30 April 2008
Ok, so I don't have number plates today
It turns out that in addition to the engineer's certificate, I also need a "blue slip" which you get from another type of authorised inspection station.
No worries, I'll get it sorted next week, and so I got bored tonight and decided to take out my jets
I didn't realise before how easy it was to take them out, so when I got the carbs rebuilt I asked the mech to write down the jet sizes for me, and they were: 145mains, 220 air corrector, F11 emulsion tubes, 32mm chokes. Now this may seem to be mumbojumbo but its actually not that hard to understand.
The way I understand it is that the chokes are what determines the airflow into the carb. The chokes are that grey "insert" you see inside the mouth of the carb, and you can see that it's smaller in diameter than the body of the carb.
This is deliberate, since the choke is a restriction in the carb, which causes the airflow to speed up, hence drawing/sucking fuel out of the jets (fuel comes out of that little tube in the middle). So my 40DCOE Webers aren't really the same as 40mm throttle bodies, they are really 32mm throttle bodies.
The choke is an airflow restriction and so the rich/lean fuelling is determined by the ratio of the choke size to mainjet size. The bigger the jets, the more fuel will be sucked out of them. In theory, once you get the jetting right, you shouldn't need to rejet the carbs for extra mods since the extra mods might suck more air into the engine but the ratio of fuel to air is controlled by the main jet to choke size ratio and so the AFR should remain the same.
Weber recommend that a jet size 4x the choke size is a good starting point when tuning. So for my 32mm chokes I would need 128 jets (but they come in steps of 5, so either 125 or 130 would do). I had 145 mains (or so the carb guy said) which sounded too rich...and given that the car fouls plugs black like it's going out of style that made sense.
Taking out the jets is pretty easy...remove the round jet inspection cap and just unscrew this long thing (there's 2 per carb).
The jet stack then just pulls apart into its component bits (from left to right): main jet, emulsion tube, air corrector and the threaded bit is just a holder for the whole shebang. So it's pretty easy to swap jets etc, and I guess you have to remember that back in the day before dynos, you would set the jets by swapping sizes, going for a blat in the car and determining if you were rich or lean by pulling a plug and reading the colour. So I suppose Webers were made so that swapping jets is more or less a 5min job you could do by the side of the road.
Now here's the funny thing...on closer inspection...I have a 125 main jet after all....which should be more or less on the money.
The air correctors are definitely 220 though.
Now as for the correctors. The main jets control fuelling from the midrange upwards, but the corrector adds some air into the fuel to lean things off at the top end. I guess any engine is going to be more efficient at high rpm, so if the fuelling is perfect in the midrange, at the top end it would have so much more airflow/suck that it would pull a proportionately higher amount of fuel thru the jets and hence run rich.
So the corrector adds some air to counteract that...Weber says that for a slogger engine, you would have a corrector size +50 above your mainjet size, and for a screamer race engine you would need more fuel and so it may only be +10. So for my L-series which might have peak power before 6000rpm something like a +50 corrector would be the go, while for say a really worked 9500rpm 4AGE it would be more like +10.
Put this all together and the ideal (or close to it) starting point for my engine should be 125 mains and 175 correctors.
Hence my 220 corrector is very weird and should make the engine fall flat at high rpm (which it does) from extreme lean-ness. But the midrange should be pretty close since it's the theoretically ideal 125. And at the moment the midrange power is crap.
I wonder why it's running so rich then...maybe the 125s were redrilled in Japan to a 145 size, which is possible........or maybe the ideal jet size is something like 115. Weber also says that when you have a big engine and small chokes, you often need smaller jets than the ideal because the big donk will get such a great suck thru the carbs that the main jet will work too well and overfuel, so it's very possible that the theoretical ideal 125 should be downsized to a 115.
I'll go see the carb guy and report back.
No Kev, you are eating a duck fetus.