Bucket jetting

[stanko]

Im trying to get my bucket sorted out for next week. Its an Aprilia RS50, it came with a 12mm carb, which is going west. Ive got a H100 carb and am looking for some Idea where to start jetting. the new carb has a 125 main. Is that a good starting point or will it drown it? Ive made a new inlet manifold from 20mm id tube and a couple of bit`s of plate. the forks are off to get new seals and its going to be a busy week.

[FROSTY:Me]

Mate --I would start with the stock jet sizing your bike came with.
It wont be right but at least its a baseline to start with.
With a bike like yours my honest advise to you is -get your hands on a big selection of jets and take the bike along to someone with a dyno
They might be $100 an hour but you will get more tuning done in one hour than you could hope to in a days practicing.
trust me Ive been there done that

[k14]

12mm carb, geez thats pretty tiny. You probably already know this but if you've added a bigger carb you would definately want to start on the rich side (too much petrol) rather than the lean side (too little petrol) or else you will get detonation and could sieze it. So for this reason I wouldnt start with the same size jet as the stock carb, because you will be giving it the same amount of petrol but probably more air, I would probably go up a few sizes.

If the carb is a mikuni I have a few jets, range from 168 to 182

[FROSTY:Me]

I agree with K14 stock jetting will be too lean but if ya just running it up on a dyno for a couple of secs itll be fine.
Just a thought here too --have ya read up on your bike??
The small carb is probably matched to a restricted airbox,exhaust and possibly exhaust port.They used to have a restricted and an unrestricted model -the restricted one was to keep the hp below a certain level for yong/learner riders.
places to look-are inside your airbox and trumpet to the carby. inside the exhaust pipe in the top end by the barrel,possibly the inlet manifold might be partially blocked.
What Im saying is just banging on a bigger carb is unlikely to give you a performance boost--You need to work right through the combustion process from end to end checking for restrictions.

[stanko]

thanks guys, the stock carb is the same model as the one on my weed eater so Im pretty sure its the restricted model. Ill whip the barrel off and have a look soon same with the pipe. ive been looking but havnt found any portmaps or timing figures for this bike. I dont want to have the perfect bike by next weekend I just need to get out there. My longer term plan is to put a FXR150 motor in this frame, so I wont loose too much sleep if this one goes pear shaped. Ill post a few pics in a day or so with my progress.
Mike

[F5 Dave]

The H carb is a bolt on affair right? (too long ago) the vibration makes non rubber mounted carbs a pain. A 24mm would have been a good start with a decent manifold. You will need a pipe, or at least hook the washers out of the std if it has them.

First thing to do whenever putting on a carb from a bigger bike is to drop the mainjet out & see if it runs on full throttle. If it dies after ½ throttle then proceed, but if it keeps reving then the needle (upstream of the main of course) is the restriction.

How can a small bike need bigger jets? Ok you have removed the airbox so it is running less restriction than the donor bike, but it is largely down to the reduced suck of the smaller engine. Less suck means less petrol being pulled into the airstream.

Though correspondingly there will be less petrol requirement.
Often this cancels out, but best to start richer.

[stanko]

thanks F5 , Ill work my way through that. Ive made a new manifold for the new carb. There is no air box and the pipe seems to be unrestricted. I took the barrel off and compard to an RZ all the ports look huge (relative to the bore size). I dont get the forks back till thursday so it may end up a bit of a blur. If all else fails I can run the stock carb. What sort of jets are the H100's normally I buy Mikuni hex jets but these are a bit different they have a screw slot, what do I ask for?

Cheers Mike

[TonyB]

Strangely enough on my GP100/125, the Hayne manual shows a bigger main jet for the 100 than the 125, despite the 125 having a bigger carb.

Oh and Stanko- found a replacement bore at Dons- it's tight enough to need honing which is being done now. So I'll be out there too- might even have one of your pipes on.

[F5 Dave]

Heck I threw the H100 carb away with my H100 so I don't know, but Mikunis have either button (flat screw) type or hex. Don't interchange.

[F5 Dave]

It's not too strange re jet sizes. Assume when you start to dick with std setup that the jet size you started with may have no relationship with what you end up with. While on an MX bike they may only be mildly modified so there might be info on what jet size might be close, that is now not applicable. Assume your bike is unique.

As far as the port sizes on the RZ/LC they are a little antiquated. The position of the bores means that the transfers travel straight up the side of the barrel (well almost, RZs are a little better) compared with a nice sweeping arc. & the inlet ports on my old RZ were smaller in the ally than they were at the bore, misaligned by ~6mm.