ariel square four carb

[john.whiting]

Mid 50s Triumph twins used a motorcycle [MC2] SU, theyre sometimes seen on ebay.Some car SUs had an oil filled dashpot inside the piston,I remember filling the Jag with oil,on occasions.I once fitted a Zenith from a Wisconsin engine to an iron sq4,you had to open the throttle slowly,or it would spit back .The SU allows for all this automatically.Regards John.

[brenton.roy]

I have just gone around to the other shed and come back with a couple of SU's. The one I am sure is Morris Minor has a high extension to the dashpot, with a brass cap. You may be able to swap it to the other type of dashpot, which is a fair bit lower and has a plastic cap. Otherwise, I think this would make it fairly unlikely to fit in the bike. I don't know if the pistons inside are different, but the dashpots are.
The other (don't know what it is from) is almost identical to the MK2 Ariel carby. The only visible difference between this carby and the Ariel carby is that the end of the choke lever has a flat on the MK2, and the other one has a hole for some operating mechanism.
The other obvious differences between the carbies are around the inlet manifold. The MK2 studs are upright, the MK1 manifold studs are across the carby. The Mk2 carby has a block at the manifold end which can be unscrewed and rotated 90 degrees, which would mean it might then fit a MK 1 motor.
The Morris carby I am sure about bolts on the right way (across) for the MK1, and this is not adjustable.
The distance between the studs is slightly different between the MK2 carby 57mm and the Morris carby 60mm, although I guess you could slot them. I don't know the measurement across the studs for a Mk1 inlet manifold.
The Morris carby also has a bolt on air cleaner, the Ariel type has a sleeve fit.
While I don't think you would have to do it anyway, the carb might work on it's side, but the float bowl would need to be remote.

Lots of cars had SU's - maybe look for one with a short dashpot and a plastic cap.
I hope this isn't all too confusing. I can post photo's if it would be any help.
Brenton

[simon.holyfield]

Mk 1 inlet manifold studs are on a 1 7/8" pitch.

[p tuxworth]

thanks , the inlet manifold is cast as part of the head on this one but i have a lath and miller so should be able to make any adapter req

[nevhunter]

Some dashpots use a spring and others rely on weight. Stronger springs /heavier weight, makes the carb richer. I have never seen one run on it's side and I think you would have trouble with it.
The SU's don't have a separate idle system and the needle and jet have to meter the fuel/air over the entire range. The needle has to be precisely set in the centre.
I would recommend a Stromberg CD, but better still something off a Jap bike as they should be plentiful. A CV carb relies on the damping of the rising dashpot to enrichen the mixture on acceleration and will work OK if the carb is a bit too big. All that will happen is the dashpot won't open fully so you tune it over the range that it moves through. Nev

[p tuxworth]

thanks ill stick with su for now and see how it goes

[p tuxworth]

got the SU carb inch and one eigth this carb has a tickler on top of the floot chamber ? never seen this on a su before , had a quike look and there is plenty of room to fit and looks to be a very straight forwards job , will turn up adapter monday and post pics when it on but all in all looks very promising , thanks for your help
ps did the su fitted by ariel have a spring in the chamber ? anyone the spec on jet size and needle number

[brenton.roy]

I'm glad it fits. Apparently the same carby was fitted to 6T Triumphs.
Both the Morris and Ariel SU's I've got have ticklers.
The book doesn't specify jet size / needle number. I can't see any stamping. It just talks about getting the idle mixture right and checking it's not too rich, which needs the motor running.
According to the book, if you get the idle mixture right, it should follow through the range of engine speeds.
Screwing the bottom nut up = lean
There is a procedure for centering the needle and you need it running to do the checking of mixture, because it is about how much the piston lifts.
Yep, there's a spring.
Brenton

[keith wainwright]

Hello did you resolve your carb problem? if not I may have the solution sitting in my workshop. Regards Keith

[p tuxworth]

yes bike ran a treet , but i like the singles far more so sold it , just working on a 500 bobber girder fork model