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Old 20-12-2004, 15:32   #1
Mike1969
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MGB Idle Problem

I am at wits end. I can not make my MGB (1969 GT) idle below 2000 RPM.
It started suddenly. I thought it was just a dirty float valve so I pulled the SU's down and cleaned them and put them back. No change.
Idle screws are backed all the way out.
Choke fast idle screws are not touching.
There is clearance on the throtle cable lever next to the choke shaft.
Engine runs great on the road, just won't idle.
Any thoughts as to what to look at next?
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Old 20-12-2004, 15:40   #2
E_T_V
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seized throttle butterfly? Carb icing?
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Old 20-12-2004, 15:42   #3
Mike1969
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Not cold enough for icing, but I will pull down the piston and watch the butterfly valves to be certain they are moving and shutting tight.

Thanks for the quick reply.
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Old 20-12-2004, 15:50   #4
Richard Moss
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It's worth checking that the piston isn't sticking - undo the 3 screws holding the chamber on, remove, clean with petrol/white spirit/carb cleaner and reassemble. Be careful with the needle and don't forget to refill the dashpot with oil (ATF - automatic transmission fluid is good).

It could also be an air leak - try squirting WD40 all over the carbs, their mountings and the inlet manifold. If the exhaust turns white then the WD40 is getting in somewhere. Also, check that the breather hoses from the crankcase to the carbs are not spilt and the same goes for the servo hose (if you have a servo - after all it was only an option that year).

Last edited by Richard Moss; 20-12-2004 at 15:55.
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Old 20-12-2004, 15:59   #5
Mike1969
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Cleaned and checked the pistons yesterday - not sticking.
Replaced dash pot oil.

Not sure what you are calling a servo. Is this the powerbrake servo?
Don't have the power brakes if this is the case.

Will try the WD-40. Thanks
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Old 20-12-2004, 16:10   #6
Richard Moss
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Quote:
Originally Posted by Mike1969
Cleaned and checked the pistons yesterday - not sticking.
Replaced dash pot oil.

Not sure what you are calling a servo. Is this the powerbrake servo?
Don't have the power brakes if this is the case.

Will try the WD-40. Thanks
yes, I meant brake booster servo.

Another thought - has the ignition timing been disturbed (advanced) and is your vacuum advance pipe for the distributor conencted and in good condition?

Lastly, a mate of mine had an MGB about the same age as yours. He stripped the carbs down to clean them up and when he reassembled them, the misfitted the linkage from the choke mechanism to the jet tube. This caused the jets to always be opened and as a result we could not get the idle speed down.

have a look into the mouth of the carb - with the choke off the top of the jet tube should only be a little bit below the level of the carb body. You should also be able to screw the mixture adjusting nut all the way up - putting the top of the jet tube level with the carb body. If you can't do this the jets are sitting too low.
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Old 20-12-2004, 16:19   #7
Mike1969
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Will check the timing with a light tonight. Never had timing effect idle before. As far as I know the distributor is functional. At speed the timing feels correct.

Looked at the jet seat position yesterday and they were in the correct location - the choke is not stuck.

I am not a novice with this MGB, I bought it new 300,000 miles ago.
I do appreciate your ideas and thoughts.
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Old 20-12-2004, 16:23   #8
Richard Moss
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It just has to be excess air going through the carb - and as it requires this air to draw the fuel through the only thing I can think of is stuck piston or butterfly.

A combination of air leak and over rich mixture could do it, too.
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Old 20-12-2004, 16:30   #9
Mike1969
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I agree with the excess air.
It must be a butterfly that is not closing as far as it should.
That is one thing I have not checked yet.
If the butterfly was completely stuck, the engine would not rev up as easily as it does.
I will have to investigate as soon as I have time.

Again, thanks for the quick response to my problem.
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Old 20-12-2004, 22:09   #10
Richard Moss
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Do the butterflies on your carbs have those silly little spring loaded valves on the plates? I don't imagine that they do, what with the age and all, but if they do then I would just take them out and replace them with little bolts or rivets to block the holes.
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Old 21-12-2004, 12:41   #11
Mike1969
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The butterflies are solid disks.

On the way home from work yesterday, the idle came back.
I had to adjust the idle screws back to where they belonged so the engine would not die.
I may never know what was sticking, but for the moment all is well.
I will check the butterflies the first chance I get.
Thanks to everyone for their ideas and help!
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Old 21-12-2004, 12:49   #12
Richard Moss
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Glad to hear that it is sorted out.

Now I have to work out why my MGC keeps overheating it's ignition coil. It has a new coil but it still gets hot and becomes hard to start and loses power. Swapping to the old coil cures the problem - until that warms up.

I suspect that it is a problem with my aftermarket electronic ignition kit (now 15 years old) - so i may go back to the original breaker set up and see what happens.
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Old 21-12-2004, 12:53   #13
E_T_V
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If the car has been stood for a while it could be gum or varnish on the throttle butterfly (from evaporated fuel). If the problem ever returns try blasting some choke/carb cleaner around the butterfly area and pivot to see if that helps.. glad you're motoring well, just in time for xmas.
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Old 21-12-2004, 13:02   #14
Mike1969
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My MG does not sit.
I truly believe driving it keeps it young.
I drive it to work everyday.
It is famous here at the plant.
It has more hours at work than all the people that currently work here except me.
I have been driving it to the same job location for 31 years.
I will use the cleaner the first chance I get.
Thanks for your help.
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Old 21-12-2004, 13:31   #15
E_T_V
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Glad to hear it gets regularly driven. Also this might amuse you if you've not seen it already. (It originated in the US but was posted on a forum in teh UK that I frequent.

*to the tune of jingle bells*

Dashing through the snow
In my rusty MGB
Down the road I go
Sliding past a tree
I need new piston rings
I need some new snow tires
My car is held together
By a piece of chicken wire


Oh, rust and smoke
The heater's broke
The door just blew away
I light a match to see the dash
And then I start to pray...
The frame is bent
The muffler went
The shifter just broke free
Oh, what fun it is to drive
My rusty MGB


I went to IGA
To get some Christmas cheer
I just patched up my left front tire
And it's gettin' hard to steer
Speeding down the highway
Right past the county cops
I have to drag my swampers
Just to get the car to stop


Oh, rust and smoke
The heater's broke
The door just blew away
I light a match to see the dash
And then I start to pray...
The frame is bent
The muffler went
The shifter just broke free
Oh, what fun it is to drive
My rusty MGB



Bouncing through the snow drifts
In a big, blue cloud of smoke
People laugh as I drive by
I wonder what's the joke
I have to get to Wal-Mart
To pick up my layaway
'Cause Santa's comin' soon
In his big, old, rusty sleigh


Oh, rust and smoke
The heater's broke
The door just blew away
I light a match to see the dash
And then I start to pray...
The frame is bent
The muffler went
The shifter just broke free
Oh, what fun it is to drive
My rusty MGB
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Old 21-12-2004, 13:39   #16
Mike1969
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Thanks for the song.
It is great.
The part about seeing the dash is too true.

Merry Christmas
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Old 21-12-2004, 14:09   #17
Gra
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I shouldn't laugh, but that's a very good song and I haven't heard it before!
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Old 21-12-2004, 15:01   #18
E_T_V
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Heck if you own and regularly drive a classic (or a nearly but not quite classic like my maestros) then you have to have a good sense of humour in order to survive!

Like the fact that that my heater has packed up (due to a blown headgasket) at the coldest time of year. If my fingers weren't so numb then I'd have to laugh at its comic timing.
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Old 21-12-2004, 15:17   #19
Mike1969
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I understand the lack of a heater problem.

Last week when I pressed the heater blower switch, nothing happened.
I flipped it off and on several times and no joy.
I left it on and used the towel I carry to help with the window defrosting.
(Doesn't every MGB driver have a towel handy?)
A couple of days later, the heater blower came on.
Now I will not turn the switch off until spring.

The heater switch for the 1969 is one of the few parts I can't get for the car from my usual parts source.
I am thinking of buying a headlight switch and using only half the contacts.
I can put a little black paint on the headlight switch front to cover up the lamp symbol. At this point, I know where all the switches are.

Driving the MB is a fustration and a joy. You must be a mechanic to own and drive one.
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Old 21-12-2004, 15:27   #20
Richard Moss
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Mike - is that heater switch the silver toggle switch or is it a plastic rocker switch (for US safety regulations)? If it is the lformer it should be available from any of the MG specialists this side of the Atlantic but if it is the latter then it may be more tricky.

One supplier who I have found to be VERY helpful is the MGB Hive in Cambridgeshire. Even if they can't help, they are quite happy to point you towards someone who can.

http://www.mgbhive.co.uk/
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