HIF44 1.75" SU
The carbie was the next on the agenda. It was been rebuilt using a Fuel Miser rebuild kit that I ordered in from the local Auto One store. The carbie is an HIF6 1.75" SU. The below image shows the original pancake K&N air filter to on the back however this would not fit so I had to purchase a cone K&N filter in the end. This filter really lets it breath and you can hear the whistle under the bonnet at high revs. Sounds great!
There is a real art to tuning any SU carburettor and I think I got it to a point where I could really the Moke up and going with acceptable performance so here is my advice:
In order to obtain the correct needle and dashpot spring, I used a program called WinSU to spec it out. In the end I purchased a BDL needle and a Yellow (strong) dashpot spring. These seem to have done the trick quite nicely. The original needle was a BER and the spring was unknown. The BER needle was slightly too lean once I had put the newer air filter on so I had to get a richer needle to compensate. My logic behind this is as follows (open to corrections/suggestions etc.):
From what I read/have spec’d up using WinSU the possible needles for my Moke are:
- BER (currently in it)
The below graph shows the differences of these needles. From what I read, when choosing a different needle, it is best to select one with the same idle figures to avoid having to reposition the jet. To elaborate on the science of this, if a new needle has a leaner idle figure, the rest of the needle will give a slightly richer mixture than the profile suggests and vice versa because you would need to re-position the jet to get correct idle mixture which would otherwise be thrown out by the needle. So immediately this rules out a BDK needle. Now when I lifted the piston in the carburettor at idle, the revs would fall indicating that it needs to be richer at low revs. With this in mind, the BDD and the BDK appear to have the same mixture at low revs as the BER which in turn won’t solve this problem. The BDL however has a richer mixture at low revs which is what I require and given this logic this is what I am now running along side of a yellow (strong) dashpot spring as recommended by WinSU.
How to tune an HIF carburettor (HIF6, HIF44 etc.) - applies to most other SU carburettors also:
- Remove the air filter and ensure that the throttle and choke move smoothly without any grabbing. If they do, take my advice and re-build the carbie now. There is no point going forward.
- Unscrew the idle speed screw until it just lifts off the throttle cam. Now turn it 1.5 complete turns in (clockwise). If you have twin carburettors, go for one full turn on each carbie.
- Raise the dash pot so that you can see the jet (this is the little hole that the needle goes into). Alternatively, you can remove the chamber so that you can see it from the top. Turn the mixture screw until the jet is flush with the bridge and not higher than it. Now turn it two full turn (720 degrees) in so that the jet goes below the bridge.
- Fill the piston chamber with with dampener oil to about 1/2 an inch below the top of the chamber. I used Automatic Transmission Fluid here with great results. Alternatively S.A.E 20 engine oil can be used.
This is the best starting point to where you should be able to get the car to start with the choke on.
- Warm the engine to normal running temp. It must be at this temp to tune it properly.
- Clean out the carbie by screwing in the fast idle screw so that the engine runs at 2500rpm for 30secs. Nb. If you have not tuned it within 3 minutes (i.e. completed the below steps) from this point, you will need to start from step one again. This is also very important.
- Adjust the mixture screw (usually clockwise at this point to richen the mix) until the engine speeds up to its fastest point and where it sounds nice and smooth (but not too far where the revs begin to fall).
- Back the idle speed screw off to about 1200rpm and when at this speed, lift the dashpot only very slightly (1-2mm) and listen for the rev change in the engine. If it the revs raise and do not fall again, the mix is too rich, if it stumbles, the mix is too lean. The idea here is to adjust the mix to the point where the the revs raise only just slightly and then fall again. Turns on the mixture screw should be minimal about 1/8th of a full turn.
- When you are happy, back the idle speed screw off to the factory specification (usually between 800-1000rpm). If it still does not idle, then something is wrong in the carbie and I would suggest a re-build.
At this point, the mix should be pretty close to spot on. To perfect it, you should probably take it to a Dyno (rolling road) tuning shop but this should be pretty close. For a good video tutorial on this being done on a twin-carbie MGB, see here: