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MG MGB Technical - running too rich
|i just put in new jets in my su's i have a 71 mgb roadster but it's running too rich, the pipe is black and the gas mileage isn't very good. can i order lean needles? do they have to be matched to the jets? i have oil in the pods but it's 40wt i'm sure it's too heavy tell me what i can do. david|
|Maybe the gaskets are thicker than the old ones, locating the jets on a smaller part of the needle. If this is the case, maybe you can move the needles down just a bit to compensate. Sounds pretty touchy, never done it, but I'll bet someone here has. I'm assuming you have them adjusted as lean as they will go already.|
|Filing the needles will make the mixture rich. |
Start by using 20w50 in the dashpots. If that doesn't cure your rich symptoms, start going through the ignition system. Weak spark is much more likely the cause.
|Did it run alright before you changed the jets? If so make sure you got the correct jets. Also check that they are not being held down by the choke linkage.|
|Did you set up the carbs from scratch after changing the jets? You must. There are only two sizes of jet for MGBs - 0.090" for the 4-cylinder carbs and .100" for the V8 carbs. After that it is all down to setting-up.|
In simple terms, the shoulder on the bottom of the thickest part of the needle should be flush with the bottom of the piston. After that it is a matter of adjusting the jets so they are initially flush with the bridge in the carb body, then turned down two full turns. That is the starting position for basic tuning. Then turn them up or down for fastest idle, then for fine tuning you use the lifting pins and very small movements of the jet adjusting nuts or screws to get the correct result, which is a momentary increase in idle speed when the piston (*not* the lifting pin, which will move some way before it starts lifting the piston) is raised 1/32". If the idle increases and stays increased it is too rich, it the idle decreases and tends to stall it is too weak. Even though you may not be able to judge the correct position (it is almost subliminal on HIF carbs, slightly easier on HS)you should be able to tell when it is too weak and when it is too rich, and position it between those two points.
Both carbs whould end up a similar distance away from the 2 turns down starting position, if not a problem with one or other carb is indicated, and both carbs should not have to be moved far from the 2 flats down position. If that gives correct mixture, but you find it is still running rich at wider throttle openings, you may have a vacuum leak.
But first you must ensure that the ignition system is spot-on as problems here can affect the carb indications.
|Paul Hunt 2|
This thread was discussed on 10/02/2006
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