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MG MGB Technical - Spark plug carbonisation & low compression

So here's the story.....

I have recently bought Monty (a '67 B) in which, in the first few weeks of ownership I have managed to rack up just over a 1000 miles of sheer unadulterated pleasure. Marvelling at the beautiful exhaust note the little beauty was running like a dream.... until a few days ago.

Halfway home on a 50 mile journey there was a sudden loss of power & the sound of the beautiful exhaust note was drowned out by the sound of a tractor as Monty started firing on only 3 cylinders.

After nursing the poor fella back home I started to use my very limited knowledge of car mechanics & the internet to try & ascertain the problem.

Here's what I've found:

My engine number is 48G 733 AH ER9117 which I believe to be a gold seal reconditioned 18V581/582/583/779/780 (not sure which).

The spark plug in the 2nd cylinder from the front had heavy solid carbon deposits around the insulator. This was the offending cylinder that wasn't firing.

I have carried out a compression test and the results are as follows: 1. 146lbf/in2 2. 140lbf/in2 3. 150lbf/in2 4. 158lbf/in2
My investigations lead me to believe that compression pressure at new should be 170lbf/in2.

I have replaced the no.2 spark plug and the engine once again seems to run like a dream however I'm worried that I have temporarily made the problem better but haven't got to the root cause of the problem & have exhausted my capabilities. I don't know whether the compression pressures listed above are within an acceptable range or what caused the spark plug to carbonise & have no idea where to go from here so any help/advice would be very much appreciated.

Thanks
Ian.
I S Russell

Ian,
Since it's running well after changing out plug, your problem doesn't sound too serious. Although your compression numbers don't seem the ideal (e.g., at 160-170 psi), they're probably in the norm. When taken, was your engine hot, warm or cold? Don't be too concerned with how much, what you want is uniformity. Take multiple readings for each cylinder. I take three sequential readings (e.g., 1,2,3,4 - 1,2,3,4 - 1,2,3,4), then average (they will change). Also recheck compression in each cylinder after adding some oil to each cylinder (e.g., 3-4 squirts from an oil can). Let us know what you find.

Check your valve clearances and adjust to specs. This might help some, but I'm guessing your cam lobes are pretty well worn.

Could be that you had a faulty plug or wire. An intermittent misfire, can progress to plug failure. You've changed the plug - consider some new plug wires.

Could be that piston rings for #2 are worn a bit more than the others. If so, you'll have to live with it until it's time (not now) for an engine rebuild.
Steve Buchina

If the plug was a sooty black, it is likely due to miss fire as stated above. Could also be a rich mixture but that will normally show up on at least 2 cylinders. If it's oily black, then it is worn rings or valve guide seals.
John H

Ian. Your Gold Seal engine was a replacement for the various Home Market/Export (Non-US) engines used from 1971-74. Certain of the original engine numbers you list are for cars equipped with overdrive units or automatic transmissions, probably as a "quick check" for factory trained mechanics. The basic block interchanges for all applications and the only change between the manual and automatic versions would be the flywheel. Hence, determining the exact specification is not a concern.

What is a concern is the fact that this engine is of the 18V conformation which means that it was set up to use the four syncro transmission and the "pre-engaged" starter. Normally, the 1967 MGB used the three syncro transmission and the older style of starter. This is something you will want to investigate as it will be of interest in the future.

For now, however, let us focus on your immediate problem. Steve has provided a good foundation on which to build, so let us do so.

My website, www.custompistols.com/ has technical articles on both the compression check and on ignition system trouble shooting which may be of use to you in your investigations.

Cylinders one and two, and cylinders three and four share a single intake port. Cylinders one and four have their own exhaust port while cylinders two and three share an exhaust port.

Thus, the mixture setting of the front carb has a more significant impact on the operation of cylinders one and two and the rear carb has a more significant impact on the operation of cylinders three and four. There is a "balance tube" between the two legs of the intake manifold which has an influence on this, allowing some fuel to reach all of the cylinders from a single carb, but this is, in operation, a minor matter.

The two cylinders sharing an exhaust port, cylinders two and three, tend to run somewhat leaner than the two cylinders, one and four, which have their own, dedicated, exhaust port. This is because the exhaust is not as efficiently removed from cylinders two and three and tends to dilute the incoming charge slightly. Thus, the second and third cylinders tend to run slightly lean if the first and fourth cylinders are "spot on" in their mixture. Cylinders one and four tend to run slightly rich when cylinders two and three are "spot on" in their mixture. You will, quite commonly, see a tan plug on cylinders two and three and a somewhat sooty plug on cylinders one and four when the mixture is properly adjusted so that all of the cylinders are running properly.

When there is an air leak, cylinders two and three, running leaner than cylinders one and four, are the first place this will show up, causing a misfire on those cylinders due to their operating at a leaner mixture.

All of this is background information which will help you to understand what may be happening as you do your trouble shooting.

Your compression figures are interesting. They show the front two cylinders as having a lower compression figure than the rear two cylinders as if something, perhaps a slightly weak mixture, has caused the valves for the front two cylinders to wear more than the valves for the rear two cylinders. Unfortunately, this can only be determined by removing the cylinder head and disassembling it for a physical inspection.

Generally speaking, the guide lines for "good" compression readings are either "no more than a 15 psi difference between cylinders" or "all of the cylinders should be within 10% of the reading of the highest reading cylinder". In your case, this means that all of the cylinders should be within 16 psi of each other, using your figures and the latter formula, which cylinder number two is not. You do, however, have sufficient compression that all of the cylinders should be capable of firing properly if the mixture and spark are there for them. This is something to observe and track in the future. I have seen compression readings increase in, when the engine was used on a regular basis, on cars which had, previously, not been driven regularly.

For the immediate period, I would suggest that you do as you have done and simply replace all of the spark plugs so that you have a set of known plugs (age and condition) in the engine. Then, carry a spare, gapped spark plug set and a wrench in the boot. If you have another problem, you have a replacement immediately to hand to correct the problem. You will, also, be building up your data base to allow you to determine what is happening. It may be, with a car that has been recently purchased, with an engine that you do not know how often it was operated or how it was serviced, that this was simply a case of an old plug that was not operating properly and fouled out because of this.

Drive the car, after replacing the plugs, and see what will happen. Doing an ignition system tune up, including replacing the spark plug wires, and setting the valve clearances properly, would help in the diagnosis. Then, if the problem should recur, we will have a better, larger, data pool to draw upon.

Les
Les Bengtson

The original factory figure for the compression on the factory high comp motor was 165lb. Your motor is getting close to the variation allowed. If the car is running well however with a new plug, its not worth pulling the motor down at this stage and there could be many years of life left in it yet. A good tune and tappet adjustment as the others have mentioned and tighten of the manifold, carby etc bolts.
Les I see it mentioned a lot in these threads about the Siamese ports and their effect on mixture and of cause you are right in regard to the inlet ports. The center exhaust port, although not the best design, is not effected as the inlets. This is because the center two cylinders never fire one after the other, unlike the inlets. The main reason for the difference in color of the center EX valves is because using the same port they run a little hotter. The long center branch manifold is designed to help overcome the short comings of this design. All the best Denis
DENIS4

Since you said you just bought the car you should consider what the
previous owner has or has not replaced recently. I would also consider how much he drove the car. You may be giving the car more of a work out than the previous owner did. You may encounter several small things that may need fixed. The compression readings do vary somewhat. I may put some Marvel mystery oil in as directed on the bottle (if available in your area). I have used it in the past to free stuck rings and it will also coat the cylinder walls and may help with the compression. As for the misfire, it may be as simple as new plug wires or distributor cap.
The Marvel is used in the fuel and crankcase. (be sure to read the bottle for how much to use) Also check for leaks around the intake gasket, that could lead to a misfire.
David

All,

Thanks very much for the advice so far. Here are the findings from today (unfortunately having to fit this around the day job).

I have re-tested the compression as per Steve’s advice with the following results. By the way, the initial tests were done with the engine warmed to operating temperature whilst in the garage. The new readings were taken after having driven the car for 15 mins. The first ones are the average of 3 tests as per Steve’s mail.

1. 153psi 2. 140psi 3. 159psi 4. 161psi

The next set of tests were carried out directly afterwards but had the oil added to each cylinder and gave the following readings:

1. 160psi 2. 153psi 3. 170psi 4.170psi

I have also used a colortune to check mixture in the cylinders with the following (interesting) results:

1. lean mix 2. correct mix 3. lean mix 4. lean mix

Obviously cylinders 1 & 2 should display the same characteristics, so does this lead to the conclusion that there is a valve problem?

I have checked the dist. cap and rotor arm & they look pretty new & don’t show signs of wear but as they are relatively cheap components I will replace them along with the plug leads, plugs & points.

My next steps are to check the valve clearances and balance of the carbs. In the meantime any further comment/advice on the above will be much appreciated as always.

Many thanks
Ian.
I S Russell

Ian,
Your results don't suggest (to me) that you have any major problems. Perhaps some wear to rings and/or cam lobes, but that's to be expected.

Compression for #2 still stands out a bit and again may suggest some wear in cylinder components, but could just as well indicate excessive valve lash. You should check and adjust (if necessary) your rocker arm to valve stem clearances. However, because you have indicated that you have "... very limited knowledge of car mechanics," I would strongly suggest you ask a local and experience MG enthusiast for some (in person) help/guidance with this procedure. It isn't difficult, but it needs to be done correctly. Note: As suggested by David, some Marvel(or the like) treatment might help on the numbers and engine performance.

I don't have much confidence in colortune results. My personal preference is to monitor the appearance of my plugs at the firing end. The MGB OWNERS WORKSHOP MANUAL by Haynes gives some good pictures of what you should be looking for in this regard (i.e., "Normal. Grey-brown deposits lightly coated core nose ..."). If you don't have it already, suggest you buy this manual along with the more comprehensive manuals by Robert Bentley. Caution: You definitely don't want lean mixtures in your cylinders (leads to burnt exhaust valves and a whole bunch of troubles).

Don't jump on the your carbs before replacing, as you say, plugs (correct for the 'B), wires, cap and rotor. You probably don't need to replace these items, but having such spares is a good thing. Run it (your 'B) and let us know how it performs and what your plug ends look like. Listen for engine ping upon (3rd or 4th gear) acceleration up a hill. Results will guide you (us) on what the next step(s) should be.

After some reading, you might be tempted to replace your points and adjust ignition timing. Don't try this (now) without some experienced help. You might end up digging a hole you don't need to be in.

Patience - one step at a time.
Steve Buchina

I agree that there are many good miles left in this engine. I had a spate of problems with no 3 plug fouling but now I have finally got the timing and carbs right for Shell V power they all keep clean. I no longer get the light brown colour but I think thats gone the way of lead tetrachloride.
Stan Best

Les-
Sorry, but this time I must disagree with you.

I would like to debunk an old myth about the Original Equipment B Series engine. When equipped with an intake manifold that gives the incoming fuel-air mixture a straight shot at the intake ports (such as the Original Equipment intake manifold), the inner cylinders do not run richer than the outer cylinders. In reality, the pressure waves in the siamesed intake port that result from the 180° throw difference of the crankshaft have a definite influence on the state of fuel-air mixture separation and fuel condensation in the arriving fuel-air charge in the siamesed port, and this is what creates the impression that the inner cylinders run rich. The so-called “rich mixture” in the inner cylinders is in reality the consequence of the problem of interplay between the resulting stuttering flame propagation and the condensed (reduced atomization) of the gasoline that are caused by the return pressure wave inside of the runner of the intake manifold. The color striations in the carbon deposited in the combustion chambers that resemble sand ripples on a beach indicate interrupted flame propagation in cylinders #2 & #3, while the combustion chambers of cylinders #1 & #4 are much more evenly colored and grade out from the spark plug to the opposite wall of the combustion chamber.

You said: "When there is an air leak, cylinders two and three, running leaner than cylinders one and four, are the first place this will show up....." In order for this to occur, two air leaks would have to exist because the center cylinders do not use the same runner. If an air leak were to occur, both of the cylinders that are fed by the runner would both run lean, such as #1 & #2, or #3 & #4.
Steve S.

This thread was discussed between 19/06/2008 and 22/06/2008

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