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MG MGB Technical - Zenith_Stomberg 175CD5T idle problem

Hello,

A friend just bought a single carb. MGB. I am familar with SU and not too bad to adjust them. But this Stromberg drive me crazy. The problem is an idle that behave curiously. If I rev. the motor to 3000RPM it idle near 2000RPM and finally it decides the return to low RPM slowly. Nothing appears to be stuck in linkage and I looked at that, "illogical" to me, coarse and fine idle screw. Both of the are screw to max position.

I have two questions:

What could cause the idling problem?

How to adjust those coarse-fine idle screw?

Thanks for four help...

Jean G.
Jean Guy Catford

Hi,

Another question: is there a site explaining basics in adjusting this carb.?

As Haynes or Bentley manuals are not clear on that topic.

JG
Jean Guy Catford

Make sure the throttle cable has some free play in it. See if pressing on the quadrant makes it return properly or not. If it does then something is sticking and preventing the butterfly returning, if not there is some other reason i.e. route for mixture to get past the butterfly. Does the butterfly have a poppet valve? I know these were used on SUs and are well known for causing idle problems.

Could also be the auto choke if it still has it. This http://www.sterlingbritishmotoringsociety.org/files/choke.pdf cover the water choke and other features of the carb.

As far as mixture adjustment goes if you can't get to the correct point within the range of the fine screw you centralise that then adjust the course nut to about the correct position, then fine tune with the fine screw. If even that doesn't do it then you centralise both then have to use the special tool in the piston to adjust the needle to about the corerct mixture, then the coarse nut, then the fine screw. Easily said I know, but if there is anything wrong with the carb, or inlet manifold, or ignition or virtually anything else on the engine like emissions or PCV system, you will have problems getting the correct mixture and idle. The carb can only be correctly setup when everything else is correct.
Paul Hunt

Thanks Paul,

In fact it has that poppet valve which could be faulty. I will check that. But it is mentioned nowhere how to adjust those coarse nut and fine idle screw.

Cheers.

Jean G.
Jean Guy Catford

Jean. I am not familiar with the Zenith but on a lot of that type of carby I have come across there is a small lever from the throttle spindle to some sort of damping device. It was meant to slow down the closing of the throttle for pollution reasons. It seems that snapping a throttle shut causes pollution problems.

Might be worth a look.

Tony
Tony Oliver

I'm attaching scans of the section in my Haynes. However it jumps straight from the fine screw to adjusting the needle in the piston there, I have seen info elsewhere about using the coarse nut between those two stages.

Part 1 ...

Paul Hunt

... and Part 2:

Paul Hunt

Thank you Paul and Tony,

Your help was appreciated.
I found a problem with a misalignment of throttle/choke lever. After a psychological treatment( I bent then to restore their interaction correctly). It was part of the problem. Following Paul advice, I decide to fiddle with coarse/fine screws to obtain someting decent.

Cheers,

jean G.
Jean Guy Catford

I forgot,
I notice a lean needle behaviour. Now if I cant solve that to enrich properly the mixture. I will have to deal with needle swapping which needs at leat 8 NASA engineer to understand what should be done.

JG
Jean Guy Catford

Too lean off-idle where it is correct at idle? You need to look for a 'richer' needle i.e. one where it tapers more as it gets thinner. It is a bit hit and miss rather than scientific, there are a couple of programs around on the net that compare needles (e.g. http://www.mintylamb.co.uk/suneedle/), as well as at least one that can be downloaded.
Paul Hunt

Paul,

Now I have a steady idle, which is a great improvement. But if I lift carb. piston 1/4" a decrease in engine RPM follows, indicating a lean mixture. I suspect needle is at richest point, I will be oblige to search for a richer needle. I am not familiar with that aspect. Thanks for the program infos.

The basic problem with old B is to not knowing the real state of engine as very often many parts have been swap with other models.

Personally I think a change for a dual intake with 2x HS4 would be the best thing do do. But it is not my car and this appears to be an heresy to the new owner.

Thank you for you input...

Jean G.

P.S. I finally found that coarse idle nut purpose is an overpass of air...
Jean Guy Catford

You *should* be able to weaken or richen a long way either side of the correct point with any needle at idle. 'Richer' (and 'weaker') needles have a different rate of taper to cater for different requirements off-idle, i.e. at various throttle openings *between* idle and full throttle. The majority of needles are the same or only slightly different for the first two 'stations' i.e. the idle stations, and then taper differently from there to give richer or weaker needles.

If you can't richen enough with the fine screw then you need to position it mid-way in its travel and adjust the needle in the piston with the special tool for correct mixture as best you can, then fine tune with the screw.
Paul Hunt

This thread was discussed between 22/06/2009 and 27/06/2009

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