Carb piston lifting

[ethel]

I'm trying to adjust the carbs by lifting the piston and listening for a fall in revs (too weak mixture) or rise in revs (too rich) as described in my manual. I think I'm getting there, but it would be useful to know that I'm lifting the piston by the right amount (about 1mm) and how sensitive the test is to the lift, in other words, if I was to lift the piston 2mm instead of 1mm would that always make the revs drop and for how long should the piston be lifted, should it be lifted and held or just flicked up for a second?
Thanks, John.

[sujitroy]

I did this exercise a couple of weeks back. I raised it about 1/2" quickly and let go. Make sure you have oil in the dampers. The main thing is to make sure you lift them at the same height,

Sujit

[KOY 23]

1/2”!!!

[ethel]

1/2”!!!

My manual says 0.03inch or 0.8mm, hence my original question about whether a 2mm lift would have a different result.
John.

[sujitroy]

oh. I'll redo mine. runs quite well

[RNF]

I'm trying to adjust the carbs by lifting the piston and listening for a fall in revs (too weak mixture) or rise in revs (too rich) as described in my manual. I think I'm getting there, but it would be useful to know that I'm lifting the piston by the right amount (about 1mm) and how sensitive the test is to the lift, in other words, if I was to lift the piston 2mm instead of 1mm would that always make the revs drop and for how long should the piston be lifted, should it be lifted and held or just flicked up for a second?
Thanks, John.

I use a flatblade screwdriver that is ~1/4" wide. I slide it under the piston and then turn the screwdriver which lifts the piston. That way the amount of lift is consistent.

[DZP]

Hi John,
You need to lift the damper piston by the least discernable amount possible.
Using a screwdriver sound like a consistent way to do this, but I have always used my finger.
Get the tickover really low.
Rev the engine a few times and then let it settle down for say 20seconds.
Lift the piston as little as possible, the rpm should increase slightly and then drop back to normal if the mixture is correct, the whole process is max 3 seconds.
Don't be tempted to keep lifting the piston without doing the whole process e.g. rev, wait, lift piston, adjust mixture if required.
If you cannot get the carb to tune, then you may have worn carbs with air leaks via throttle spindle etc.
i think someone on the forum suggested putting some oil on the spindles and listening for rpm change to debug this issue.
Den

[ethel]

Thanks RNF and DZP, it sounds like I just need to be a bit more methodical and make my process a bit more repeatable.
John.

[jbuckl]

As Dzp says any unmetered air through worn spindles, seals or throttle disc eccentricity will make the piston lift exercise void inconclusive to tuning.
when that happens , thoroughly check for unmetered air, including loose breather hoses, oil filler cap gasket & dipstick felt.
imho these being overlooked leads to people ditching their strombergs.

[ethel]

I probably ought to block off the small hose that goes to the underside of the air cleaner box before I try next time.

[Lingen]

Ethel.
That shouldn't be necessary as it should be a "Closed" system unless something is broken. However, if the flap valve in the inlet to the air filter box isn't working anyway, you might as well block off the spigot on the inlet manifold.
Mike.

[thebadleys]

Use a 1mm drill bit for consistency in piston height

[ethel]

Ethel.
That shouldn't be necessary as it should be a "Closed" system unless something is broken. However, if the flap valve in the inlet to the air filter box isn't working anyway, you might as well block off the spigot on the inlet manifold.
Mike.

But I've got the air cleaner and elbows off the car so I can get to the carbs......

[Lingen]

Now I understand. Block it off then.