Need some advice please
 

If you are reading this, then thanks for taking the time to help me out. Any advice or suggestions will be much appreciated.

A few weeks ago I picked up an old GY6 125cc running scooter for a few €€. It was riding ok, but was leaking oil through the 2 case halves. Took long to reach 80 km/h (49mph) on speedometer and did not go any faster. I decided to do a total rebuild on the engine incluiding bigger piston ( no case boring needed ) and bigger crankshaft. According to calculator, I should go from 125cc to 163 cc. After rebuild, I notice more power on take off but no increase in top speed and still takes long to reach the 49 mph. Still does not go faster than that

This is all that I installed:

SKF bearings and oil seals
58.5mm big bore kit
+3mm crankshaft
A10 cam
Blue racing electronic cdi box
Orange sparkplug lead + better spark plug
Free flow cone air filter direct to carburator
Re-jetted carb. Stock carb is pd24
Free flow exhaust
Belt 835/20/30. Stock was 842/20/30
Racing variator (Bigger than stock)
Racing cluth with red spring (2000rpm)
16/37 +25% Final gear combination. Stock was 13/40

I have tried different weight rollers ranging from 8g to 14g. I tried 12g sliders. I tried different jets in carb until I found the combination to not stutter or back fire. Valves are set to intake 0.3 and exhaust 0.4. Spark plug is right colour so there for not running to lean or excessive

It gets very quick to 60 km/h (37mph) but then takes long to reach the 49 mph. Rev counter is almost at 6000 rpm. Does not go higher than that. Red line starts at 10000 rpm and max is 12000 rpm

Back wheel is rolls free by hand and tyres are at correct pressure

A friend of mine has a stock SYM Fiddler 125cc. Side by side he falls back on start but then easily overtakes me and leaves me behind. I tested it and it clocked a little over 105 km/h (65mph) on speedometer.


Any ideas where or why ? Any help or ideas are appreciated.

What top speed do you want? Work back from there to keep the engine in the power band. It sounds like it's going into higher "gear" to soon for what you want. This link is for cars but the principles are the same. Acceleration vs top end vs cruising. The faster you go the more wind resistance or drag and that the major limit to how fast a scooter can go. Here ya go get ur slide rule out ... or one of those new fangled calculators. https://www.racingjunk.com/news/2016...r-performance/

Thanks for response Roscoe:

Im not looking for anything major on top end. But surely after all the mods I have done it should easily reach the 60/65mph range and feel that little extra power on say an uphill or overtaking a car or wind resistance. I know it is a small engine and you cant make miracles happen. This is a project to me. I know that the amount of €€€ spent so far on engine is ridiculous and does not compensate. Could just get a bigger cc bike. But this is a project and I learnt a lot in the process. This has now got personal and from what i have read, it is possible to achieve this. So where or what am i doing wrong is the question. >:>:>:

I thought it too was shifting maybe to early. I installed stock clutch combo and there i noticed a lack of power and rpm took ages to climb. Red spring combo is 2000 rpm. Yellow 1500rpm and blue 1000rpm

If i rev the scooter on the bike stand, then revs go up to +- 8000rpm. Just when I ride it, it stays just below the 6000rpm

I also checked for air leaks in carb and exhaust leak but there is none.
This is eating me up :hmm::hmm::hmm:

have you tried to go back to the original gears? my 150 goes 55mph at 10000rpm

Thanks for reply 2Dongfangs

That is exactly why i decided to change the gears as original gear combo did not go any faster.Also went down to a 835x20x30 belt.
I also tried to install stock clutch and bell.It was worse. Only thing I have not tried is installing yellow cluth spring 1500rpm. But then theoretically it will shift earlier.Dont think it will make much of a difference

Could the A10 cam be your problem? ... http://scootdawg.proboards.com/threa...ms-a10-a11-a12

Have you done the sharpie test on the variator and clutch pulley? If not I would start there. I could be wrong but I would say you’re going to need more than 6000 rpm to get 60mph. Does your scooter have the short case or long case? If you have the 13” wheels I think the 16/37 gearing is to much. Your engine probably doesn’t produce enough power to pull those gears. Your stock carb is probably your biggest hold back if I had to guess and are you running your stock air filter and air box? If so I would at least put a uni filter on it and go with a bigger carb if you can.
I too am intrigued by my little project scooter at the moment. I done the 180cc big bore kit and added the 2.5mm stroker. I used a 28mm oko carb with a uni filter. I used 15/37 gearing and a dr pulley variator with 14gr weights. I still have a lot of tuning to do and only about 300 miles on the engine. My top speed so far on gps is 70mph and I believe it was a little over 7200 rpm. It gets up to 60 pretty quick and cruises 55-60 at about 1/4 throttle

Also not to sound stupid but did you check to make sure your carb is opening up all the way when your throttle is twisted to wide open?

Thanks Roscoe for reply

Interessting piece of info. Over the week end will have a proper read.

Thanks JJT for reply:
I have not done the sharpie test. Over the week end will do it. It is a long case with 13" wheels. From what I read the 16/37 was the best option for a 150cc. I moved to the next available gear combo. This way i should have not lost alot of power and would increase speed a little.I do have a free flow cone air filter direct to carb. It is the stock pd24 carb. My next option is indeed to get a bigger carb. I would be so happy if I could just get it to 60

Hey something else I would do for sure is replace all of your vacuum lines and your fuel line. Also make sure when you do the sharpie test mark the front and the back so you can make sure both are working properly. I’m going to keep posting anything I think of so if you get sick of it just tell me to stop. Lol.

And don’t forget to check that your carb is actually open all the way when your throttle is twisted all the way

Hey JJT. All help is certantly wanted. That is the idea of exchanging ideas. I really appreciate all help and ideas. I have to work this week end so will only check next week on bike.

Once again , thankyou