MGB: SU Carb wanted

My 73 SU carb has a stuck piston - tryed everything - anyone got one to sell?
Thanks

You really want a matched set!! They are matched up at the factory and really should always be used together!! GOOD LUCK!!!

What have you tried? Usually it's the needle stuck in the jet from gummed up gas. You might try spraying carb cleaner in and around everything.

Have you tried a little emory cloth on the inside of the dashpot? Just today I went out to my parts guy shop. This other guy out there couldn't figure out why the car was running rich. I checked the carbs and both pistons wouldn't move! I took them apart and cleaned up the dashpots with the emory cloth and that helped a bunch. Both the pistons and the dashpots were pretty scratched up. He had bead blasted them a long time ago before he rebuilt them. I figure there must have been some remnants of the beading material left.

If you don't have any emory cloth try some high grit sandpaper, maybe 220+ (if you do, pour a little liquid soap on it for lubrication-then rinse super well).

I bought a 1 gallon can of carb/parts cleaner fromAuto Zone - it is wicked stuff - will burn holes in skin! However, it works wonders on carb parts. I would put the whole carb in it, leave it for a day or two while the gasket/rebuild kit arrives, then wash it off and GENTLY try to release the pistons. If you resort to the emery paper - use VERY fine (500 grit wet or dry will work) and go very carefully. The pistons are a very close tolerance fit in the dash pots and will not work properly if that fit is compromised.

You can probably get a pair of carbs fairly cheaply but be prepared to do some work on themto get them in shape - in the end it may be easier to buy two new ones!

Good luck

they do come completely apart so if you did want to replace any piece individually that could be easily done. u can break that puppy down like a rifle :)

Don't dump the carbs in carb cleaner with the floats still in them, that stuff will desolve them.

And everything else!!

here's a possibility
assuming you have HIF4s, but HS4 will do just fine, too

For 56 bucks it would be worth the gamble, but not a whole lot more, in my humble opinion!!!

I ahve read about people swapping out the slide and dashpots from earlier carbs onto HIF to get the fixed needle setup. Might just be worth the few bucks to try, Or install the whole setup as is

also that stuff evaporates like..... -insert weimerism-

or this

I've always heard that you NEVER want to use an abrasive, or even a metal polish, on the inside of the dashpot. The reasoning is that the fit of the piston to the dashpot is critical to the proper operation of the carb, and that changing that fit by removing metal (or swapping parts around) will upset the behaviour of the carb....

Rob,
You're correct, however if the inside has been damaged then it will work until a new set can be obtained. It's not the best solution, but if the piston doesn't move freely inside the dashpots, and you can feel where it's binding, then it's a good solution for a temporary fix.

That's why it's important to use the highest grade (like emory cloth) to limit the amount of material taken off, and not leave scratches on the inside.

Good point Tom - I forgot about that one! It also makes ANY plastic or paper part unusable, but it does get things clean!

Kind of makes me wonder why SU did not design them so they had a replaceable seal of some type, like a hard plastic wear ring or something

the pistons aren't identical for say.. all H24's?

Nope. The early ones used a fixed needle system and the latest ones used a biased needle, as do the HIF4s. Parts and not interchangeble between these two systems but as matched parts(slide, needle assembly and dashpot with damper) the can be swapped I understand. Oh, along with the fixed needle comes the need for jet centering so the needle does not rub the jet every time the throttle cycles. The biased needle rubs all the time but the spring loading allows it to slide over the surface of the jet under light spring pressure. This is part of the reason I favor the HIFs

My understanding is that the piston and dashpot are a matched set. I am not sure that the carbs are sold as a matched set but since they are "handed" I doubt if anyone will sell just one. If you cannot get the piston unstuck, I do have a pair of HIF4's that I would sell. They were working when I took them off the car about 9 months ago but they are in need of new bushings and probably a complete rebuild. Let me know if you are interested.