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MG MGB Technical - Engine Temperature fluctuations

Following replacement of electronic ignition and timing adjustments, engine temperature goes up with higher rpm and goes lower when rpm is reduced. My engine always had the temperature reading right in the middle at all speeds, now it reads half way between normal and hot when I run at 55 or 60 mph.

Any suggestions?
A Tavss

What year i.e. electric gauge or capillary?

Timing may be wrong, could be retarded or over-advanced, both can result in higher running temperatures. Did you record the timing at, say, 1k and 2k before you started and reset back to that?
paulh4

Early '74. Chrome Bumper. I didn't set the timing, my technician did. He has since retired, following my requesting his attention to the timing. I suggested that he set the timing at 3K rpm. Whether he did so is not available.
A Tavss

An early 74 CB for North America would originally have had an 18V672/673 low compression engine with a 25D4 41491 distributor.

Static timing was 6 degrees and strobe was 11 degrees at 1500rpm - manifold vacuum advance source disconnected and plugged.

You or someone should probably recheck it. Whilst the original figures are not totally valid with todays ethanol fuels (if that is what you use), even if low octane (high octane is preferable) it makes little difference with the low compression engine, high compression engines probably need to be retarded from that to control pinking.

paulh4

Thank you for your comments. I will check further into the issue with another competent mechanic, when I find one.
A Tavss

Maybe I misunderstood the original post but it sounded to me like the symptom was that the temperature gauge more or less followed the engine speed - temp up with higher rpms and down with lower rpms. To me that sounds like unstable voltage. It might be interesting to know if his lights get brighter and dimmer with higher and lower engine speeds.

It seems to me that the temp sender is just a variable resister that relies on a fairly stable voltage to be accurate. Of course, I could be all wet.

Jud
J K Chapin

That's why I asked if it was electric or capillary.

If electric it should have a voltage stabiliser in circuit, and for the fuel gauge, so if that were faulty both would go up and down.

Lights probably do get brighter from idle to even a slightly increased rpm as they are not stabilised.

If the temp gauge has somehow been connected direct to unstabilised voltage but the fuel gauge was still stabilised, and it was purely voltage causing it, it would happen at a standstill when raising the revs not just when driving. It would also vary between 'ignition on engine not running' and 'running and charging', assuming the engine temperature is the same. The attached shows a _fuel_ gauge working normally on the left, unstabilised ignition on engine not started in the middle, and unstabilised engine running and alternator charging on the right.

I also assume the temp gauge only rises gradually as the engine warms, and doesn't go straight up to a reading when it is cold.

All questions that we don't know the answers to.
paulh4

I had the exact same symptoms that you describe with my '66 B. Turns out it was the radiator. It could not dissipate the additional heat generated at higher engine loads. When the temperature got too high, I just took my food of the gas, and it cooled right back down. Eventually, I replace the radiator and never had the problem again.
bhall1

The Robert Bentley publication containing the factory workshop manual and driver's handbook is the basic reference for any MG owner. It will give the exact ignition advance curve that your engine originally had. That curve can be compared to the curve your "electronic ignition" currently shows. I put "electronic ignition" in quotation marks because you have provided us with an undifined term--there are several different forms of such devices and we cannot offer opinions on any of them until we know, exactly, what you mean by that term. An "electronic points replacement module" (e.g. Petronix Ignitor or Allison Optical-Electric systems) use the original distributor body with a replacement for the factory points. Other systems are entirely new and do not reuse the original distributor body. With a points replacement system, I would expect the basic advance curve to remain the same. With any other system, you have no way of knowing if the mechanical advance curve is in any way correct for your engine. All of this needs to be checked out, on the engine, with a good quality tachometer and a "dial back" strobe timing light.

I have some questions about your "competent mechanic". How do you know the individual is competent? Do you have sufficient knowledge to make this determination?

Mechanics working on modern autos have a vast volume of knowledge to obtain and have a constant need for regular training to keep up with the latest changes. A modern mechanic may spend a few evenings a month (every month) in training or may spend a month (sometime more depending on the brand of vehicle) in refresher/upgrade training. It is a far more complex business, requiring a much higher level of intelligence and ability to learn complex systems than was true a half century ago. It is also a very different form of mechanical repair with the modern mechanic using computer diagnostics to evaluate the problems that an older series of mechanics had to evaluate by trial and error testing. As a result, finding a "competent" mechanic, one who can and is willing to work on older cars, can be an expensive business.

Many of us have, out of necessity, been forced to learn to do much of the basic, sometimes the advanced, mechanical repairs ourselves. There are, in some areas, classes hosted by the local high schools or community colleges, on auto repair. If so, they are work looking into. A basic knowledge of automotive mechanics might well help you find the elusive "competent mechanic".

Otherwise, a copy of the Bentley manual, a dial back timing light, an accurate tachometer (not the one on the dash), an infared thermometer, and an assistant would allow you to do sufficient diagnisis to either correct the problem or to describe it in a manner that would allow us to help you correct it.

Les
Les Bengtson

I totally agree with everything Les said and want to add: If you haven't already, please join your local MG or British car club. You'll find many helpful hands and a wealth of knowledge immediately at your fingertips. British car nuts are born to be helpful.

I know that my club is too far away for you but take a look at this https://fbccsc.org/ anyway, especially the latest newsletter. Hopefully you will find a similar club nearby.

Jud
'72 MGB
'62 MGA Mk.II
'53 MG TD
J K Chapin

Installing an aluminum radiator made a huge difference in running temperature, both at speed and at idle only 1970 B. The car is used in Arizona, New Mexico and Chihuahua summer heat with A/C. If electric fan(s) have been added, the best solution is to remove them and to install a 7 blade plastic fan. The "pancake" fans can block air flow at speed.

For voltage stabilization, I installed a second, adjustable voltage regulator after the solid state replacement for the original bi-metal regulator that powers fuel, temp and (some years) oil pressure gauges. I used a full tank of fuel and the fuel gauge to trim voltage so that the needle is right on the "full" line. This brought the temperature into a stable range.

Although it should be noticeable, I have experimented with timing settings and mixture and have never been able to detect a difference in running temperatures on either the B or the midget. I have the impression that the timing would need to be set way out of range to create an engine heating problem at speed.
Glenn Mallory

It's not the Full line that is important but the E line. Run it out, put a gallon back, then adjust to E and it gives you a psychological reserve. Full is irrelevant - when you have filled it at a pump you know it's full.
paulh4

This thread was discussed between 29/07/2024 and 16/09/2024

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