Aprilia rs 125 issues

[Ross1992]

What it has so far :

154cc big bore
34mm carb
Modified cdi unit
Rave power valve
Carbon reeds
Lower gear ratio for faster acceleration

..

Issue:

Bogs out and dies quite often, struggles to run idle and cuts out eventually. Other night riding back it cut out and then would go for a bit and die again like it was struggling to produce power

Any ideas ? Jet blocked? Carb needs cleaning and tuning ?

Also exhaust is leaking from seal.. Will re seal soon but could that cause this?

[RhynoCZ]

How did it run before or did it ever run alright after the mods? This could be anything from water in the fuel, faulty valve, runing too rich (now the plug is fecked) and so on, even the leaky exhaust.

By the way, use a proper exhaust sealant, not the red stuff you rub on your engine covers instead of a proper paper seal.
____________________
'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Kickstart]

Hi

Rotax / Aprilia list to use high temp silicone sealant to seal the exhaust to the barrel stub.

As to how the bike runs, is there any pattern to it? Does it play up at certain rpm? At a certain temp? After a certain time running? Is it better or worse at certain temps?

The cdi mod (assuming this is a mid shape RS - from ~1998 to ~2005) only affects from 4750rpm to 6000rpm. It does nothing outside this rev range. It works by breaking a bridge on the circuit board to trigger the unit to use a different ignition timing map. But it is possible to damage the cdi when doing the mod with odd effects. However most of the time I would expect it to either work or not work.

Can be lots of odd problems. A problem at certain throttle openings / rev range combinations would suggest a problem with a jet; possibly just the wrong size. If the bike cuts out after running for a while (maybe a bit longer if taking it easy, quicker if thrashed) then it suggests an issue getting sufficient fuel to the carb (possibly a blocked fuel filter, blocked fuel tank breather, etc)

All the best

Katy
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Pjay]

I had exactly this same issue, turned out to be the float needle had worked its way loose with the vibration.
Float chamber would fill up when sat still and run for a bit until the chamber was empty and then lose power as the fuel couldn't get in quick enough to refil it.

It's quite an easy fix.
To check if this is the problem, turn off petrol, remove the tank and the airbox, unscrew the clip holding the carb to the engine and spin it so you have access to the nut on the underside of the carb, remove it, open the carb and check the float pin is tight.

3 Reassemble.
4 Thank me.
5 ????
6 Profit.

[saikouslx]

clean out abd inspect the carb, and change the spark plug (check the strength of the spark)and then do a plug chop over the rpm to give you a good idea of how your mixture is varying. Sounds like your mixture at idle is off so when you shut off throttle it's not getting the correct amount of fuel. Maybe cause of your big bore you need to change the pilot (I think that's what it's called) jet to allow more fuel. Due to the incomplete combustion to deposits have accumulated the bee timed and fouled up the spark plug? An educated guess

[Ross1992]

[stevo as b4]

You've spent time and money on fitting a load of expensive bits to a fussy hi-po two stroke, yet you come on BCF asking what jet sizes to use??

Take the bike to a dyno and get it set up properly with the different carb, before you trash that expensive BBK you've fitted.

I've played with a fair few 2strokes, but would not from my limited wisdom attempt to try and dial in a new bigger carb perfectly on a performance engine.

Lots of reasons why, but basically most people on here or generally would only be able to tell if it was very rich or very lean from messing with jetting and reading plugs etc. Also unleaded fuel does not make plug chops anything like as conclusive as old leaded stuff. Its also open to variables in the test and if done incorrectly you might pass through several carburation circuits that could go from rich to lean etc before you get to read the plug.

You've paid for the parts, now pay to get a good professional set up done that'll be reliable IMO.

[Ross1992]

[RhynoCZ]

[Ross1992]

[Pjay]

[stevo as b4]

exhaust will need to be a really good seal at all joints, to get a both an accurate mixture reading and avoid running lean from an exhaust leak.

If it was a newly re-built engine, it's often worth rigging up a kit to do a leak down test to see if all joints are air tight, as even new builds can have air leaks, at gaskets, manifolds and seals etc.

What are you doing about the exhaust?
If your using a std or aftermarket pipe you should still get some fair gains in the midrange at least over std. But it will probably run a bit flat on top or lose 500rpm or so of useable power, (not a massive issue on a road bike).

To get the most peak power from a 154cc cylinder however you'd want a one off custom pipe made to suit the BBK and its porting.

[iooi]

[Ross1992]