Technical Carb

[Kiltedboarder]

Hi all
Just been looking over the car again and saw my carb is a 24IMB

Is this normal, I thought most 500s had the 26 carb

It’s a 1974 model by the way

Would a 26 carb also fit a run correctly?

Thanks

 

[smart51]

The 24 IMB was fitted to the earliest cars in the 50s and to the 500R from 1973 onward. It looks like you have the original carb.

The 595 engine in your car was designed to use the 28IMB. If you fit one, you'll up the power from 18 to 23 BHP. You'll probably need to rejet your new carb though as most you'll find are jetted for a 650cc engine. I have the 28IMB on my 1975 500R. It didn't work well with any of the standard jettings. With a bit of trial and error, I found a setting in between that works really well.

 

[Kiltedboarder]

Thanks, great info. Like the idea of that massive power boost . Out of interest how do I rejet the carb? Thanks

 

[smart51]

The jets are brass screws with holes in them. They're sized in increments of 0.05mm. The main jet sits at the bottom of the float bowl inside the 28IMB. The Air Corrector Jet is also inside. The idle jet is behind a screw on the outside of the carburettor. Replacing them is just nuts and bolts, though it helps if you have a tiny stubby screwdriver.

Knowing which jets to fit is more tricky. You can take the car to a rolling road where they measure the exhaust gasses at different engine speeds and suggest jetting changes. Or you can do what I did and try different combinations unit I found a good one. I don't recommend it. You might find this thread interesting: https://www.fiatforum.com/threads/correct-jetting-for-594cc-engine.504490/

 

[Kiltedboarder]

Cheers mate, very useful. I’ll dig into a bit and might try to get hold of one and play with it. No rolling roads here at least my lack of decent enough German language to get myself understood on such a topic as I live in Austria