Zenith 28IF-running too rich

[angel]

My '75 R1123 needed to pass the emissions control test (once a year here) but it won't! CO is arond 2% at idle (OK) but goes up to 6,5-7,0% at 2500 rpm (second part of the test). The carb is a Zenith 28IF,cleaned and fitted with new gasket and needle valve. Needle valve height is the correct 8.4mm. Main jet is correct (I think so), it has only an "E" stamped, no dimension, but I have seen normal CO values with this jet before, furthermore I have two more same jets, which I tried without success.I also tried putting an "old style" SEV-Marchal fuel pump on, which,from my experience, gives lower fuel pressure than the sealed type that I had on. Still high CO. I tried lowering the float level a bit but this results in fuel starvation when I keep the throttle mid-open. Engine has covered 50.000 kms since major rebuild, air filter is new, ignition timing is spot-on. I think it is clearly due to over-rich mixture, but I cannot find why,especially since this carb is so simple. Does anyone of you have any ideas? Or maybe a similar experience with that carb?Thanks!

 

[malcolm]

I've been pondering over this one all afternoon but don't have an answer. Surely wear in the carburettor, leaks or blocked jets would make the mixture lean. The only wear that would cause richness is wear in the jets/needles and that shouldn't happen in a year.

If anyone else was asking this question I'd suggest blocked air filter, choke slightly closed, timing retarded, vacuum advance broken (do they have one?), distributer advance springs past their best/missing/broken (theme of retarded ignition here). The manual will surely list the correct ignition advance at 2500rpm - have you done a strobe test?

The only other thoughts I have are the breathers from the engine block. If they were blocked then less air would get into the carb and potentially make things a little rich.

Can't think of much else. Would the testing garage be happy if you took along a spare carb and distributor and did some testing with their carbon monoxide measuring equipment?

 

[angel]

It sounds strange to me that a jet can wear so much that its increase in size can alter things so much. We discussed it with the guys at the garage, and they had the same opinion,too. So I fitted two more jets of the same size and I saw no change. Next step will be fitting another 28IF I have and I think is in better shape (not that mine is bad...),there is no problem,the guys that run the garage are close friends of mine.
There is a possibility that the breather system is blocked at some point, I usually find the small hole at the inlet manifold to be blocked so I will check this out.
I have checked timing with a strobe light at idle,it is correct 6 deg. BTDC. Didn't check at higher rpms though. As it is my daily car I guess it is running fine,otherwise if the advance mechanism was sticky I could certainly tell a difference in performance.But you can never be sure,I will check this out,too.The distributor is old syle, no vac advance. Side note, stretched springs would have steepened the advance curve (more advance, not less).
Does anyone know what "E" stamped on main jet means? Normal size is 90 or 94 but it doesn't appear on anyone of the 3 main jets I have.