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MG MGB Technical - Throttle butterflys not sealing?

Hi,

I'm still struggling with the tuning of my '66 GT. I'm trying to set the carbs (twin SU HS4, K&Ns and richer needles) but can't get a nice idle. The car's also underpowered compared to how it used to be.

I've removed the air filters and noticed that even with both idle screws completely clear of the throttle cam the engine still idles around 1000rpm. Surely if both butterflys are firmly shut the engine should stall? How do I go about checking to see if the butterflys are still open with the linkage shut?

Thanks!
Tim.
Tim Jenner

Tim: The "best" way would involve removing the carbs and shining a light through the throat from the engine side and observing the light around the edges of the butterfly.
However, a somewhat easier thing would be to remove the dashpots and pistons from the carbs while still on the engine and look at the butterflys close-up. Also test the fall of the pistons in the dashpot to see if they are binding at all as they fall.
Disconnect the pinch bolt that locks the two throttle shafts together and manipulate the shafts, and look for excessive play. A proper fit of the butterfly in the throat of the carb should be obvious, I think. Another, likely, cause of your problem are badly worn throttle shafts and elongation of the holes in the carb bodies. These can be rebushed, reamed, and new or oversized throttle shafts can be fitted.
It sounds as if you have richened the slow running mixture so much to compensate for air leaks that you are now too rich for normal driving at speed.
Andrew Blackley

Thanks for the reply. I've tried spraying a bit of WD40 onto the ends of the throttle shafts to see if they are leaking, and they don't seem to be. The pistons rise and fall nicely and the dashpots have 20w/50 engine oil in them. I set the mixture using a ColourTune plug, so I think they're roughly correct.

I suppose the next question is how obvious is a proper fit of the butterfly? I haven't taken the carbs off, but I tried shining a torch into the throat with the air filters removed and it looks to the naked eye like the butterflys are both shut. They're obviously not, because covering the throat with my hand stalls the engine! Is the butterfly movement adjustable or is it fixed by the way the butterfly is attached to the throttle shaft?

Cheers,
Tim.
Tim Jenner

Tim: There is some slight adjustment available by slackening the two screws that attach it to the shaft, then move the disk ever so slightly by trial and error for the best fit. The butterfly has a beveled edge, and this should be in contact with the carb body.

Since these little screws are slpit like a cotter pin I would first obtain new ones. They are prone to breakage and should not be reused, IMHO.

This is a carb off the car job tho'. At least with the little beastie in your hand you can get a better "grip" on the problem...
Andrew Blackley

Hmm...carbs off = hard! Well it's going to the garage in a couple of weeks to get the tracking done, I think I'll leave it for them to sort out!

Thanks again,
Tim.
Tim Jenner

Tim, it is not hard. Just get yourself a Haynes or a BMC workshop manual, and follow the steps set out there. Haynes is particularly good for first timers. And remember that there is a large willing battery of help available as close as your keyboard. There are some very talented people on this net, who are excited about helping others and getting others hooked on the marque. Linkages can often be the problem, and if you undo the inter connections between the carbs - slacken off the pinch nuts, it will help you to isolate your problem. With these slack, if it makes no difference to idle speed, check to see if the return springs are the culprits. Lift up on the operating levers attached to the throttle shafts, and check if this makes a difference. It is simply one step at a time. Do get yourself that workshop manual, as you will need it to see what I am trying to explain, and in order to put things back right. Above all, remember it is not a big deal to get into this very basic carb adjustment/tuning. After all this, you could discover it is something else- eg timing, which of course should be checked first in a complete tuning session.
Ken Rich

Well, had another look under the bonnet and discovered the reason why the butterflys weren't sealing - the fast idle (choke) screws were holding the throttle open! I've had a quick play and the idle is much smoother, just a case of setting the mixture properly and balancing the carbs, hopefully I should be all set! Will report back, but thanks for the suggestions so far. But I still subscribe to the theory that most problems on old cars can be fixed by giving them a good wash and wax, they always run better after a bit of TLC...

Cheers,
Tim.
Tim Jenner

This thread was discussed between 05/04/2003 and 06/04/2003

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