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Dominant Carb
https://www.ausmini.com/forums/viewtopic.php?f=2&t=58093		 Page 1 of 2
Author:  Mick [ Tue Oct 20, 2009 4:27 pm ]
Post subject:  Dominant Carb
I've been tuning the twin SUs on my mini over the last couple of days, and have had a bit of an issue...

I undo the linkages, reset the jets to 12 flats from closed and try to tune them in. I can't seem to get the normal rich/lean signals from the carb as I lift each piston to determine the mixture.

From one carb I get a reasonable raise/lowering in revs, but from the other I only seem to cause the engine to nearly stall.

I have tried setting it by ear, and also by colortune. The colour tune shows a really rich mixture yet the behavior is the same.

Both needles are straight and true, and I have tried using another set as well. Both float bowls are equal, and I have been equalizing the airflows accurately. PCV has been blocked off as well.

Is there anything else I can look at??
Author:  Blokeinamoke [ Tue Oct 20, 2009 4:39 pm ]
Post subject: 
are you tuning with air filter on or off?
Author:  MG Rocket [ Tue Oct 20, 2009 5:06 pm ]
Post subject: 
Had a similar problem....it turned out that the needle on one carb was binding ever so slightly in the jet.
And as dumb as this might sound.....the carbs was easier to ballance with
the oil cap off.... obviously something wrong with the crankcase ventilation......I'll get aroundtoitoneday.
Author:  Mick [ Tue Oct 20, 2009 5:46 pm ]
Post subject: 
Air cleaner off to make it easy to get to everything.

I used a new pair of needles to try and clear the fault, but no dice, the needles also seem to be nicely centered in the jet as they fall with a nice 'clink' each.

thanks
Author:  Blokeinamoke [ Tue Oct 20, 2009 5:53 pm ]
Post subject: 
vacuum leak? Is the piston moving when you push up the pin?
Author:  Mick [ Tue Oct 20, 2009 6:08 pm ]
Post subject: 
Yeah they're both moving nice and smooth...and shafts are fine as well.

I just played some more using the colortune and at 800 rpm, with the 3/4th cylinder burning just over rich (orange) lifting the carb induces a near stall. Lifting the 1/2nd carb gives a near perfect rise fall.

With the bowls removed I get a fuel level of 23.5mm from the top on both..
Author:  Blokeinamoke [ Tue Oct 20, 2009 6:12 pm ]
Post subject: 
Does the jet move when you turn the screw? I have had one carb (out of three) three flats different to the other two because the jet stuck at the bottom of its movement.

Does the richness decrease if you lean the jet off?

After all - you only trying to get a equal starting point for both carbs - after that the turns should be the same.
Author:  GT mowog [ Tue Oct 20, 2009 6:13 pm ]
Post subject: 
Are they both taking the same air at idle?

Have you also checked that the jet is the correct size (0.090") and that it is not blocked?
Author:  Morris 1100 [ Tue Oct 20, 2009 6:24 pm ]
Post subject: 
Are the jets returning all the way? Are they sitting against the nuts?
Author:  GT mowog [ Tue Oct 20, 2009 6:27 pm ]
Post subject: 
Morris 1100 wrote:
Are the jets returning all the way? Are they sitting against the nuts?


Hey Morrie, don't go there, we all got in the crapper the other day with stuff like this :lol: :lol:
Author:  Morris 1100 [ Tue Oct 20, 2009 6:35 pm ]
Post subject: 
GT mowog wrote:
Morris 1100 wrote:
Are the jets returning all the way? Are they sitting against the nuts?


Hey Morrie, don't go there, we all got in the crapper the other day with stuff like this :lol: :lol:

GR started it. :mrgreen:
Author:  GT mowog [ Tue Oct 20, 2009 6:37 pm ]
Post subject: 
Morris 1100 wrote:
GT mowog wrote:
Morris 1100 wrote:
Are the jets returning all the way? Are they sitting against the nuts?


Hey Morrie, don't go there, we all got in the crapper the other day with stuff like this :lol: :lol:

GR started it. :mrgreen:


Mmmmmm......true
Author:  sgc [ Tue Oct 20, 2009 6:47 pm ]
Post subject: 
Do the revs change if you spray carb cleaner around the throttle shafts and/or gaskets?

[edit] Something I discovered on my car (and have yet to fix): Are the springs equal length and strength?
Author:  Mick [ Tue Oct 20, 2009 7:37 pm ]
Post subject: 
Morris 1100 wrote:
Are the jets returning all the way? Are they sitting against the nuts?


[/quote]

Yeah I made sure they were pushing all the way up...

and I don't believe its a manifold leak...As the idle sits just right, and the mixture on the pair of cylinders is coming up bright orange..but there's not much else it could be apart from something fundamental..

Twin SUs make for interesting physics experiments sometimes...who could handle triples? :roll: :lol:
Author:  Morris 1100 [ Tue Oct 20, 2009 7:51 pm ]
Post subject: 
Mick wrote:
Twin SUs make for interesting physics experiments sometimes...who could handle triples? :roll: :lol:


Me. I used to race Holdens with 3 x 1¾" SUs or 3 x 1¾" Strombergs. 8)
The Strombergs were nicer! :lol:
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