MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - lift the piston method for zs carb

hi
79mgb, zs carb.
i have been using a 'colortune' to adjust the mixture with success ,imho.
i would like to try the lift the piston method and listen for engine speed changes but i don't know how much to lift. to much and the engine dies.
manuals seem to be of little help.
unlike an su carb there is no attached lifter.

so, how much should the piston be raised and what is a convenient tool to use for the job.
any help/tricks would be appreciated.
john
john sutter

John,
The mixture is tested by inserting a long thin screwdriver into the air intake and lifting the air valve approx. 1/4 inch.

If the mixture is correct the engine speed should be steady or increase slightly at the 1/4 inch lift point. If the valve is lifted further, the engine should slow down and possibly stall.

If the mixture is too rich, the engine speed will increase maybe 10% or more, and may not tend to slow down until the air valve is lifted much more than 1/4 inch, depending on the richness of the mixture.

If the mixture is too lean, at the 1/4 inch lift point the engine speed will decrease and possibly stall.

Before each test the throttle should be opened increasing the engine speed to about 2000 RPM for a few seconds to clear all fuel from carb and manifold.

Taken from Part III Carb Overhaul by Nelson Riedel found at the Buckeye Triumph tech pages.

Hope this helps
Dennis
Dennis Silance


thanks dennis. exactly what i wanted to know.
john
john sutter

The method I picked up from Tom Josefek of British Motor Classics here in Chicago on my midget when it still had a Zenith is to unscrew the cap and rod from the top, pull it up and tilt it to the side so it gets a grip on the piston and pull it up the 1/8" or so needed to check - about the same lift as afforded by the lift pins on the S.U.'s. This seems to be more accurate to me because it can be done with the air cleaner assembly in place and no obstruction in the carb entry by the screwdriver.
Kim Tonry

FWIW the lift is only 1/32" on SUs. Much more than that and it gives incorrect indications.
Paul Hunt 2

Paul is correct on the lift - 1/4" is too much, on either carb setup. No more than 1/16". The engine should stall immediately at 1/4", if it is not set way too rich.
Kim is correct on the requirement of having the filter in place on ZS. On the ZS MGBs, the filter box is so restrictive that the mixture is substantially disturbed, even at idle, if the filter is not in place. Use a NEW filter; they are prone to clogging from fine dust or soot if you have even the slightest exhaust leak at the manifold (many cars do!). Given the goofy backwards flow on this filter, you cannot judge the condition of the filter, since the dirty side is inside the element.

FRM
FR Millmore

This thread was discussed between 06/11/2007 and 12/11/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.