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MG MGB Technical - Pinking... then dead engine

Car - 77B, mostly standard with DGV.

The car was running great for about the last year. Drove it this morning and nothing was wrong. Started it up after work and it would immediatly start to ping under load - only up to 3000 rpms - well before it even started to warm up. Strangly enough, it would run quiet above 3000. As long as I kept rpms above 3000, it wouldn't ping so I went home (6 miles).

Thought I might have broke a spring in my distributor, so I replaced it with a spare (including new cap, rotor and wires) But, it continued. I retarded and advanced the timing, but the problem persisted each time I drove it down the block to test it. On about the third try, the engine quit altogether - suddenly quiet. The engine would turn, but it wouldn't fire.

I checked the spark, and it seems fine. I looked at the plugs and they are ashy and whiter than normal (just changed plugs about three months ago).

I'm tired and frustrated, so I'm going to sleep. But, I was wondering what would be the next thing I should check.

Tomorrow I'll check compression. Would a plugged jet cause a lean mixture that would lead to pinging? If so, why would it run fine under load at 3000 rpms?

I submit myself to your collective thoughts.

Tony Calleja

The way it suddenly happened and the symptoms suggest an air leak somewhere on the intake to me Tony. Maybe a split hose, loose connection, or even more obscure, leaking/stuck anti-run-on valve, servo, etc. Rich

Tony - Do a compression check or a leak down test. It sounds like you may have a blown head gasket (or a significant leak) between two cylinders. Good luck - Dave
David DuBois


Plugged jets cause a bucking or surging, a lot like running with only three spark plug leads. I'm with Rich, the sudden appearance of symptoms sounds timing/ignition related. I assume you have replaced the Opus electonic ignition with something else. Do you have a spare module or a points-type distributor to see if the problem is in the electonic ignition?

D. Cook

I appreciate the responses.

The car has been desmogged for the last six years. So I'm limited to the number of hoses still available. But, I'll check for an induction leak when I get home.

Then, I'll check the compression. Blown headgasket wouldn't be as bad, but I would think that the car would sound like it would try to start. Instead, I'm just spinning the crank.

Finally, the points were replaced by a pertronix Ignitor about two years ago. I have some points lying around and I'll check them after the other two items fail.

I completely forgot to mention, when I opened the hood, I noticed that the oil cap was hanging off to the side. I just checked the belts about a week ago, so I believe that it was closed. I didn't think much of it, but could something related pop the cap off the valve cover?

I'll post results as soon as I can.

Thanks guys,
Tony Calleja

I replaced my Eurospec 45D with a functioning points 45D, and it still wouldn't start. But I was getting a spark.

I pulled all four plugs and ran a compression test, throttle full open:
#1 - 111 psi
#2 - 108 psi
#3 - 210 psi
#4 - 211 psi

(I ran the test two more times and got within 2 psi of these figures - but didn't want to kill the battery)

So, Dave, it looks like you were spot on. I'll have to pull the head this weekend, but do you think it would only be a gasket? Unless my head cracked (doh!)

If it turns out to be only a gasket, should I go back to copper (as it is now), or should I use another material?
Tony Calleja

Tony - Looks like you just have a leaky gasket between cylinders 1 & 2. If it was completely blown, the compression readings would be a lot lower and the engine would have made much more noise (when I blew the head gasket on our Mazda truck, the shop I took it to could hear me from a block away). I would stay with the Pella gasket, it seems to be the best, but I would look into replacing the head studs, at least the short ones (our local shop said that those are the most likely to streatch, although I always replace the whole set). Further, even though it says that you don't need to retorque the nuts when using the Pella gaskest, I always do after about 500 miles. Good luck - Dave
PS - you might want to check the head and block deck for evenness (is that a word?).
David DuBois

Just as an aside, because the diagnosis has been made. Aren't 210 psi readings exceptionally high for a 77B? Could the high pressure be a reason the gasket blew after only a year?

If so are there alternatives other than a new head to bring compression down? Doubling up gaskets, perhaps? (I'm anticipating for my own future woes, here)

Tony. I have used the Fel-Pro upper gasket set which has a resin type, "Made in England" head gasket. They have been holding up well for several years now.

Like Philip, I am somewhat confused about your compression readings. You say you have a basically stock 77, which should be a low compression engine. Over 200 PSI would be on the high side for a high compression engine. For an older, stock engine, I get about 150 psi and the engine will start and run if you have 100 psi or more. I also wonder about the "copper" headgasket you mention. These are normally used on racing engines rather than stock engines. I think I would cross check my compression readings with another gauge and do some more trouble shooting before pulling the cylinder head. If you are still getting more than 200 psi readings, you do not have a stock engine.

I would also remove the valve cover and turn the engine over by hand (spark plugs removed) and watch the valves move. Wiped lobes on a cam (and the stock cam was known to be soft) can also give you low compression readings. Les
Les Bengtson

I'm with Les on all that and have seen cam lobes wiped off. You'd still expect it to fire, the low compression engine was only 130 anyway.
Strange, those high figures, but if the oil cap was blown off it would also point to crankcase compression from a blown gasket.
Standard is 160 and low 130. Mines a little tweaked and is 180. The only car I've checked over 200 was a Volvo 240 which was a pretty soft engine. The figures don't tell the whole story due to valve overlap but 200+ in a B is high. Rich.

The engine was rebuilt about six years ago. Since this was my first rebuild, I had a mechanic do most of it. I pulled the printout from the shop this morning and things to note were:
+020, small dish, decked surface

The head also had minor work done: valve job, shims, angle grind, clean the ports, resurface

Since then, I've replaced the cam with a stock unit. And replaced the distributor with an electronic eurospec unit.

I have over simplified when I stated "stock", but I didn't ask the shop to do anything other than rebuild to specs.

The only two problems I had were 1) rich emissions and 2) missing idle.

After replacing the cam, and sealing the induction the car has been running perfect for the last few.

Do you believe the car should still fire with these compression readings? If so, what else could cause this problem - especially the sudden onset of ping and finally a dead engine.
Tony Calleja

Given those compression readings I'd have a good look at the head to check for any damage. I'd guess running that sort of CR isn't too healthy.

Theoretically, the higher the cylinder pressure, the more horsepower you can produce. By timing the cam with the crankshaft, you are looking to produce the highest possible cylinder pressure. It looks like his cam timing is perfect, but the headgasket is blown. Have the head checked for cracks and don't use the copper gasket. You are more likely to keep a good seal with the composition-type gasket Moss and others sell.
Jeff Schlemmer

These pressures are at cranking speed not max power speed and do not relate to power output. They are an indicator that the engine is not standard in some way (200 is too high) and that it has a fault (all cylinders not similar).

Powerful engines may produce low cranking speed pressures due to wide valve timing angles. The inlet of a wild cam doesn't close till well after BDC to make use of gas inertia at high speed. At cranking speed though the inlet charge is not moving fast and compression cannot start til later so it produces less pressure even if it has a higher compression ratio (this is why compession ratios are sometimes termed theoretical). A powerful engine and a stock one could both show around 160 at cranking speed and it would mean nothing power-wise. Rich.

I have a few updates.

The gauge is off by 32 psi. Apparently, the needle wasn't setting back to zero. The shop replaced it, but I'll have to check compression after everything is reassembled.

I pulled the head. It popped right off.
There was a 3/8 inch piece of the copper gasket missing between cylinders 1 and 2.

Cylinder 1 was clean, The piston had a brass color to it. 2 was almost as clean, with a darker color and a little carbon. Both sets of valves were greyish tan.

3 and 4 had carbon deposits on the piston face, but not much. The valves were sooty, between black and brown.

All cylinders are smooth to the touch, and the pistons don't feel loose. The mechanic sent the block to a shop that builds racecars. I'll venture to guess that the cylinders are in very good condition.

I have a gasket made of a greyish resin like material. It is stampped "Made in England", "AK660", "Top" and "Front". It has metal rings around all the openings (some silver colored, some copper), and what appears to be brass between the cylinders. Shall I use this one? I'd prefer to get the best - so any input would be appreciated.
Tony Calleja

Yes, that is probably the best head gasket for your application. Clean the ring around the cylinders with a scotch-brite type pad before installing.
The brass color on your piston is probably from the long-term erosion of the head gasket. Everything else sounds normal.
Jeff Schlemmer

Tony, Just checked a premium quality headset I have here and the headgasket is exactly as you describe, quite complex construction and it also has some red sealer around some of the holes.
As I recall these are designed to go on with all surfaces clean and dry (no sealer or oil) and the gasket surface then molds as sealer. Others may be more up to date but that's how I'd instinctively use this type.
I'd check/skim the head if needed and retorque cold after a couple of hot/cold cycles. Rich.

What type of gasket was the blown one?

I think the head gasket your are describing is a payen this is a good head gasket set and i was supplied one of these by chris @ octorane services {when i had a similar problem} when you replace the gasket read through the archives for some interesting facts on which way round to fit the manifold gasket i.e foil side to the manifold or block makes great reading
Richard H
Richard H

Rich, thanks for the info... I was going to buy gasket sealer, but will hold off on that.

Sorry, Rick, I don't know the brand for the blown gasket. It was a solid copper type purchased locally several years ago. I tossed it in the garbage yesterday, and it's burried under grass clippings.

Richard, I was reading that the shinny side goes out. But, I'll read up on the debate... thanks.

Now that the head is off, I'm going to drop it off at a shop to check for cracks and warpage. The gasket blew for one reason or another, and I'll try to see that it's not the head.

I'm also buying replacement studs. The one directly above the water valve was stripped where it went in to the block. I moved another one in to it's place, and the block wasn't stripped - luckily. It wouldn't torque down (the stud came out when I removed the nuts). I don't know if this contributed to the problem, as this was towards the rear of the head, and not where the gasket blew. But, I'm sure it couldn't help.

I'll update if anything new happens.
Tony Calleja

One thing I've noticed with water in the combustion chamber is that it cleans the carbon out. It sounds like you may have gotten a little water in the combustion chamber with the blown gasket
Barry Parkinson

Final post... just as an update.

The head was cleaned, checked for cracks, resurfaced and a 3 angle valve job was done.

While this was being done, I took the time to flush the radiator, inspect the oil pump, redo the oil pan seals, inspect the cam, change the plugs, replace the rocker shaft, and upgrade all studs.

My head was full of some grey substance that plugged the heater valve area. It looked like wet cement. The radiator flush had some white substance come out, but not too bad - I think it all built up inside the head.

After everything was reassembled, torqued and clearances checked, I tried to start the car. But, it didn't. After a night's sleep, I came back and did some trouble shooting. The coil was firing but no spark made it to the plugs. Just short of replacing the distributor, I bought a new cap and rotor (which I should have done with the new plugs)and it started up.

The rotor was bad. Can't figure out how a simple piece of metal on plastic goes bad.

I retarded the timing a bit from what I had it at before - a bit cautious. I'll wait another week to retorque the head and re-check valve clearences.

It was a lot of work, but it sure makes driving it away from my garage that much more satisfying. I thank all those that helped.
Tony Calleja

"Can't figure out how a simple piece of metal on plastic goes bad"

You get tracking from the metal to the spindle which shorts the HT. Can sometimes be internal if there is a crack.
Paul Hunt

Only one thought to add.

Make sure you examine the center stud hole on the spark plug side of the head. Its ID is slightly smaller that the others and it is close to an area where the gasket frequently fails only enough to slightly leak coolant. As a result, the inside of the stud hole can get a bit gunked up over time. Make sure the stud is a sliding fit in this hole as a tight fit will cause incorrect torque readings and ineffective clamping of the head and gasket.
Richard Smith 1

This thread was discussed between 03/03/2005 and 18/03/2005

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