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| Su tuning 6 flats instead of 12? https://www.ausmini.com/forums/viewtopic.php?f=2&t=91398 |
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| Author: | rapa59 [ Tue Dec 29, 2015 4:52 pm ] |
| Post subject: | Su tuning 6 flats instead of 12? |
Tuning a set of su's which have been rebuilt and 1 seems happy at 12 flats or 2 turns but 1 doesn't like it and runs real rich according to plugs. It's happier at 6 flats of tuning so I have 1 carb at 6 and the other at 12?!?! Is this normal to have a big difference of is it not right. Seems to drive smooth now after coughing and popping a bit at the start. |
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| Author: | goodie [ Tue Dec 29, 2015 6:24 pm ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
Have you got the air flow right ? |
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| Author: | TK [ Tue Dec 29, 2015 6:40 pm ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
Same needles? |
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| Author: | rapa59 [ Tue Dec 29, 2015 7:17 pm ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
yep both airflow the same, brand new and same needles and springs |
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| Author: | gtogreen1969 [ Tue Dec 29, 2015 7:22 pm ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
Were the jets centered? |
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| Author: | rapa59 [ Tue Dec 29, 2015 7:30 pm ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
yep |
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| Author: | Goldbrocade_62 [ Wed Dec 30, 2015 4:48 am ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
Sucking air somewhere |
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| Author: | rapa59 [ Wed Dec 30, 2015 8:48 am ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
If it sucking air from somewhere wouldn't it need to be tuned more richer instead of leaner? |
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| Author: | Bill B [ Wed Dec 30, 2015 9:09 pm ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
When you started to count to 6 or 12 flats down, did you check that both jets started at the same height. I use the end of a steel rule across the land on either side of the brass jet. Screw the brass jet up/down till it just touches the ruler for both carbs. Then count down equally. This also assumes the float levels are as close as possible. After running engine, remove dashpot, piston and look down jet. Fuel level can just be seen down the jet hole - should be vaguely same. |
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| Author: | drmini in aust [ Wed Dec 30, 2015 10:04 pm ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
Did you push both needles right in, or set them flush? ie. are both set the same? Also, a difference of .001" in main jet bore size would cause this. SUs are not precision instruments...! |
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| Author: | rapa59 [ Thu Dec 31, 2015 3:25 am ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
Yes I used a steel ruler to get it as flush as possible then unscrewed it 12 flats. Also the needles are set flush to the bottom of the piston on both carbs. Tried to get everything as close as possible for both. |
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| Author: | Bill B [ Thu Dec 31, 2015 11:35 am ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
Sucking extra air will only affect idle tune - once above idle, the balance tube balances the air flow equally. But if one carb is seriously different in richness, they will remain different through rev range. I have had SUs that tune a long way apart and seemed to work. It is the float bowl level- check float and pin for unequal wear? If you set the idle to around 1,500 rpm by pulling the throttle cable by hand, using the dashpot damper or a finger under the piston (if there are no filters), gradually lift the piston of one carb up all the way, so that the airflow is not drawing petrol from that jet and the engine is running only on the other carb. The engine will chugg, but if tuning is close it will keep running. Do the same to the other carb and the chugging should be about the same. This is fairly accurate and gives a quick check that they are close in richness. |
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| Author: | rapa59 [ Sat Jan 02, 2016 5:12 pm ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
I changed the float pin needles. But would the float bowls have that much affect since the su adjusts how much fuel goes in by the piston |
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| Author: | drmini in aust [ Sat Jan 02, 2016 9:37 pm ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
Are you using the new 1 piece nylon floats with no adjustment provided? They are rubbish. The older ones with the metal strip on top are better, you can adjust them. |
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| Author: | carbon [ Sun Jan 03, 2016 3:50 am ] |
| Post subject: | Re: Su tuning 6 flats instead of 12? |
Also try swapping the dashpot springs around, these make a big difference to jet height at idle. Even if you have a pair of red springs or pair of blues, there is no guarantee that they have same downforce. Only way to be sure is to take measurements, I had to sort through a whole pile of old red springs to get two that matched well. |
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