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MG Midget and Sprite Technical - Am I missing something?

Hi, I have just rebuilt the su's on my 1500 as mentioned in a previous thread. When I reassembled the carbs I checked that the jets were level and then turned them out the standard 12 flats. I have balanced the carbs by ear after realising that they were way out. My question is how do you tell if they are running lean, rich or are ok? I have been out for a drive and it seems fine (far better than before the rebuild) plenty of smooth response from the throttle, no juddering or anything similar and it idles at about 600rpm. Does this sound right, am I just lucky or am I missing something? Would a gas tester help if so can anyone recommend a resonable one.

Thanks
Kevin
K Grice

Sorry Kevin but when I read your other thread I got the impression that you were au fait with the simple technique for setting up and balancing SU carbs. So are you now saying you are not aware of lifting the piston by a tiny amount and monitoring the affect upon engine performance. If you lift either piston by say a millimeter then the engine should speed up momentarily and then settle back down if the mixture is spot on. If it dies a little then the mixture is weak, alternatively if it speeds up and remains so then it is rich. Do the same for both carbs.

Normally better if you watch a competent person perform it before trying it yourself. Normally a competent person should be able to balance and tune twin SUs in about 10 to 15 minutes start to finish.
Bob (robert) Midget Turbo

Thanks Bob, Su's are new to me, I found the balancing part easy enough and I have managed to get the same tone of hiss! I will look at adjusting the pistons tommorrow & see how I get on.

Thanks
Kevin
K Grice

Kevin,

By your response, I'm not sure you got exactly what Bob said. You aren't going to adjust the pistons. You are going to lift the pistons as Bob describes. This is the diagnosis part. Then, based on the engine's response to lifting the pistons, you may decide to adjust the mixture by giving it more or fewer flats than your starting point of 12. There is a spring-loaded pin on the outside of the carb below the dash pot that you can use to lift the piston (at least there is on the HS, I'm not sure about the HIF)

Charley
C R Huff

<<<<Sorry Kevin but when I read your other thread I got the impression that you were au fait with the simple technique for setting up and balancing SU carbs>>>

Bob, you need to be careful how you phase things sometimes. Do you realise this appears quite rude?
Nick

I have a HIF44 on my frog, and there is virtually no adjustment, no lift pin, just a tickover speed screw and idle mixture screw.
Graham P

Possibly right Nick but I can't see it unfortunately. When Kevin said in the early thread words to the effect of is there a better way than the old listening down the pipe etc, I naturally took that as him knowing how the old (best) technique was performed. Sorry if I have upset you Kevin.

Graham apart from the lifting pin the adjustments you have available to you are the same as those on the earlier SU.
The mixture adjustment sets the mixture (assuming the profile of the needle is correct). This is exactly the same as the adjusting nut at the bottom of the early SU. And as you stated you also have the idle adjustment for setting the speed of the tickover.
The technique is different for setting it up, it is in my opinion easier. Simply Rotate the mixture screw very slowly and listen for the engine to speed up or slow down. You are looking for the point at which the engine runs best. This will be a wide range and you can choose to set it at the rich end or the weak end of that wide band of adjustment. Off the top of my head I have forgotton the direction for richening or weakening. :-)
Bob England

Bob,
The bit that I always found to be less-than precise is judging at what point the spring-loaded lifting actually starts to lift the piston. I could only tell by taking the air cleaner off and watching for piston movement and of course removing the air filter does itself have some effect on the mixture strength.
Any tips?
Guy Weller

Yes I know thaT Guy and it is why I suggested having a dabble with someone who has done this before, because it can be a bit of a feel thing.

The other thing is that some people I have gone to help out have pushed the pin completely up moving the piston about 10mm! not what you want. I must admit sometimes I have the air cleaners off and do the same as you and think the bit of enrichment when the filter is put back on is not such a bad thing.
Bob England

Thanks for the advice guys. Don't worry Bob, no offence taken. I merely asked this question as the car does seem to be running well and I wondered if there was anything that I need to do, to fine tune it.
<<< I will look at adjusting the pistons tommorrow & see how I get on >>> What I meant was lift the pistons as Bob described.

Thanks
Kevin
K Grice

This thread was discussed between 07/08/2008 and 08/08/2008

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.