k+ n lovely sound

I found bigger jets got rid of hesitations too. That's no surprise, these things are always a compromise and Fiat were trying to achieve good fuel economy, emissions and torque at part throttle openings and full throttle; putting bigger jets in swings the compromise towards torque and response and away from fuel economy; I've had 45 mpg out of a car with standard jets (albeit a lightened car with no bumpers) but struggle to reach high 30s with big jets.

Rolling road data tells you nothing about drivability (hesitations). On a rolling road power run you're at full throttle all the way with both chokes on the carb wide open. The hesitations people experience on the road are at part throttle openings and transients, especially when the second barrel of the carb opens up when the throttle is floored. It takes a real expert on a rolling road tuning session to get better low speed response; and that expert will probably do it by feel and not use the dyno other than as a load.

It's easy to achieve more power on a rolling road by optimising the timing to achieve higher torque/power at high engine speeds; that timing modification is likely to reduce your torque at lower engine speeds which will equate to slower acceleration. Show me a torque curve from a dyno run which shows increased torque from low engine speeds as well as a higher peak and I'll be very impressed!!

Another one of my soapboxes! I'm not normally this bad, honest!!

PS my airbox isn't standard... because I love the sound. Each one to their own!

i had my cambelt replaced and the timing set up by olly vayle and it was vastly improved ....he knows his stuff!

Always a pleasure reading your comments Martin.
So whats the best Jet size option for standard setup, for driving fun style and not driving miss daisy eco style?

:roll:
Mark

i have fitted 120/135 which i believe is best

I think it depends... I can only go by my personal experiences.

I used 120/135 for a while on the VS with a standard air cleaner, it did get rid of hesitations and felt good to drive. I then cut off the venturi on the air cleaner inlet and it drove just as well but sounded much better - the venturi is only there to act as a silencer. I also cut away the slats on the engine lid to let the hot air out and get cold air into the carb.

I then had the same set up on the GF except for not cutting away the lid slats. This is a compromise because of the trapped hot air getting into the carb; but the car lives outside some of the time so I didn't like the engine getting soaked everytime it rained! It still sounded good, but hot starting was difficult, it often stank of fuel and the fuel economy was low 30s. I tried a 112 primary but it didn't make much difference. I then went back to the standard 107/130 and it runs really well with no hesitations! The difference with this engine was that I'd had the head off, decoked it, reassembled very carefully to ensure no air leaks, cleaned the carb thoroughly in cellulose thinners and tuned it carefully with new consumables and electronic ignition.

So I guess a well sorted engine will run well on std jets; anything less than that or an engine ingesting cold air with a low restriction engine may need bigger jets.

The thing is, jets are so cheap and easy to change you might as well buy a selection and play around! You need to leave them in for a couple of weeks and optimise the timing and idle mixture on a day to day basis to get the best performance, but it's quite satisfying doing that. Just make sure you mark the position of the mixture screw so you know what the original set up is - you can twiddle it in 1/4 turn increments.