Brabham Posted January 15, 2011 Posted January 15, 2011 Hi, the large piston (around 50mm) in one of my flat top carbs is jammed. How can I free it up? Someone suggested blowing compressed air down through the top of it, is this wise? Otherwise can I remove the cylindrical housing for it or something? The carbs are off the car and I am trying to fix them up. Cheers guys. Quote
Riceburner Posted January 15, 2011 Posted January 15, 2011 Try cleaning the pistons in kero with very light grade wet and dry, then rinse in methylated spirits. Is the needle straight as they are easily bent, also check the plunger rod for straightness( it's the removable dipstick on the top of the dash pot/piston cover). Are the covers round without dings or damage and Is the oil level ok in the dash pot and the spring ok? Quote
Brabham Posted January 16, 2011 Author Posted January 16, 2011 Hi Gareth, can I take the large cylindrical housing off (the flat top casting) that the piston is jammed in without disturbing any settings? Plunger rod is ok. Covers are undamaged. There is no oil in dash pot as carbs have been sitting in a shed for years. Have put a little bit of WD40 on the piston to try to get it to free up. Cheers. Quote
Riceburner Posted January 16, 2011 Posted January 16, 2011 Yeah, you can undo the screws and remove the cover, this will give you access to the piston, it's probably a combo of carbon and dust/dirt making them sticky. I know mine were very dirty when I stripped them down. When you have the cover off you'll be able to see the needle better and check it's straight, just dont knock the needle and it'll be fine. There'll be no changes to the settings. Also you can use ATF as oil in the dash pots Quote
RB30X Posted January 16, 2011 Posted January 16, 2011 Make sure you use the right size screw driver for the screw and turn while pressing down very hard, as the heads can get very weak and strip very easily. Quote
Brabham Posted January 16, 2011 Author Posted January 16, 2011 Thanks guys, pulled the cover off this morning, put in a little more WD40 around the piston, let it soak in for a while and then pulled it out. Cleaned up the cylinder and the piston and then reassembled. The needle is good. Looked over at table and saw I had left the spring out. Put this back in and now they are both working fine. I gave the cylinder inside and piston a light oil, should it be oiled or is it supposed to run dry? Cheers guys. Quote
zeds4ever Posted May 1, 2011 Posted May 1, 2011 G'day Alex, according to my SU carburettor book it is recommended that 20 grde oil be used in piston tops & to cover no more than 11mm above piston top for best performance.By the way how did the A/C cmprsr go. Regards: Alan. Quote
lightmaster240z Posted May 1, 2011 Posted May 1, 2011 I use Singer sewing machine oil in mine when peter cleaned them or any light oil will do the job Quote
Gordon Dobie Posted June 17, 2011 Posted June 17, 2011 The clearance between the piston and carbie body is critical to performance and under no circumstance should any abrasive be used which would affect this clearance. Oil should not be applied to the piston bore. Auto transmition fluid is the correct viscosity for the piston damper. Cheers Gordon Dobie Quote
zeds4ever Posted June 18, 2011 Posted June 18, 2011 According to my SU bible, if you use a lighter gde oil in the "Dashpots" they will react quicker under acceleration ,which stands to reason as there is less restriction thru the dampers & the reverse applies with heavier grde oil as there is more restriction. The down side with the lghtr gde oil is that you use fuel quicker, but the up side is that the motor will respond better thruout the rev range. Regards: Alan. Quote
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