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MG MGB Technical - Carb icing??

I run a 72 Roadster as a daily driver. During the winter months (not necessarily freezing) I observe the following effect. My journey to the salt mine is about 8 miles, the middle section of which is generally at about 60mph at a steady cruise. On approaching the ensuing roundabout I ease off the accelerator but the revs want to stay up! Blipping the pedal in neutral eventually gets me back down to idle. Carbs are HIF4's with K&N pancakes (Don't recall the problem with the original air filters). Any ideas?

Thanks in anticipation,

Keith
Keith Yates

I had a VW once with a carb icing problem. When it happened, you could actually see frost on the outside of the carb body. (Interesting coincidence - the VW had a Zenith carb.)

I don't know if SU's would do the same, but I suspect so. Even if they don't frost, you would notice by touch that they're remarkably cold. Since you only drive eight miles, the motor probably isn't warmed up, so the carb bodies aren't that warm. I think icing is a possibility.

If that's not the case, a dirty cable will stick in cold weather. The grease and mud inside can get thick and sticky. The accelerator cable is so easy to remove and replace it would be worth taking it out and cleaning it.

Brake parts cleaner with a little tube on the nozzle is good for cleaning out the housing. If you put a rag under the housing when you spray it you can use that to wipe down the cable itself part. Oil gently with WD-40 or light oil. Make sure you don't have any burrs on the cable. They'll hang up inside the housing.


Hope this helps.

Matt K.
Matt Kulka

Here in Canada there was a factory buletin that was issued to prevent carbs from icing . It requires to swap the side tap covers.
As you know ,the front cover has pipe that is connected to a hose that connects to the carbs.
That pipe is exposed at the front to the cold air incoming and pushed by the fan.This causes the vapours to enter the carbs at very low temp causing micro particles of ice to form. If you swap the covers,the exit pipe will be at the back of the engine which is much warmer , so it will avoid icing.
When you swap the covers you will have to bend the pipe towards the back ,along the top of the cover and add an extension to it (weld it) and then turn the pipe up next to the exhaust manifold . You will then connect the hoses that go to the carbs , and voila.
Jose Santos

There are 2 types of carb icing both of which lead to loss of power or complete shutdown AFAIK. I have not heard of icing where the rpm stays high.
Daniel

Could be the throttle shaft butterfly is binding up at given heat level causing some expansion of the metal. If you've done any work on the carb recently you might check that the butterfly was centered properly.
Bob Ritter
Ribert Ritter

I had a serious icing event once in northern Minnesota in December. I had 30 minutes to make a 45 minute drive so I was flying down the highway wide open (79 F100 V8). All was well untill I came to my exit. When I hit the clutch to downshift the dang thing went to redline and beyond. I have never had such an adrenaline rush, it was snowing at the time and getting slick. I quickly put it back in gear and lugged it up to the stop sign and shut it off. My tiny throttle return spring was intact but the carb was extremely frosted over. As I was pondering what to do, the engine heat made its way up to the carb and the rest of the journey was uneventful.
I fixed the problem by getting a factory air cleaner with a provision for ducting hot air to the carb in cold weather, and installed a healthy spring on the linkage pulling in the opposite direction of the throttle cable - any where else will accelerate bushing wear.

This event is what kept me from going to a weber carb setup - no provision for warming the air.
Carl Holm

This thread was discussed between 14/01/2002 and 15/01/2002

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