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MG Midget and Sprite Technical - SU carb Tune

midget 1500 1977
Having had a few issues lately: fuel problem/missfire. The plugs looked sooty & the idle speed is about 500rpm!! I cleaned the carburetors a few weeks ago. I thought I'd have a go at tuning or at least checking using that guide the someone posted not long ago. The question is, listening to the "hiss" should be the same for both. When I listened to mine, the back one (nearest the bulkhead)was very very quiet, where as the front one there was a very definite louder audible hiss - is this right? They were "tuned" about 18 months ago (approx 2000 miles) when the engine was rebuilt
Nigel Axtell

yepp...thats correct

im not sure why, but I notice that is the norm for out of balanced carbs... more his at the front less in the rear... why is that...haha

im guessing a modern day shop did the carb tune...if so, thats to be expected...its old tech only old farts can understand... you need.a classic brit shop... normally run down wrong side of town hole in the wall shop that time passed by and forgot about

a better method is the ciggerete ash try carb balancer...you just put it on the carb venture and watch the little ball move up and down and adjust according

another tip for carb adjustment.it you dont have a classic.shop....try a motor cycle repair shop...they deal with duel similar carbs fairly often on old bikes

prop
Prop and the Blackhole Midget

it's not right, they're out of balance

2000 miles in 18 months will do the whole car no favours

servicing is time related and not just miles traveled

idle speed seems very low but you also have to allow for inaccuracy of gauges

have you cleaned air filters, checked dashpots - in fact have you serviced the whole car in 18 months

you should have a least changed and the oil filter twice by now (I'd had 4 changes by 2000 miles but that's me)

if you've not already done so service the whole car (Driver's Handbook has full details)

set tappets, CB points, plugs and timing and then mixture (carbs) in that order

some John Twist videos to help you with carbs to follow

setting carbs and getting the engine running right are not the important matters on a car so deal with them once you're certain other items and systems are fully set up and in good condition such as - brakes, tyres, suspension, lights and windows

These John twist videos are excellent at giving information, some may not be exactly for the model of carbs in your car but the basic principles apply and all full of useful information

44 Secrets of the SU Carburettor Part 1 of 2 - http://www.youtube.com/watch?v=7GRAcqDySog&feature=plcp

43 Secrets of the SU Part 2 of 2 - http://www.youtube.com/watch?v=60Bj_2cZQnc&feature=plcp

29 SU H-Type Carburetters - http://www.youtube.com/watch?v=GHNlT5yHDxk&feature=plcp

41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA

147 MG Carburetter Tuning - http://www.youtube.com/watch?v=4nvGLgO6pj0&feature=plcp

35 Tuning HIF Carburetters - http://www.youtube.com/watch?v=ASeMfXfjNpw&feature=plcp
Nigel Atkins

sorry...I meant thats correct in that the hiss needs to be the same for both carbs

not a louder in front and softer in back

do you have a local classic brit car club in the area... thats a GREAT source of help for getting this sorted...go to a meeting/get togather and im sure they will have you sorted before the beginning of the 2nd round of drinks are needed

prop
Prop and the Blackhole Midget

ETA: I meant to put after you've fully serviced and set up the car and carbs start enjoying it go out for drives frequently if possible, if not regularly

now you've got 2000 miles on the engine you can be pushing it, drive it as it was designed, once fully warmed have the rev needle passed the upright a bit, go between gears more, just have fun

ETA to add to ETA: - Prop,
both times I was typing whilst you posted so I wasn't referring to anything in your posts, although the post about the Frogeye being OK made me smile
Nigel Atkins

Nigel
Thanks for the info. I looked some of the J Twist video's last night - very useful.
The car was fully serviced last May. I had fuel issues last August & replaced a defective fuel pump. I also replaced the points, condencer & rotor arm from the Distributor Dr. I had an odd missfire at the beginning of December. Going through the proceess (slowly, as time permits) of eliminating the problem. As stated, I looked at the spark plugs - sooty. Low idle, smelt rich.
I cleaned the pistons and suction chambers in december. As I said, I thought I'd have a go at tuning as per that useful tuning guide that was recently recommended/posted.
Trying the hose to check the air is where I am now. In the meantime, I'll watch some of those video's you suggested that I haven't watched yet.
Regards
Nigel
Nigel Axtell

Nigel,
see how you get on with set up the mixture then, those videos can give useful background info as well as specific info

the Driver's Handbook (of course) tell you about setting carbs too and will be specific to your model

if for some reason you can't get it to tune properly then it could be a repair/clean/lube/adjust issue but that bridge can be crossed if it comes up

once you got it sorted keep it sorted by frequent or regular running, even a 20-30 miles run once every few weeks would benefit the whole car and your driving experience and fun with it no end

good luck
Nigel Atkins

There's a lot to recommend an "Italian Service" once in a while.
Guy W

there's also another name for it that I can never remember

I do remember when my wife got her new Fiat Cinquecento we asked how the engine should be run in and he said to drive it like the Italians would, we couldn't be that harsh with a new car so she just drove it normally without labouring it
Nigel Atkins

Nigel Axtell.

Use a longer hose, to get your head out of the engine bay. It's easier to hear the venturi over the engine that way. Esp if you've got dodgy hearing ;)

Get one of these. Very useful and what Prop described I believe.

It's adjustable via the knobbled wheel in the centre. Lets you control the airflow, so the pellet stays in a useful part of the indicator tube.

I can't remember where I got this, or how much I paid. But it's probably still on sale somewhere.


Lawrence Slater

it's use is demonstrated in one of those JT vids (too)
Nigel Atkins

Anyone knows where to get one from thats what I want. (I don't have dodgy hearing honest - speak up there!) TYPE LOUDER. :-)
If I find out 2 night I will post it. Back to workshop.
Dave Squire - Notts

Is this it?

http://www.thepartsbin.com/cartools/speciality_tools-synchrotest_carburetor_synchronizer-d.html

No pricing online but will give it a go 2 morrow.
Dave Squire - Notts

probably the drawing isn't the best is it

I'm not sure you need to buy, as Lawrence has put a longer pipe with possibly a small funnel at the ear end should be fine for home use

one you've got your car fully serviced, sorted and set up you should need to fiddle with the carbs unless the carbs are worn in which case they'll need dealing with

perhaps a few photos of your engine bay might help with this and future matters, many cars have very restrictive air filters fitted too

even worse if they're dirty too :D
Nigel Atkins

This is the gunson version. Not as nice as Lawrence's Antiques Roadshow version. And for what is basically a few simple bits of plastic its a rip-off price. But seeing the pictures, it would be easy to make up something to do just the same job.

The bit of the process which gets easily forgotten, is to slacken off the interconnecting clamps on the butterfly spindles before attempting to balance the carbs!
Guy W

Sorry, here's the missing link:
http://tinyurl.com/b9rysof
Guy W

Thread about the Moto Meter Synchro test here. http://www.british-cars.org/mgtd-mgtf1500-bbs/info-on-carbs-synchro-gaget-2011092023344927464.htm

Or get one of these at,
http://www.retrodubs.co.uk/Carb-sync-tool/air-flow-meter

Or the original SU tool kit, if you can get one, discussed here. http://www.spridgetguru.com/TA0053.html





Lawrence Slater

Carb balancer here plenty of others as well.
http://www.ebay.co.uk/itm/CRYPTON-CARB-BALANCER-/281049877344?pt=UK_Diagnostic_Tools_Equipment&hash=item416fe0cb60
mark (1977 1500 Midget) Preston Lancs

in the thread Lawrence put up this is the favoured one but their adapters for for Spridgets are no longer available but I've seen them used on SUs as they are - http://www.amazon.co.uk/Synchrometer-Tuning-Weber-DellOrto-Throttles/dp/B008BQV5HM/ref=sr_1_1?ie=UTF8&qid=1358106212&sr=8-1

Nigel Atkins

What I suspect is a Chinese copy of the original SU kit [as per Lawrence's posting] is available from MGOC:

Tuning Tool Kit Z015 £14.95

and very easy to use it is.

HTH
Doug Plumb

Just out of interest, I see that Moss have the "Uny-syn" balancer as shown in Lawrence's earlier post and as used by John Twist in his video's. Cost £29.95 + P & P. ref: MM386-200.
I may invest in one, probably a bit more accurate than a piece of tube & 55 year old ears!
Nigel Axtell

Buy a flow meter... they cost pittence and save all this listening for hiss nonsense.
C L Carter

If you have good ears Christian, it's not really nonsense. It actually works as near as matters, and is easier than using a tool, -- other than a bit of hose to poke in yer ear.

Having a single on the Sprite now, and being a bit mutton anyway, I'd use my tool on the Sprite these days and will use it on the Midget when I get around to it. Lucky I didn't sell it as it happens, having bought the midget that is.

Anyway, how was America?
Lawrence Slater

This thread was discussed between 12/01/2013 and 14/01/2013

MG Midget and Sprite Technical index

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