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MG MGB Technical - carb tuning

I have just finished rebuilding a 72B and have been attempting to tune the carbs. I followed all the instructions but just cant get it to idle decently. If I set the carbs where it idles OK then the car "stumbles" and hesitates when accellerating from low speeds.If I richen the mixture slightly (say 1/2 turn) it idles roughly.
If I set the mixture to where it idles OK (lean) and I use the lifting pin on each carb the engine will almost stall which according to my Haynes indicates a rich mixture. I am not sure which dizzy I have but at idle or low speed I wouldnt have thought it would matter.
The carbs are good and clean and float level is correct per Haynes. Any suggestions would be welcome.
B R pasmore

You did remember to put oil in them?
Steve Postins

Have you checked everything else before tuning the carbs? Particularly, timing, points, plugs. Is there anything obviously sticking - a butterfly or a piston? Is there an air leak in the manifold (are the gaskets OK)
Andrew M

Hi B R,
Actually, have a look again at the Haynes manual. If, when you lift the piston with the pin very slightly (0.75 mm), the engine speed decreases, then the carb is lean, not rich. Lifting the piston leans the mixture by reducing the vacuum across the jet. If everything else is right -- and that's an enormous "if" -- then you should be able to get the carbs tuned by successive approximation. But as Andrew rightly states, everything else must be right first -- points gap, timing, plugs, valve adjustment...

HTH!
Rob Edwards

Are the butterfly spindles worn? If so you may have to accept a compromise setting until you can fix them. Try a dollop of ordinary grease on each end of both spindles and see if that helps to seal them.
Miles Banister

I have a similar lumpy idle problem...the car runs great but idles like it has some monster full race cam in it, (which it does NOT); and it's making me crazy. I have a lot of years with SUs but can't seem to fix this. I'm tempted to go back to the Weber 45DCOE type which I've been running for 14 yrs. without all this foolishness. I just thought I'd try to do it the factory way for a while.
The only change from normal on the SUs is that the pop open valve on the butterflys (it's a '72 partial anti smog car) have been replaced with non-poppet valved butterfly plates to try and stop the tendency to backfire on the overrun.
Any suggestions??? I've even retarded the spark a bit (Webers like a lot of lead) to get it back to near factory specs.

Wayne Hardy
Wayne Hardy

I have a rough idle too if my idle is below 1000 rpms. I'v been playing with SUs for almost 30 years and for whatever reason I can't get a clean idle below 1000. I checked the SUs for leaks and they're fine as well as for vacuum leaks on the manifolds and they're OK too.

I used a Colortune and my a/f meter and all is well as well as a vacuum gauge and a compression test. I have just come to accept the problem.

Advance boosts the bottom end and hurts the top end.
Mike MaGee

Mike: I am in the exact same boat (my first LBC was purchased in 1974). I recently replaced the stock air filters with K&Ns (in the stock air cleanner cans)and my idle speed dropped from where I had set it previously at around 1000 rpm to about 600-700 rpm. When stoppe din traffic unless I blipped the throttle every so often it would not be happy. So tonight when I got home from work I popped the hood and tweaked each carb evenly to get back to 1000 rpm. Didnt take much fiddling, and the engine definately idles smoother. My carbs are the later HS4s with the poppet valves, and are not rebuilt. I know there is some play in the throttle shafts, and a carb rebuild is planned, but otherwise the car runs very sweetly and gets about 24 mpg with hard use. So I am in no rush yet to tear it down.
I wonder if modern fuel formualtions are to blame?
Andy Blackley

Andy, I'm not an expert on this one but I'm sure I've read that K&N filters have different airflow characteristics and therfore need different SU needles to change the fuel flow. Can't remember if it is more or less air. Suggest search of the archives.
Richard Evans

Hi Andy,

Yes Richard is quite right. Converting from standard
air filters to K&N implies you have to swapp the carb needles in relationship because of a poorer mixture.
Check on this BBS archive to get the correct needles/carb combination.
Good luck .
Renou

Something strange going on here. K&Ns are freeflow filters and will allow more air to pass through the induction system therefor the richer needle is necessary to compensate for this. Std needle is AAU, fit the AAA for richer. What I cant understand is why the idle speed drops when you fit them when in actual fact it should maks no difference as the volume of air passing through them is minute at idle. There seems to be a restriction somewhere. Check to make sure that the filters are clean and that they have been properly drained after coating in the special oil.
Iain MacKintosh

The flow through the K&Ns is DIFFERENT, therefore the mixture will be as well; which way depends, but either too rich or too lean can cause the idle speed to drop. Too rich probably won't cause a stumble on acceleration, but lean will.
I suggest that most or all of the problems on these various UNSPECIFIED cars may result from timing errors caused by vacuum advance difficulties. There are a lot of different spec units to match various engine setups; I have found very many cars to have the wrong ones,or wrongly connected vac, especially modified or desmogged cars. Rebuilt distributors frequently have the wrong mechanical and/or vac units. Lastly, the vac units are getting old and failing at an ever increasing rate. I have found drastic improvements in "desmogged" cars by playing with these advance/initial timing parameters, even with pretty badly worn throttle shafts and needle/jet. The idle is much influenced by basic timing, and off-idle acceleration is critically dependent on the transitional advance characteristics. Attempts to "correct" by fooling with the carbs cause it to be wrong everywhere else.
Despite the myths, SUs are not fussy, and even worn ones can be set to get a decent idle pretty easily. Worn throttle shafts will result in some variation of idle speed, but it should still idle reasonably well at whatever speed it settles to. If mixture is set correctly at idle with worn shafts, it will be slightly lean at running speed, so set it right, then richen a flat or two, depending on the degree of wear.

FRM
http://www.usachoice.net/gofanu
FR Millmore

FRM has a very good point here. The very last thing you should fiddle with on the engine is the carbs.
I have found that performing a complete tune-up (and I mean complete, with all new parts) nearly always restores my sanity and the car to a decent base-line. I trouble shoot from there with far more success than trouble shooting from scratch.
One exception was the Spitfire 1500 with the flat cam, but that's another story (needed VERY tight valves to run at all, factory specs resulted in a non-overlap period of 10 degrees!).

Mike!
mike!

This thread was discussed between 15/06/2004 and 22/06/2004

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