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Hi guys!

I have an 1992 AF1 futura , rotax 123 engine. This model has a 34mm Dellorto, just like the first rs extrema.

It currently has an 128 main jet in it wich is waay too rich. To me that sounds weird cause after some research i've found out most of you guys are running with 155 to 160 main jet.
My bike is all stock, recently rebuilt the engine myself. Changed all bearings etcetera.

I checked the chocke, it's not stuck or anything.

The probs i'm having is it wont rev higher than 5500~6000rpm and the plug keeps going bad (short circuit or something).
Today i tried running it without the airfilter and guess what, runs way better. Revs nicely past 10.000 rpm.

Does anyone have some advice for me? It seems to me 128 main jet is too small but then again this is my first 125cc twostroke engine.
 

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hi fella,

you got any pics of your bike ?

there are several types of dellorto 34mm carbs around - it could have the wrong

parts fitted ie slide and needle

it could be worn

have you set the air screw and the mixture screw correctly ?

are you pre mix or tank ?

is your air filter clean or aftermarket......they all have effects

regards
 

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125cc Sportsbikes Boffin
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Hi

128 main jet is WAY too lean for the 34mm carb. Need something around a 150 main jet. A 128 would be within reason for a 28mm carb.

Main jet is the important one at high speed / revs, so might be the issue. But those revs are a bit low for it to be the whole cause.

What plug are you running? What is the plug gap? What have you set the ignition timing to? The Futura (like the early RS) was set up to run on high octane fuel as standard.

All the best

Keith
 

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Hello Again.

First of all, many thanks for the quick replies!

The carb is a VSHB 34. K56 needle (top or bottom notch doesn't seem to have any effect) but the needle itself doesn't seem to have any wear on it, I haven't got a clue what type of slide it has though.

Air screw doesnt have any effect on this matter.

At first I used NGK BR10EG spark plugs. Those died rather quickly because of the rich mixture I guess. Then I used a BR9ES wich has a bigger centre electrode, those lasted a bit longer. Before they gone bad the spark color was black. Haven't changed the spark gap by the way, it went in straight out of the box.

The airfilter is a new original foam filter wich I soaked in oil, it sort of clamps in the airbox between two plates. When I bought the bike a while ago the old filter was gone.

I use 95 octane fuel, which should be sufficient.

Ignition timing is 'somewhere' in the middle. Since I don't have a proper gauge to measure TDC it's a bit of a guess. I tried different settings, doesn't seem to have much effect.

Pics:




I've been struggeling with this fore quite some time now. I bought the bike 2 years ago, thought it would be an easy fix. I was wrong.
 

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125cc Sportsbikes Boffin
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Ignition timing is 'somewhere' in the middle. Since I don't have a proper gauge to measure TDC it's a bit of a guess. I tried different settings, doesn't seem to have much effect.
You set it to TDC using the hole under the crank, just forward of the starter motor. There is a matching hole in the crank web, and you put a bolt through the hole into the hole in the crank to lock it at TDC. Then you set up a dial gauge , remove the bolt and turn the crank back the correct amount and check that the stator is in the right position relative to the flywheel.

Ignition timing is very critical on these.

All the best

Keith
 

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As all has said 128 way to small


Make sure the choke is off and there is free play in the outer cable, also check the choke plunger has its rubber coating on the tip
 

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Discussion Starter · #9 ·
Well there's something didn't think of. I guess it could be the main bearing seal, should be the clutch side then. It would be a disappointment since I recently changed the clutch plates and springs, if I knew the oilseal was leaking I was allready halfway there. Now I have to take it all apart again, bummer.

I figuered there could be a blockage inside the carburettor which would cause for a faulty mixture. I plan on having the carb cleaned in an ultrasonic thingy. Perhaps that'll work. If not, I'll take the clutch off again.

However.. Come to think of it. I don't think my new airflter is an original from Aprilia. It is cut to fit the airbox but it came in a clear packaging, no aprilia stickers or anything. I think I bought that filter at 2rad-sausenthaler.de
Maybe the airfilter is too thick which would cause for less air to go through and therefore a too rich mixture?

Other than those three possibilities could it be anything else?

I knew the ignition timing is crutial but since it does rev nicely without the airfilter in I figuered the ignition isn't causing problems in this matter.

What I didn't know was that these bikes require a higher octane fuel than RON 95. I assumed there would be some americans here, they call RON 95 'premium'.

Thanks again for your support! I really appreciate it.
 

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What I didn't know was that these bikes require a higher octane fuel than RON 95. I assumed there would be some americans here, they call RON 95 'premium'.
The US uses pump octane ratings (PON). This is the average of the RON rating and the MON rating for a fuel. As a rule of thumb the MON rating is about 10 points lower than the RON, as a rule of thumb the PON figure will be 5 points lower. So US 90 octane fuel would be about the same octane rating as UK 95 RON fuel

All the best

Keith
 

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Discussion Starter · #12 ·
What about a k&n filter in stead of the original box and foam filter?

The k&n rc-9360 should fit and it's not too expensive. I have got k&n filteroil on the shelf.
 

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Keith,
What plug gap do you suggest for 06 model, running the hpi cdi? I was using the recommended gap from Haynes & Aprilia Serv manual. Thanks,

Rich
 
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