Aprilia RS125 carb issues?

Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

Hi everyone.

I have an Aprilia RS125, 2005 model, full power (derestricted arrow exhaust, modded CDI, powervalve with RAVE unit, Pipercross panel filter).

I'm having real trouble getting the bike to start, and even more trouble getting it to run. The bike starts of its own accord maybe 50 percent of the time. The other 50 percent, I have to use a syringe to inject a couple of millilitres of petrol straight into the barrel before it will start.

When it does start, it struggles to rev past ~5k, and on its way to 5k its very spluttery and weak.

The bike is sparking just fine. I have had the reed system, carb, and air filter out and cleaned thoroughly. The spark plug is oil-fouled, which I believe (and I could well be wrong) is because the bike isn't getting enough fuel? I took the card off again today, gave it a good clean, and put a larger jet in it (140) and my problem persists. I have played with the idle and mixture screws for hours, and have tried the needle on all of it's notches, to no avail. The filter is brand new, as are the reed petals and spark plug. I changed the coil recently as I wanted to be sure that it was sparking correctly, and it is.

I'm at a complete loss as to what to do next! Do I need a 34mm carb? The weird thing is that when I inject petrol straight into the head, I'll get one perfect rev, where the bike will willingly hit the redline if I let it, and then it 'bogs' down again, and won't do it again unless I inject a relatively large amount of fuel directly to the head. I have checked the fuel line, it's free and clear. The float bowl fills with fuel as it should. But even with the 140 (I tried a 145 and it was no better) jet it doesn't want to play ball. The carb had a 120 main jet as standard, and I've tried a 130, 135, 140, and 145 and none of them have helped. I've tried setting the carb back to stock with the 120 main jet and the screws set to around 1 and a half turns out.

Am I missing something? Is it possible that I just have a duff carb?

I've spent the last week reading posts on various forums, especially the ones marked as solved, and nothing seems to have worked for me. I'm new to carbs and 2 strokes. I've had plenty of 4 stroke bikes, and only ever had an issue with a carb on one of them and it was easy. But the carb on my Aprilia is baffling me. I just can't seem to get it working properly. It's a Dell'Orto PHBH28 BD, by the way.

I'm not looking for someone to serve me a solution on a silver platter, but even just a hint as to what could be going wrong would be a godsend.

Old Git Racing · World Chat Champion Joined: 08 Aug 2009 Karma :

Think there's a mesh filter on the carb somewhere where the fuel goes into it, cant remember really but my mates superteen race bike had a problem that was down to this

OGR
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Grubscrew · Scooby Slapper Joined: 23 Dec 2018 Karma :

Return it back to a stock bike. Then you’ll be able to ride it and sell the mods on eBay .
No grief, a rideable bike Very Happy

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Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

Robby · Dirty Old Man Joined: 16 May 2002 Karma :

Does it have any 2-stroke oil in, is the autolube working, and when was it last rebuilt?

andy_uk · World Chat Champion Joined: 13 Aug 2011 Karma :

Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

Nobby the Bastard · Harley Gaydar Joined: 16 Aug 2013 Karma :

Was it working properly before you changed the plug?

Has it ever worked properly?
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Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

Easy-X · Super Spammer Joined: 08 Mar 2019 Karma :

Oily spark plug, bogging down, only runs well if you squirt fuel in the carb?

Sounds like it's not getting enough fuel! You've cleaned the carb? Great! Clean it again Smile

Also have you pulled the fuel pipe off the carb? Is fuel pissing out of the pipe... basic stuff Wink

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Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

Nobby the Bastard · Harley Gaydar Joined: 16 Aug 2013 Karma :

It stopped working when you changed it for a different pipe?

Clearly needs rejetting to suit the new pipe.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

P. · Red Rocket Joined: 14 Feb 2008 Karma :

How long has it been standing and just to humor us, whats the compression figure on it now.

stevo as b4 · World Chat Champion Joined: 17 Jul 2003 Karma :

Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

stevo as b4 · World Chat Champion Joined: 17 Jul 2003 Karma :

It's a tuning mod, and you need an engine that's already 30bhp+ to even see a tiny benefit from a larger carb, as 28mm can support over 30bhp.

If you're trying to tune an already well modified 125 two stroke to the ragged edge then it's fine, but the std carb isn't a restriction part as long as the slide is opening fully.

But please don't listen to me, better go check with Andy as he seems to know something even the factory doesn't know?

andy_uk · World Chat Champion Joined: 13 Aug 2011 Karma :

Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

P. · Red Rocket Joined: 14 Feb 2008 Karma :

Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

P. · Red Rocket Joined: 14 Feb 2008 Karma :

So it's not run in at all, what figures do you have on compression tests, without putting stuff down the bore.

6bweeks probably isn't enough for anything to really dry out seal wise but it doesn't sound like it's been used much to really say...

Mine was fine at low rpm and then was shite afterwards, maybe up to about 9k, then it shifted but was always crap. Then it shat itself.

Danonymoose · Derestricted Danger Joined: 03 Aug 2019 Karma :

Robby · Dirty Old Man Joined: 16 May 2002 Karma :

The reason you're getting lots of tricky questions is because its an RS125. People cock around with them, so there could be a bigger underlying problem.

If the bike is in otherwise good and standard-ish condition, then the only change required should be setting up the fuelling to match the new pipe. A bigger carb is not necessary to make it run right, but may (or may not) be required to get maximum power.

What is necessary is getting the carb set up to match the pipe. Sometimes the pipe comes with recommended carb settings, sometimes you can find it in forums. Either way, if you've gone up from 120 to 145 on the main jet, you should raise the needle height at least one position as well. The needle is really controlling the fuelling between one quarter and three quarters throttle opening.

Of course, it's an RS125, so someone could have messed with it. Common things include messing with the oil pump, disconnecting the autolube entirely and running pre-mix, and poorly done top end rebuilds.

So you want to do a compression check (on wide open throttle), and really make sure that the oiling is working properly. If it is blocked off, disconnected, or has air in the lines, problem.

Finally, if the trouble started just after filling up with petrol, that's a sign that it was running on pre-mix before and you've just trashed your engine.