TR6 Carb air piston test, still results in stall even when adjusted rich

[Steve1970]

Hi, I've got a stock ZS carb set up. Car is slow on take-off, so I am playing with the timing and tuning the carbs.

Haynes manual says that if I use a screwdriver to slowly lift the air piston 1/4" and it stalls, that I should adjust the mixture to run richer.
But even when fully adjusted to rich, the air piston test STILL results in stall.

How can I get the air piston to operate correctly?? (I.e., use screwdriver to lift air piston 1/4" should result in slight increase in idle followed by return to normal idle). What else impacts the air piston?

thanks, Steve

 

[Merlin63Tr4]

The Tr6 ZS's might be different, but at least on the Zenith 175CD on the Tr4, the air valve is only lifted 1/32" with a thin screwdriver to check the mixture.
The air valve is lifted 1/4", with the engine not running, to check for free movement.

M.

 

[Geo Hahn]

Yes 1/32" inch only and careful to not let the screwdriver block much air flow.

I prefer to raise the piston using the lift pin on the carb (if TR6s have this, TR4s did).

It is a subtle change you are are looking for - it works but takes some practice.

 

[poolboy]

1/4" is too high and leaving it up longer than 2 seconds while you're trying to decide is too long, just about guaranteed to give make the engine begin to stall.
That technique requires finesse and experience.
Reading the spark plugs after a spirited 30 mile drive is better.
And here's the important thing to remember...No amount of adjusting is going to compensate for carburettor components that need replacing....and if you have ignition timing problems no amount of fiddling with the carb will compensate for that.
Get the ignition timing at idle close to the sweet spot first.

 

[glemon]

That technique requires finesse and experience.
Reading the spark plugs after a spirited 30 mile drive is better.

Agree 100%, I have never been real confident with the lift the piston test. Too rich will sound "fat" deeper, with an occasional miss. Too lean will be sharper, crisper sounding. Get it running well, do final tune with the plugs, pics in every Haynes manual or online if you don't have one.

 

[Steve1970]

Ok great, I'm not going to pursue it then. Car seems to be running much better on take-off now with the timing reset to 4 deg ATDC and the carb float levels re-adjusted to 17mm. Only problem seems to be difficulty cold-starting the car, I am in process of troubleshooting this now. Thanks guys

 

[poolboy]

Forget about those marks on the damper, the engine will tell you when you have it right.

 

[glemon]

Does it need some choke for a while on a warm day after start up? Say more than 30 seconds or so. By needing choke I mean can you goose throttle without it stumbling or stalling with the choke off? If not it may be a little lean, and that may affect the cold start. I would still pull a couple plugs and take a look, may tell you something.

 

[pdplot]

In over 60 years of fussing with LBCs, I've never had any luck with lifting the piston. I've used a uni-syn but the best thing is to listen with a stethoscope, try to get the same hiss out of each carb, take the car for a 10-mile ride taking it up through the gears, pull the plugs for a look and if the color is grayish brown you're ok. Set it and forget it. Most owners spend much too much time adjusting and readjusting SU's and Strombergs. In over 60 years and 4 MGs, a TR3 and a TR6, the only trouble I've had is a sticking float on the TR6 a couple of years ago. I rapped the float bowl with a wooden screwdriver handle and cured the problem. And - make sure your choke cable is adjusted properly if you have trouble starting.

 

[Brinkerhoff]

You are listening for an immediate, very brief and subtle bump up in idle speed or smoothness , by just lifting the pin ever so slightly. Once you can hear it then it will make sense. Turn off the ball game or the Pink Floyd and just listen to the music !

 

[Geo Hahn]

You are listening for an immediate, very brief and subtle bump up in idle speed...

And if you have trouble hearing it (too many Pink Floyd concerts) you can use a dwell/tach and watch for the change in idle speed.

 

[pdplot]

dwell/tach? what is this high tech stuff? All you need to work on an LBC is a dirty rag, a screwdriver with a wooden handle to rap hard on the SU fuel pump with stuck points and a stuck float, a Lucas point gap tool and a can of 30 weight oil. Maybe a stethoscope although some old-timers used an extension from a 1/2" SK socket set. If you have no point gap tool, use a matchbook. Some baling wire and a roll of duct tape and you're all set for that cross-country trip.