Carburetor mixture screw

[mojobaby]

I've just failed my second MOT due to my exhaust emissions being to high.

The first time I went, I had a 6,40 reading when I needed a 4,40

My screw was actually about 4,50 turns outwards and Bobbie suggested that I turn it 1,50 turns out, which I did.
After that, my car ran perfectly; no smoke at start up, or after the engine had warmed up and even if I revved the engine, there wasn't a trace of smoke.

So I was VERY surprised when my new reading yesterday was 8.50!!! While the test was being done, I opened the bonnet and screwed the screw completely in and it made no difference at all to the reading.
Luckily I know the MOT guy; I've been going there for 12 years so he said that I can come back later.

I came home, removed the carb and cleaned it thoroughly and also checked the mixture screw. I found that there was damage to the tip. I don't even think that the needle was entering the small hole in the carburetor wall, so that's why it made no difference whether I screwed the screw inwards or outwards.

I cleaned up the tip of the screw with 1500 sandpaper and now if I look to the interior of the carb I can see the pin protruding through the little hole.

my wife took the macro shot (3rd photo) with her Iphone 6

 

[mr-reno-139]

Depending on age pre 73 I think uk mot is no visible smoke
Even Co reading of 12 won't give visible smoke

 

[mojobaby]

So pre-73 I would have passed my MOT
I suppose in that time there were no computer assisted exhaust analyzers. I wonder when they first started to use them?

The only visible sign that something is not correct, is that my exhaust is black.with carbon.

At least now that I have the tip of the mixture screw going through the little hole, if I screw it either in or out, there is a change in engine tone.

 

[Niels Svane]

Here in Denmark they don't bother with emission tests on any old vehicles. Not those I use anyway

 

[harbourseal]

Isn't there supposed to be a spring on the mixture screw of a zenith 28IF
I do hope you get through this time Mojobaby.
My second chance will be Thursday.

 

[mojobaby]

I can't believe that you spotted that!!!! Yes, I took the spring off when I was trying to get the screw to fit correctly. It's back on again now.

 

[mojobaby]

Happy to say that I passed my CT/MOT today.

My emissions reading had gone up again above 4,40 even though I hadn't touched that mixture screw since the last time.

While the car was running and being tested, I opened the bonnet and screwed it all the way in.

It couldn't go any further but at least the emissions reading went down to 4,01 and that's all they wanted to see.

Renault don't have that mixture screw they said I should buy a new carb!! and I've tried some places online as well but no luck.If anyone has a spare, I'll be happy to buy it .

The carb is Zenith 28IF V05 078A