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MG MGB Technical - Engine Misfire and Overheat issue

I have a early 74 MGB which is running terrific but my brother has a 74 that has been plagued with issues. First off there is a miss in the engine at all RPMs. Here is what has been replaced. There is a new electronic distributor, new coil, new wires, new distributor cap and rotor, new wires and plugs. The miss is still there. Reinstalled old wires temporarily to make sure we didn't get a bad set, no change. Checked firing with a pencil ignition tester and each wire at the plug is firing. I'm stumped.

There has also been an overheating problem. Replaced water pump (small leak) and thermostat, Test drove for 20 minutes at which time overheated and boiled over. Replaced electric fan (old one was noisy) at the auto parts store with the biggest fan that would fit. Temporarily wired hot all the time. Refilled radiator and drove home 20 minutes and boiled over again just before reaching home. Replaced radiator with 4 core aluminum one and removed thermostat. Drove again and found what we thought was the real culprit, head gasket on passenger side spewing antifreeze. Removed head and had .006 removed by machine shop. Reassembled and overheating problem remains. I don't know if the engine misfire is related to the overheating or not.
TPM Timothy

First off is the tach jumping about with the misfire? If so then it's an ignition LT problem, if not then HT, fuel or if it had points then condenser.

Is it backfiring in the exhaust? If so that tends to indicate an ignition problem with a good combustion igniting unburnt fuel in the exhaust.

The two could be associated, how long between the to symptoms first starting? An electric fan already fitted indicates the cooling system problem may be long-standing. An engine overheats if it is generating more heat than it should or the cooling system can't get rid of as much as it should. I tend to think that the root cause of the head gasket problem is still there. Does the temp gauge rise steadily into the H zone or swing about? How does the rad outlet temp compare to the inlet? Is there any sign of water/coolant in the oil or vice-versa? Any signs of bubbling in the radiator with the cap removed? Had the rad cap pressure tested or replaced? If the system is getting up to the correct cap pressure then there is almost certainly a combustion leak, under normal circumstances it should only rise a few psi and be well below cap pressure.
paulh4

All the above, plus
Check the voltage at the coil, tired ignition switches can gobble up a volt or more, and when running electronic that can be a real issue.
Check the coil is the right type, ballasted system (needs 1.5-ish Ohms coil) or not ballsted (3 Ohms).
If ballasted, the electronic distributor power lead must go to the fuse box, not the coil (which sees only 6-ish Volts thanks to the ballast).
And check the electronic distributor itself. The cheapos usually dont have anywhere near the right advance curve and can end up with 15 deg too much advance in the mid range. That would certainly raise the heat dumped into the cooling system.
Paul Walbran

Good point about coil and ballast. As a 74 it could be either - CB would originally have been non-ballasted i.e. 12v 3 ohm coil, RB would originally have been 6v 1.5 ohm coil with 1.5 ohm ballast in the harness. However often when changing to electronic ignition purchasers are nagged into buying a new coil and it could be the wrong one. Could also have been changed to the wrong one without that. Getting that wrong can contribute to misfiring, less so overheating/coolant loss.
paulh4

I've just seen an old photo that reminded me of something. I'd been up Shelsley Walsh hill climb and the roadster was pinking noticeably (as were a number of others of that era I noticed). At the top the temp gauge was right round in the H zone, so while we waited to tour back down in convoy I ran it for a bit till it cooled down.

Assuming yours is low compression I don't think they pink/ping as such but can still be over advanced. I realise you have replaced the distributor and probably retimed it, but maybe something to eliminate, especially that it's not going too high at higher rpms.
paulh4

Thanks fellas, lots of good info for my brother to check. He is in Buffalo NY and I am in Midlothian VA. Forgot to mention that the compression test we did revealed 90 PSI for all 4 cylinders. We also had replaced the radiator cap with 16 PSI cap at the auto parts store. There may be something to the coil issue since it is actually a 74 engine in a 79 MGB. I'll have him check the voltage at the coil.





TPM Timothy

90psi would be very low, if another gauge shows the same given the rapid boiling I'd suspect incorrect valve timing. But how did you check it? One person here had low figures but he was only cranking it to get one compression on each cylinder whereas it needs at least half a dozen to give the correct reading, stepping up with each pulse until it stabilises. Normally done hot but cold still should not be that low. Some say throttle wedged wide open but having done it both open and closed I've not found any difference.

To check the coil and harness combination connect what should be white/light-green wires to the coil +ve and earth to the coil -ve having removed any other wires. Turn on the ignition and the correct coil and harness for a 79 should show about 6v on the coil +ve. If you see about 9v then you have a 12v coil in series with the ballast that will give a weaker spark. If you see 12v the harness ballast has been bypassed and it needs a 12v 3 ohm coil, disconnect the wires from the coil and measure between the spades and that will show which coil you have.

If you don't have white/light-green wires at the coil +ve then the wiring has been changed, but do the same test with what you have.
paulh4

As Paul notes, 79 originally had a ballast, it was fitted internally in the wiring loom. As well as yhe checks he outlines, make sure your electronic ignition power is drawn from the fuse box white terminal or other full 12V source. Less than 12V will provide exactly the misfire you are experiencing.

Agree on cam timing. To check, set #1 inlet temporarliy to 60 thou, insert a 5 thou feeler and slowly turn the engine over by hand until the rocker only just nips the feeler. At that point the engine should be at TDC. BUT iirc some US spec cars had timing a few degrees different (5 deg?) so a small variation may be down to that, or a worn chain.
It takes a big difference such as a tooth out (= 18deg) to vary the compression by that amount so you should get a good guide as to whether this is the cause from the above test.
Paul Walbran

Timothy. You need to start with the very basic things, like finding out what engine you have (your photo shows the aluminum engine number plate is still on the block) and how it is currently set up (e.g. what carb(s) are on the engine?). Then, you need to investigate how the wiring and engine compartment are set up (e.g. the master cylinder reservoir is from a model earlier than a 79). You seem to be working on a car that has been changed over the years. The cylinder head does not seem to have the holes for the air rail, which a 74 model cylinder head should have. So, you do not, really, know what all you have and will have to build a picture of what you are actually working with before you can intelligently trouble shoot the system.

Things that might help to know include knowing what carb(s) are installed on that engine. The Z-S carb is a known problem and may require frequent rebuilds. (My original 79 required the diaphragm to be replaced twice in five and one half years of ownership. Modern, "Made in China" rubber parts do not seem to hold up as well as the older parts did.)

What is your exhaust system? The original had the catalytic converter installed just off the exhaust manifold with the down pipe below it. It was both a restrictive and a dangerous system. (Dangerous in that a rich fuel mixture will cause the cat to glow bright red from excessive heat and it is sitting right under the carbie. Several engine fires traced back to this poor engineering.)

When you had the cylinder head skimmed, did you have it crack tested? A number of years ago, some friends who owned and MG wrecking yard and I, who had a non-automotive machine shop, kept track on the number of cylinder heads that were cracked when inspected. More than 50% of all the cylinder heads inspected had some cracks and of the later model cylinder heads, over 80% were bad.

What form of "electronic ignition" do you have? The factory used two models, the Lucas 45DE4 and 45DM4 distributors. These distributors had the mechanical advance curves tailored to provide the best emissions results possible. Many people replaced them with other, earlier model, distributors which provided better mechanical advance curves for power and performance. (Remember, if the ignition timing is over advanced, the piston is fighting the expanding, burning fuel causing over heating of the engine. If the ignition timing is overly retarded, the fuel is ignited too late, does not burn efficiently, and may still be burning when the exhaust cycle begins causing excessive heat to be injected into the exhaust system and may cause over heating of the cooling system.)

So, four basic areas that need check out: basic engine (what is it, how has it been changed?), electrical system (has it been modified, especially as regards the ignition system), the ignition system (what is it, how is it connected up), and the fuel system (both the carbie and the fuel pump need to be identified and inspected/tested for proper functioning).

After you have the above information, and still have a problem, it would be time to inspect the cooling system in an orderly, intelligent manner. A simple system pressure check would have told you (and shown you) that you had a head gasket leak, without the risk that driving the vehicle until it over heats will cause new problems to go with the existing problems.

My website, which I will be dropping in a few months, has some tech articles on crack inspecting cylinder heads, testing cooling systems, and checking out ignition systems which might be of use to you in your trouble shooting. It is: www.custompistols.com and you might find the MG section useful.

Les
Les Bengtson

I recently replaced spark plugs and had a terrible miss afterward. One had a cracked insulator out of the box. Holding it with the electrodes up it looked fine, turning it over revealed that the insulator tip slipped down and touched the electrode. It must have been damaged in shipping but could also fail under the right running circumstances. Also had a pertronix distributor fail mechanically and this was causing an occasional misfire, especially at speed. The tachometer needle jumped up with every miss. I pulled the distributor out and found that there was enough play in the shaft for the pickup to touch the lobes. Replaced the distributor and transferred the Pertronix and problem fixed.

90 PSI compression might sound low but this is what I have read on mine, especially right after a valve job. It should be OK, even though we would all like to see 135 PSI even if it required to pay for Premium fuel to avoid knocking. Yours should be a V block engine which has a lower compression ratio. As Les observes, the lack of air injection ports implies that the head would be from an early engine that was intended to have a higher compression ratio.

Regarding the overheating problem, make sure the fan you adapted is pushing or pulling air in the right direction. It takes 20 minutes or so of driving to build up enough heat to stress the cooling system. I have been experimenting with electric fans lately as alternatives to the mechanical fan that came on the 1970. Ambient temperatures, speed, stop and go, etc., have had a huge effect on running temperatures and the results have been clearly observable after about 20 minutes of running. The car boils over in 95 - 100°F ambient temperatures in traffic. The mechanical fan avoided this so I have determined that two electric fans obstruct air flow to the point that it runs hotter in high ambient temperature conditions.

And, as observed by Les, there may be a crack in the head.
Glenn Mallory

I recently replaced spark plugs and had a terrible miss afterward. One had a cracked insulator out or the box. Holding it with the electrodes up it looked fine, turning it over revealed that the insulator tip slipped down and touched the electrode. It must have been damaged in shipping but could also fail under the right running circumstances. Also had a Pertronix distributor fail mechanically and this was causing an occasional misfire, especially at speed. I pulled it out and found that there was enough play in the shaft for the pickup to touch the lobes. Replaced the distributor and transferred the Pertronix and problem fixed.

90 PSI compression might sound low but this is what I have read on mine, especially right after a valve job. It should be OK, even though we would all like to see 135 PSI and be required to pay for Premium fuel to avoid knocking.

Regarding the heating problem, make sure the fan you adapted is pushing or pulling air in the right direction. It takes 20 minutes or so of driving to build up enough heat to stress the cooling system. I have been experimenting with electric fans lately as alternatives to the mechanical fan that came on the 1970. Ambient temperatures, speed, stop and go, etc., have had a huge effect on running temperatures. The car boils over in 59 - 100°F ambient temperatures in traffic. The mechanical fan avoided this so I have determined that two electric fans obstruct air flow to the point that it runs hotter in high ambient temperature conditions.

And, as observed by Les, there may be a crack in the head.
Glenn Mallory

Glenn. A few more photos of the engine compartment would be of use. If the vehicle is a 79, as has been stated, it should have twin electric fans in front of the radiator (US version--Canadian had only one fan). Thus, additional fans, e.g. one behind the radiator, might well be blocking air flow. I have run three RB roadsters over the years and have always found the factory twin electric fans to be sufficient for driving in the hot Arizona summers, both in stop and go and limited access highways. Hence, there may be quite a number of problems associated with this vehicle. That is why it is important to find out what it actually is, how it has been modified, and what systems are currently in use.

Les
Les Bengtson

This thread was discussed between 17/08/2022 and 20/08/2022

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