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MG MGB Technical - How to set the choke

Can someone tell me the correct method of setting up the choke mechanism on the HS carbs on my '68 roadster. I have set up the balance/mixture but when the engine is cold it runs poorly (sound like a rhythmic misfire about twice a second). When the engine warms up it runs fine.
Steve Church

Make certain that both jet tubes are moving up and down the correct distance and the same amount. It sounds like one is lower than the other and is running too rich. Also, be sure to check that the needles are properly held in place, by their grub screws. RAY
rjm RAY

Steve, as you probably know the choke is a two stage device ie pull half way to engage the fast idle before the mixture enriching starts by moving the jet down and there's not really much to adjust but here goes..
First you need to do a few checks
1, check the operating cable has about 5mm free travel before it starts to move the choke linkage.
2, make sure the fast idle screws are NOT touching the cam on the carb at the fully closed position.
3, check the linkage is correctly engaged in the cams on each carb and the operating levers from the cam to the jets is correctly assembled.
4, ensure the jets start to move down at the same time as the cable is operated. All that just makes sure everything is installed correctly and to be honest I'd be very surprised if they're not.
The only other adjustment is the fast idle screws which in effect keeps the balance of the carbs off idle. Do these like this.
1, with the engine warm pull the choke cable out to the point just before the jets start to move.
2, balance the carbs by adjusting the screws against the cams (NOT the throttle screws!).
3, once balanced adjust both screws until the engine speed is about 2-2.5k rpm
4, release the choke and ensure the screws remain clear of the cam. If not back them off the same amount until the worst one is just clear.
Job done.

MGmike
M McAndrew

Thanks for the info guys.

Mike, just what I wanted, a step by step guide. Just a couple of questions though. It sounds like first half of choke travel engages the fast idle cams. Then past half way the jets start to move. Is this correct? Your (4) says " ensure the jets start to move down at the same time as the cable is operated". Would seem to contradict this.

Should there be a spring attached to the choke mechanism that effectively closes everything when the choke knob is pushed in? I did find one reference to this (centre spring) but all three springs attached to the heat shield are operating on the throttle.
Steve Church

There is a choke return spring (AEC2075)Item 3 on http://www.mgb-stuff.org.uk/choke.htm#11 Connects to heat shield and a small plate on the linkage

Bernie
B Anderson

Thanks Bernie,
Is this in addition to the three I have attached to the throttle mechanism, do you think? There are certainly four holes at the bottom of the heat shield which would indicate that there should be four springs.
Steve Church

Yes it is, there are 4 in total. 1 each carb end of linkage, one in the middle of carb linkage and one for the choke. They are all the same part number
Bernie
B Anderson

Fast-idle travel of the choke control is only about 1/4", not half-way. Fast idle should be 1100-1200rpm on a hot engine, not 2-2.5k.
PaulH Solihull

I decided to check the jet settings first to make sure they were at least close to each other. Front 0.052 down and rear 0.072 down. Thinks I need to get that right first! No wonder it hunts a bit. I believe I read a thread somewhere about setting the jets a set distance down (rather than 12 flats). CAn someone remember the distance? Also should the jet outer tube be flush with the carb bridge. I think one is a little low and I cannot get it tightened up more. I'm d*mn sure I set them flush when I rebuilt the carbs prior to the engine rebuild.
Steve Church

Steve-
"CAn someone remember the distance?"
>>I start at .065, about 10 flats, and rarely go more than one flat either way.

" Also should the jet outer tube be flush with the carb bridge. I think one is a little low and I cannot get it tightened up more. I'm d*mn sure I set them flush when I rebuilt the carbs prior to the engine rebuild. "
>>This is non adjustable on HS, entirely down to the carb machining. Doesn't matter, as long as they are not above the bridge. (never saw one).

FRM
FR Millmore

I do have one of Peter's fast road heads and BP270 camshaft. Carb is fitted with No.6 needles and using standard filters with cans removed. I'm not sure what effect these mods will have on the stock settings, but I'll start with your suggestions FRM.
Steve Church

Hi Steve

When you come up for tuning we will go through all the settings with you. As long as all works smoothly it is the first 1/4" of choke which needs fine setting, this should just raise the idle to around 1200 rpm when warm before pulling the jets down to richen the mix. It is set with the two screws ( one on each carb) on the throttle cams. The screws are set with a little clearance to the choke cam, pull the choke cable a little until the revs pick up without the choke mechanism pulling the jets down.

Peter
Peter Burgess Tuning

"Front 0.052 down and rear 0.072 down ... No wonder it hunts a bit"

My roadster carbs are near enough identical, my V8 quite a distance apart. Put to the same *that* hunted, whereas it doesn't when adjusted to the lifting pins which puts the jets at different heights. The whole point of using the lifting pins is so that you can take dynamic account of any differences between the carbs that a static measurement won't. How much more you have to adjust one compared to the other also tells you that you do have a difference between the carbs, that may well need further investigation. Again setting the jets with a static measurement won't tell you that.
PaulH Solihull

This thread was discussed between 27/06/2012 and 29/06/2012

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