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MG Midget and Sprite Technical - SU jetting etc

I put a set of recon HS4 carbs onto the midget (1500) a while back and I’m still struggling to get them set up correctly.

They were bought from a guy on ebay "SU-Carbs" who reconditions them; they were done to suit a spitfire/midget 1500. With “fixed standard SU jets not waxstat”

I’m starting to think perhaps he’s put the wrong jets and needles in.

Every time I’ve played with the carbs I’ve ended up having to have the mixture on one carb adjusted alot more than the other. I’m also getting alot of popping on the overrun, which I don’t mind but I’m guessing its carburetion.

The cars a standard 1500, but with a LCB and peco exhaust. Can someone tell me what needles and jets should be fitted, I’m thinking it would be worth changing them just to make sure.

Also the jet-locking nut on one carb doesn’t "lock" the adjustment, the other side it does.

And also, Haynes says the top nut is the jet-adjusting nut, the bottom being the locking nut. I thought it was the other way around, i.e., you wind up/down the lower nut to adjust?

Dave - in a confused state
D Tetley

Are the needles seating properly, (when you "drop" the piston into the jet, does it fall into the jet without any binding) If its binding in the jet, then the needle isnt seating, Id look to see if the 2 piston springs are the same color (red,blue,green or yellow...I think) I wonder if there is some kind of wear in the piston/inside dashpot area. not sealing the same amount...common problem with rebuilt carbs because some will use sand paper to clean out the varnish build up on the piston or inside dashpot. (NOT GOOD) Maybe the butterflys need readjusting, , did you loosen the linkage nuts before making adjustment...must do. Is the Dissy vac port hooked up, closed off, or free balling...(open to atmosphere).


A real good possiable area to look at is to much wear in the butterfly shafts/bushings
Prop
Prop***The End in 2012

Dave,
When i rebuilt my HS4s for my 1500 I converted from Waxstat to solid jets, and i still have not cured the problem of the jet sticking in the guide when the choke has been used. In a nutshell, when you use the choke the jet does not reliably return to normal causing excess fuel consumption, rough running etc. I have just finished mt 3rd attempt at gettting the jets to move smoothly and reliably when the choke is operated and released. If this attempt does not work I am seriously considering returning to waxstat jets. Everthing works sweetly on the bench but after a few runs and heat cycles they start to stick again.
This problem also shows up as you adjust the mixture, as you weaken the mixture, then jet does not follow the adjusting nut ( the bottom one) you could try pushing the jet upwards by hand, you might find it moves !
As for needles, my completely standard car uses ABT needles (FZX 1258) in the Moss catalogue it is suggested that with a standard head and tuned exhaust AAQ needles or for a lean mixture AAT, another thing to check is, are both needles the same ?

HTH
Alan
A Pritchett

Dave,
I should have added, that I have no experience of the alternative needle suggestions they are only what i have read, so I can't say how good they are.

Alan
A Pritchett

Alan, I have that exact same problem aswell, But ive been trying to set them up first. Im not that impressed really. Im tempted to try out a single stromberg/su or maybe a webber
D Tetley

It sounds as if you both have jet centering issues. Just to clarify - the top lock nut is for the jet bearing only - and you tighten this once you've centered the jet. The lower nut is the mixture adjustment screw which alters the height of the jet assembly relative to the needle, but you cannot lock the mixture adjustment. I'd recommend you both strip the dashpots and pistons and clean them out in cellulose thinners and reassemble with which ever dashpot gives the most equal drop time. Check the needles are both a matching pair and not bent,then reassemble and recenter the jet. With clean assemblies you should be able to raise the pistons inside the bell chambers and see and hear them drop in unison with a soft clonk onto the bridge. Do this several times and if one is a bit faster than the other you can tweak the viscosity in one damper to get them the same. If the jets still get stuck when warm it's sometimes due to the choke linkage connecting rod expanding and binding up in the bushings,so you may need to loosen the carbs on the inlet manifold and give them a wiggle to bring them in line with one another.
f pollock

Dave,
What method was used to convert your jets ? The first method i used was a right angled bracket screwed to the jet and used the existing choke operating rod, this did not work and i did not try and get it to work so i returned it to Burlen and refitted the old waxstat jets and everything worked perfectly, they then sent me another kit which used a new operating rod and a new cam to control the choke, this was better but still not as good as the original setup, I have made several attempts to get this to work, but it is still unreliable. On reflection, i think the right angled bracket would have been better to adjust, mainly because you could do it without removing the carbs.
The problem is nothing to do with how clean the carbs are or the rate at which the pistons drop as suggested earlier, I reassembled my carbs using anothe set as a guide, so i am confident everything is assembled correctly.
I think the problem lies with the angle that the choke operating rod is made to go through and most importantly the length of the fuel pipe to the jet, the length of pipe on my jets seems to be slightly shorter than the original so the jet has a small amount of sideways pressure exerted on it and also it is made to turn slightly, which then affects the angle of the operating rod again.
I have thought about going the single carb route, but so far I have not come up with a suitable manifold to use.

Alan
A Pritchett

This thread was discussed between 22/03/2009 and 29/03/2009

MG Midget and Sprite Technical index

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