Bike bogging (derbi)

[Toppingcat]

Hi i have just bought a 50cc derbi senda sm xtreme 2008 model. when i bought it there were a few problems such as exhaust leaking, air filter being in pieces and jets were too big (82). i put on a "big one" aftermarket exhaust and downjetted to 72.The guy i bought it from said that he had recently had a top end rebuild on the bike. when you get up to about 35mph it bogs out, it also sometimes has trouble starting up.any advice on how to fix this will be much appreciated

[Mr Fro]

Is 72 the standard size jet and do you have a rev counter?

[Toppingcat]

im not entirely sure what the standard is i think it is 68 but with the aftermarket jet i thought up jetting was the right thing to do, i do have a rev counter but it dosent work properly it sometimes works sometimes dosent im not sure why

[Mr Fro]

Stick the standard jet in if you can. High speed bogging is a symptom of too big of a main jet.

[Toppingcat]

Ok thanks i will try lowering the jet, im fairly confident this isnt the issue however as i think around 70 is the standard, any other advice will also be greatly appreciated

[Toppingcat]

lowered the jet to 72 and it ran like crap, so put the 88 back in and it worked better so we went up to a 95 and its the same as an 88, dont think the problem is jet related we are thinking maybe cdi related?

[Mr Fro]

72 to 88 is a big jump for a 50cc - I only do steps of 10 when tuning superbikes!

I would look at jets in between those before thinking about the CDI.

[Guest]

Those Dell'Orto PHVA 17.5 carbs are an atomiser / needle taper sensitivity nightmare. Also' the auto vacuum fuel tap diaphragm brakes and starves the engine of fuel.

Here's a few part numbers..

Derbi 00H00404181

Polini 201.1700

Polini 343.0009

Polini 343.0023

Polini 372.0004

Polini 370.0002

Adjust the oil injection pump nut to its maximum adjustment..

http://www.polini.com/wp-content/uploads/2016/05/cp_polini2ing.jpghttp://www.polini.com/wp-content/uploads/2016/05/cp_polini6ing.jpg

Slice those 2 pointy bits off the manifold rubber so the Polini carb fits properly..

Leave the needle clip on stock setting (middle notch)

Use pilot jet size (49)

Turn air screw fully in. Unscrew anticlockwise by 1 1/2 turns

Set the throttle stop screw so the engine stalls within 20 seconds

HFFS (How far from 4 stroking)

Most important rule: Don't inspect spark plug porcelain color. Leave that plug in the cylinder head.

Start the main jet rich (94). It'll fire on every 4th cycle at full throttle. Lower the main jet number 1 increment each time until it stops 4 stroking.

The engine should 4 stroke slightly under little engine loads around 1/4 throttle. If it 4 strokes like mad, lift the throttle needle clip to the 2nd notch.

https://www.youtube.com/watch?v=PZnRhJocqCQ

[Toppingcat]

hey sorry i completely forgot about this forum i actually got a polini carb about a week ago and fitted it as the phva was leaking anyway, just wanted to thankyou for your reply i did cut those bits off the manifold and im glad it was the right thing to do thankyou very much for the reply and sorry for the late reply