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MG MGB Technical - Running and Idling Hot

I have a rebuilt 1979 MGB. Engine rebuilt with a new head, header, HS4 conversion, cam and electric puller fan. It has about 250 miles on the rebuild. I recently installed an insulation kit from Moss on the underside of the hood. I started running it after the winter a few weeks ago. I've experienced the following the last two of the three times I drove it. The car runs great until it heats up and is allowed to idle for more that a few minutes. It then runs rough and will die if I don't give it gas. Today I manually ran the fan so it didn't get to the auto turn on point. It didn't die. When I opened the hood I noticed that the coil was very hot. In the past when it dies the fuel filter is full. Today it was passing gas and I could see the fuel going through in spurts.

I rewired the car and keep the resistor wire to the coil. I think I need to check that the coil is a ballasted coil. Any other ideas?
Dick Field


Sounds like it could be fuel vaporisation.
Check the obvious;
The carb heat shield is in tact, the bulkhead fuel pipe shield is in place. The insulation you installed could be preventing the bonnet/hood being used as a heat sink, depends on the outside ambient temp.
If you have a ballast system but a 12 volt, 3 ohm coil, the spark wouldn't be good but don't think that would overheat the coil. They do get hot, especially when ambient temp is also up.
Allan Reeling

I've never understood how vapourisation can cause a problem with SUs, with the possible exception of in the exposed jet pipe if the heat shielding is faulty. With the pump applying a steady pressure to the fuel line 99% of the time, if the level in the float chamber drops the float will drop and open the float valve. Any air or vapour anywhere in the lines will be pushed through that and out of the overflow, to be replaced eventually by liquid fuel. The fuel would have to be vaporising at more than one Imperial pint per minute to defeat that, and would be accompanied by much chattering from the pump.

Given what you observed in the filter you need to do a delivery check. Remove the supply pipe from the front carb - be aware that it will spurt if the ignition has been on recently - and direct it into a container. Turn on the ignition and it should deliver a minimum of one Imperial pint per minute and in practice more than double that, in a continuous series of pulses with minimal bubbling.

HS carbs will always slowly choke up if left idling, especially in warm weather, which is why the setting-up instructions say to run the engine at a fast idle for a few seconds every couple of minutes or so, to clear it. HIFs suffer less, if at all, and are what a 79 should have, FWIW.

Coils will always run hotter than the surrounding air. From my measurements in an ambient of 10C mine were about 40C which is barely warm to the touch. But in ambients of 27C the ballasted coil was 62C and the unballasted 67C which is almost too hot to touch. Ballasted coils will always run cooler than unballasted as only half the energy is being dissipated in the coil, the other half is in the ballast resistance. Having the fan running will almost certainly make the engine compartment and hence the coil hotter than without.

However my measurements were using points, electronic systems will usually be different. This can range from being HIGHER with some after-market systems which use a fixed dwell higher than for points. But variable dwell systems such as the original 45DM4 system used on later North American cars will run much cooler under anything except maximum revs, as the dwell is only a fraction of fixed dwell system, especially at idle.

You do need to check your ballast and coil and make sure they are matched. As Allen says a 12v coil in series with a ballast will run cooler than normal, but you will be getting a weak spark. But a 6v coil intended to be used with a ballast, but without it, will run very much hotter than it should. A 12v coil measures from 2.5 to 3 ohms, and a 6v coil from 1.4 to 1.5 ohms, with the ballast measuring a similar amount. But voltage is a better indication, with the supply wire(s) connected to the coil +ve, and an earth on the coil -ve (i.e. ignition on but engine not running), a ballasted system should show about 6v on the coil +ve. If it shows 12v the ballast has been bypassed, and anything much more than 6v could mean it is partially short-circuit. However a 12v coil in series with a good ballast will show about 8v on the coil +ve, which is why you need to measure the coil resistance as well, with all wires off the terminals.
P Hunt

Paul, Interesting you mention the jet pipe. The only time i had this same problem as Dick, what I maintain was vaporisation, was a B with HS4's and an iffy heat shield, but always idling in traffic on hot days. As Dick says it cleared if I "give it gas ". In other words upped the fuel flow and introduced some cooler fuel to the carbs. I cured the problem by improving the heat shield and, as a belt and braces effort, insulated the bulkhead fuel pipe, hence came to the conclusion that the problem was heat related, either rising heat from the manifold or "exiting" heat through the transmission tunnel, or both, especially the latter when the fan was running.
Allan Reeling

Allan and Paul,

Thanks, I'll work this this after noon. The heat shield under the carbs is in good shape. It was installed when I did the carb conversion. The bulkhead might be another problem entirely.

Paul - I'll make the measurements you suggested and see where it goes.

Dick
Dick Field

Just had another "theory", could be the heat expands the fuel in the line which builds up enough pressure to overwhelm the float chamber jets..............flooding! Giving it gas clears it.
Allan Reeling

I found one problem today. The coil is a 6v coil by markings and the resistance is ~ 1.4 ohms. The voltage into the coil is 12v. I installed an Advance Auto-Wire system when I rebuilt the car. I didn't find a ballast resistor in the old wiring harness but there was a resistive wire which built into the new system. Obviously this wasn't enough. The 79s specify a 6v coil. I have two questions:
1. Why do the 741/2-80 cars have a 6v vs 12v coil? Does it have to do with the electronic ignition system? The car has the original C.E.I. system with the external amplifier.
2. The wiring diagram that I have ( Advanced Auto-Wire system) shows a ballast (if required) in the power lead from the fuse panel to the coil. What size ballast is required?

Allan - I don't have any heat shield material on the bulkhead. Any chance you'd send me a picture?

Dick
Dick Field

Dick,
The "resistor wire" IS the resistor/ballast. It should be pink, but confusingly the ends aren't. Being white at the fuse box and white/light green at the coil. The pink resistor wire goes across the slam panel loom and back again. The reason for installing this system was to compensate for some cold weather starting issues. i.e., Drop in battery voltage when cold; the extra drain caused by the starter having to turn an engine with cold thicker oil; maybe because the 12 volt batteries of that era didn't have the CCC of the twin 6 volts.
The difference was firing available battery voltage, maybe 7 or 8 volts, to the starter AND a 12volt coil produced a very weak spark, whereas using a 6 volt coil gave a much healthier spark. But the coil only got the available wack when starting. As soon as the key was returned to "run" the feed to the coil ran through the resistor wire effectively reducing the feed voltage to 6. There should be 2 W/Lg wires at your coil +. The full battery voltage was supplied from a spade on the stater solenoid W/Lg again. On later B's the ballast wire was fed by a White and brown off the fuse box.
Dick the bulkhead heat shielding was a piece angled, painted steel which was screwed to the bulkhead with the heater box screws and the fuel pipe tucked behind it. Haven't got a picture, sorry, my V8's don't use it. It's easy enough to insulate anyway.
Allan Reeling

Allan,

Thanks, since I've rewired the car already. I guess I'll go with a ballast resistor. I just need to know an approximate resistance to purchase. Any idea of the appropriate resistance level. The ballast resistors seem to be readily available at the auto stores.

Dick
Dick Field

"The voltage into the coil is 12v."

How did you measure this? The coil has to be carrying current i.e. with an earth on the -ve in order to see the true voltage on the coil +ve. If the coil isn't carrying current then you WILL see 12v on correctly ballasted wiring.

If you did do that and still saw 12v, then your ballast has been bypassed, the wiring should be as Allen describes. If you can't find out where it has been bypassed then as mentioned earlier you need a 1.5 ohm ballast resistance, between the fusebox white/brown (probably on a 79) and the coil +ve. However that will disable the coil boost function, so you would need to run another wire from the coil +ve down to the boost terminal on the solenoid. Originally this was smaller than a standard spade, but rebuilt units seem to have both the same size in my experience. It's a bit mucky but I'm attaching a picture of the boost solenoid. A is the battery cable stud, B is the large spade that used to be for the brown wires. C is the standard-sized spade for the solenoid operate wire (white/brown), and D the small spade for the boost circuit - present unused on CB cars, one white/light-green wire on RB cars. You will have to remove any white/light-green wires from the boost terminal and the coil +ve or they will be bypassing your new ballast resistance. As belt and braces you should locate the white/brown that should be feeding the harness ballast and disconnect it in case it is damaged and shorts to earth at some point. You should have three white/browns on the front of the second fuse up - one is from the ignition relay, one is to the coil ballast, and the third is to an in-line fuse feeding the cooling fan switch. Two will be in one spade connector - probably the first two, but you will have to check which circuit or circuits go dead when you pull the single white/brown off the fusebox. If I'm right then short of cutting off the harness tape you have a 50:50 of chopping the right wire and leaving the spade on it!

Having said all that the coil boost (used on all RB cars regardless of points or early or late electronic ignition) only makes a difference under very adverse conditions. I only discovered recently that my V8 had been running without boost for many years - parked outside and used daily in freezing conditions in winter - as the contact inside the solenoid was bent over (from the rebuilder) and never received 12v on cranking. I'd never noticed.




paulh4

Paul and Allan,

Thanks. Paul was correct. I measured the voltage at Ve+ without the engine running so there was no load on the system. V=IR still applies. I measured from the Ve+ to a ground point on the body of the car. I remeasured with the car running and got a little bit of confusing data. If I measure across the coil Ve+ to Ve- on the coil, I get ~ 3.6 - 4.2 volts. If I measure Ve+ to the ground point on the body I get 10 volts. This tells me that when I grafted in the old resistance wire, I probably got only a portion of the wire ( the basic wire run is new from the Advanced Auto-Wire kit and was just standard copper wire).

I'm not sure what the Ve+ to Ve- on the coil means.

I think I have two options.

1. Pull out the wire I ran from the new fuse panel to Ve+ and install a new wire with the ballast resistor and keep the 6v coil. I already have a wire run from the starter solenoid to Ve+ on the coil.

2. Pull the wire I ran from the new fuse panel and just use a 12v coil. I don't know how this would effect the electronic ignition.

Thoughts, please and the voltages measured above (i.e. the Ve+ to Ve- on the coil vs the Ve+ to body ground.

Dick

Dick Field

"I remeasured with the car running and got a little bit of confusing data. "

Still wrong. This test needs to be done with the ignition on but the engine stopped, and an earth on the coil -ve.

With the engine running the current through the system is being turned on and off by the points or other trigger, and so the voltage across the coil and the ballast resistance will be going up and down in time with each cylinder firing, and hence difficult to work out what is happening.

Without knowing exactly what you have it is difficult to give detailed advice, but the Autowire Powerblock instructions for 74-80 MGBs show the ignition feed coming from W12, which then goes via an external ballast resistance (if required) to the coil +ve. If you have the original harness then W12 should be connected to the built-in resistance wire which is fed by the white/brown. But if you have an alternative harness, or the wiring that came with the Powerblock on W12 has been taken straight to the coil +ve, then you have bypassed any ballast that may bge present elsewhere. It doesn't help that the Powerblock diagram shows a white feeding power to the coil +ve after the ballast, and a white/brown going to the bypass terminal on the solenoid. Both those differ to the original harness.

Strictly speaking you need to buy and mount a ballast resistance of the correct value, connect in series with the feed wire and the coil +ve, and take another wire down from the coil +ve to the boost terminal on the solenoid. But unless you are comfortable doing all that it'll probably be easiest to just go with a 12v coil.
paulh4

Paul,

Thanks.

Dick
Dick Field

My B and the midget become very grumpy when ambient temperatures reach 90 F. The alcohol in U.S. fuels causes vapor locking and makes the S.U and Zenith carbs especially prone to rough or difficult idle conditions over 90F. The alcohol has a low boiling point, to the degree that I have heard it boiling in the tank when the car is parked over extremely hot pavement.

I recently made a 3,000 mile trip into the interior of Mexico and despite relatively high ambient temperatures, this problem went away. Apparently Pemex is not formulating fuels with the 10 - 15% alcohol content mandated in the U.S.

I added an electric fan to the Midget hoping that increased air flow would keep the carb bowls cooler. It has had no effect other than the noise created by the fan. The B has A/C so it gets really unhappy if stuck in traffic in the heat.

Another problem is the voltage stabilizer that regulates voltage to both the fuel and temp gauges. I checked mine and found that it was putting out 12 volts, not 10V. This made the gauge read hotter and also left the fuel pump clicking just before the fuel gauge hit empty. The electronic Moss stabilizer replacement is temperamental. Heat and fuel levels will increase with RPM so regulation is not accurate with this "upgrade" either.

The cars were designed to run on leaded high octane fuel in the days before Global Warming. I suggest pulling the choke handle slightly to increase idle but before mixture levels are raised when caught in the high temperature, slow going/idle situations.

Glenn Mallory

Dick
If you don't have the problem when you leave the fan switched on maybe your fan temperature sensor is switching on too late and causing a overheating problem

willy
William Revit

Thanks to all,

I'm still playing with this. I just put in a new coil and removed the ballast wire after putting in a ballast resistor from NAPA. I'll test it this week and then move on to a complete check out of the fuel delivery system. I'm also looking at measuring the temperatures at various points with an electronic thermometer. Willy's comment is greatly appreciated. My temperature gauge runs at the high end most days. Since the radiator was serviced, and other cooling system components were just put in with the engine rebuild (< 500 miles), I think they are good.

Dick
Dick Field

This thread was discussed between 11/05/2016 and 22/05/2016

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