I`m still at square one with a couple of things I don`t understand. You say the stock pistons were 9:1 compression ratio. I`m not aware there were different types in the Paso engines. Never read anything else than 10:1. The Supersport came with 9:1 CR.
Is yours still the original Pantah/Paso engine?

You write you have this filter and Laser exhaust pipes but are using a main jet that`s smaller than the original and an air corrector jet which is larger than the original, hence leaner. I`m no carb expert but to me that doesn`t sound right.


Figuring out the compression ration doesn`t need any equipment. With bore and stroke you calculate the cylinder volume. What can be more tricky is to figure out the volume of the combustion chamber. This is the inner volume of the cylinder head. This volume is impacted by the piston position at TDC (deck hight). If it`s lower than the cylinder top this volume ads to the cylinder head volume. If the piston is higher it decreases the volume. It gets more complicated if the piston has a dome and the piston is lower than the cylinder top while the highest part of it is above.
The area between the top land (the part between the upper piston ring and the piston crown) and the cylinder wall also is a volume that`s added to the combustion chamber volume.
The volume of the cylinder head is determined by turning it upside down and filling it up with fluid and a syringe.
With these numbers you can calculate the current compression ratio.
If you know the compression ratio you`re aiming for you can calculate the required combustion chamber volume.
https://app.box.com/s/69k6z5jjv7s8ze6pnjn9mqgfpemgmel7

This a 2 stroke book but the cylinder head info is worth reading
https://app.box.com/s/oigjlb9f7y2gnhrmm50ainhrv7dtd3mj

If you really had high compression pistons you would have to install colder spark plugs. (NGK 9 instead of 8 ) but that`s not necessary if you`re using original Paso pistons.

My paso is a cali model with a few changes like the 906 is equiped with. Probably telling you nothing new, but it has the digiplex with matching flywheel, pickup etc. Also it has different gearbox bushing, exhaust headers, heads slightly different etc. On top of my head the pistons have a CR of 9:1 see picture. I installed the stock Borgo. High Compression if you compare it with the cali pistons

The jet setup is wierd, I admit. I have driven it for ten years with the 145 main jet and it run ok. No burned valves, brownish color on the plugs, some hesitation around 5k rpm. Couple of years ago I started to have a go at the Weber. First step was the setup of Jay according his topic "Weber what if". I even bought a LM-2 and installed a dung on the rear header. With the 145 main etc setup it run around stoich. Only still lean and rich spots at transition.

Tx for the links. Always fund of some literature For calculation of the CR I would prefer a meter at some time now it has a dome on it..

cali piston

Just two tips: About the idle, I don't think it's important to set the balancing @1200rpm, better to handle throttle @2000 and work on this before than lower range. Have a look at my suggestions here: .
About loss of power @5000 rpm Someone said that it's because of the cross exaust pipe, that's too mutch small and rebuilt it with larger steel folds. I can't remember who, but he did it cutting and welding four pieces like this

and he said he solved the problem.
About jets: Low compression pistons can run with lean mix and small jets. European high compression require mutch more fuel, so
Main 150
air corrector 170-165
idle 57-60

was from a disscussion between Brad and a few others on another forum

YESSSSS!!!

Here're some solutions:

4 valve info,can't say if its relevant

a little more

Hi guys. I'm brand new on these forums as well as with my "new" bike, a 750 paso 1988.
After 18 months of putting up with a mechanic who tinkered around with it, am about to give up on the carburetor, a double body Weber; it won't stay at idle and does not stop pumping gas. Any suggestions for fixing or replacing it completely?

Hi Dendo. Setting correctly the Weber carburateur is one of the best matter for Paso owner.
In the F.A.Q. section of this forum you can find my suggestions for your mechanic.
Otherwise there're suggestions to install Mikunis from Monster/Supersport or Dell'Orto from elder bikes or Cagiva Elefant in the same post