Mechanics Odd Diagnosis


(Tigger) #1

Posting on behalf of a friend –acquired 3.8 about a year ago.

Generally all well but recent running problems – mechanic diagnosed sticking carb piston – further advised can happen if engine has overheated and distorted the dash pot (?)

Goes on to say said mechanic has tried ‘skimming’ the chamber but without success…

I’ve never heard of this as a cause of a carb problem, nor a solution – comments?


(Andrew Waugh) #2

It can happen if you fire the bell across the room while buffing.

I’ve seen dampers stick a couple of times after they’d sat for a long time without oil.


(Tigger) #3

Love this.

I think said friend and mechanic are about par on knowledge!


(Geoff Allam) #4

Difference is your friend just paid the mechanic to ruin his expensive SU carburetor. I usually prefer to do the ruining myself. That way all you have to pay for is the replacement.


(Robin O'Connor) #5

Talking from experience Andrew? :slight_smile:


(Tigger) #6

Quite. The thought of skimming the piston chamber left me cold.


(Ian) #7

I can’t see overheated engine distorting the dash pot , skimming is the last thing to do , if the carbs have high miles on them , the centre tube is probably worn , allowing the piston to touch the suction chamber side , hence sticking !

Be quick at this price :grin:


(Tigger) #8

Agreed - cannot see heat distortion being the issue and ‘skimming’ will wreck the job. My gut is long term storage gloop/varnish in the damper tube or ham fisted handling of the needle.


(Andrew Waugh) #9

I’ll take the 5th on that.

Actually, you can distort them just by buffing so that they build up a huge amount of heat on one spot.

Skimming the ID of the bell sounds like a nightmare to me.


(tony) #10

there is a precise test for this issue, outline in the official SU Carb manual…and other places

the piston is placed within the bell, a passage is blocked, and then how long the piston takes to slide out is timed. Importantly, each piston must be the same as its brother/s, and within a specified time.

can probably excerpt that and edit it in here, if u wish.

I believe simply dropping the bell onto concrete can do it, although I have never done this


(Pete55Tbird) #11

I saw a video on YouTube produced by I believe MOSS Motors or University Motor or such and he went into a lot of detail regarding SU carbs on British cars. It helped me setup my two SU`s on my 420. It seems the bell and piston were a matched pair and MUST remain together as a PAIR. Pete


(The Jag Man) #12

Old grumpy mechanic I knew had a child’s size mattress underneath his buffer just in case he lost control of the bell and it took flight I bet one could also rig a cable though the center and around your waist to prevent derail acrobatics? Just don’t put that cable around your neck!!

Gerard


(Paul Wigton) #13

Not that either of us has ever done such a thing.

:grimacing:


(Paul Wigton) #14

No: if the engine ever got that hot, warped bells would be the least of ones’s problems!


(Tigger) #15

Agreed.

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