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Taking the mystery out of SU jet heights.
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Ray White



Joined: 02 Dec 2014
Posts: 6319
Location: Derby
PostPosted: Fri Jul 21, 2023 12:57 pm    Post subject: Taking the mystery out of SU jet heights. Reply with quote
Fuel should be visible in the jet tube (remove the carb dashpot and piston and turn on pumps to see where it settles. Set the jet tube height relative to the top of the bridge and NEVER subsequently touch the jet height (i.e. don't mess with raising/lowering the jet with the screw at bottom of the carb). The top of the jet should be between .060 and .065 below the top of the jet bridge.

Best atomization happens with this "jet drop." With the jet higher than that, the fuel actually "mists" too much, and with the jet lower than that, it actually "puddles" and doesn't atomize well enough.

Once the jets are set relative to bridge height, the next step is to set the fuel level in float bowls. You should see the fuel about 1/8" to 3/16" below the top of the jet tube. Any higher and it's likely spilling over when the motor fires/vibrates. Blow down in jet tube (with pumps running) to see where the fuel level settles.
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Rich5ltr



Joined: 28 Mar 2008
Posts: 678
Location: Hampshire, UK
PostPosted: Sat Jul 22, 2023 2:35 pm    Post subject: Re: Taking the mystery out of SU jet heights. Reply with quote
Ray White wrote:
<clip> NEVER subsequently touch the jet height (i.e. don't mess with raising/lowering the jet with the screw at bottom of the carb)...
But when you (or the article you've quoted from) say never touch the jet height surely that's counter to how one adjusts the mixture with the jet adjusting nut under the carb. I don't see how you could set the carbs properly without touching the jet height. Question
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Ray White



Joined: 02 Dec 2014
Posts: 6319
Location: Derby
PostPosted: Sat Jul 22, 2023 5:05 pm    Post subject: Re: Taking the mystery out of SU jet heights. Reply with quote
Rich5ltr wrote:
Ray White wrote:
<clip> NEVER subsequently touch the jet height (i.e. don't mess with raising/lowering the jet with the screw at bottom of the carb)...
But when you (or the article you've quoted from) say never touch the jet height surely that's counter to how one adjusts the mixture with the jet adjusting nut under the carb. I don't see how you could set the carbs properly without touching the jet height. Question


You are probably right. I should not have posted it.

This is the complete article taken from an MG racing site.


[Taking the Mystery Out of SU Jet Heights
—by Manley Ford/MGVR Yahoo E group
Below is advise I received from Craig Chima several years ago. Craig holds several SCCA National Champion- ships in numerous cars running SU carbs – all of which he constructed and maintained personally. Lately he’s moved to vintage racing, both supporting his brother Derek’s very quick Midget and preparing Loti and other very fast cars.
‘Fuel should be visible in the jet tube (remove the carb dashpot and piston and turn on pumps to see where it settles. We actually set the jet tube height relative to the top of the bridge and NEVER subse- quently touch the jet height (i.e. don't mess with raising/lowering the jet with the screw at bottom of the carb). We've found that top of the jet should be between /.060 and .065 below the top of the jet
bridge. Best atomization happens with this "jet drop." With the jet higher than that, the fuel actually "mists" too much, and with the jet lower than that, it actually "puddles" and doesn't atomize well enough. Once the jets are set relative to bridge height, the next step is to set the fuel level in float bowls. You should see the fuel about 1/8" to 3/16" below the top of the jet tube. Any higher and it's likely spilling over when the motor fires/vibrates. Blow down in jet tube (with pumps running) to see where the fuel level settles.’]
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