Modern Vespa

Hi fellows!
Need your help again.
I noticed that my 50 Special have revs problem.
If I quick WOT it wont rev at all. If I quick open throttle very close to WOT but not full it revs much higher.
Same story when I ride on it.

It is VMC GS 125cc kitted
Mazzucchelli ETS crankshaft
Rotary intake
SIP Vape Road ignition
VMC Tork pipe
PHBL 24 carb
D22 - clip 2nd from top
AQ 264
MJ 100(tried 105, 103, 98, 97)
IJ 55(tried 52, 50)
Mix 2.5 out(tried 3, 2)
Float needle 250(tried 400, 350)
Throttle slide 40
Floats weight 6.5 gr.

I tried new spark plug, new spark plug cap, new HT wire, new SIP Vape CDI.
Nothing changed.

Here is video and how spark plug looking.

How's the vape attached to the engine? Resistor spark plug cap?

Sorry but can't understand. How stator plate attached to crankcase? It was full Vape kit from SIP with all hardware. I think yes. It is NGK and very similar to this one.
When I get home I check it again.
Spark plug also with resistor.

I don't know PHBL carbs, but that spark plug is dangerously lean. It looks like there's some detonation evidence on it, too, I think.

Also, what's your ignition timing set to? Could be too much advance, but definitely too lean.

Yeah attaching with the SIP kit didn't work for me, removed all the brass and rubber pieces.

I don't remember correctly but it is for sure 18º or 19º. I adjusted with degree disk and checked with strobe. My is attached same as on your photo. But after last engine rebuild I noticed that killswitch button not works. I added extra ground wire between CDI mount and chassis and then killswitch button work again

Any progress?

PHBL on a 112cc rotary should be in the range of:

50 pilot
75 Main

So I'd guess for a 135cc

Would just be a 85-90 main.

But you plug looks pretty lean.

Air leak?

I found new not used Vape ignition for tests.
At first changed only flywheel. Not helps.
Then changed stator and flywheel - nothing.

Then I had long speak with my friend. He is mechanic in racing kart team.
And he sure that my problem is causing by D22 needle.
He suggested to try D21, D27, D28 or D29 needle.
Now I need to find it somewhere. Or to order from SIP. And wait for 2-3 weeks. I'm pretty sure there no air leaks. Engine is dry and more or less clean everywhere.
The problem appeared after carb changing from PHBH 28 to PHBL 24.

Hmmm not sure about that… all I've read about is using the D22 needle. And using it on my terrible Polini 112cc build, it's seems in good shape.

It's kinda like the x2 needle for the PHBH. It's pretty standard… You might only use a x13 depending on air temp and humidity. But usually back to the x2.

Able to swap back to the phbh and see if it's changed again?

Yes. I'm reading a lot of similar setups and lot of them was with D22. It also was suggested by one British guy Henri who is smallframe guru in my opinion.

For sure I can swap back to the PHBH 28, but I need to change intake manifold. And now I'm bit tired of playing with Special. So maybe later.
But I also can say that with PHBL 24 I like it more.

Air leak at the intake manifold?

Yeah I can imagine the PHBL is probably more responsive.

Tested it with spraying carb cleaner. All ok.

Odd…

If it is fuel related, what happens if you do the same test, but raise the needle one clip and then lower it? That would affect the 1/4-3/4 throttle acceleration.

If no change I'd suspect ignition

I played with needle clip. It was on 2nd from top. Tried all positions and best was 1st from top. But not affect on the problem at all.

And I tried new stator, flywheel and coil - no luck. Also I changed spark plug, it cap, HT wire.

Ok so if youve replaced all the electrical I'd eliminate that.

Now that I've read your response, I'd say it's only fuel related. My guess is when you slowly accelerate, the needle limits the fuel so the motor can rev up. When you give it wide open, it is just full 100% main jet. It's too much fuel to burn cleanly so it sputters.

Would I change anything, no. I'd leave it how it's set up so it doesn't seize under load.

I think that problem is solved!

Thanks for responses friends!