Hi Mitja...No...Your M20 won't need a bigger main jet...
Pilot air screw and needle adjustments should enable you to get the right fuel/air mixture up to 3/4 throttle (assuming you have the right slide cut away) and the 170 main jet will be OK from there to full throttle...
If have all the standard parts fitted, the carb is internally clean and not worn out but the mixture is still weak at the top end or through the rest of the range then you probably have an air leak elsewhere which is causing the weakness...
For example...
Worn exhaust valve seat/valve face.
Defective gasket between the carb and barrel.
Distorted carb mounting flange.
Worn carb body and/or slide.
Leaking head gasket.
Incorrectly set valve lifter.
A leak under the head of the timing hole blanking bolt in the cylinder head.
Incorrect petrol level in the carb (is the fitting on the bottom of the carb body the correct one?)...Ian
email (option): ian@wright52.plus.com
Hi Mitja,
With a 83mm piston on an M20 you should have 508cc
The formula is πR^2H, so
3.14 x 4.15^2 x 9.4 = 508.3cc
Or you can just use-
http://www.mk5cortinaestate.co.uk/calculator2.php
Anyway,
All of Ian's good points apply also to a new 276 carb.
Actually, by mentioning the float needle position and so on,
This is the carb he was referring to.
If it is a new carb, it may not we worn,
But air can be vacuumed through the gasket,
(On barrel side or carb side), cracked spacer and so on.
BR,
Noam.
email (option): noam10@gmail.com