MIXED CARBURETTOR FEELINGS

A friend wrote to me, seeking advice about his m75 (single) carburetted Saab 99.
He could not get the carburettor setting right. If adjusted to < 4.5% CO at idle, the mixture leaned out on acceleration and the car ran horribly-hardly drivable. If adjusted so the car ran decently, the CO went up to 7% and the car idled horribly.

I told him that although I have experienced those very symthoms on Zenith Stromberg carbs before, "fixing" a carb via e-mail is always difficult. A mechanic needs to see and feel the "patient". But I could always have a go.

The needle and jet was new, but the float level had not been adjusted when he changed them.
He'd also recently installed an electric fuel pump and pressure regulator, but mixture adjustment problems predated those modifications.

I GAVE HIM A BRIEF CHECKLIST TO ELIMINATE THINGS:
-El-fuel pump and pressure regulator? Definately not. They cant do that.
-Air filter? No, but I assume you've renewed it anyway.
-We need to look at the carburettor itself.
First of all, when you've bought a new needle and jet, a diaphragm and some other pieces, you tear the carb fully apart and clean all the parts. -You've done that, haven't you?
-And in the assembly-process, you MUST adjust the float level. It is vital. (and check the intake-valve for wear) On other carburettor-designs, it is the float level alone which determines the mixture .
-Then there is the matter of installing the needle in the piston. It must be adjusted to the right height.
-The piston itself can be rotated, and therefore installed the wrong way. But I assume you knew that and checked before reassembly.
-Check for vacum leaks. Scrutinize all hoses. Plug the brake servo hose and try again.
-IF YOU'VE DONE ALL THIS, I would inspect the needle and jet. WHERE did you buy it? Are you sure it's for your car? -Those symthoms really indicate a worn needle and jet...

AT THIS STAGE MY FRIEND WENT THROUGH THE CARB AGAIN,
readjusted everything to specs and replaced the throttle shaft and seals. The needle and jet had been bought from a well known British carb company. The car ran better, but still would not idle well and hesitated when starting from a stop and under acceleration when hot.
He decided to leave the car alone for a month, this was driving him crazy...

WELL, HOW LONG DO YOU THINK THAT LASTED?
Precisely! The very same week he swapped the carb for anotherone. He had just remembered that the carb which was giving him problems did NOT have a new jet afterall!
m75-carbs are of the old "adjustable jet, fixed needle type" and very hard to source parts for.
I'll just repeat the conclusion I wrote in the first place:
"-Those symthoms really indicate a worn needle and jet."

SUM UP
I love troubleshooting via e-mail. My hands dont get dirty :-)