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MG Midget and Sprite Technical - Lumpy Running

The car has started running very lumpily (is that a word? It is now).

Plugs 1 and 2 look fine, 3 and 4 are very black.

I've recently replaced the floats and float needles and the throttle discs and balanced everything up. This was 4 weeks ago.

I've done about 200 miles since then and all was well. We were coming back from Oulton Park a week ago when it was noticed that tickover wasn't as smooth as it should be.

I've subsequently checked that the rear choke wasn't sticking on and that there wasn't any muck holding the needle open and all was well. Cleaned the plugs and went for a spin. Alas black plugs on the rear two again.

I haven't checked the electrical side of things, I suppose I ought to check for a decent spark.

Am I missing something obvious here?

Thanks in advance.
Ian
I Ball

Hi Ian

First of all, don't adjust anything - the settings are all fine - it's just that you have a fault that needs fixing - not something you can 'tune out'.

By all means check for a good spark on 3&4 - but I can't see how 2 or 3 wouldn't be affected as well, unless a dissy cap problem (has to be a common fault between 3 & 4).

Almost certainly the problem is in the rear carb... check needle-valve and so on again... muck moves around; check needle hasn't moved (should be same as front carb); and so on.

Don't do anything drastic... it has to be something simple based on the carb.

A
Anthony Cutler

Thanks Anthony I'll look at both carbs side by side and see if there's any difference.
I Ball

Just to be sure switch the plugs around.
2 duff plugs is uncommon but not impossible
Onno K

I have found tiny bits of fuel hose and such in S.U needle valves. It is worth removing the assembly and checking each surface carefully. Certainly sounds like the rear carb is flooding.
Glenn Mallory

Rear carb suspect No1.

Easy things first

Check all hoses and connexions for leaks, remove and blow through main jet etc. thoroughly,- simply looking is not enough - check damper oil, check mix/balance again.

Failing that

Could be flooding from changing out floats - sometimes a poorly seated float needle - had very similar recently on my old bike. Flooding gives a rich mix and rubbish idle, hence black plugs. Check if float height is set correctly first though.

Mark O

Assuming Matt Black soot and not Gloss Black oil, then all the above stand as good suggestions.

My bet, flooding due to a poor sealing float needle valve.

Best of...

MGmike
M McAndrew

You didn't replace the float chamber needles with the ball type did you? If you did, chuck them out. My experience and I believe that of others, is that they are ineffective and will allow fuel to bypass. You can't beat the pointy ones!
Graeme W

Graeme,
That's exactly what I did, having had them recommended. I'm popping into Moss's tomorrow for float gaskets so will enquire about getting a replacement while I'm there.

All being well I'll get this sorted tomorrow.

Thanks to everyone for your input, I'll let you know how I get on.

Ian
I Ball

comparison of needles & seats at the start of this video - 222 MG SU Carburetters: Setting the Float Height - http://www.youtube.com/watch?v=82YNx-RkGNI&list=UU40j4KqUJPMVv4FQ29ro-xQ&index=11

and this video shows you something to check when buying the needles & seats - 232 MG Carburetter Needles and Seats (Needle and seat) - http://www.youtube.com/watch?v=pVX_JClyeDQ&list=FLr4Udxf9e9Cq7fxH4V23MWQ&feature=mh_lolz


Nigel Atkins

I had similar issues with needle valves sticking which was due to deterioration of fuel lines, specks of rubber holding valves off seats fit new lines all ok .
mark 1500 on the road Preston Lancs

Thanks again everyone, I've just ordered some bits from http://sucarb.co.uk/, so all being well I'll get it sorted and get the old girl out driving again.

Of course we'll both be in the midget :-)
I Ball

I saw what you did there ;)

do be aware the SU web site parts catalogue like all catalogues contains errors and omissions so do cross reference any information you take from there
Nigel Atkins

Its your balls

Get rid of em and you will be much happier...

Prop
Prop and the Blackhole Midget

It took Prop to find it, but we found the level :-)
I Ball

New needles and seats arrived and fitted, buggered about with carbs, and went for a blast.

Everything is better, but not best. Tomorrow I'll be going around making sure that we're not sucking any air in where we shouldn't be and having another tinker.

We're not far off now and I'm feeling more confident that I'm not going to break anything...famous last words ;-)

Ian
I Ball

How bad where the grouse jets (aka your balls :-) ) ive heard it dosnt take long for the balls to get nicked and scared up

Some times it just takes alot of fiddling

Are your float the soild plastic with no adjustment, or are they the HS4 that have the plasic float and the attachments are all brass

The reason I say is maybe the float level is set to high, I learned that this spring and saw the importance of this little tid bit real quick...something I was only mildly aware of and gave little thought to...but was huge in the end.

Prop
Prop and the Blackhole Midget

This thread was discussed between 12/08/2014 and 15/08/2014

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