Gilera Runner SP50 tuning questions


New Member

I'd like to use your wisedom and experience to get ahead of my problems. Or maybe issues. I'm lacking both knowledge and money, so I have to precisly plan everything to buy, and can't risk any misteakes. My current setup is :

SP 50, 2008r.
Streetrace 70cc
LeoVince RR
Malossi Multivar (old, 16x13), 3x4g+3x7g
Polini Air Box
21mm PHBG, main jet 80.
Vmax aprox 95km/h.

While I was on 17.5mm, stock airbox and 6x7g rolls I could go even 115km/h
But I was affraid of the engine being too hot, so I bought 21mm.
I started with 92 main jet, but it was hell. Had to drop to 72 jet to get it going.

Now I bought S6 Pro Replica, as Yasuni R and S6Pro are to expensive here.
For now my issue is : Runner starts at 6.5k, accels nicely, than on "cold" start (when I just started it after night)
it gets strange rpms lost at about 7k till 9k, it seems like it has too much fuel, it makes strange sound from the exhaust, and I feel on the handles that it can't rev up. If engine is hot - this issue does not happen.

My brother says that this is 2 stroke engine, and that's just it's way of working while it's cold.
Second thing is that I know I should start it with choke on (I have manual now) but although it ONLY starts
with choke ON, I have to always turn it OFF 5-10s after start, if I won't do it, it will fall off, and die on me. Usually I try to sit on the bike after it has 4 stripes of temp on the screen. I know that switching from stock airbox onto polini air box, I had to upjet. BUT what I don't know is about how much? When I bought Gilera, it had 70 jet on it. And it was 50cc, all stock. Then I went from 70cc streetrace, and with stock airbox I was getting best results with 72 jet. More was giving me stuttering effects on 8k rpms when I would violently pull to full throtle. Now I changed to 80 jet. Should I go higher, or can I try lower?

Does little gold screw I belive it's called pilot - affect only idle engine speed, or also whole throttle operation?

I know these are quite specific questions, but these are only those I can't bare to find any answers for.

Thanks in advance,
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Happy to help!
Malossi Multivar (old, 16x13), 3x4g+3x7g
That's a bit odd, why don't you run 5.5g rollers, instead of the mix? The 7g rollers will wear down quicker than the 4 grams ones!

For now my issue is : Runner starts at 6.5k, accels nicely, than on "cold" start (when I just started it after night)
it gets strange rpms lost at about 7k till 9k, it seems like it has too much fuel, it makes strange sound from the exhaust, and I feel on the handles that it can't rev up. If engine is hot - this issue does not happen.
To be honest I would go with a modified stock airbox on the Stage6 Streetrace. The powerband and target revenue isn't high enough to justify a racing filter and smaller airboxes are much easier to jet properly. You will also save a lot of fuel!
Also, with your setup the 17.5mm would have been sufficient actually...

The other thing is the cold behaviour. 2 stroke carb engine are in general difficult to setup, especially when considering the cold start. The choke is just a help to get the engine running so it can warm up properly. No one will ever tell you that their scooter is running equally good cold and warm once it's tuned significantly. Moreover pushing the cold scooter to it's limits is just not the way to do it. If you are short on money go slow and warm the engine up first. It will last you ten times longer!
It is also normal that you are required to use the choke when starting the engine, especially with a racing airfilter! Once it's warm tho, it should be possible to restart it without for the time it has it's target temperature. Is this the case?

The carb has two screws: One is for the idle speed and the other is the idle mix
Have a look here, this is a great resource on setting up the defaults for your carb:

For your engine setup the default settings of the carb should be good. The only adjustment necessary is the idle speed and air screw as well as the main jet. For the main jet you simply overjet (fail explosions whilst riding) and then jet down until you found the perfect balance. With 21mm and race filter i'd say it should be around 100 in your case. with stock airbox I'd say 80-85...

Hope that helps!


New Member
Strange thing is, yesterday I've installed S6 Pro Replica, expecting some boost. It's worse than Leo RR! Engine don't want to go higher than 7.5-8k. I can go top speed aprox 50-60km/h
Is this a symptom of some restriction? Where should I look for it?

edit. I called seller, he says it's already derestricted. I will try to put 6x5g rolls.

edit2. My bad. I haven't given a sealing damper between exhaust and cylinder! That was making the noise. Have to now wait for the wheather and than try with 6x4.5rolls.
Last edited:


New Member
Ok, just tried it and it goes like this:
Engine has troubles to reach 9k, clutch connects at around 6,5k (white s6 springs),
it struggles through 7k-9k, rpms drops, 2-3x than it climbs into 9-10-11k and goes nice.
Ofcourse scooter as a whole makes sounds like it would want to blow itself up or smth.
Is it possible that belt is slipping? How can I be sure? The sound that is coming out from the carters (where CVT is)
is like psssssssssssssssssss, it's similar to a turbine in a jet engine, and I belive it starts where engine starts to rezonance with the exhaust. Maybe I'm confusing facts, I'm still learning.


New Member
Hello again, I tried installing back standard Dellorto 17.5mm carb and airbox, but wasn't happy with the results. Turned back to 21mm, and now I have problems... Look at video I made, first is with current performance,

and second shows how it was not so long ago.

What can be the cause? Can you help me out? Too much air, too less air? I haven't changed sizes of any jets inside carb, one thing I did though was cleaning it. No part was lost during the procedure, I made it on clean, white cloth without any fibres. I haven't made pictures, and now want to kill myself. Bike is very loud and struggles to get into revs,


New Member
Hello again, the reason behind my problem was inproper needle instalation. I installed it in such a manner, that I could change its position without dissambling the spring.... That was dumb, and my brother helped me a ton with this. I could than lift it up via roller on my handlebar, but spring wouldn't push it back, so it freely jumped along the engine vibrations, thus making all the noise and scooter completly unrideable.

Since that time a lot of things happend. I was using my bike all along even in winter here in Poland.
Now after reading a lot of articles from many web pages I started to understand how things really work.
So I refitted s6 pro replica, adjusted contra spring, to yellow, +1000rpms, droped rolls to 6x5.5g, went back for original 17.5 carb and reffited original airbox, derestricted. I found myself running preatty good with 75 main jet.

Also during winter with hight snow, modded original mudshield with some pices of gum, to fully cover the carb area.
Now I'm just waiting to get gearup kit, 13:39 from s6. Top speed went up to 115km/h for now, with great accel almost from the bottom of the revs.

My only question now is that I have a trouble with original dellortos electric choke. I've bought brand new carb and genuine choke. With it engaged, my engine dies quickly after starting it cold. It seems like choke jet would be too big. As it's brand new, Im sure it size is 34. As stated in all guides, I adjust idle speed and idle mixture screw after warming up. If I choose nice 1200rpms with this method, it will acctually step down after being warm. But just seconds after cold start it dies in the same settings. I have to manually adjust idle speed each and every time. That after 5mins of warming up, idle speed goes high, up to 3k. Than i have to get it back. What could be wrong here? Is a sign of too much air? There is not a possiblity of airleak, as manually moving all elements doesn't make any diffrence in idle speed (and it should change even a litttle if airleak would be present I think) plus, almost all parts are brand new, thus no airleaks again. Should I sell electric choke, and switch for manual one? If so, for how long should I keep it turned on when starting the engine? Also, Daniel - what do You think about gear up kit? Is it a good solution for me (i haven't paid yet)

Best regards,
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