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HS 6 Carb problem

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markclark1556

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I am rebuilding my carbs (on a BJ7) with a Moss Kit and have a problem BUT it's a holiday weekend so they are closed until Tues. UGH! My problem, on the front carb the piston is sticking when I try to lift it it and will not move up and down inside the chamber. It is clean and seems to only move up and down when I remove the domed chamber. There is a brass pin that is held in place on the throttle body by a set screw that lines up with a grove on the piston and it might be sharp and sticking into the piston's groove. But the Moss cat. doesn't even show the part I am discussing. Any ideas? The needle slides easily into the jet when the chamber is removed. Thanks and happy Labor day.
 
Matt it sounds like the jet is not centered properly. Refer to the shop manual but its the same procedure for all SU models. With the piston and dashpot assembled on the body , loosen the large jet securing nut slightly and wiggle the jet back and forth. Raise and let the piston fall ( without the dashpot cap), retighten and recheck until the piston falls and raises freely. If you can't accomplish , your jet may be bent slightly. Kevin
 
Mark--

The needle may seem to fall free through the jet with the chamber removed but it must be accurately centered with the chamber on. Also the piston can bind if the latter is scratched, dented or bent or misoriented. All three ears on the chamber must align with the tapped holes in the carb body.
 
In Addition to Michael's remarks all the referenced parts must be spotlessly clean.
 
Any chance you inadvertently 'switched' the pistons when you had both carbs apart? That could conceivably cause your problem as I believe the pistons and vacuum domes are machined to each other.

Also, resist the urge to 'polish' the cuff of the the pistons or the inside of the domes with, say, very fine sandpaper as the clearances are critical and extremely narrow and you don't want to change them even the slightest.

I don't think the brass pin is your problem--unless it's been damaged or bent--there's plenty of clearance in the groove.
 
I will double check that the parts are clean and the pistons did not get switched as I never had both carbs apart at the same time. In re the brass pin: it seems to have a sharp edge protruding a bit - I will get a magnifying glass into the barn and look at it. What do you call this part? and has anybody had any problems with it before? Thanks, have a great holiday
 
You might try removing the needle then checking if the piston moves up and down smoothly. It should make a clicking sound if it's striking the bridge correctly. I always replace the needle when rebuilding the carbs.
 
What do you call this part? and has anybody had any problems with it before? Thanks,

:savewave:

A " Needle "

If the needle is bent there will be a worn shiny line on it.
 
markclark1556 said:
... What do you call this part? and has anybody had any problems with it before? Thanks, have a great holiday
Click to expand...

Don't know the 'official' name, but things like these are usually called a 'locating pin' or something similar. AFAIK, it's there to keep the vacuum piston from rotating in the dome. Why that would be a problem is a mystery to me, though I'd guess that, again, it's there to preserve the important clearance between the vacuum dome and the piston cuff.
 
Actually, when I asked "what do you call this part?" I was referring to the small brass tab held in place by a screw that is the "Guide" for the piston and is mounted on the edge of the throttle body., Mark
 
--??????????- OH!!!--"-Piston Key-"-----Keoke--- :laugh:
 
Thanks, I worked on the edge of the slot that go up and down the piston with a file because there were a few nicks that the piston key seemed to be sticking to. I was very careful not to work on the outside of the piston itself and the piston seems to work (go up and down) much better. The only thing is I do not hear the clicking noise that Johnny mentioned above ( I do hear it in the other carb) when the piston lands on the bridge. When the needle is removed the piston drops freely - so I am thinking I might have a bent needle. Once I get the car started (its been in storage since 1984) I'll probably replace the needle. Can anybody tell me how to ensure that the needles are centered on the jet once the chamber is screwed into place?, Thanks, Mark
 
Can anybody tell me how to ensure that the needles are centered on the jet once the chamber is screwed into place?,

If the piston drops freely with the typical clunk it is probably in pretty good shape.

However, to be absolutely be sure of the alignment between the needle and the jet there is an alignment tool you can purchase to do the job..

If new needles are required the standard needle is identified as a " CI ". But look on the shank of the needle it may have it's code stamped on there--Keoke
 
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