SU 1" - difference between jets?

[JonE]

Can anyone tell me if eyebolt and standard SU jets are interchangeable... and what the differences are between them?

I've got the parts from a UB to fit the OM (which better fits the standard Austin sidedraft manifold without simple adapter), but I note my present jet is an eyebolt one.
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[Jeff Taylor]

Jon,

As far as I'm aware the eyebolt jets were invariably fitted to much earlier bronze bodied / bronze float chamber SU's - the type of choke lever linkage requirement needed at (1) is obviously different. All the alloy bodied OM's I've come across have taken the (A) Standard Jet at 0.090" - having said that, Burlen do supply new H Type eyebolt jets at 0.090". I doubt if there's any real difference between the 0.090" jets themselves, it all depends on whether your OM has the correct style of choke lever to accept an eyebolt jet or has the choke lever for an (A) standard jet. I've some photos on file somewhere of early bronze bodied SU carbs with eyebolt jets - I'll try and find them.  

Here's a photo I've just found on the net which shows the design of the choke lever attachment to the eyebolt jet - note that this arrangement doesn't have provision for a return spring between the choke lever and carb body - presumably the choke linkage would have been by rod rather than by wire. 


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[JonE]

Thanks Jeff. I've also just spoken to Burlen who suggest that there is no difference in operation of the two jet types. So I should just go with whatever is on my present linkage - the (slightly more expensive) eyebolt one.

I wondered what the extra drilled 'oles might do in practice...

[Jeff Taylor]

A photo of your carb / present linkage would be useful Jon. Does your present choke lever / carb body have the necessary provision for a return spring? I suspect the additional holes in the jet are because of differences in the individual components making up the jet bearing kit of parts between bronze bodied carbs and alloy ones and possible differences in the way in which the fuel flows from the bronze float chamber to the jet in the bronze body - though having never dismantled a bronze bodied carb, I can't be absolutely sure about that. The photo below shows the jet bearing components of a Morris 8 Series E H1 SU carb which are the same as the earlier Morris 8 Series I / II  UBA carb.


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[JonE]

thanks Jeff - brought them home from garage today but now I'm working away for a few days so will photo when back. I won't order from Burlen until after then anyway... their postage is so exorbitant for small packages I don't want to miss anything out! (Need to find the right brass damper cap too)

[garryruck]

Sorry to give a different slant to this topic but I have an SU carb on a high compression headed in a 19 29 fabric bodied saloonwhich floods badly to the point of causing the engine to stop. Once it sits for a while it starts again but floods to a stop again. There is no spring or damper in the carb - just a simple brass screw in cap with a screwdriver slot (not knurled). Question is should there be a spring or a damper. It has an electric fuel pump but the float in the bowl works fine as after a couple of clicks the pump stops prior to starting. Any answers as to spring and damper much appreciated.

[Jeff Taylor]

Garry,

You'll need to give us more information regarding the model of SU carb you have - there should be a body number on the side of the main body casting - a photo of your carb would also be helpful. It does sound as if yours is probably a pre-war carb.
Generally most if not all pre-war side-draught SU's had neither a spring nor a damper - the dashpot piston being weighted should fall on its own accord without the need of any spring assistance. Dampers were a post-war development. It's possible you may have an early 1930's SU OM - 1" carb - body number 617 - these were originally fitted to early MG's - no spring and no damper. The SU H1 - 1.1/8" carb - body number 3457/1 fitted to 1948 to 1953 Morris Series MM Minors (a popular carb fitted to a number of Sevens) did have a dashpot damper but definitely no spring.

Have you tried fitting a brand new needle valve in the float chamber. Also check that the float fork is correctly adjusted using a 3/8" diameter bar or dowel - illustration to follow. I'd also carefully check the brass float for any pin prick holes around the soldered joints allowing petrol to get inside and weigh it down. Check that the 0.090" jet is correctly centred and that the dashpot piston rises and falls back cleanly without any restriction.

Is your electric fuel pump also an SU ? if it is, then new diaphragms are available from Burlen that are suitable for modern fuels.

If you require any spare parts then Burlen stock almost everything you might need to refurbish your carb - link below.

http://sucarb.co.uk/?SID=qpkjp3c2kb5u2f0...__store=su

[mk1-mark]

Flooding isn't usually a concequence of anything to do with the Jet, it is a concequence of a poorly seated needle valve.  The valves on pre war carbs as illustrated above are particularly prone to leakage.

If your carb has the very early flat top float chambers fitted the only thing you can do is make sure that everything is in tip top condition & keep your fingers crossed.  If anything is worn or damaged then there is very little that can be done.  Some bits are available from Burlen Fuel Systems, but not everything by any means.

For day to day driving the very best solution is to replace the flat topped float chamber with the later "H" series domed float chamber that has a MUCH better needle valve fitted.  While this is not 100% correct, it will undoubtedly solve your problem permanently.

[garryruck]

Thanks for the advice. Things are looking much better now - car runs and does not cut out. It now starts without any choke and have the feeling it is still an over rich mixture but much improved. Have tried screwing up the mixture nut but with little change. I am unsure if the needle valve in the float chamber should be adjusted ie is the level there too high? There is no spring on the piston and the piston drops quickly when the throttle is released. Should the dashpot have oil? So a couple of issues still, but feel I am getting close.