SU jetting for 3.9
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SU jetting for 3.9
I am fitting a 3.9 into my 110 v8 and want to retain the SU carbs for reasons of cost.Can anyone tell me what jets would be the best for this set up?The 3.9 is a bog standard 9.35CR unit.Cheers,Nick
Hi
Without putting this on a dyno, or atleast sticking a lambda probe up the exhaust pipe anything recommended is judt going to be a starting point from where you have to select the needle/spring combination to fit your engine. Having said that I would start with what you have, take the springs out of the pistons, pull them to increase their length by about 10% this should get the pistons lifting about the right amount for the bigger engine (if not chop off a coil or two and restrech the spring to original length +10%). Put the existing needles in on the lower indent in the mounting (not the middle one as they are likley to be in now) and tweek the main jet height (usual mixture screw) until the mixture is about right at Idle (use the lift pin technique), it should be on the rich side of Ok but should run. I would then get it on a rolling road, set up the carb piston travel properly and spend some quality time selecting needles.
Best regards
Miike
Without putting this on a dyno, or atleast sticking a lambda probe up the exhaust pipe anything recommended is judt going to be a starting point from where you have to select the needle/spring combination to fit your engine. Having said that I would start with what you have, take the springs out of the pistons, pull them to increase their length by about 10% this should get the pistons lifting about the right amount for the bigger engine (if not chop off a coil or two and restrech the spring to original length +10%). Put the existing needles in on the lower indent in the mounting (not the middle one as they are likley to be in now) and tweek the main jet height (usual mixture screw) until the mixture is about right at Idle (use the lift pin technique), it should be on the rich side of Ok but should run. I would then get it on a rolling road, set up the carb piston travel properly and spend some quality time selecting needles.
Best regards
Miike
poppet valves rule!
