Bike idle drops and dies when hot

[DucksEverywhere]

Hello all,

Turned 16 a few months back, starting out on a little two stroke Aprilia RS50 2005.

The current issue is that when I have ridden the bike for around 5-10 mins, it will not idle when I come to a stop. It's very irritating for traffic lights.

It seems to come down to 1K RPM and then just konk out. The idle speed screw is turned all the way in, and the air screw is 2 1/2 turns out. The maximum idle I can achieve like this is 2000RPM when cold, which drops down to nothing when hot (when the bike is first started up, it idles fine).

The problem is the bike has been derestricted, so jets were different to stock setup. I have since reinstalled the stock size of pilot jet though, so idle should be same as stock, right?

The plug is quite rich-looking, but I've never done a proper plug chop so I can't be sure.

Another strange issue which could be a key clue is that the bike's idle speed always decreases by around 250-500RPM when it is upright - as opposed to sitting on the sidestand. I assumed this was just the float doing it's job... Any ideas?

noticed last time the air box was apart, the filter is looking a little old. It's quite flakey, could this impact air sucked into the carb?

I might just buy a new filter and a float kit from Racing Planet - just to be sure.

When I had the carb apart, I checked the float height - it seems to be slightly below level (this is holding carb so pin is just seated, not upside down). However, there is no adjustment on this plastic float. Perhaps the float needle spring is not stiff enough? Perhaps it has worn over time?

The float kit costs £6 and includes the float, pin and needle. Do you guys think it is worth a try?

Maybe the float being a little too high has an impact on idle performance on/off sidestand?

Thanks,

Callum

[fastbob]

You already seem to have an excellent understanding of all the probable causes, at least where the carburetor is concerned. I have almost no experience of two strokes but the words Reed Valves and Crank Seals do come to mind . What about the petrol tap , if it has one , or the fuel filter perhaps ?

[DucksEverywhere]

Hmmm... yes it has a tap and fuel filter, but that was replaced around 6 months ago, when the bike was bought. It's barely been ridden since.

[Guest]

The clue is the idle adjustment... it should not be screwed fully in. There are tons of you tubes on how to set up a 2 stroke carb. Yes, replace the filter. It sounds as if it could be blocked. When you put the original jets back in, did you replace the restrictors?

All the float is doing is to make sure there's enough fuel in the float bowl (there's a clue!) to feed the rest of the gubbins in the car.

Replace everything to stock, watch the videos..... then report back.

[DucksEverywhere]

Here's a recent development:

I've just realised the bike only dies down when in gear with clutch pulled in. Perhaps the clutch is slipping/worn, giving it the tiniest amount of drive - enough on a 50cc to bring the revs down? I can feel the bike pull a little when in first with clutch lever in.

Afraid I cannot go back to stock - the jets on the stock RS50's are not big enough for the unrestricted bike. The only jet that was returned to stock was the pilot jet, which is 32 instead of 34 a week ago. Main jet is 86 as opposed to stock's 76-ish. The last thing I want to do is downsize and restrict this little bike. Even though the bike's max idle is 1500RPM in neutral now, it just drops off when I kick it in gear. Maybe my low idle speed and dying bike are seperate issues? It will happpily sit at 1500RPM in neutral, just not in gear.

Messing around with air screw still only gives me maximum idle of 1500RPM (when hot). Idle screw is still turned right in.

I have tested the bike at idle in neutral with and without airbox on - inside of it is my air filter. This had no effect on idle speed. I don't think my air filter is blocked, but may replace anyway.

[TimR]

unrestricted 50cc at 16 ...

uninsured in the eyes of law

otherwise in accordance of licence ...

potential 12 points on a provisional licence

seizure of bike £150 release fee plus £12-£25 a day storage

is it worth the aggro for a 50cc bike which will being unrestricted be ragged to death anyway

[Tango]

Old fuel?

[DucksEverywhere]

Old fuel?

 

Nah, fresh in a few days ago. I'm in the UK too, so our fuel is generally pretty good.