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MG MGB Technical - Choke setting

I recently reset the HS4 carbs with Colourtune on my 71 B, with the choke disconnected, as usual. This time I followed the advice to reconnect the choke such that the jets start to move after 1/4" or so of cable travel, with some considerable take-up of the choke return springs (I have previously just reconnecte the cable to take up slack). This results in very rich running, with sooty plugs and bad hot starting.

For some reason, therefore, the carbs seem to be over-choked, even when there is no apparent movement of the jets. What have I done wrong? Was I right before and wrong now? Should amateurs be allowed to touch such delicate flowers?

Help, please

Tony
Tony Bridgewater

Tony

I suggest you disconnect the cable and see if it cures it

If not , there is a very nasty little thing that can happen - the spring which is wrapped round the choke shaft can slip off so that instead of holding the jet UP it pushes it DOWN

with the cable disconnected, try pushing up on each jet - you should not be able to achieve any movement

if you can get significant movt then this is what has happened and the only practicable thing to do is to take the carb off and do the very fiddly job of putting the spring back as it should be

what usually causes it is that the brass nut which holds the jet operating apparatus to the carb body unscrews itself allowing the spring to come free

Chris 0151 342 3175
chris

Another, odd, item to look out for (tho it is seldom a culprit) is for the choke linkage attachment on the body of the jet (the plastic base of the jet) to have broken, possibly crumbling away from overzealous screw tightening, and allowing the jet to float freely in its holder - not being drawn back up by spring pressure as it should when the cable is released.

This is probably not your problem, but I post it every so often as it does happen and those with similar problems to yours may benefit from being aware to look for this too.
Bob Muenchausen

How did you set the mixture with a Colourtune? You either need two or keep moving it from cylinder to cylinder because the carbs are interdependant. This means that when adjusting one the other will need a smaller compensating adjustment and so on. You can't just adjust one, then the other, then finish.
Paul Hunt

Thanks for advice. Yes, Paul, I should be a bit more patient with Colourtune. I sense you don't approve or it. I have never found the lifting pin method easy to use, perhaps because I don't have have a rev counter meter, and I find with the overall noise level impossible to judge small changes in revs.

I think I shall go back to my previous setting, whereby I simply took up the slack in the choke cable when I was satisfied with tuning. I alwaya got good plug colour this way, no hot start problems, and reasonable cold starting.

Repeat after me, "Leave well alone"

Tony
Tony Bridgewater

Not so much "don't approve" as "never liked it much on a single carb and don't see how one can be used on twin carbs" - V8 excepted as that has completely independant inlet manifolding. Still use lifting pins for balance and a gunsons for CO on that, though.
Paul Hunt

This thread was discussed between 14/09/2002 and 17/09/2002

MG MGB Technical index

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