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MG Midget and Sprite Technical - Note of Caution cc Needles

Dear all,

Just a note of caution. I have just comeback from getting my midget setup on the rolling road to try and solve an over richening of my front carb (as a previous BBS).

We found that the problem was with the needle. The needle shaft was not secured to the "top hat" section that sits on the needle guide. Thus causing the needle chaft to move up and down of its own accord and altering the mixture dramatically. The engineer at the road sugeested it may be a manufacturing fault with the needle(which was not more than six months old), so just be careful to check needles.

I am glad to say the problem was solved with a new needle and she is running well now.

Regards

Jim
Jim W

was the needle NOT tighten down with the little set screw on the side of the piston?.....or was the small piston spring weak that the needle carrage rides on?.....

I dont understand why you had to get new needles???

seems to me you just needed it tightened or a new piston...(not sure if the spring loaded needle carrage can be replaced).....

are you sure the that both needles where the same...or had one of the needles worn more due to not being centered properly???

prop
Prop

Prop,

Needles for HS4 SU are two pieces pressed together a top hat section and the needle shaft (Both Brass), together forming the "needle". The needle shaft was loose in the top hat section.

The needle shaft presumably sliding within the internal diameter of the spring.

Regards

Jim

Jim W

Ah I got ya, I thougt you where talking about the HS2,which most midgets are running, vary few are doing the HS4....But your correct that the HS4 dosnt have the spring loaded carage....I am in the middle of rebuilding a set of HS4s myself and I noticed the same thing, I figured the set screw wasnt fully tight and the needle head just wore down.

prop
Prop

Prop and Jim. There are two styles of needles which can be used in the HS-4 carbs. The first is the original solid needle which is held in position by a set screw and has to have the jet centered to it. The second is the "biased type", which has a spring on top and fits into a needle holder which is secured to the carb piston by a set screw.

With the original style, solid, needles, you keep the set screw tight and little can go wrong--if the jet is aligned with the needle properly. If not, you will wear a flat along one side of the needle, altering the mixture characteristics. The jet may also be damaged by this contact.

Later, the HIF was produced with a "biased" needle which has a limited ability to self center itself within the jet. The spring attached to the top of the needle pushes it to one side of the jet, allowing for more rapid needle and jet wear than was found on the original style needles used on the HS and older series. But, the biased needle set up is quicker to manufacture and does not require that complicated centering of the jet, hence allowing less well trained people to both assemble and to work on the carbs.

The current production (Burlen) version of the HS carbs uses the biased needles. My experience with the current production is limited to two examples, not a "scientific" sample. (Actually, not a sample size large enough for statistical methods, but large enough to qualitative research techniques.) Significant wear is notices on the needles and minor wear on the jets in about three years of use as a daily driver. Sufficient to cause the needle to need to be replaced and the jet seems to need replacing about every second needle replacement.

Les
Les Bengtson

thanks les,,
thats some good info.

its amazing how much you can learn from just tearing down a set of carbs.


prop
Prop

Prop. It takes a little more than that.

Les
Les Bengtson

This thread was discussed between 14/02/2008 and 16/02/2008

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