Archived Message
Idling problem MGB GT V8
The idle on my V8 is generally fine around 500 - 800 rpm but I have recently found that the idle revs start climbing when the engine is hot -particularly when the fans are kicking in. Blipping the accelerator to try and settle things down doesn't work and the revs pick up even more on idle - sometimes as high as 1500rpm. I had the local garage have a look but they couldn't identify the problem ....which of course didn't manifest itself when they drove the car. Does anyone have any suggestions? Thanks in advance.
Thomas Thain
21/08/2012 @  18:28
Under topic: V8
 
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Idling problem MGB GT V8
First point of call would be to check that the throttle mechanism is actually returning to the idle stop. Investigate that the throttle linkage moves smoothly and doesn't bind up. You can take the opportunity to lube everything and grease the throttle cable. If the throttle cable is new they can be a bit sticky at first.

You should also check that the dampers are returning properly and not sticking in the up position.
1) Unbolt the main air box.
2) Unscrew the black cap on top of the carbs and withdraw the piston and spring.
3) Next, undo the three screws holding the top of the dashpot cover to the carb body
4) Withdraw the dashpot top
5) Withdraw the piston CAREFULLY - don't bend the needle!
6) Using a can of carb cleaner give everything a good clean. You can use a finger to remove any stubborn stains but under NO circumstances be tempted to use any form of abrasive material.
7) Spray a bit of carb cleaner inside the carb too to get rid of any deposits.
8) Reassemble in reverse order.

With a bit of luck it's just a bit of solidified gunk holding the piston open ever so slightly. It'll certainly be something relatively simple.
Jon Moulds
22/08/2012 @  05:06
 
Idling problem MGB GT V8
The throttle cable and linkage is worth checking as it may not be moving smoothly and possibly could be sticking in some way which might give rise to the elevated idle speed difficulty you mention. In the second year of my owning my V8 my wife experienced an alarming event when the throttle stuck open. On inspection I found one strand of the multistrand wire sheathed throttle cable BHT1120 had broken and had started to unwind from the other strands and had eventually unwound to such an extent that it jammed in the throttle slide or adjuster BH1059 leaving the throttle wide open! This tip is in fact V8NOTE4 in Volume 1 from July 1979. So I would check that or better still take the throttle cable off and lubricate it with graphite grease. It's also worth reading V8NOTE202 in Volume 5.
Victor Smith
22/08/2012 @  19:49
 
Idling problem MGB GT V8
Thomas,
Worth a check to see if your SU carbs are fitted with the poppet valve in the butterfly. These have a habit of not sealing properly and allow air to pass through the disc. This will cause the intermittent "climbing idle" problems that you have. If you have them fitted, the best solution is to remove the butterfly and solder the valve shut, or change to the earlier type of butterfly. Regards,
Jim Robinson
23/08/2012 @  05:55
 
Idling problem MGB GT V8
With Jim Robertson's help an illustrated note on his suggested check has been uploaded to the V8 Website as V8NOTE457 which should help to identify the valve for fellow members not so familiar with the HIF carburettors. Jim Robinson runs JRV8, a V8 engine specialist in Northern Ireland. JRV8 is in the V8LIFELINE, our listing of specialist MGV8 spares and service providers.
http://www.v8register.net/subpages/websiteupdatelog.htm
V8 Webmaster
23/08/2012 @  22:05
 
Idling problem MGB GT V8
If you haven't cured it yet you might have a look at Note 262. I had this problem and found that the throttle cable did not have enough slack in it and as the temperature rose under the bonnet the cable outer expanded and shortened the cable throw increasing the tickover.
Geoffrey Hall
22/09/2012 @  17:42
 
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