Aprilia RS 125 - powerband narrowed

[DEADmetal]

I've got a new shape 2006 RS 125 fullpower. Got the bike when it had only 2k miles clocked. Now it's nearing 3.5k miles.

Before this problem occured, it pulled very strongly, up to 11k RPM (after which the power drop was very sudden). It had a solid top speed of 100 mph indicated. After riding it some (just about 200 miles), it lost power at 11k RPM and started pulling only up to 10.5k RPM (in higher gear), after which, it basically had no power. This powerband difference was very evident. It would not reach 11k RPM in 3rd and higher gears, when before, it reached 11k RPM explosively.

To solve this problem, I've tried:

1. Cleaning PV multiple times.

2. Keeping PV open permanently by using zipties.

3. Using blanking plate instead of PV.

4. Decatting the exhaust and completely emptying the expansion chamber, as well as cutting off the exhaust appendix (which, as I was told here on the forum, acts as a restrictor).

Only the exhaust mod made any difference in powerband. It now pulls up to 10.6k, instead of 10.5k, but it's still evidently lacking.

The power at say, 9k RPM, 10k RPM, or 10.3k RPM seems to not be lost, compared to how it was when I first started riding it (when it had high power up to 11k RPM). The problem is that it doesn't have power up to 11k RPM any more. The top speed has also suffered, obviously.

I'm thinking, if the carb would be clogged, the symptoms would be much more restrictive. Correct me if i'm wrong though. Could it be an electrical problem? Perhaps I should also mention that the CDI is taped over. Could it be so that something has been done to the CDI (a mod) and after I've ridden the bike for some time, this CDI mod failed?

While the symptoms of this problem are requiring a detailed explanation, hopefully, this post doesn't confuse anyone. In case something needs clearing up however, I'll try to rephrase it.

[P.]

2006, so 12 years old. 3500 miles.

Under 300 miles a year, so it's going to have sat around for a while. When was the rebuild done and what's the compression like on it now, use a decent compression tester.

[Pete.]

[Yeti]

I think your Purple Power band is worn out.

Try upgrading to the Blue Power band..... They are hard to come by but when you find one you will love it!
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Lifes a Beach!

[skatefreak]

Not cleaned out the carb?
Clean out the carb before making permanent changes .
A minor fuel restriction can effect high RPM only so it's not unlikely.

It doesn't sound like you've introduced new problems yet but you need to improve your trouble shooting approach.

What could have changed to alter the performance of the bike?
The cat? Not likely. Stripping and cleaning the PV Maybe... but pinning it?
Failed CDI's usually stop a bike...

It sounds like you've ventured down the route of 'improving' the standard setup than trying to fix the problem now?

The thing with carbed bikes though is the first step to any running issue...

Service The Carb(s)!

[DEADmetal]

[stevo as b4]

I'd check compression and leak down figures first for at least some indication if it's healthy. Its a two stroke single so a really simple engine to work on.

With a bike like this I will assume your not a skint teen trying to run it on a tiny budget and not using crap chainsaw oil etc.

Strip and ultrasonically clean the carb and jets as it's not going to hurt and if it's had old fuel sat in it for years, it'll be gummed up to some extent.

I'd personally order up a new piston, rings, bearing and top end gasket set and do a piece of mind re-build now so you can log it's maintenance/hours or miles under your ownership too. If you've got a good cylinder bore surface you want to keep it that way by changing your piston instead of replacing your cylinder.

[Triton Thrasher]

[Pete.]

[Old Git Racing]

Whats the plug like if you do a plug chop? The freed up chamber could be causing it to run leaner. These bikes were pretty tight on jetting with the std pipe.

OGR.
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2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[Fin]

[P.]

My point was... I had one, it ran great for about a month or two, 1500 miles.

It had been standing for about 8 months. It munched itself as every seal has pretty much dried out from lack of lubrication and Pete ^^^ rebuilt it for me.

I'd highly suggest getting the basics checked on a 2 stroke before fiddling more. Smashing the mesh out of the exhaust and welding it back up should have been done before hand, or at least been swapped out to a decent full power one. Now you are likely having to rejet and other things to get it running nicely.

[-]

Any slight blockage in a jet or passageway in the carb will have a profound effect on power. Now you've gutted the expansion chamber, it will probably need re jetting to suit too.

To give you an idea on sensitivity, on the TZR250 race bikes (essentially 2 X 125 cylinders, carbs and exhausts), just removing the inner "trumpet" spout from inside the airbox required main jets to go from 230 to at least 320 in each carb.
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Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[skatefreak]

Strip and clean carb, if no improvement plug chop and post a pitcher.
It could just want a fresh plug thinking about it... my NSR we t through a couple...

[MCN]

[Polarbear]

I would get hold of a standard exhaust and fit that THEN start looking for the problem but Pete already basically told you that.

From my knowledge of 70's and 80's 2 strokes one of the worst things you could do was fuck around with the exhaust unless you knew exactly what you were doing. Rejetting was de rigueur when you altered the exhaust or put expansion chambers on it. I presume you understand on a 2 stroke it's not just about flow through the exhaust.

I cannot understand the logic of something was running well, it isn't now, I'll change a fundamental part of the engine.
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Triumph Trophy Launch Edition

[iooi]

[Polarbear]

^^^

[ThatDippyTwat]

You need to understand that there is a precise science to the size, shape, volume, hell, everything to do with a 2 stroke expansion chamber.

Think fluid dynamics. but with exhaust gas.

How the gas travels in that chamber is critical to the bikes performance. And you're just hacking away at it.

Put a stock one on, or buy a proper, tuned exhaust and make sure your carb is set up for it. Hint - They're not cheap.
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'98 VFR800 (touring) - '12 VFR800 Crosrunner (Commuting) - '01 KDX220 (Big Green Antisocial Machine)

[MCN]

The expansion chamber controls the way that the cylinder is sealed.
Exhaust gas pressure prevents the fresh charge of air mixed with atomized fuel from being pushed out the exhaust port on compression.
The shape of the chamber can be fine tuned to change some of the engine characteristics.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[stirlinggaz]

hi,
seeing as youve 'modified' the expansion chamber, its probably fucked.
fit a better one, like an arrow or similar. proper sports exhaust innit.

carb : easiest carb to clean out. simply remove the fuel line banjo from the side of the carb (8mm bolt iirc) under there lives a 2p coin sized gauze filter, these get clogged up easily. clean it with soapy water & refit.
if your feeling ambitious, strip the carb down & clean out with carb cleaner & compressed air. (seen a couple of grains of sand play havoc with an rs, found under needle jet)
theres a very good guide to your carb in the sticky setion. i suggest you have a look.
while your there, take a note of what jets are fitted, seen umpteen fp bikes fitted with wrong jets.
clean air filter.
do compression test & then fit new (decent) spark plug.

let us know how you get on.

cheers,
GAZ

[stirlinggaz]

hi,
I should have added, most importantly, "modifying" the exhaust has likely f*cked up the jetting as well.
these wee dellorto carbs are sensitive enough so the change to the expansion chamber will result in rejetting being necessary
check the official aprilia derestriction guide for the size of main jet suggested.
guaranteed you don't have the required jet kit fitted which will result in it being unable to rev out to 12000rpm at wot.

cheers,
GAZ