29 replies to this topic

[paulrockliffe]

As the title really, I've just rebuilt my carbs and got them nice and shiny, but one of the pistons isn't coming allt he way down and closes slower under gravity than the other one, you can see what I mean here:



I rebuilt them because I was having problems getting the carb that has the open piston to draw enough air at idle speed compared with the other and the idle was erratic. I think one of the manifold gaskets was a bit suspect - blackened right through - so thought I had an air leak there, but now I think this is going to be a problem for me.

Soooooooo........what's the solution?

If it's not an easy fix is there anyone near Manchester with a spare carb and manifold I can borrow until these can be sorted out? NEED to MOT to get to Bingley!!!

Cheers!

[minimender]

I spose you tried swapping the piston dampers.
Take the tops of and see if the piston is catching anywhere, it should leave a mark on the casing.
Try a thorough clean with carb cleaner, might have a little dirt in there. Dosn't take much !
If you need to clean them up inside use brasso but very little at a time.

Also check that the keyway in the piston isnt damaged and that the needles are seated propperly.

Edited by minimender, 13 January 2008 - 09:47 PM.

[blacktulip]

i bet the needle locking screw is too tight? which is pushing the needle over so it dont go down into the hole straight. just loosen it a turn and see then.

[paulrockliffe]

I've oil the piston/housing sliding surface and undone the needle half a turn and it's closing all the way now, but still seems a little stiff and closes a little bit slower than the other piston.

[paulrockliffe]

I've oil the piston/housing sliding surface and undone the needle half a turn and it's closing all the way now, but still seems a little stiff and closes a little bit slower than the other piston.

Is my problem something that is less of a problem under operating conditions and when the dash pot has oil in it do you think?

[blacktulip]

what about your springs in the dashpot? also, there might be a little tab on the main body when you lift the dashpot off that sits in the grove on the piston that can cause resistance, it has a little screw that you can undo and push the tab back a bit. im not sure if thats just on the hif models though. you will want it moving freely otherwise you might find yourself stalling at the lights.

[mk1leg]

Hi..!..this is what you should do take tops off carb bodys 1 at a time use autosolve polish and polish pistin tops and covers once done rebuild and top up dashpots with sewing machine oil as its the lihgest and will hepl pistons move up and down quicker aids faster throttle response.............good luck...................

[minimender]

i bet the needle locking screw is too tight? which is pushing the needle over so it dont go down into the hole straight. just loosen it a turn and see then.

I think mikecmini is right....try this
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[paulrockliffe]

what about your springs in the dashpot? also, there might be a little tab on the main body when you lift the dashpot off that sits in the grove on the piston that can cause resistance, it has a little screw that you can undo and push the tab back a bit. im not sure if thats just on the hif models though. you will want it moving freely otherwise you might find yourself stalling at the lights.

There's no tab on the HS2s and the springs are both fine/identical. It's better, but gets worse when i tighten the dashpot down fully, just seems like it's tight somewhere, but no obvious places!

[paulrockliffe]

what about your springs in the dashpot? also, there might be a little tab on the main body when you lift the dashpot off that sits in the grove on the piston that can cause resistance, it has a little screw that you can undo and push the tab back a bit. im not sure if thats just on the hif models though. you will want it moving freely otherwise you might find yourself stalling at the lights.

There's no tab on the HS2s and the springs are both fine/identical. It's better, but gets worse when i tighten the dashpot down fully, just seems like it's tight somewhere, but no obvious places!

OK, jet removed and no more problem! SO centre the needle in the jet........

[paulrockliffe]

OK, jet removed and no more problem! SO centre the needle in the jet........

Except that diagram isn't the HS2 and I don't really know what I'm doing!

[blacktulip]

do the needle screw up so it holds it only. it dont have to be tight. the needle should kinda float about a bit.

[paulrockliffe]

do the needle screw up so it holds it only. it dont have to be tight. the needle should kinda float about a bit.

I've got it set so that it's floating, but then the needle can move up and down vertically too? Should the needle be as far up as it can go, or so that the cylinder at the end sits flush with the piston body?

[blacktulip]

i do it so the metal sleeve that the needle goes into is flush with the bottom of the piston.

[minimender]

OK, jet removed and no more problem! SO centre the needle in the jet........

Except that diagram isn't the HS2 and I don't really know what I'm doing!

Sorry wrong carb but almost the same

There is a paragraph here which explains it a bit.
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