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MG MGB Technical - Help understanding compression test results

I drove my GT into work today. She's been a bit ignored while I work on my roadster. Car ran great except I noticed on the drive the oil pressure would occasionally drop from 60 to around 20 then jump up again. When I got to work I checked the oil and it was really low so at lunchtime I went and bought some oil and topped her up.

On the drive home I developed a misfire. The car was only running on three cylinders. When I got home I pulled the plugs and found No. 3 was fouled. Black gunk completely bridging it.

I decided to check the compression thinking maybe I am getting oil in there. My results were all over the place at first since but that was because I didn't spin it over long enough. I now do it long enough to start showing some movement on the oil pressure gauge and that seems to even out the results. I also do it with the throttle wide open. I did them dry first then with some oil squirted in the cylinders. This is what I got:

1: 150/180
2: 150/175
3: 150/175
4: 150/180

Now I know this means the bores/rings are worn. But why would the #3 be fouling? Could I be losing oil though the valves too?

Is it worth me pulling the head off for a look? The 1 and 2 plugs were nice and ash brown coloured. 3 was awful and 4 was sort of half brown but not oily at all.

Simon

Simon
Simon Jansen

Simon. You have an excellent compression test, or a bad guage. The readings are some of the most consistant I have ever seen--150 across the board. This is more than sufficient to indicate a well broken in engine which has plenty of life left in it. The somewhat higher readings when performing a "wet" test are also very consistant. The general "specification" for a compression test is that all cylinders should be within 10% of each other. You are doing much better than this with your readings. (I might point out that you will show about a 20 psi reading increase, dry to wet, with a brand new, just broken in properly, engine. This is because the oil seals the gap between the piston and cylinder bore and the gap between the rings slightly.)

Thus, I consider your engine to display excellent compression test results. But, remember that a compression test only checks the compression rings and the ring to cylinder bore condition. It does not do anything to check for a stuck oil control ring. Good compression and stuck oil control rings are perfectly possible, especially on a car that has not been driven for a while. Stuck oil control rings could be part of your problem. These might be freed up by removing the spark plugs and spraying some good quality penetrating oil into each cylinder, some Marvel's Mystery Oil (or locally available equivalent) or some ATF, into the bores and letting it soak for a day or so. Then, turn the engine over on the starter, plugs out, to blow the excess out of the cylinders and all over the engine bay--good show. Install a new set of plugs and drive the car a lot to see what happens. Best case would be a stuck oil control ring freed up by the penetrating oil and the driving. Worst case is no change.

Yes, the intake valve is another source of possible oil contamination of the combustion chamber. Broken/missing oil seal and/or worn valve guide could give you this indication. If the problem does not get better after having used the penetrating oil and driven the car regularly for a week, the head is the next easiest thing to check.

After that, it is time to pull the pistons to check the condition of the oil control rings. Much more complex job. Les
Les Bengtson

Thanks Les. I should say those results are probably +/-3 since my gauge doesn't have a very accurate scale (increments in 5). But the readings were very consistent once I turned the engine over enough. I think the turning till I saw pressure rise meant I got the same cranking time for each test.

That's interesting what you say about a 'brand new' engine. I will actually be able to test that on my roadster once I get to the point of starting it. That engine is rebuilt with new pistons and overbored cylinders. I'll make a point of doing a test once I get it running and post the results.

Never seen Marvel Mystery oil here but we do get some stuff locally that might do the trick. It's called Moreys (http://www.moreys.co.nz/) and they have what they call an upper cylinder lubricant. The blurb makes it sound like it should do exactly what you talk about here. As you say it is worth a try.

Thanks again!
Simon Jansen

Simon, I agree with Les and also wonder, if it's on SUs, if the rear carb is a bit rich,

<<The 1 and 2 plugs were nice and ash brown coloured. 3 was awful and 4 was sort of half brown but not oily at all.>>

Once a plug misfires, for whatever reason, it fouls and may eventually stop firing completely. There's always some oil gets into the chambers of these engines and it will no longer burn it off so it will look oily.

There could be other reasons such as duff HT lead, diz cap or plug cap. I'd check and clean these, fit new plugs, reset the carbs and timing, then take it for a thrash.

On a general note I find NGK BP6ES very good. I run vintage trials bikes which are oily and run slowly, and they rarely miss. Also a detergent engine oil SG or later to help keep the rings and valves free. Rich.

Rich

Maybe I should not say this in public, but my experience has been that an engine can be quite worn , but will still run fine if you get the ignition and carburation correct . With the compression tests here which show lots of life remaining I would vote for the rear carb being rich , but would also do the dizzi cap plug leads etc . I also quite like the gunsons colourtune for setting carbs , but please don't lets start that one again .
I have used Redex at the start of the season to make sure the rings have not gummed up over the winter,on Chris of Ocataine's advice .
S Best

Simon,

The fact that you took a lot of cycles to reach the full pressure is indicative of worn rings I believe (depends just how much you had to spin it over to get to the full reading!). In a really good engine it should only take about 3 cycles, with the first cycle jumping 2/3 of the final total (Source: http://www.pumaracing.co.uk/comp.htm). Also a leap of ~20% for the wet test to me would suggest some wear (my new engine jumps <5%). But, the readings are so consistent that this would just indicates nice even wear, and not a problem that would lead to fouling.

Could a sticking oil pressure release valve cause the erratic oil pressures seen?

Iain
I D Cameron

Sorry, I should have added at the start that the dizzy cap, rotor, leads and plugs (which are BP6ES) are all new (stolen off my unfinished roadster). I think this has been happening for a while since I found a few weeks ago that the number 3 lead was bad and the cap and rotor showed signs of arcing. The plug I removed then was also oiled up. That's when I replaced them. I thought then maybe the bad lead caused the plug to stop firing so it gummed up. Perhaps it was the other way around?
Simon Jansen

Worth swapping the coil too if it's same type. If something's marginal it may not fail all together but may start showing problems at the weakest point first.

Then back to a rich carb or oil fouling Simon.

Resetting the carbs is quick and cheap so I'd go there first. Remove the float bowl tops also and check the floats and shut off heights.

Worth checking the tappets as it's not unknown for a cam lobe to wear down, a friend's went onto 3 cylinders before he found out!

It could be general wear, or valve stem seals though you'd really only be looking at No3 inlet with these symptoms. I had a C which ran very well yet consumed a pint of oil every 50 miles. The only thing wrong was oil control rings worn nearly flush. You may have 1 broken. A new set of rings in de-glazed bores and it ran as new. What's oil consumption like?




Rich

I concur with only a handful of compression cycles being needed for maximum pressure. Did you remove all plugs and crank with the throttle wide open? If you drive for some time with a non-firing plug you won't know if the fouling caused the misfire of vice-versa. But if you check the plug *immediately* it starts firing on just three and the plug is fouled, then it is the fouling that has caused the misfire which indicates oiling, which could be from valve stem or rings. If valve stem this normally reveals itself by a cloud of blue smoke when accelerating after a long downhill section on the overrun, or a long idle. If rings I'd expect much more of a difference between cylinders than you have. With clean plugs do you get an even drop in revs when disconnecting each plug in turn? One cylinder giving significantly less of a drop than the others indicates weak firing, which is probably ignition. Two weak ones indicate unbalanced carbs. If the rear carb was rich I'd have expected more fouling on 4. But after making sure all the ignition system is setup correctly, and the valve clearances for good measure, definitely setup the carbs from scratch to make sure they are right too. Look out for one carb needing more turns than the other to get it to the right mixture point when both were set to the same starting point. Make sure the choke mechanism is fully shutting off on both carbs, and operates equally on both
Paul Hunt

OK, thanks for all the answers. Looks like I have some more playing to do. I am not sure what the oil consumption is right now since I haven't been driving it regularily enough to notice but I will start making a note of it now.

Paul, the plugs and leads were new a few weeks ago and I it only started missing again yesterday on the way home. I pulled the plugs as soon as the engine was cooled down a bit so I could touch them! I have previously tried the disconnection leads trick too and to tell the truth I find it hard to hear any difference in revs. I have a meter though so I will try that again and use the meter to measure.

Haven't noticed any smoke during normal driving. Did make an impressive cloud though when I restarted the car after the wet test of course!

When I didn't turn the engine over as much the readings were between 130 and 140 dry but I could never get consistent readings. I'd re do the same cylinder and get a reading +/-5 from that.

I'll start looking at the carbs. I have never done that bit before but I need to learn for getting the roadster going soon. And if I completely bollocks it up my mechanic isn't far away!

The only other thing I have noticed is the car usually runs great except sometimes I have an ever so slight hesitation when opening the throttle. Usually when the engine isn't fully warmed up. I checked the carb dampers and it's not a lack of oil. It's only very slight though. Just enough to make me feel a little nervous it might stall but it never has.

Simon
Simon Jansen

Hesitation when not fully warmed up, if you have pushed the choke fully home a bit early, is normal. Which is why choke return is a gradual process as the engine warms, one learns to feel the length of the exposed metal rod with a finger and compare it with the temp gauge reading. If it doesn't hesitate when fully warmed then damper and mixture are correct - or at least the mixture isn't weak.

Ah yes, the cloud after the wet test :o) On a V8 you get an enormous clatter as well because the hydraulics cam followes have all 'deflated' after all that cranking. Thought I'd knackerd the engine!

Only tackle the carbs after everything else is right, some apparent symptoms of an incorrect mixture can be caused by ignition problems.
Paul Hunt

For now I need ot recharge the battery. I went out for a play this morning and it was flat! It is a bit weak that one actually so I should replace it soon. Or at least get a trickle charger for when I am not driving it all the time. I might try the ATF in the bores trick since as Les says there is nothing to lose there. I had a look at the autoparts store but I don't like the look of all the 'snake oil' products. They never say what's in them and I suspect most of them are just kerosene dyed a pretty colour!
Simon Jansen

This thread was discussed between 14/04/2005 and 17/04/2005

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