MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Setting warm idle, single ZS carb, 1980 B

So I changed my coolant today. It's been put away for 7 years until just recently.

Today was the first time I let it actually warm up. I was happy to finally see the temp gauge come to life as it hasn't budged the last couple times I started it for the couple short test drives I've taken.

I read that you should not adjust the automatic choke high idle until you've properly adjusted the warm idle.

To adjust the warm idle, I read that you should have the motor completely warmed up so you could be sure the choke was off, then turn the warm idle screw till you reached a desired idle.

With the aid of my girlfriend on the throttle keeping the motor running, it took quite a bit of turning (lets say 3 full clockwise turns) before the car would idle on it's own. I found the car to be fairly steady at about a 1500 rpm setting, but after about 30 seconds, the idle would drop to about 500. I was worried it would stall, and since it won't start when warm, I did not let that happen.

I pulled a plug before starting the car this evening and found it was black, I assume from an overly rich mixture, even though the car does not smoke once warmed up.

So I'm thinking that the automatic choke is possibly stuck in the ON position which would explain why it refuses to start once warmed up and also explains why the plugs are dark from it's last semi-warmed up test drive (which was ended when I let it stall and it would not restart).

I'm looking for feedback as to if my assumption that the auto choke is likely stuck. I wish I could put a Wideband O2 in the tail pipe, but the car has a cat so anything after the cat would give an incorrect reading.
Jeff Grant

Plugs will be sooty after fiddling about, only cleaning up on a good run when they get fully hot.
Have you adjusted the mixture? That needs to be correct before you can get the correct hot idle speed. If you can't weaken the mixture enough the auto choke could be stuck, you could remove the heat mass from the carb body, then you will have access to both the mixture and fast-idle mechanism, see where the heat mass bi-mettaic spting loop and operating peg are at, and see what happens when you twiddle those. This http://www.sterlingbritishmotoringsociety.org/files/choke.pdf should tell you all you need to know about the water choke.
Paul Hunt 2

Thanks Paul for the link. I believe you (or someone) has given it before and in fact I have already printed it out.

Unfortunately, the PDF is more of an explanation of the parts, not really explaining how to adjust.

I'm thinking my best bet is to pull off the auto choke section and confirming the parts are in a relaxed off choke state. Then, once I confirm this, which will be difficult as I will have to deal with the carb while the motor is fully warmed up, go ahead and adjust the warm idle mix.

I did not see in the PDF where I can adjust the warm idle mix. Can you tell me which picture shows it?

Also, can you confirm that I can remove the choke assembly from the side of the carb without coolant coming out? I assume the coolant passage is conceiled from the choke parts.
Jeff Grant

That PDF describes the water choke, it describes how to setup the choke i.e. adjustments on pages 13 and 14. And yes, you can detach the heat mass and bimetallic spring assembly from the body of the carb without losing coolant.

You will have to consult Haynes or other workshop manual for information on basic mixture and idle setting. Basically you tweak the fine idle screw, or coarse idle nut if the range of the screw isn't enough, or in extreme cases use the special tool to adjust the needle in the piston, until you get the highest idle speed. Either side of the ideal mixture the revs will start to drop off.
Paul Hunt 2

Thank you Paul for the quick response.

I had actually used Picture 20 (item #2) on page 13 to adjust my warm idle rpm already.

When I had anything that resembled a non-stalling idle, I had a good 1/4"+ of thread of the idle screw sticking through the bottom.

I had also confirmed the heat mass index points were aligned.

It's a shame the PDF doesn't have a warmed up view of the choke alignment.
Jeff Grant

BTW, it is the three screws round the edge of the heat mass you loosen to adjust the amount of cold-start enrichment, and remove if you want to detach the heat mass altogether. If you undo the central screw you will get coolant loss. The alignment marks should be regarded as a starting point, you may have to move one side or the other to get the right amount of enrichment in individual cases. 1/4" of the idle screw sticking through does seem quite a lot, but it bears on a system of levers to open the butterfly and that is where the variation can come in. It also depends on timing, including whether manifold vacuum is connected or not.
Paul Hunt 2

there is a good video on rebuilding the water choke on the stromberg carb on John twists you tube website. the site is www.youtube.com/universitymotorsltd
kra Karl

When I was picking up my '71 B parts car yesterday, the gentleman told me about the youtube videos! I checked some out last night but didn't notice the automatic choke one.

Thanks for the heads up Karl, I'll check it out tonight! Sounds like exactly what I need!
Jeff Grant

Yup the John Twist Youtube videos are great, you should watch all of them...
james

WOW. So I finally finished watching all 3 videos about rebuilding the Stromberg carb.

The two primary things which was said to watch for were the following.

1. The mating surface of the choke body to the carb should be perfectly flat. That can be determined by hitting it with some sand paper and shaving down the high spots. I'm not 100% on what impact this has, but okay, I can probably do that.
2. The heat mass changes it's warmed up temp rotation as time passes, so it's important to calibrate it yourself. That it likely won't be where the OE lines are (which is where mine was placed before I began my latest attempt at starting the car).

Something else I noted was that at the end of the last video (video 3), he could rotate the water jacket freely. That's very interesting! I would imagine that even if the coolant lines were stiff or too short, this could impact the automatic choke's operation. Can anyone comment on this?

Now it's time for me to try and find a rebuild kit so I can possibly see about salvaging my auto choke until I can find my manual conversion, instead of just buying another manual conversion kit.
Jeff Grant

That's why the datum lines are only a starting point in practice - you then have to tweak it to what works best in your conditions. That makes it trial and error, and it did take me several goes on another car with water choke, but I ended up with one that worked as it was supposed to.

Not sure what your comment on video 3 is referring to. When you slacken the 3 screws in the locking ring it allows you to rotate the heat mass and bi-metallic spring which is what you have to do to change how much enrichment is applied with the fast idle. This is the tweaking I refer to above. Once you have tweaked you tighted the three screws again, which locks the heat-mass and spring in that position ready for the next cold start. If it still isn't right, you slacken the three screws, rotate the heat mass again, then tighten the three screws again ready for the next cold start.
Paul Hunt 2

Okay, I think video 3 left out the tightening of the 3 screws so that's why I thought it turned freely.

Do you think my MAPP gas torch is too hot for quenching the aluminum part with steel bolt? I don't have access to a regular propane torch.
Jeff Grant

I took my auto choke off the carb today, leaving the carb installed on the motor.

The fast idle pin does not have a cap on the exterior of the choke body like pictures and the videos showed it should. Instead, the pin is just sitting there. The pin is free flowing, I can move it either way. But if there's no spring on the outside of the choke body to push the pin inward, I don't know how the idle will ever go back down regardless of blipping the throttle.

Can anyone help on this?

Everyhting else seems to be right. My axle was loose so I tightened that up. Nothing else seems odd so far.

I'll definitely do the boiling water test for a new alignment mark calibrated for the heat mass.
Jeff Grant

I took my auto choke off the carb today, leaving the carb installed on the motor.

The fast idle pin does not have a cap on the exterior of the choke body like pictures and the videos showed it should. Instead, the pin is just sitting there. The pin is free flowing, I can move it either way. But if there's no spring on the outside of the choke body to push the pin inward, I don't know how the idle will ever go back down regardless of blipping the throttle.

Can anyone help on this?

Everyhting else seems to be right. My axle was loose so I tightened that up. Nothing else seems odd so far.

I'll definitely do the boiling water test for a new alignment mark calibrated for the heat mass.
Jeff Grant

I was discussing this issue with someone today and I think I figured out the fast idle pin.

Normally, when the cam is in it's "cold" position, the pin sticks out of the housing causing the throttle linkage to be pushed open slightly. Then, as the engine warms up and the heat mass tries rotates, the cam is held in place by the fast idle pin until you blip the throttle. When you blip the throttle, the pressure against the cam is released and it can finally rotate to where the heat mass wants it based on temperature. As the throttle closes from the blip, the linkage pushes the fast idle pin inward.

So this being said, I don't think that I am missing anything on the outside of the choke housing afterall.

Someone please correct me if I'm wrong on this.
Jeff Grant

When you switch off from hot the cam will be in its hot position i.e. a low part under the fast-idle pin. As the car cools, this stops the cam from rotating to its cold position. You have to blip the throttle before a cold start, which lifts the fast-idle pin off the cam, which flips into its cold position, and when you release the throttle the pin now rests on a higher part of the cam to give you the fast idle. Can't speak for missing bits, but you should be able to see if you you have got works OK by switching it off hot and leaving overnight, then next day carefully nore where the *outside* of the fast idle pin is and the lever, blip the throttle which will move the lever off the pin, then see if the pin is sticking out further and hence moving the lever.
Paul Hunt 2

But what causes the fast idle pin to push out further. There is no spring mechanism that I've seen which will push the pin outward when you blip the throttle.

Is it just gravity that causes it to move away from the cam when you press the throttle?
Jeff Grant

As to that I don't know whether it is simply gravity or the pressure of the cam that can now move it when the 'weight' of the throttle butterfly and return spring is removed from it, the important question is *does* it move outwards when the pedal is blipped and does it hold the throttle open for fast idle? Can you move it in and out by hand while the throttle is held open?
Paul Hunt 2

This thread was discussed between 13/04/2008 and 24/04/2008

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now