Moped still lean after 30% upjet?

[simont123]

Hi all

I have finally got round to asking my local garage to install my new parts on my 1991 Yamaha BWs. Below is the set-up

70cc Stage 6 Sport Pro Cylinder
19mm Stage 6 Dellorto Carb (5mm main jets)
Stage 6 variator
Stage 6 Sport Pro clutch and drive belt
Stage 6 Primary gear up
Yasuni R Exhaust
Standard Air Box

The garage had to remove the oil pump as it was not strong enough for my setup. They set-up a 50:1 oil to fuel ratio that I mix directly in the petrol tank but they told me the bike was running very lean and I need to up-jet.

I started with the factory 92 main jet and have tried 100, 105,110 and 120 jets but I still cant fully open the throttle without the bike bogging down?

The bike gets to 50+ mph at 1/2 throttle but as soon as I open it fully the bike bogs down and starts to die. Opening the choke (to let more fuel in) doesnt help either?

Am I still running 'lean' despite a 30% upjet? Can anyone suggest anything as a 120 is the largest jet I can find and am out almost out of options.

Would a 33:1 oil/fuel ratio be a better choice with this set-up too?

Thanks for any help!

[Kickstart]

Hi

Probably rather more than a 30% upjet. Not sure on your carb, but often the jet size is related to the diameter of the hole in the jet while the actual size for flow is related to the area. If so a change from a 92 to a 120 is about a 70% increase.

However as pulling the choke on makes no difference it suggests this might not be the problem. The main jet is meant to be the ultimate limit of fuel, but possible something else is limiting it. Assuming the fuel filter, etc, are fine then possible that the combination of needle and nozzle is limiting things. As a quick check raise the needle to the highest notch (ie, notch nearest the sharp end) and if already there try shimming the needle up a touch.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[deeds]

As a very rough guide My Sons first ped (AM6 engine) was enlarged to 70cc and that needed upjetting from low 90s to 142 before it would run right.

this lot have higher than 120s:-

https://www.psntuning.co.uk/scooter_parts.php?prt_id=21

[YBR Ric]

Adding more oil by swapping from 50:1 to 33:1 will dilute the amount of petrol present in each litre of liquid presented to the engine to burn, so overall things will be even leaner.