Flat Spot

[stephen1967]

hI , i HAVE A BETA EVO 250 2011. tHE BIKE HAS DEVELOPED A FLAT SPOT JUST AS YOU OPEN THE THROTTLE. tHE REST OF THE REV RANGE IT RESPONDS FINE. i AM USING AN NGK BPR 5ES PLUG AND AFTER EVERY USE I CLEAN THE AIR FILTER AND CARB.. i HOPE THERE IS A SIMPLE SOLUTION TO THIS PROBLEM. MANY THANKS.

[pindie]

Sounds like your pilot jet may be blocked or you have a float height issue. Have you had it long? Has it got an inline fuel filter?

I'm certain it is a simple fix.

[stephen1967]

i HAVEN`T HAD THE BIKE FOR LONG. i DON`T KNOW IF IT HAS AN INLINE FUEL FILTER. WHERE SHOULD I BE LOOKING IF THERE IS ONE ?

[overdale]

The flat spot just as you open the throttle sounds like a pilot jet/airscrew problem.

First check how far open the airscrew is. Screw it in clockwise counting the turns until it seats, could be anything from one turn to three turns out.

Then remove the airscrew and tension spring.

With the carb up upside down remove the float bowl and remove the pilot jet with a screwdriver, and the main jet assembly with a 10mm spanner.

With these out blow down the holes where the jets have been removed, when you blow the pilot hole air comes out the airscrew hole and a small hole at the air inlet end of the carb. If not get some carb cleaner in there and try again.

Be careful if you turn the carb over as the bush which the needle passes through has been held in position by the main jet assembly and can fall out.

Blow through the main and pilot jets before replacing. Replace the airscrew and spring to the same position as before, Down to the bottom and open by whatever it was before - but is is usually around 1.5 turns open.

Make sure that the float bowl is clean.

Float height can be checked using the pinned description for Beta Keihin Carb.

Any new spark plug comes set at around 1mm gap, it should be 0.6 mm or 24 thou, - that can effect performance, and the little screw cap on the top should be tight.

When you run the engine and it is warmed up, you can always adjust the airscrew a quarter turn at a time until it gives smooth response under throttle opening.

Try that for a start ,and report what you find.

[stephen1967]

thanks for the advice. I intend to have a go at your tips on Saturday and will report as to what rhe outcome is.

[stephen1967]

took the carb and air filter off this morning, gave them both a good clean, removed jets in carb, cleaned with carb cleaner and sprayed in every orofice ( even got some in my eye !! ). Put it all back together and it was marginally better , adjusted throttle cable and it seemed a little bit better and also adjusted the air screw..There seems to be a tiny bit of a flat spot and the noise ( intake of air ) is coming from the air box. Any ideas what could be causing this to happen?

[dan williams]

Usually the pilot jet circuit. Clean with compressed air. A damaged reed will also cause a flat spot off idle.

[stephen1967]

.What do you mean by the pilot jet circuit Dan ? How can I tell if a reed is damaged and are they easy to gain access to ?.

[dan williams]

Sorry, didn't mean to be cryptic. Your carburator has several "circuits" for controlling mixture at different throttle settings and these all affect each other as they transition from one to the next. By circuit I mean a metered fuel oriface (jet) and various air passages in the carb body.

From idle to 1/8th throttle the fuel air mixture is done by a combination of the idle air circuit and the "pilot" jet circuit.

From 1/8th to 1/4th throttle mixture is controlled by a combination of the pilot circuit and the angle of the throttle slide cutaway.

From 1/4th to 3/4th throttle the mixture is controlled by the "needle" and "needle jet" with influence from the slide at 1/4 and influence from the main jet at 3/4 throttle.

From 3/4th throttle to full throttle mixture is controlled by the "main" jet.

As you can see a fleck of dirt in one of these jets will affect performance in just one area of throttle. Cleaning with compressed air is really the best way to make sure the circuits are clear. One thing I have noticed about the Keihin is the size of the pilot circuit exit behind the throttle slide is extremely small. Much smaller that the old Mikuni and I have had instances where a bubble of water will sit in and block this exit hole bunging up the way the bike runs off idle. The hole is so small that the water won't evaporate because of its surface tension. It has to be blown out. I now carry a can of compressed air from a camera shop just in case I need to clear a jet in the pits at an event.

Even though this is for a Mikuni it is still an excellent guide to carb operation http://www.mikuni.com/pdf/vmmanual.pdf

As for the reeds they sit in that square block between the carb and the cylinder. You can take the four bolts off and the assembly comes right out. Just do a quick visual to make sure all the reed petals are in one piece and that there isn't any dirt or debris on the seats that would hold the petals open. Don't worry if they look like they are slightly open. Air pressure closes them so they don't need to lie completely flat to work. Make sure you get a good seal when you bolt it all back together as an air leak can mess with carburation. Cleanliness IS the rule of the day with these fiddly little bits.

Happy hunting.

Edited August 20, 2012 by dan williams

[skyline1]

I had the same flat spot, cleaned my carb for the first time as above. Pilot jet was blocked & a little dirt in the main jet. the bike is running great now. Thanks to all for the info above, worked a treat for me.