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MG MGA - SU Needle Selection

I have a 3 bearing early MGB engine in my A. The carb is standard MGA H4s fitted with the 'standard' MB needle for this 1800 engine. My problem is even with the adjustment completely up the engine is still too rich at the normal tickover adjustment speed, i.e. engine carries on increasing in speed when I lift piston with the pin. In terms of on the road running the engine is fine, although exhaust pipe is black rather than grey.

Clearly I would like to get the mixture right and I could fit the alternative correct '21' 'weak'' needle for this engine but before doing that my queries are. Surely the engine should be adjustable on the standard needle, so any ideas why it won't? Also going to the weak needle will this affect my on the road performance. Any other comments.

Paul
Paul Dean

Paul

I run the 18V with the H4. Mine were originally fitted with No. 6 needles when I bought reconditioned carbs during my rebuild in 1997. I have always tended to run rich the way you describe. I tried FX needles but did not like the performance curve. Few weeks ago I fitted MGA standard GS needles. Tuned up perfectly. Performance excellent (as was previously) but this time lots of adjustment still left on the jet nuts and the plug colour is correct.

Steve
Steve Gyles

Thanks Steve. I have a pair of GSs so I will give them a try. It won't be for at least 2 weeks as we are going on hols today.

Paul
Paul Dean

Going back to my thread on always running a bit too rich I have just solved my issue by taking Barnie’s advice. I dropped the needle by about a 1/32 and I now have the adjustment I need. I will run like this for now but replace both jets and needles in due course as this indicates wear somewhere.

Paul
Paul Dean

Paul

Your remedy I think confirms my similar experience from about 6 months ago. Like you I had always run a tadge rich. I had tried everything over the years except for one item - the jets. I had never considered them to be the issue as I assumed they had been changed in the 'new' reconditioned pair I installed during the rebuild. However, they turned out to be the cause. New jets and needles sorted her at a stroke. I can only assume (no proof) that they were never replaced during the overhaul. Someone will say I never had the jets centred and that caused the wear in the jets, but I can assure any doubters that was never the case.

Steve
Steve Gyles

Paul,
It could be your jets are worn. Have you checked their diameters with a known diameter rod (drill bit), microscope inspection? Is the needle catching because not centred properly , does piston drop with a clunk?
My MGB maual shows no 5 standard, no 6 rich, no 21 weak (not sure how relevant they are with today’s fuels).
Lifting the piston only confirms mixture at idle. What is it over the rev range - air/fuel ratio measurement is needed to confirm, especially if your engine varies from standard.
My modified 3 brg mgb engine runs MME needle in H4 carbies (about 20% richer at top end).
Mike
Mike Ellsmore

They usually run just fine with #5 needles.
Bill Spohn

Reviving this thread after taking Steve’s advice. I have now changed jets so will see how I get on in due course but a couple of comments in the meantime.

Firstly reading the original workshop manual it comments that worn jets were common in the day due to jets not being properly centered. A friend of mine who has just retired from running a workshop said if someone brought an SU car in running badly he always changed jets and needles first and this was usually problem. So yes this shouldn’t happen but it does.

Secondly I had bought the rebuild kit from Moss but in a sealed SU bag so can’t blame Moss. One of the most important bits is the big cork seal but it wasn’t the expected O ring but a disc of cork of correct diameter as they appeared to have forgot to stamp the hole out of the middle. When phoned Moss immediately shipped correct seals for free.

Thirdly another problem with SU kit. When all assembled I had problem turning mixture nut to top. This wa being caused by the new spring spreading itself over the nut hence locking it. Once I worked out what was happening I easily solved issue by refitting original springs.

But why 2 faults in SU kits! I do worry when you tell Moss about issues like this do they do anything about it or just keep selling. Did they tell SU/Burlen.

Paul
Paul Dean

Now that I know that I have forgotten most of what I ever knew about tuning SUs and the rest of the system, I go to experts for help. I had mine done recently on a rolling road. It was running rich and had uneven performance over the rev range. Tom changed the needles and reset the jets. He ran it up again and identified the flat spots, which he corrected by shaving the relevant part of the needles. The end effect is a massive improvement on my 1500 engine bored out to 1600 and rebuilt just a couple of thousand miles ago. 64bhp at the wheels, which is enough to be going on with. The journey home was a real pleasure. By coincidence, my neighbour took his Bristol 411 to the same place and is equally pleased. Both maroon cars were on show at the village fete. The tune was not cheap but was done in a couple of hours while you wait. Now needs the choke for about 15 seconds (no need for a handbag to hold it out...)
Steve: he was impressed with the carburettor thingies (told you I was forgetting) you designed a few years ago. I'll remember what they are called once I have pressed the send button!
Shane
Shanerj

Shane

Good to know he liked the Stub Stacks.

Steve
Steve Gyles

A related question. What method do you guys use to check SU carb needles and jets for wear and possible need of replacement. I'm thinking about micrometer needle diameter measurement, but it's tapered? Or maybe it's a visual check for obvious wear and ridges down the taper? Similar re the jet diameter measurement - with a drill shank? I would just like to see if there is a good case for new needles and jets whilst changing seals.
Bruce.
B Mayo

Not a way of measuring it but a symptom of there being a problem. Having just fitted new jets one benefit I have noticed is that my tickover speed is consistent. Before it was quite variable so I think this may be a tell tale sign that needle isn’t sittting correctly in jet.

Paul
Paul Dean

This thread was discussed between 23/06/2017 and 02/09/2018

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