Tuning 500 udgraded to 540, do I need a new jet?
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Tuning 500 udgraded to 540, do I need a new jet?
the hobbler
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Initially I would run it 'as was' and see how things go. keep an eye on spark plug colour and make sur that it is not running too lean. What might help to give you the full benefit of the extra engine capacity would be to fit a "28IMB" off a '126' and leave that on standard settings.
I am pleased to hear that the kit made a difference as I have a Nanni 540cc kit also which I may use. The gain will have come from the capacity increase plus the high compression pistons if it is like mine. Altering jet sizes would not be expensive but there are no comparisons to go by so down to trial and error.
The 500 Sport model used a Weber 26IMB26 3 that had an enlarged Venturi, the same spec as the 26IMB26 5 which was fitted on the sportier Autobianchi cars. Both extremely rare carbs.
The cylinder head inlet port on a 499cc engine is 26mm in line with the carb. On the 126 this goes up to 30mm so if you fit a 126 carb you need to use a 126 carb base , open up the cylinder head inlet and change the inlet elbow also.
the hobbler
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Thank you for that input David---always ready to learn new details about the various versions of the 500.
My car is a Autobianchi Cabriolet, the 540 udgrade made my lttle car accelrate uphill in 3 gear, before 3 gear uphill was a no-go. Before 0-60 kph on 16 sec is now 12 sec and thats with a lower rpm. I allso installed a 123ignition wish I set on program 1, normal curve with improved idle.
My carb says 26IMB 10, do you know anything about that?
Now I have search through the complete internet at least twice 
what I found:
- the fiat 500 sport versions used weber 26imb2 or weber 26imb3
- the autobianchi cabriolet or at least some of them have 26imb5(I also have the original italian manual wich says 26imb5)
I undstand that these carbs are quiet rare, but why can't I find a picture of at least one of those 3 carbs, are they soo rare what none of them have a visual presentation on the internet, I guess the name is writte the same way as on the others? I found a 26imb4
what I found:
- the fiat 500 sport versions used weber 26imb2 or weber 26imb3
- the autobianchi cabriolet or at least some of them have 26imb5(I also have the original italian manual wich says 26imb5)
I undstand that these carbs are quiet rare, but why can't I find a picture of at least one of those 3 carbs, are they soo rare what none of them have a visual presentation on the internet, I guess the name is writte the same way as on the others? I found a 26imb4
Now I have search through the complete internet at least twice
what I found:
- the fiat 500 sport versions used weber 26imb2 or weber 26imb3
- the autobianchi cabriolet or at least some of them have 26imb5(I also have the original italian manual wich says 26imb5)
I undstand that these carbs are quiet rare, but why can't I find a picture of at least one of those 3 carbs, are they soo rare what none of them have a visual presentation on the internet, I guess the name is writte the same way as on the others? I found a 26imb4
You must have missed my post of a few days ago about a Weber 26IMB 5 carb that I have just finished repairing. I will continue that post which will probably answer your questions but here is a picture.
