SQ 4 running rich

[jmoseley]

My 59 SQ4 with fully restored SU carb runs rich at tickover, which sounds lumpy. Low speed running on light throttle openings are not smooth. The jet nut has been adjusted from two turns open plus or minus one turn in stages. The engine starts first kick without choke. The ignition is by electronic box and points. The plugs are black and sooty.

[harry.thompson]

If your carb has just been put back together it could be that the needle isn't centred correctly in the jet. Whenever I have messed with the carb I always find this has to be done just before it goes back on the bike. You can tell by lifting the piston and dropping it. It should clunk. Tighten the adjusting nut to the top, loosen the securing nut half a turn, give the jet a bit of a tap, lift the piston and drop the needle into the jet. Tighten the securing nut without disturbing the jet bridge and drop the needle again. If it doesn't clunk, do this again, and again. When its on the bike check for the clunk (I'm paranoid). If its not this the needle may be pushed too far into the piston or be the wrong one. If you turn the adjusting nut down more than two turns to get a weaker mixture its probably something to do with the needle size/position. Don't adjust the mixture until the bike is warm

[jmoseley]

Thanks Harry for quick reply. Burlen did the job and it looks very good. The piston clunked when I fitted carb some while ago. I will remove filter and have another look. I have had the head refurbished and it now runs on unleaded petrol. Perhaps I need a different needle jet combination ?

[Geoffbrown]

Hi My Sq4 also runs rich at tickover, yes the jet is new and centered, new needle, good clean airfilter, small amount of light oil in the dash pot, yes its got the right top on the dash pot (no hole), no air leaks, timing spot on (just slightly advanced), tappets spot on etc. etc. I have tried everything I know and have got it running as smooth as I can but at tickover but it just isn't perfect. As you say it should run perfectly smoothly. I have concluded it is the petrol, I run the bike with an additive which ups the octane and provides protection for the seats. I am thinking about putting it on a rolling road to set it up properly and get the correct needle to suit the fuel - but maybe that's for another day. Anyone else have a rich Sq4?

By the way I run it on fully synthetic oil (it has a cartridge filter) and it is nice and smooth, quite, starts easily, and runs cool - mind you leaks a bit. Have ordered a Morgo oil pump as well that'll really make it leak!