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MG Midget and Sprite Technical - AUD502 adjustment on MKIII 1275
|In my recent post is asked info about what kind of Carb my midget has. On the Carb is AUD870/871 casted. But as turned out the label on the floating chamber AUD502 seems to be the right number. |
With this info i ordered new needles (AAT), new Jet assy (AUD9141/42), new Damper Spring (Red) and new Jet Bearing, according the "SU Carb tuning tips & techniques" book.
Strange thing is that the hayens workshop manuals writes that a blue Damper spring is needed (page 81).
So i used the holliday weekend to place the new parts and today is had to adjust the carbs. The manual says to set the jet flat with the bridge and turn the jet nut 2 whole turns down. After starting, warmin up it would not run without choke out. with no choke it died, so i turned the nut another turn and it still died. Only on turn 4 i was possible to keep it running and was able to adjusted the carb's. Whats wrong here?, 4 turns cannot be correct i think, although the engine runs fine at the moment. But what will happen if i drive on highway, full trottle, are the jet's not to far down?
|the principles on this video might help plus there are more videos on the subject of SU carbs - http://www.youtube.com/watch?v=4nvGLgO6pj0&feature=plcp|
|Thanks Nigel. I know how to adjust the SU HS2 carb, still it is nice to see it been done. Youtube shows an other MG carb then the midget, but principle is the same. My "problem" is that my adjustement nut setting for the new Jets is 4 turns instead of the 2 turns that is wrote in the books|
|sorry I meant John Twist has more videos about SU carbs on his site not just general YouTube stuff|
there used to be at least one other John Twist video about setting the carbs on there butsorry I've lost all the shortcuts so you'll have to look for it and the others
sorry but I'll have to say this, not the Haynes or other manuals but the Driver's Handbook would give you a step by step set up of your carbs on your model
sorry I can't help further as I don't really know and will confuse myself and you
my Spridget came with red damper springs I think as a tuning step but I replaced them with blue which suited my car more, how much this is relevant to you I don't know
1275 are just on the border between blue and red springs. Some listings, as Moss US, show all HS2 1098/1275 as taking the same (unspecified) springs, but these cars came with both. A low tune 1275, as the late US car, came with blue, but a higher tune as UK cars came with red. It will have some effect on adjustment, but the big change is at top end. So, not your problem.
As I said earlier, I have never had need to replace the jet bearings on these, I have heard that Burlen now supply them with metric threads, but I've not seen one. That would change the "turns down" somewhat, but not that much. The definitive setup is to measure the distance the jet is below the bridge. About .065-.070" (1.5-17mm)is a good starting point. I rarely have to change more than two flats from this.
If you are that much lean, then you have a fuel level problem or an air leak someplace. Big problem here is that while you can compensate an air leak by richening idle, it will go way rich at speed. If it is a fuel level/feed problem, then it may go way lean at speed.
|Was it running O.K prior to rebuilding the carbs ?|
What needles were in it when it was stripped ? same now ?
If it is an ex USA car without the smog kit etc what is left on it ?
As FRM why did you need to replace the adjusting nuts/tubes ?
Can you change back to the original ?
Check for air leaks if bits removed and is manifold gasket good and sealed?
Timing about correct with correct points gap ?
Assume needles are set at correct position at base of piston assembly ?
If it is a high compression unit you could use the U.K needles.
|Was it running O.K prior to rebuilding the carbs ? : **(It ran fine before te rebuild,only changed jet assy, jet, needle, spring)|
What needles were in it when it was stripped ? same now ? **(there were AAT needles wich replaced with new AAT)
If it is an ex USA car without the smog kit etc what is left on it ? **(Nothing is left on it exept crank case ventilation to carbs)
As FRM why did you need to replace the adjusting nuts/tubes ? **(engine was running not regular for some years, as i don't know what the condition state of parts were, i decided to change them with new, so i know the condition is good)
Can you change back to the original ? **(i can change back, because all parts are still there)**(the original adjustment nut was metal, the new is brass)
Check for air leaks if bits removed and is manifold gasket good and sealed? **(will check on this)
Timing about correct with correct points gap ? **(timing has not changed/adjusted before/after changing carb parts)
Assume needles are set at correct position at base of piston assembly ? **(followed the procedure in manual, edge flat with bottom of piston. Same procedure as i did always with old needles)
If it is a high compression unit you could use the U.K needles.** (how to determine if it is high compression unit?)
|One more thing to check: if your US Carbs have the original throttle plates with the little spring loaded "pop off" valves in them, the spring looses strength over time and those valves open up too easily, leading to an over lean mixture. Their purpose was to lean the mixture on over run only, but when the spring is weak they lean it out all of the time by leaking.|
either replace them with solid throttle plates
or solder the poppet valves shut for a quick correction
|The trottle plate is the solid kind. 4-5 years ago i had the shaft and bushes renewed by BCCP in holland, because the shaft was completly worn. I checked the shaft and there is no wear and leaking|
|The manifold gasket is a good place to check for air leaks, it fails easily where it is thin immediately above or below the inlet ports.|
|Today i replaced al the gaskets (6x) and did the carb setup again. I also measured the old and new Jet and the old and new needles. They all have the same values.|
I started with the jet flat with bridge an then 2 turns of the nut, engine will only run on choke, when choke is in the engine dies. After turning nut another 2 turns it is possible to setup basic carb seting and idle speed. (choke is in).
tommorow i wil do the setup with the old parts and try again to get to basic setup at 2 turns of the Nut.
|Today i replaced the old parts (Jet, Jet Assy, Needle, nut) and went to basic setup (2 turns of nut) started with choke and when engine was warmed up, i could close choke and engine kept running. I made final adjustment from this 2 turn setting and only had to go 2 flat richer. |
Tomorrow i wil try this setup but with the new needles, lets see what this brings
|Seem to remember the carbs you have have the 'biased' spring mounted needles.|
If so both the needles and jet tubes wear as they are always in contact at low rev settings.
I would replace both the needles and jet tubes.
Never had an issue changing just these.
|Yesterday i changed back to the old parts and basic setup was 2 turns of nut and 2 flats. Today i only changed to new needles and i am at 3 turns of nut and 1 flat to get to normal idle run|
|Today the first nice summer day in the dutch mountains :-), so i did my first long-run this year after passing MOT one month ago.|
Haynes tells to put 20W40 oil in the carb damper and i was advised to use a thinner oil. Just before leaving i changed the oil in the carbs to shock absorber oil and replaced the red damper spring back to blue. In a earlyer post i mensioned that the red spring and 20W40 damper oil let the car smoke black when letting it run on high RPM. The reply whas that the spring was to "heavy" and the oil to thick. After warming up i find the engine somewhat hessitating when accelerating and going trough the gears, but she runs nice when cruising at constant speed. I just checked the spark plugs and they are (as the manual says) light grey/brown, meening that the mixture should be correct.
|The difference you report on the 2nd June is the normal difference between old needles and new. Old needles will have worn in the idle position so require the jet to be wound up a bit further.|
Your last post indicates that the combination of damper oil and spring you are now using is allowing it to lean off too much while accelerating. I would suggest changing back to the red spring as this is what is specified for the AAT needles. (And they are the only standard 1275 needles that had the red spring according to the SU listings)
Care is needed when using plug colours as a guide for mixture. In the first instance, the colour check reveals only the last running conditions of the engine, so if it was allowed to cruise or idle before checking them then they are indicating the mixture under those conditions. To get useful information from the plugs you need to hold the engine speed & load at the misfire combination for a few seconds (10 is good but hard to achieve) and then immediatley cut the engine without lifting off the accelerator and coast to a stop.
The second qualification about reading plug colour is that the colours in the manuals were based on leaded fuels. For unleaded fuels the situation is less clear, certainly here in NZ the tan colour is not seen and the plugs look black at correct mixture so we get to interpret different shades (densities) of black!
|Paul, thanks for the reply. I will change back to the red springs next time when i go for a drive and see what it brings. Just for the record, i dont have a misfire problem, just that it is not evenly responding on the trottle, its holding back a litle. Unfortunatly the weather forcast is not good for topless driving the next 3-4 days :-(. Here in the Dutch Mountains it is al unleaded fuel to. I always put in a little mixture of lead subtitude to compansate for unleaded fuel.|
I also ordered a 123 ignition and are very courious in what that brings :-).
|after you put the red springs back and before you fit the 123 set/check tappets, points, plugs, timing and mixture (in that order)|
then take the car for a run as this will check you've cured the problem and give you a run to compare the 123 against
fit 123 do same run in same conditions
if you're not pleased with the improvements I'd be very surprised
or you've put the leads back on in the wrong order :)
|Hi Niek, sorry to go back to Pauls point about changing the manifold gasket, but have you? I had a very similar problem to what you describe. There was no noticable blowing/sucking noise from the manifold gasket, even on hard reving, but when I took off the manifolds the gasket, as descibed above, had split top and bottom on both inlet ports. I wish I had replaced it first it would have saved me many hours retiming and carb tuning, I had no idea a small leak could make such a huge difference.|
|I changed/renewed all the manifold gasket's (2x3=6 in total).|
|Noticed something else today. When engine is on temperature and i stop after a nice drive and let loose the trottle it seems that the engine is stalling (RPM drop below 700RPM) but just before it cuts out, its going to 1000 RPM and ideling is fine........|
Dave is speaking of the manifold to head gasket - a common fail point, as the manifolds are often loose or have been.
I once had a young female customer call me in desperation from an unsavory area of town, late at night, because her Midget wouldn't start. I went to look and found that all the manifold nuts and big washers were missing, and the manifold was well off the head. I have no idea how it ran that long, but it did - she admitted it had been hard to start and erratic in operation for some time.
I found enough nuts and washers here and there to get it going again, and she dropped the car off for a proper fix a couple of days later; she said it was running much better thank you!
|I don't have a problem with starting, she starts within seconds. I didnt disasamble the inlet manifold and the last time i checked all the nuts were fastend, but i wil give it a proper inspection|
|>>she starts within seconds<<|
Niek your expectation of a Spridget in good condition may be too low
a standard car in very good condition may start in well under a second (warmed up, no choke)
maybe a few seconds is very optimistic, but i don't have problems starting
|The gasket leak which is sufficient to cause your sort of trouble may be small enough to have no other noticeable sign, and it can only be found by inspection. It could have happened years ago. In 50 years of servicing these cars, both A & B series,I have decided that checking those nuts is essential at any service, and unless it is a car I have worked on before, they are almost always loose. It is simple enough to pull the manifold, and I always do if I've gone over the rest of the usual easy fixes.|
I think you may have misunderstood me
a standard car in very good condition may start in well under a second (warmed up, no choke)
the car should start within a second, not longer, not seconds
|@nigel. The car has been sitting for months and last year starting took lots of second, but i think it was dry and took longer to get fuel. Now it starts fine, just a couple of turns of the starter, maybe a couple of seconds, but that does not worry me.|
|if it takews a couple of seconds to start when it's warm it means something isn't right|
even on choke if you get it right and the fuel is through it should start straight away unless the starter motor is missing a turn
|I mean starting procedure when cold. When engine is warm it starts immidiatly, with just a few turns of the starter.|
|I just took of the inlet manifold. All the nuts were tight and it seems there are no leaks between manifold and engine, so this looks ok.|
|"it seems there are no leaks" so Niek, did you replace the manifold gasket? the gasket has a metal centre with soft material either side, if you can see any metal, the gasket is probably worn letting in air.|
|I did not replace the gasket, i dindt see any metal and gasket looked ok|
|Yesterday i did a new check of points, valves, timing, mixture etc. Also put in back the red spring and filled up with shock oil. Today i did a run of about 30min and the "hessitation" on acceleration is stil there, although sligtly less. The engine is running fine at cruise speed and has misfire. On my other thread (AUD502 change manifold to port vacuum) i placed 2 videos of engine run and exhaust. the engine is running a little not regular and the exhaust is "splassing" (is that correct english??). I adjusted the carb as by manual and running 2 full turns and 2 flats rich on both carbs, any more flats to rich side and RPM are dropping, so that should be ok. I run the new needles (AAT) and new jet's wich i ordered 1 month ago.|
|Forgot to mension. Did a dynamic timing on 10-12 BTDC vacuum off, manifold plugged at 1000RPM. When putting back the vacuum the timing got to 35-37 BTDC, meaning the vacuum advance is 25-27 degrees, is this a correct value? seems like a lot of vacuum advance. i use a 23D4 dissy.|
|sorry for the type, but i use a 25D4 wich came of my donor car. When i bought it 13 years ago a 45D4 was fitted.|
This thread was discussed between 28/05/2012 and 04/07/2012
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.