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su carburetor tuning

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M

mapleaf

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Need some help on tuning the rear carb on my TR3A to stop the fouling of sparkplugs #3&4. I've followed the book by turning the adjusting nut out 2 turns (12 flats) then screwing it back in 1 flat at a time aiming for slight or no increase in speed when raising the piston. With it screwed all the way up (or in) there is still significant engine speed increase. BTW the needle is properly installed and centred, the jet is fully home and the piston moves freely. What else could it be? Any tips, suggestions or ideas will be greatly appreciated.
TIA
 
Hi there.

So with the nut adjusted all the way up - as snug as possible to the main carb body - she is still running rich?

Sure it is the right needle?

Are the floats correct?
 
Is the dashpot oil at the correct level, and SAE 20 viscosity ?.

Float level can be set easily using a 7/16" diameter drill bit as a bar across the brass forks.

If the jet hasn't been centred properly, the needle wears the jet hole slightly oval, and the only cure for the resultant over-richness is a new jet and needle.

Needles should be SM for a standard 1991cc engine, TW for a larger bore, and installed with the shoulder exactly level with the bottom of the dashpot.

Viv.
 
Needle and jet wear does occur. If these are not new ones, that might be the issue. The idle setting is the thing most difficult to achieve, when they get worn. As you're not complaining of lean mixture, I won't direct your attention to possibly worn throttle shafts.
 
Moseso said:
Needle and jet wear does occur. If these are not new ones, that might be the issue.
Click to expand...
:iagree:
Also check that there is no fuel in the float, and the float valve seals completely with minimal pressure.
 
Check to make sure it is not just running on the front carb. Piston should raise up the same amount at idle and at speed.
Good luck,
With the SUs, adjust, then repeat, again until good.
Scott in CA
 
Thank guys. From your tips I rechecked the carbs and found that when I adjusted the float level to 7/16" I should have also checked the floats. Gas in the rear carb float. Replaced it with a SU spare which is actually slightly shorter than the original but seems to work alright. Also checked the needles and found that although they are both stamped SM they are not identical in size. Probably different suppliers. Anyway, I found a matching set, installed them and I can now get the proper adjustment at about 4 flats out and it sounds good. On the weekend I will replace the sparkplugs and change the dashpot oil from #30 to #20, take it out for a good run and hopefully that will be the end of carboned up plugs. BTW I've ordered a new set of needle and jets.
Thanks again.
 
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