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MG MGA - Problem when temprerature up

On the way home after a 30 mile drive today, I hit the dreaded north circular at rush hour in the warm sunshine. Sitting in heavy traffic, my temperature gauge went up from 190 to just over 212. What worried me was once the temperature was up, the idling, which is a normal steady 1,100 rpm starting alternating between 500 and 1500 rpm, until it eventually stalled. It continued like this after restarting the car and moving along very slowly in traffic in 1st and 2nd gear was what I can only describe as "bunny hopping".
Eventually, I was able to escape down a side turning and within four or five minutes the temperature was back down to 190 and the engine was thankfully back to normal. Then I hit more traffic and the exact same pattern repeated.

My car is a standard set-up. Would a Kenlowe fan solve the problem do you think? Is the problem from fuel vapourisation? I should add that I do not have the hose intake behind the radiator grill to cool the carbs as I cant seem to get the grill (Mk2) off to fit it.
Thanks, Graham
Graham M V

Hi Graham
What will your car do when it gets hot in summertime???

I would first ensure my radiator was up to scratch, mine was recored by a rad specialist some almost 10 years ago and it was money well spent!

If you are really confident about your raiator then an electric fan will help when in heavy slow moving traffic. My car struggles if the ambient temp is above 34 degrees and car idling in traffic. The temperature would rise to about 210 and I would also experience a bit of erratic idle. Not often we get 34 degrees but have fitted a secondary electric fan should those conditions ever occur (again!)
Robert (Bob) Midget Turbo

Hot carbs by the sound of it Graham. I had the same thing in the Cambridge rush-hour a couple of weeks ago. Stuck in near stationary traffic for at least half an hour (electric fan only), the temperature stayed at around 190 for quite a long time but then started to creap up to just north of 212. The tickover, normally 750rpm, started to oscillate but not as violently as yours. Once moving again I had a bit of a mis-fire, but it quickly sorted itself out and the temperature dropped to normal again. When I got to my destination, I lifted the bonnet and nearly burnt my hand on the stay! I don't think MGAs like to be stationary. The heat seems to get stuck under the bonnet when the car is not going forward and the radiator fan just keeps adding to it. If you could hear your fuel pump I think it would sound like it was doing a tango!
Lindsay Sampford

Graham - Exactly the same behaviour as mine when it gets hot and mine has the hose leading to the carbs. i think the vapourisation is more to do with the fuel we are getting now than anything else ( 5% ethanol with 10% to come)- I reckon an electric fan might help and I am considering the Revotec - just waiting for feedback from a friend who has just fitted one to his MGA -
Cam Cunningham

I have experienced this problem also. (1962 MK-II) I am convinced it is fuel vaporizing. We are stuck with 10% ethanol mix here. I had my exaust manifold ceramic coated inside and out. That helped a little, but not enough. Then I wrapped my float bowls and fuel lines with the tape used for wrapping headers. I put the tape under and around the bowls several layers thick. I used wire ties over the tape to keep it from comming loose if the fuel enviroment loosened the glue. That has been a great improvement. It will still start to sputter if I am stopped in traffic long enough but it takes a long time for it to happen. I have considered adding an electric fan, but am not convinced it would help this problem. The best solution seems to be using enough fuel to keep cool fuel comming into the carbs. I have not cured the problem, just given myself more time before it happens.
Ed Bell

Cam, I don't think adding an extra electric fan would help. It seems to me that the problem lies with hot air not getting OUT of the engine compartment. Maybe an extractor fan is the answer! Louvres in the bonnet would probably do it. The Morgan has louvres in the top as well as the sides of the bonnet, so I dare say that car would suffer the same way as an MGA if it didn't have them.
Lindsay Sampford

Sounds like vapor lock. I've had it many times in extreme heat, such as the Key West Parade. I've often used Barney's advice and richened (cooled?) the mixture by pulling out the choke to minimize the effects. Also, during hot weather I've wrapped my carb heat shields in aluminum foil (original heat shields in the car).

Also, I've added a pusher fan for better air flow and rev'ing the engine to force more new petrol into the floats also helps.

- Ken
Ken Doris

I had all these problems and it took a lot of the fun out of driving it in town. I tried some of the above ideas and others besides.The easily reversed modifications (image) coupled with a manually operated electric supplementary fan solved them and allows me to use it as a daily driver. (spare bonnet-original in the shed.)


John McMaster

Okay, it's summer again in the north, so it's time to let my cat out of the bag on this subject. I hope you all don't mind my rambling.

I have known for a long time that this bad running condition with hot carburetors is caused by the 10% alcohol content of modern fuel boiling in the carburetors. Vapor bubbles contain far less fuel than liquid fuel, so the engine goes lean when the fuel bubbles. Pulling out the choke enriches the mixture to restore decent running until the overheated conditions have passed. I have been doing that for years when stuck in stop and stop traffic, or when trying to restart five minutes after shut down on a hot day.

Furthermore I have always figured that the hot carb problem has three sources of heat contribution. The source mentioned most often is hot air from the radiator. However, with coolant temperature limited to about 220dF I figure that the air exiting from the radiator is not high enough temperature to cause this problem. In fact the warm air from the radiator may actually be cooling the carburetors which are hotter than the warm air at the critical time.

The second heat source is the exhaust manifold heating the intake manifold by convection and radiant energy. The hot intake manifold in turn heats the throttle bodies by conduction. This is reduced somewhat by the thermal spacer between the intake manifold and carb. Many years back I doubled up on the thermal spacer blocks, seeming to do a little bit of good, but definitely not curing the problem. Several years ago Gregory Brown fabricated a fairly simple sheet metal shield to place between the exhaust and intake manifolds. I never got around to trying this one, but is is reported to do a lot of good (possibly even curing the problem). For details see here: http://mgaguru.com/mgtech/carbs/cb202.htm

The third source of heating is the exhaust manifold heating the heat shield mostly by radiant energy. The heat shield in turn may get hot enough to heat the carb float chambers also by radiant energy. Original insulation on the heat shield on the inboard surface in the vicinity of the float chambers is supposed to reduce this avenue of heat transmission, but apparently it is not sufficient to cope with modern fuel containing alcohol. There is no insulation in the center of the heat shield, so the center branch of the exhaust manifold can heat the metal there rather a lot, and heat can then travel along the heat shield by conduction.

About five years ago I got some information from Patrick Mulholland in Australia about some special heat shield material he added to his original carburetor heat shield. He claimed at the time that this has been completely banished the hot carburetor issue. Details of his modification can be found here: http://mgaguru.com/mgtech/carbs/cb203.htm

Subsequently Patrick sent me a piece of the same heat shield material (which languished on my desk for some time). Last January I had the carburetors off while replacing engine mounts, and I took the opportunity to reattach loose ceramic pads on the carburetor heat shield. While I was at it I wanted to install the new "magic" insulation on the heat shield for a personal test. I went a step farther than Patrick and installed insulation on both sides of the original heat shield, and not just at the ends but all over it and extending somewhat above and below the original plate. This installation is documented here: http://mgaguru.com/mgtech/carbs/cb204.htm

By now I have finally had the opportunity to drive for a few hours at a time in 90dF weather, some at expressway speed, some puttering around town at slow speed in lower gears, and some standing still idling for prolonged periods. So far, even when coolant temperature may be 220dF, it always runs and idles fine and dandy, and I haven't touched the choke knob once in hot weather. I am beginning to be real optimistic about this mod, still waiting for a some scorcher day when we get stuck in really hot and slow traffic conditions, but I already know it has made a huge improvement.
Barney Gaylord

Hi Graham
As you know my MK2 has a Kenlowe Fan as well as the standard yellow mechanical fan- I also have the vent pipe from the Grill to just before the carbs
My guage generaly reads 175 on normal running and rises to 190 on long fast 70 mph runs - but I get similar symptoms to you when idling in traffic for some time guage goes up to 200+ and the kenlowe cuts in but only dropping the temp marginally idle is then a bit lumpy and slight misfire until able to give the old girl a blast which seems to clear her-Seems par for the course with these 60year old style motors that lack todays electronic gizmos to keep everything normal
Paul
P D Camp

Has anyone tried using two of the stock heat shields? I have considered adding a second one seperated from the original by the thermal spacer.
Ed Bell

All very interesting. I had thought an electric fan would be the solution but it appears not.
Has anyone fitted the radiator shroud (available from Moss)? I wonder if this would help but am guessing it will do no better than an electric fan?

www.moss-europe.co.uk/Shop/ViewProducts.aspx?PlateIndexID=20068&SortOrder=1
Graham M V

I've been running a radiator shroud since 1997. It is a nice improvement for puttering around at low speed or idling in hot weather. The fan shroud is essentially a vacuum box. It pulls air through the entire surface of the radiator rather than just through a circle in the center. Above 30 mph I'm pretty sure the fan shroud makes no difference one way or the other, and in particular it does not impede air flow at road speed. On the flip side, the fan shroud has no effect on hot carburetors and boiling fuel.
Barney Gaylord

I tried a fan shroud when I first had this problem, (Fuel vaporizing), but I couldn't see any improvement. I finally took it off as I didn't like the increased fan noise it created.
Ed Bell

This thread was discussed between 14/06/2011 and 15/06/2011

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