I'm trying to sort a lack of power on my 1975 Fiat 500. It should make 23 BHP but I suspect it's only making 18, mostly because the top speed it will make is 62 instead of about 67 MPH. I've asked on a Facebook group and some of the Italian members say the standard jetting is a bit lean.

The Weber 28IMB comes with a 115 main jet, a 190 air corrector and an F8 emulsion tube. I've done a back to back test today swapping the 190 for a 185 and a 200. Top speed with the 190 is 62, with the 185 is 61 and with the 200 is 60. It says that the 190 is the best match for the 115 main. Does it also say that the engine is neither rich nor lean at 60 odd MPH? Do you think it worth my time trying a 120 main, or am I barking up the wrong tree here?

Have you done a plug chop after a run? Check to see if it is lean, I’d be tempted to try a 120 before changing the air jets.

Air corrector jet - Determines the premix ratio of the petrol and the air and enriches the petrol in the jet block with oxygen. It primarily effects the lower and middle rev range, meaning in the area of the 1/4 to 3/4 of the opened throttle slide. I'm assuming the smaller jets are letting less air to fuel. On my Prefect the air corrector is an adjustable screw used to adjust the idle and pick up under load. When its too lean it hesitates badly when you accelerate, I have it pretty rich

It's 49 years old and you've lost 5mph. Plus how light was the original test driver? I personally would not be pushing such an old car to the limit anyway, Mines meant to go 66mph when new but I never push it past 55 even on a good day as I know it's not good for it. If you look at the plug after a good run they will tell you if its running rich or lean. Google "spark plug colour chart". They never lie.


[Edited on 31/3/24 by Mr Whippy]

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Originally posted by Dingz
Have you done a plug chop after a run? Check to see if it is lean, I’d be tempted to try a 120 before changing the air jets.

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Originally posted by Mr Whippy
Air corrector jet - Determines the premix ratio of the petrol and the air and enriches the petrol in the jet block with oxygen. It primarily effects the lower and middle rev range, meaning in the area of the 1/4 to 3/4 of the opened throttle slide. I'm assuming the smaller jets are letting less air to fuel. On my Prefect the air corrector is an adjustable screw used to adjust the idle and pick up under load. When its too lean it hesitates badly when you accelerate, I have it pretty rich

It's 49 years old and you've lost 5mph. Plus how light was the original test driver? I personally would not be pushing such an old car to the limit anyway, Mines meant to go 66mph when new but I never push it past 55 even on a good day as I know it's not good for it. If you look at the plug after a good run they will tell you if its running rich or lean. Google "spark plug colour chart". They never lie.


[Edited on 31/3/24 by Mr Whippy]

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Have you checked the auto advance ignition timing?

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Originally posted by rusty nuts
Have you checked the auto advance ignition timing?

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I would just try a 120 main for a start, if you are sure it’s lean at wot.

Swapped for a 120 main today. Sounds better than the 115 at all revs, but noticeably down on power mid range power. Top speed down 1 to 61. So I changed the 190 air corrector for a 200 and retested. A bit livelier in the lower mid range than the 120 + 190 combination, but still down on power. Top speed 60. I'd have tried 120 + 195 but I don't have a 195. I wonder if a dyno session might be a good idea.

OK, so what's the deal with air jet sizes? Weber say going up 3 air jet sizes leans out the mixture by the equivalent of going down 1 main jet size, but more at the top than the bottom. So a 115 + 190 combination is like a 120 + 205 is like a 125 + 220. So what would the difference be between them? They're all similar at the bottom, but the 115 + 190 is leaner at the top and the 125 + 220 is richer?