Aprilia RS125 with 154 bore kit

[kmeister]

Hi guys, recently picked up a 2003 RS125 where the previous owner has fitted a mitaka 154cc big bore kit to it. Was an unfinished project so I'm currently in the process of sticking it back together and one thing that came across my mind is the jetting. It's the 32mm carb and has a 162 main jet, anyone know a rough starting point with the 154cc?

I've also got the old 125 barrel with it which is in great condition so not put off by perhaps refitting it with a fresh piston. From reading around there seems to be mixed opinion on fitting the bigger bore kits.

[Clutchy]

[P.]

32mm carb is small. The standard derestricted RS one is 34mm.

Unless you know exactly what you are doing (evidently not as you are asking on the forum...) I wouldn't even attempt it.

[kmeister]

Sorry it's 34mm. I understand jetting, I race a kart which uses the same engine and carb so I know what to look for there. Just wanted a very rough idea of where to start with the jetting. Thanks anyway, probably just going to restore it to the original cylinder as I'd rather seek reliability than performance.

[evoboy]

[Clutchy]

Well if he races a snr Max, at least the unreliability of the rs won't come as a shock
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[kmeister]

Haha, I mean a reliable-ish toy! I won't be relying on it as a commuter, just using it as a small footstep into the bike world for a bit of fun on a nice summer's day. Plus I like to tinker so it's a nice little project to work on

Previous owner has faffed with the exhaust, it's all welded up so I assume it has had some gubbins removed. Is it safe to assume that the 162 main jet in the carb is probably what was fitted when it had the original head? I've a selection of jets in this range from my kart so can tweak from there.

[P.]

Probably had the cat and shit pulled out. Would have been easier to grab an old 2000 models exhaust or an aftermarket one.

[stevo as b4]

If it's not your only road bike, blowing it up isn't a big worry, and you wanted to throw good money after bad on a 2stroke project bike, then I might consider using the 154 Polini kit. If the kit is new and the crank has been recently re-built properly, you might get 8-10'000miles from a well set up 154cc conversion. It will be a ballache and need dyno time most likely to set up the jetting.

From what I've read the 154cc Polini is claimed to give they say about 37bhp, so low 30's at the back wheel. They never do by the sounds of it, as the kit increases the mid-range power and has a silly high compression ratio, that actually reduces top end performance. The 125cc suited expansion chamber won't ever ideally suit the 154 kit, and you would probably end up with a more vibey rougher motor, that is better in the midrange but not much if any better at the top end. You lose revs with the Polini kit, possibly due to factors such as the porting design and compression ratio, but also because the exhaust isn't 100% suited to the bigger cc top end. Maybe you can gain the speed back with higher gearing, but the fastest tuned bikes seem quicker at 125cc with professional tuning.

Would think the 34mm carb is big enough, and that as baseline maybe 2-3sizes up on the main jet would be ample to ensure rich running until properly tuned.

Good luck whichever way you go with it, but having a decent 125cc top end spare and being able to re-build it properly with decent quality parts seems by far the best way to go.

[kmeister]

Thanks for all the replies, good to get some input

I've decided my plan of action is to get the engine back in as is, get her all wired up and make sure it's going to turnover and run. My gut feeling at the moment is once it's running, swap the head over to the 125 with a fresh piston and rings. As I say won't be relying on it as a daily commuter but would like to have some confidence that I'll make it A-B and back to A now and again I'm not overly fussed about extra performance, it's my first bike so I'm sure even in standard form it would have enough power for me!

Looking forward to getting it back together!