMBER to put a support block under the rear wheel so it's not pulling on the rear brake hose!!!)

8. Unscrew the carb throttle top and work it out.  A flathead screw driver tapped on lightly with a hammer will loosen it.  Careful not to bend the needle jet. 

9. Loosen the hose clamp on the front of the carb on the rubber reed block.

10. Now you can work the carb loose.  It should still have the fuel hose and rubber airbox hose connected to it.

11. Loosen the four screws on the bottom of the carb and separate the float bowl from the carb. Careful with the gasket.

12. There's two jets in there, the main jet and the pilot jet.  main jet removed with a 6mm (?) wrench.  Pilot is recessed in a tube and can be removed with a small flathead screw driver.  Unless you absolutely need to, don't mess with taking the needle out of the carb throttle top.  The TK's have a weird needle/ cable holder arrangement. Just clean it good with carb cleaner.

13. After you've got the jets and float out, clean it good with arrisol carb cleaner and blow out all of the orrifices with with compressed air.  Make sure the holes in the jets are clear and not simply scaled over with varnish from the gas.  Good reason to not use gas with alcohol in it.

14. Put it back together in reverse order.  There is a vent overflow nipple the bottom of the carb.  It causes a lot of muck in the space under the exhaust.  If you run a small rubber tube from that nipple down behind and under the engine, it will keep the carb from venting onto the top of the engine and will keep things a lot cleaner.  Clean the air filter while you've got it apart.  Hot water and dish soap works great.  Let it dry and oil it up lightly with motoroil.  Squeeze out the excess.

15. It's a good idea to put on one of those small inline fuel filters while you've got the thing apart. Keeping crap out of the carb will help ensure you don't have to get back in there very often.

I hope I've remembered everything.  If not, you should be able to figure it out.  Good luck.

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Once you've cleaned out the carb,  replace all of the fuel with some branded fresh stuff, as the latest E5/E10 petrol doesn't last very long. Supermarket fuels tend to go for cheaper blending runs too. If its E5 or E10 & more than a couple of months old, chuck it out & get some V Power or similar. If you aren't using the bike regularly, empty the tank after each use. Ethanol causes plastic tanks to swell & GRP tanks to turn to mush. (That's one of the reasons why the stickers/ decals bubble up & fall off.  Old school O rings & seals also prone to swelling. A new spark plug wouldn't be a bad move either

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