Tuning Carburettor
- Thread starter Phild76
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Tuning Carburettor
smart51
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Weber still make and sell the jets, and they're not expensive. The standard jetting is right for the standard engine. You're not going to make power gains just by rejetting. But if you've modified the engine in some way, you need to rejet the carb to match. Is your engine stock or has it been breathed on?
smart51
Established member
You've gone for a 540cc conversion? I considered that. There are 3 ways you could go.
First, and best, is to take your car to a dyno. They'll measure the air fuel ratio at different throttle openings and different speeds. They'll tell you it's rich in one place and lean in another and will take a guess at what size jets you need. If you have the jets with you, swap them out and do another run to confirm.
Second is to find someone who's already done that and use the same jets as them.
Third is what I did, but I don't recommend it. I tried loads of different combinations, tested to see if they seamed lean or rich, and tested the top speed at a fixed point on the dual carriageway. Standard jetting for my 594cc engine was too rich at the top. Standard jetting for the 652 was too lean at the bottom. I found the setting half way between the two works very well, and that one main jet size either side gave a lower top speed.
First, and best, is to take your car to a dyno. They'll measure the air fuel ratio at different throttle openings and different speeds. They'll tell you it's rich in one place and lean in another and will take a guess at what size jets you need. If you have the jets with you, swap them out and do another run to confirm.
Second is to find someone who's already done that and use the same jets as them.
Third is what I did, but I don't recommend it. I tried loads of different combinations, tested to see if they seamed lean or rich, and tested the top speed at a fixed point on the dual carriageway. Standard jetting for my 594cc engine was too rich at the top. Standard jetting for the 652 was too lean at the bottom. I found the setting half way between the two works very well, and that one main jet size either side gave a lower top speed.
Fiat were quite consistent with the jet sizes for the 26IMB carbs keeping to the same sizes throughout the range of standard models. Only the carb fitted to the rare 500 sport models was different and included a slightly larger Venturi. You could experiment with a 28IMB carb from a Fiat 126 but ideally that would include other modifications.
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I’d have thought that gains from 540cc might be a bit limited on the standard 26mm carb?
As mentioned, an IMB28 might be the way to go if you’re after more performance.
Not to be patronising, but if this is a freshly built engine then it will likely feel a bit ‘right’ for the first couple of hundred miles till the rings wear the initial roughness off the freshly honed bores. The engine might not show its full potential till its run in a bit more?
As mentioned, an IMB28 might be the way to go if you’re after more performance.
Not to be patronising, but if this is a freshly built engine then it will likely feel a bit ‘right’ for the first couple of hundred miles till the rings wear the initial roughness off the freshly honed bores. The engine might not show its full potential till its run in a bit more?
smart51
Established member
From the Weber carb manual, the 26IMB is pretty much ideal for 500cc of engine, and the 28IMB is ideal for 600cc. Either is fine for 540cc. I wouldn't bother swapping regardless of which I had.
The 594cc and 652cc engines use the same sized carb. The 652 uses a main jet 2 sizes smaller than the 594, because the bigger cylinder pulls more air through the same sized venturi and so draws too much fuel. It uses the correspondingly smaller air corrector jet with it. At a guess, I'd expect the 540 with a 26 IMB to need a main jet 1 size smaller than a 500, with a correspondingly smaller main air jet, but based on deduction rather than knowledge.
The 594cc and 652cc engines use the same sized carb. The 652 uses a main jet 2 sizes smaller than the 594, because the bigger cylinder pulls more air through the same sized venturi and so draws too much fuel. It uses the correspondingly smaller air corrector jet with it. At a guess, I'd expect the 540 with a 26 IMB to need a main jet 1 size smaller than a 500, with a correspondingly smaller main air jet, but based on deduction rather than knowledge.
