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Expert
Posts: 4458
  
Location: Jelling, Denmark | can someone tell me at what point under full load in the rev range or what ever the bell is supposed to be fully drawn up by the vacum in an SU carburetor?
I was looking down the throat of the HIF38 in my slightly tuned 998 when it was making a pull at the dyno and the bell is allready drawn full up at 3500 rpm which seems way to soon as it pulls up to 5700 rpm.
Edited by DotMatrix 24/7/2016 11:55 PM
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Regular
Posts: 112
Location: Hamburg, Germany | Burlen Fuel says:
Piston springs
On 1 1/4 in., 1 1/2 in. and 1 3/4 in. diameter horizontal carburetters, the red (4t ounce) spring is normally used for initial testing and in most installations this load of spring will be effective, assuming that the size of carburetter has been correctly chosen. When the correct spring is fitted it is usual to obtain full piston lift at full throttle and at approximately three-quarters of the maximum rev/min. If the absolute maximum of power is required, however, it is more usual to choose a somewhat larger carburetter, which will not obtain full piston lift until nearer maximum rev/min.
http://sucarb.co.uk/technical-su-carburetters
i would have said, at peak torque, but i haven't touched a carb since years |
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Expert
Posts: 4458
Location: Jelling, Denmark | KLAS - 26/7/2016 2:13 PM
Burlen Fuel says:
Piston springs
On 1 1/4 in., 1 1/2 in. and 1 3/4 in. diameter horizontal carburetters, the red (4t ounce ) spring is normally used for initial testing and in most installations this load of spring will be effective, assuming that the size of carburetter has been correctly chosen. When the correct spring is fitted it is usual to obtain full piston lift at full throttle and at approximately three-quarters of the maximum rev/min. If the absolute maximum of power is required, however, it is more usual to choose a somewhat larger carburetter, which will not obtain full piston lift until nearer maximum rev/min.
http://sucarb.co.uk/technical-su-carburetters
i would have said, at peak torque, but i haven't touched a carb since years
so what they are saying is that if I want to fix this "problem" I need a larger carburettor for which I probably can get the right needle or a heavier spring in my current HIF38 and then make a needle my self as I cant get a richer needle than my current one. thank you |
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Regular
Posts: 112
Location: Hamburg, Germany | long time ago i had a similar engine, but with a 12G295 head with 33/29mm valves, and had to play with carbs. after some time i thought i should use a HIF44 just based on the expected HP, just like the Rover Mini Cooper (in fact a MG Metro engine) from 1990 or so. think the 60HP Metros uses this carb, too. in spite of SU claims a 1.5" carb can work up to ~75 HP.
worked great, but after trouble with carb freezing in winter i switched to EFI and never looked back |
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SU carburetor spring advice
