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MG Midget and Sprite Technical - beginner carb advice
|Help please to make sense of carb problem. I have been trying to tune my carbs which are ok but the car always has more power with a slight application of choke during acceleration. I have used colortune and both carbs seem fine and have balanced them using the tube and ear method. I can lift the pin on the rearmost carb and get the slight rise and then fall indicating what I understand is a mixture close to correct !, the front carb lifting pin appears to have no effect whatsoever ?|
I know this is a huge and complex subject but as a beginner could some one help me make sense of what is happening ?
|Setting up SU carburettors ought to be a fairly easy operation however it never seems to be.|
Firstly and most important is carb balance
How confident are you that you have set each carb exactly the same?
For those of us who have done this for many years it is easy but I find that begineers do seem to struggle with this until they have done it with someone experienced and become competent.
Also after they are balanced both must lift (TURN) exactly together and this is adjusted by the linkage and is a critical adjustment.
The oil in both dashpots must be the same, I have seen oils in dashpots where one is a light oil and the other something different.
You appear to understand the principle very well but if you raise the pin and nothing happens then only 2 things can be happening
1 the idle could be too high causing the dampers to be rding high in the carbs and out of reach of the pin (very unusual and unlikely bearing in mind one carb is OK
2 The carb with no effect is therefore not doing anything (Not balanced) and so when the pin is raised no effect is seen.
|Bob Turbo Midget England|
|Also, how are the needles? Perhaps the PO had different ones in - on this forum, nothing surprises me any longer!|
|What air filter are you using?|
|What condition are the needles in? They always wear most in the idle position, which changes their profile. When you adjust the mixture on worn needles so it is correct at idle, you end up screwing up the jets to compensate for that wear. As a result it is a bit lean everywhere else because the needle hasn't worn there. This is most noticable at lower to mid speeds rather than flat out.|
|How thick is the oil in the dashpot damper? Too thin an oil could explain leaning out on acceleration.|
For more info you can read this MGA thread titled "dash pot":
|Willem vd Veer|
|Along the same heading|
Dash pot oil and hot weather
I wonder for some of us here in the heat stroke belt of the planet if we shouldnt use a thicker oil then the 20W, after all its another barn burner dayhitting the 100 degree + temp by noon, and Im wondering with the engine heat and the air temp if that dosnt play havoic on the oil in the dashpots making it thinner then normal thus making the carbs run leaner
Just something I was thinking about last night on the way home and passed the bank sign of... 99 freaken degrees
|Still waiting for lift -|
I use SU Zenith Damper oil all year round (is that wrong?)
This winter it dropped to arond 10F and so far this year highs around 83F (far, far too hot for me)
I think I've heard of 3-in-1 oil being used but this could be wrong
Owner Handbook, of course, says to use engine oil
|balancing carburetors: I could not understand how to do this until I bought a sensitive balancer for my motorcycle. Far, far more accurate, and more sensitive than the uni-syn that I had been trying to use (and I never could hear anything with a tube to my ear). |
Before trying a motorcycle balancer, I took my car to several "experts", who "balanced" them by ear, and, typically, it ran no better, or a little bit worse after their adjustments(!).
But when I finally tried this electronic motorcycle balancer and dialed in the balance between my two carbs (they were way off). THEN, the rest of the tuning went super smoothly and the car has run wonderfully for years since (with no additional adjustment, just oil and filter changes).
So, my advice on twin carb tuning: buy the best balancer you can find, a really sensitive motorcycle one (meant for small displacement engines). Get them balanced at several throttle openings (as noted by Paul, worn needles will make some averaging necessary for this).
Regarding damper oil, the correct oil is "what works best for your car, in your environment". There are folks who swear by 3-in-1, or ATF, or proper SU oil, and the thing is that these are mostly 20W ~ 30W oils, so not very different from each other, but priced very differently. Choose a lighter weight oil to get the damper to rise faster, and vice versa.
A quickly rising damper will allow in more air, and will open up the jet. But because more air is going in, and with less pressure drop, that makes it more lean, right?
And a slowly rising damper will delay that increased airflow, creating a larger pressure drop at the jet, richening the mixture?
Norm "I think I got that right" Kerr
|>>> And a slowly rising damper will delay that increased airflow, creating a larger pressure drop at the jet, richening the mixture? <<<|
Right, that's why it's not necessarily a good thing to use a lighter oil in an attempt to get faster damper response. All you're getting with that, really, is a quicker return to equilibrium, and therefore, less enrichment during acceleration, when you need it the most. A heavier oil would prolong this enrichment, again due to the venturi effect at the jet.
The damper valve (that little sliding piston at the end of the rod) is designed to resist oil flow by a specified amount during damper rise, and is calibrated for a particular weight of oil. Again, a lighter oil will shorten the duration of the enrichment period, while heavier oil will lengthen it.
Personally, I use damper oil from Moss, assuming that the engineers know more than I do. ;-)
Just my 2p,
|Hi many thanks for guidance to answer a few of the points raised, the dash pots are using engine oil and I drained and cleaned and re filled both. The air filter is a KN one for both, supposedly designed for the midget. I am going to buy a carb balancer this morning as after what you have said I probably have got this nowhere near !|
I'll have another go when I get one and post back.
Many thanks, I have a lot to learn !! LOL
|K&N's need a needle change.|
|Yes, K&N's definitely run lean on std needles.|
|Norm - where did you get this magical device???? Sounds a dream to do it like that ... - I would love to have it for my nice new carbs :)|
|I think the PO just added bits as he saw fit and had a lot of cosmetic work done without doing any of the stuff that actually needed doing.|
Could you advise me which needles I should run with the K&N twin air fliter ? I might as well change them in any case.
|From the top of my head H6, but i could be wrong.|
You might want to have a look at e-bay item no 280521367724
|Thanks Carl - it's safely on watch and will divert my attention during Taggart - and the murrrrrderrrrrs :)|
I assume you just stick it on the same place you do with a colourtune or pipe and it tells you? Oh the joy of saving time ...........
|carburetor balancing tools:|
this is good: gunson's Carbalancer
this is the cat's pyjamas: twinmax
The key point is that the moving part in the carbalancer is super light weight and the seal to the carb throat is tight (rubber cone). The Twinmax does require vacuum ports be added to your phenolic insulators, but is well worth it (minispares sells little vacuum bits to use for this).
In their print catalog there is also just the brass fitting bit available separately, but I can't find it in their web site just now, but you get the idea.
|Thanks to all, as I do intend to change the needles in any case can anyone give me a definitive answer as to which needles would be most suited ?|
Thanks Brad for H6 advice but need to be sure.
|Paul - honestly - there is NO definitive answer on anything with Midgets - you decide what you want from YOUR car - and go with that! Yes, get opinions, read what's said, then go with your own mind - coz you're the only one who's going to drive it - and you're the only one who knows it's exact set-up - and you're the only one that matters :)|
Thanks Norm - that Twinnmax is really rather nice - perhaps when I move, I shall get one of them ...! My new carbs arrived yesterday, so I'm rather hoping that when they are in and set up, I won't have to spend quite as much time of my life levelling them up all the time - my last ones really were pants, and I probably should have given up and bought new ones sometime last year!!!
|H6 from the Moss performance guide download no.3|
This may help:
|Thanks to you all the car is running much better now I have balanced the carbs I used the simple gunsons unit but it seems to have done the trick. The mixture is still lean under acceleration but better than it was, both carb pistons are moving together and both carbs are by virtue of the lift pins not too far away at idle mixture. I have been told by a guy today that there may be merit in using thinner oil in the pots before I change the needles ? would this help the lean under acceleration ?|
I have bought two new needles from Moss the AAQ needles for K&N shall I just bite the bullet and put the new ones in and if so where do you start the mixture setting from ?
sorry if my post exposes the gaps in my knowledge but hope the questions aren't too daft LOL
|Paul, thinner oil will make it even leaner on acceleration. See my post in "little things"|
|Alex G Matla|
|Many thanks all, the needle change together with balancing and colour tune has transformed the car runs really nice now, no flat spots and good power !! yipee,|
Got my enthusiasm back to renew all rear suspension now !!
This thread was discussed between 23/06/2010 and 05/07/2010
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.