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MG MGB Technical - Running hot - stumbling idle

I am finding that properly adjusted S.Us start to run rich when the temperature in the engine compartment becomes excessive. This is a problem in late afternoon city driving when ambient temperatures are above 80°F or so. I believe that the alcohol in the fuel bowls begins to boil and this pumps excess fuel through the jets. Idle becomes rough and it is necessary to maintain a higher idle using the pedal (or first notch of the choke).

My 1970 B has A/C and an oversized Nissan alternator to power the evaporator blower fan, external condenser fan, compressor clutch, halogen headlamp bulbs, Etc.

Airflow across the radiator is compromised since the condenser is mounted in front and at an angle. The radiator is a high capacity adaptation with additional core tubes, apparently to no avail.

I have set up a thermistor to run the electric fan mounted to the condenser to bring the radiator temperature down to 90°C once the engine is turned off or during slow traffic conditions (I run two 12 volt batteries in parallel). This helps clear excess heat from the engine compartment so the car will start and run smoothly at first. The fan is insufficient to move enough air to keep the carburetor fuel bowls cool. I pulled the carburetor piston chambers off, removed the pistons and needles and was able to observe fuel boiling and surging up through the jets following a high ambient temperature/slow traffic run.

There is a tremendous amount of heat transferred by the A/C from inside the car to the condenser and across the radiator so that slow driving engine temperatures run between 85° C and 110°C. The idle gets progressively less stable as engine temperature rises when the car is in slow traffic. Exhaust smells rich although mixture is adjusted to run lean in moderate ambient temperatures.

S.Us were clearly not designed for this type of running condition. The B compartment is tight so that heat rising from the exhaust manifold is available to superheat the carburetor bowls. The MG-C U.S. version had a squirrel cage blower and ducts to direct air over the carburetor bowls and that was prior to the advent of alcohol laced fuels.

Question: Would using a Weber or Mikunis stabilize the hot idle running condition?

Glenn Mallory

You should consider having your exhaust manifold ceramic coated inside and outside.

It is amazing how little heat is emitted.

My ceramic coated manifold is cool enough to touch within a few minutes after a run on a hot day.

As a plus, you will also get better performance from your engine.
Steven Rechter

With the bonnet closed all the heat from the radiator etc. has to go downwards, which is counter to what it wants to do. Very little goes down the tunnel, so the vast majority has to come from under the inner wings, and when stationary in still air it is also coming forwards and being recycled through the rad. RV8s had holes in the inner wings for the exhaust to exit each side, you could try that. But bonnet louvres should make the biggest difference, at the expense of it going into the heater intake if you have that open.

110C is very high, I've measured my V8 in a UK summer at 58C when running, and on switch-off that is enough to boil the fuel in the float chambers, I can hear it. I'm not surprised you get problems with your funny fuels. Do you have HSs? They start to choke up when left idling on my roadster, HIFs with the thermal compensation on the V8 don't seem to suffer so badly. But I did find those getting lumpy when the temp gauge was on the edge of the red zone on one occasion.
paulh4

1) Is this a GT?
2) Create some more air flow into the engine compartment, other than through the rad, either through the radiator diaphragms or by ducting from the valance. You might also consider louvres in the inner flitches. The wheel arches are low pressure areas, but only when moving of course, but will give the fans an easier job of moving air.
3) Also insulate the fuel pipes to the carbs, colder fuel will help.
4) As Steven said above, coating the exhaust manifold makes a great deal of difference to under-bonnet temps, and help keep the float chambers cooler.
5) Check the heat shield hasn't lost it's insulation
6) I have seen insulated float chambers, which is said to have some effect.
7) Doubt whether a Webber would show any improvement, they would be just as susceptible to the heat!
Allan Reeling

Glenn. Living in Florida,I've had a similar issue with my 79. One item to check is vacuum leaks. Check the manifold nits to the cylinder head.

Once we return from holiday, I will wrap the the exhaust manifold and install the insulation spacer under the Weber carburetor.

Also working an ignition issue, but that will be the other story

Cheers

Gary
79 MGB
gary hansen

Gary
If you're still running a cast iron manifold I would caution against wrapping it. Several people have reported cracking after wrapping theirs. Tube manifolds seem all right, my V8 headers have been wrapped for 12 years without problem.
Allan Reeling

Glenn,

Two things come to mind that may assist. First is dynamic timing as Gary suggests. Below (you may have copy & paste) is a short Youtube demonstrated by John Twist at one of his (rolling) Tech Sessions. He also discusses Weber carburetors in that clip.

Second would be a fan shroud as sold by Moss USA (and others) to help pull air through the radiator. Not sure if this will fit on your non-standard radiator, and is a lot of work to install. If you do elect to install the shroud be sure it fits tightly against the radiator (and radiator support). I had to use plastic tie-wraps to pull the bottom flange of the shroud tight against the bottom tank of the radiator.

https://www.youtube.com/watch?v=NdPNzvoveqE

Regards,

Larry C.

Lawrence Cordeiro

The problem seems to be the amount of heat already in the engine bay, rather than the cooling fan not getting enough heat out of the coolant. Also those shrouds significantly reduce flow at slow to moderate speeds.

I fitted a 25% uprated rad to my V8, and was very disappointed with the results. But of course, once three rows have heated up the air passing through, a fourth row (or whatever) isn't going to extract much more. The biggest improvement was to the electric fans. The earth is just a standard gauge, shared with the headlights, going to the back of the engine compartment. Adding a local earth at each fan bracket i.e. in addition to the factory earth made an audible difference to fan speed and hence cooling. A additional improvement was to connect the spare output spade on the alternator direct to the fan relay, again in addition to the factory wire.
paulh4

Gary,

I agree with Allan. Do not install a wrap. It will certainly warp and crack the manifold.

Google "ceramic coated exhaust manifold". Engine compartment temps will be greatly reduced and the carbs will not get heat soaked.

And as I stated above you will also get a slight HP increase as a bonus.
Steven Rechter

If you haven't already, remove the seal across the back of the bonnet (hood) and fit two short bits of it in the two corners only to stabilise the bonnet(hood) when shut.

That will let hot air out at slow speeds and push cold air in at moderate to high speed as in front of the screen is a high pressure area.
Chris at Octarine Services

Steven,Allan: my manifolds are a Piece aluminum intake, and the exhaust is not cast. It's a Peco header.

I also will go with Chris' suggestion of removing the sealing strip at the rear of the bonnet to allow air flow.

Cheers

Gary
79 MGB
gary hansen

Thank you for the comments. I do worry about cylinder head cracking with such variable and high temperatures.

The problem must be solved through better ventilation of the engine compartment. Paul is correct that the heat gets trapped and only reluctantly moves downward when forced out by the electric fan. Since the fan runs with the engine off until the temperature in the radiator reaches 90°C, I can feel the warm air being pushed under the car when I fuel it. Even so, there is a concentration at the carburetor level that is difficult to move.

I have insulated the fuel line and this seems to help. Insulating the fuel bowls (HS4s) did not seem to help and may have kept some of the heat retained. I haven't noticed any insulation provision on the heat shield.

I assume that Mikuni carbs would suffer as much as the S.Us and Weber. Fuel injection makes modern fuels tolerable in newer vehicles. These were not designed for it.

PD: Have tried to upload photos (JPEG) of the crowded under bonnet ware but they do not seem to take. Is there a trick?
Glenn Mallory

I seem to be OK now with electric fan, but was thinking of a fuel return line. With HS4 you could T into the pipe on the jet assembly. Fuel tank connection more challenging you could return to the flexi connection from the filler cap to the tank inside the boot, trunk, to try it out. Your risk and careful it can't get dislodged etc.
If it doesn't work All u are out is one jet and one trunk flexi
Stan Best

Thank you for the comments. I do worry about cylinder head cracking with such variable and high temperatures.

The problem must be solved through better ventilation of the engine compartment. Paul is correct that the heat gets trapped and only reluctantly moves downward when forced out by the electric fan. Since the fan runs with the engine off until the temperature in the radiator reaches 90°C, I can feel the warm air being pushed under the car when I fuel it. Even so, there is a concentration at the carburetor level that is difficult to move.

I have insulated the fuel line and this seems to help. Insulating the fuel bowls (HS4s) did not seem to help and may have kept some of the heat retained. I haven't noticed any insulation provision on the heat shield.

I assume that Mikuni carbs would suffer as much as the S.Us and Weber. Fuel injection makes modern fuels tolerable in newer vehicles. These were not designed for it.

PD: Have tried to upload photos (JPEG) of the crowded under bonnet ware but they do not seem to take. Is there a trick? Maybe reducing them to minimum size?
Glenn Mallory

Stan,

The return line would be to keep the fuel flowing and therefore cooler? This would be in line with modern vehicles that keep fuel flowing tank - injectors - tank and this would help avoid vapor locking.

My B is a U.S. version with the S.U float chambers vented to a charcoal canister. I need to experiment and vent them to the atmosphere (like my 1971 Midget). This may lead to more stable idle if the canister line is not purging properly.

The bowls (and fuel tank) are vented through the canister over to the valve cover. The gases are drawn through the crankcase to the timing cover and from there to the carbs where they are burned along with oil blow by. The vapor line from the fuel tank could be used as a return line.

Glenn Mallory

"The return line"

Would circulate cooler fuel through the lines, but that would only go into the float chambers when the level dropped to open the float valve. At low speeds/idle that would be a tiny volume in comparison to the amount sitting in the float chamber and already hot.

The pump would also chatter away continuously. No different if it were an after-market that chatter all the time anyway, but an SU might not like being operated at that rate all the time.

The canister is purged by the crankcase ventilation system by drawing fresh air into the canister. The float chambers vent by pushing fumes out through the canister, but only when the float chamber is filling. In normal use the float valve should be trickle-refilling the float chamber as fuel is used, so the level should be virtually constant, so no flow in either direction through the float chamber vents. However a blocked canister could cause the crankcase ventilation system to lower the fuel level in the jets, which could cause running problems, although weak (it's how the later anti runon-system worked by blocking the canister and pulling fuel out of the jets altogether) rather than rich. Easy to check - just temporarily disconnect the vent hoses at the carbs and see what happens. There should be no other discernible effect on running.
paulh4

Hi Paul, a return line for the fuel feed only would help. I was thinking a restriction in a return line would be a good idea to keep things quiet and ensure enough fuel is available for the float chambers.
If you restrict the flow then you could draw via the float chamber, possibly by a T piece in a jet assembly, these are HS4s in this car. A solenoid to turn it off when you are out of traffic would be a nice addition.
I think a louvered bonnet will probably solve the problem, there is no point pushing the air in if you dont let it out. however if that doesn't work this might just turn the tide. i was considering this as I dont want to modify the MK 1 bonnet.The Revotec seems to have done the job for me.
Stan Best

Maybe installing a bilge blower fan that you can turn on at low road speeds will help you. They move a serious amount of air, and this is a popular mod in the MGA world where people have the same symptoms of overheating (albeit with different root causes).
Dominic Clancy

This thread was discussed between 13/05/2017 and 17/05/2017

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