The Age Old Question Which Main Jet to Start With

[christopher]

Where to start at with jetting, working on getting an aluminum cylinder setup together.

Current setup is
Minarelli 50cc

Carburetor - Mikuni VM16, 80 main jet, factory pilot jet.

Factory air cleaner with snorkel removed.

Tecnigas chambered pipe, not sure of exact model.

4.5 gram Dr Pulley sliders.

Spark plug is a DCPR7EIX, nice cardboard color.

Upgrading to
Aluminum cylinder stage6, it is a mk2 sport pro.
Plan on running 91 octane with a squish around 0.9mm. Really depends on how the cylinder sets up. Don’t plan on removing any metal from head starting out.

Any idea on upsizing main jet as a safe starting point? I was reading a bit and saw recommendations of upsizing 15 % around 90 or 95 main jet.

['O'Verse]

Rule of thumb 👍. Always start higher than you think and work your way down till it runs right. Rich mixtures don't kill engines. Lean mixtures do. After playing with carbs and main jets long enough you start looking at carbs with easier main jet access. Dellorto , keihin, etc. Fuel shut off valve and one nut on the bottom of your carb makes it a helluva lot easier when your trying to dial in your main jet sizes.

[aeroxbud]

Always bigger. If it runs terrible, it's easy to go down a few sizes.
Every bike is different. And sometimes needs quite a different jet size to what you think would be normal.
On my cast iron cylinder I started at 100. Being aluminium I would go higher still.

[shoom]

I've never seen even the cheap chinese iron 70cc cylinders take less than a 102. just hopefully you got at least a 43 pilot jet, as those are harder to find for the dellorto PH series than other makes..

[christopher]

95 was so rich it would cause missing under full throttle, 90 is better, getting a new plug as the current plug is black and hard to read.

[ThaiGyro]

Hmmm...put the snorkel back on first. No snorkel would seriously restricting your air flow. You are choking the box at it's inlet, creating a lower box pressure, thus tuning to less air mass. For torque, you need the opposite. Plus, you will find tuning easier.