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MG MGB Technical - compression

I have measured the compression in my 76 mgb with a simple handheld tester.
the values are for the 1 to 4 cilinder:
173-168-168-163 psi. I have measured twice with an interval of 2 weeks, the results are very close the same.
The engine is running very good, but after aprox. 100 miles the no.4 sparkplug is getting problems with firing due to the fact it is very dirty of oil and coal remainings. when I put in a clean sparkplug then the engine is back in realy good performance.
What to do now?
Jacob Halma

Jacob. You need to find out where the oil is coming from since that is what seems to be causing the problem. The two most common problems are either bad valve oil seals/worn valve guides or bad oil control rings on the pistons. I would suspect the oil is coming from the valve area myself.

To check and replace the valve oil seals, you need to remove the valve cover, then either remove the rocker arm or move the rocker arm sideways to clear the valve, remove the spring and valve keeper, then remove the old oil seal (if it is still there), replace the oil seal, then install the spring and keeper. You also have to do something to make sure the valves do not drop down into the cylinder. Others have done this procedure this way and their full recommendations are in the archives.

When I have done this, I have always removed the cylinder head from the engine. This allows me to use my valve spring compressor, inspect the condition of the valves, measure side clearance of the valves, clean out the combustion chambers (called de-coking in England) and lap the valves or have them refaced as necessary. Les
Les Bengtson

I hear people saying, your cilinderhead is mostlikely cracked.
But I think when I have such compressions in the cilinders this is not the case. (Or can it still be the case?.)
What are the signs of a cracked head?

Jacob Halma

A close relative of mine (VERY close) once used the wrong gaskets and covered holes in the outside of the carbs. Air that couldn't make it through resulted in an extremely rich mixture and black plugs.
Dan Robinson

Dan, if this is the case then I would have the same on the no. 3 plug to? because the carbs are doing 1&2 and 3&4.
Jacob Halma

Jacob. Yes, it could be a cracked cylinder head, but, all of the cracked cylinder heads I have seen have been cracked between cylinders 2 and 3 or just in cylinder 3. The problem they showed was leaking coolant when the head was under pressure, not leaking oil into the number 4 combustion chamber.

As to Dan's comments. The number 4 and number 1 cylinders run richer than the number 2 and 3 cylinders because numbers 1 and 4 have their own exhaust ports while the number 2 and 3 cylinders share an exhaust port. Thus, it would, in theory, be possible to have a too rich mixture in number 4 while still having number 3 fire. But, my experience with this has been that the number 3 plug also becomes carboned up, just not quite as much. My experience is also that the soot is dry, not oily.
Since your plug are clean on number 3 and oil fouled on number 4, I would be more inclined to suspect either a problem with the valve guide or oil seal on the intake valve for number 4 cylinder.

There are now better oil seals available to fit the MGB engine than back when they were originally made. The original oil seal was a simple, rubber O ring. You can now get an "umbrella" oil seal which provides better oil control. Les
Les Bengtson

Les, OK, your comments make me less worried about the head.
Now I have one other question about how to ID the engine when the id-plate is not on the engine.
My mgb roadster is an original direct from the UK imported to the Netherlands car in 76. car ID GHN5 405815 G.

I figured out (more a wild guess) that related to the carbs and distributor among other stuff, wich are fitted on my engine (SU HIF4 and Lucas 45D4),my engine number is according the Haynes owners manual something like 18V/846F/H......? Could that be right?

Is it very importend to know exact your engine number for ordering spareparts?
Or do the taken out parts from the cilinderhead such as valves and seals e.g. show the way them self.

Jacob
Jacob Halma

Jacob. Most of the engine parts are completely interchangeable. Those that are not, such as motor mounts and intake valves, will be obvious when you view the old and new parts. If the engine does not have the ID tag, it may have been rebuilt in the past. This also indicates it may have a fair number of miles on it. Hence, valve guides and oil seals are the first place to start looking. If, however, you decide to pull the cylinder head from the engine to check valve guide clearances and replace the oil seals, it would be an excellent idea to have the head checked for cracks and for warpage while it is off. Do not worry about what may be wrong. Worry will not help fix it. Make your plan about how you are going to check out the problem and what other checks you might perform while investigating the oil fouling problem. Les
Les Bengtson

Jacob and Les A good way of holding the valves up while changing the seals is to weld a pipe into an old spark plug and with the valves closed apply compressor pressure to the cylinder. It works great and you would be amaized at what it takes to force a valve open. I have used this system for many years with no problems. Though it does pay to put a bit of pipe over the collets onto the cap and shock the collets loose before trying to remove them.You could get years ago a special spring compressor that sat over the valve and hooked into the head,then you levered down on the other side but you can usually get by using the valves beside the cyl your working on to lever off, being carefull not to bend anything. Denis
DENIS

Les,
Typical condescension.
Help and assistance is much more appreciated if it doesn;t contain paternalistic and patronising comments.
As an "educator" I would have thought you'd know better.
Then again, arrogance is your style.

Go polish a gun or something.

Phil Liggget

What is the gun reference?
Les is helpful but not always the nice poster. He can be rough but he is giving information. I don;t like the rough style. I don't like guns. He is rude but this is nformation for all.
Alan de Cubber

Les, a appriciate your helpfull comments very much. This will help me when I do my thing needed for keeping my MGB on the road. (I will check the oil seals first)

Then to Phil, as you are so rappid (like a gun)with comments why did you not give me some advice on this matter? criticize someone else from a far and save distance is easy thing to do. IMHO this is a technical forum where people with the same interest and hobby share there passion for, in this case an old car. Excuse me for not using such nice and beautiful words as you like to use. best regards Jacob

Jacob Halma

Is this Liggget Real or a virus?
DENIS

Just like our MGs, people are different. We have different views, styles and temperament. That adds spice to life. I always find Lesī comments very helpful, and enjoy this BBS enormously.

May be I miss something because English is not my native language, but I canīt help feeling that if you have a bad day, this is not the right place to spend it.

Tore
Tore

I've seen people that were actually bad enough to be run off the board. Les isn't and he has generally got very good advice earned through years of having multiple MG's and a certain technical proficiency most of us don't. If you have a problem with someone then deal with it directly..email them..which we would with you if you had the guts to post your email address.

J.T. Bamford

Sorry to see this sort of abuse on this great board. Les, this guy is clearly a headcase and I am sure you will shrug off his flames and keep posting your useful and well received comments. "Liggget", get some professional help.
Steve Postins

Oil may also enter the cylinders via the breather system. I do not know what the setup is on your year of car, but on some MG engines I have seen the breather going into one end of the inlet manifold. It could be possible that the majority of the oil available from the breather pipe is making its way into just one cylinder. It's not terribly likely, but disconnecting the breather system for a few miles is worth a go considering it costs very little (you have to buy a can of pop, so that you can empty the can and then redirect the breather hose into it).

Most likely explanation though, as stated by those above, is a worn or broken valve stem seal.

Good luck with your repairs,
--
Olly
Oliver Stephenson

A simple way to keep form losing the valve when performing a seal replacement on the car is to use rope.

Feed nice soft poly sash-cord into the cylinder you will be working on (piston starting compression stroke) through the spark plug hole. Rotate the engine until you feel resistance (with a wrench you can rotate until it just won't go w/o causing trouble).

Now you can remove the keepers with no worries.

p.s. careful of those little keeper buggers. plug up your push-rod tubes or Murphy will certainly send one down there!
mike!

Mike

I have seen or tried just about all the recommendations above on doing a valve job with the head on, but your's is an original for me. makes sense and is easy.
Bruce Cunha

To all. Sorry that the pissant is back. He has popped up, from time to time, and seems to have developed a personal hatred for me. All I can say is that I pity him. He has been here under a number of names, but, always the same--he does not have the balls to address me personally, but only does so using an assumed name and on the BBS. One of the reasons I publish an e-mail address. If you have a problem which you think I might be able to help with, or, if you have a problem with me, I am perfectly willing to discuss this off line.

Back to the important stuff.

Olly. Have you actually seen a PCV system failure cause this type of problem? I realize that, in theory, a failure of the PCV system could cause oil fouling of the plugs. My reservations about this stem from several sources. First, in theory, if the PCV system failure could cause oil fouling of the spark plugs, I would expect it to take place on all the plugs, not just number 4 plug. Second, I have had two such failures in my life (some 54 years). One was on my 68 GT with the "Mushroom" type PCV valve. I did not know the problem even existed until I could not pass emissions testing with my own efforts at tuning and took the car into a professional mechanic for his assistance. He found a bad diaphram in the PCV valve as part of his diagnostic testing. But, the car ran fine and passed the emissions testing with the same spark plugs. (Root cause of the problem was the carbs--a three year old set of Burlen SU HS-4s.) The second example was on my Ford Bronco with the 300 CI, six cylinder engine. The replacement engine had been machined with two of the cylinder bores too small which caused two of the pistons to collapse. As a result, it was blowing oil out through the PCV system and front and rear seals. However, the spark plugs never fouled out and I was able to drive it over 1K miles, at 55 mph, to get back home. So, if you have had this form of problem and it produced a result similar to what we are discussing, please let me/us know as it is beyond my level of experience. By the way, I am getting ready to modify the transmission cross member following the pictures and instructions on your website. I thanked you, personally, before. Allow me to thank you, publically, again. The photos made, to me, all of the difference in understanding what needed to be done.

Mike!. From reading your posts over the last several months, I realize that you have a signficantly better technical background than I do. I am self-taught, with a little help from professional mechanics over the years. The "old rope trick" has been advised for both holding the valves in place when working with the cylinder head in place, and for breaking the cylinder head loose when the nuts have been removed from the head. I have tried the latter and it works quite well, but have no experience with the former, but believe it should work. However, my question is, "Should you do this to replace the valve oil seals?). It would seem that, if all is well, this system should work fine and save a great deal of extra work. My experience with engines burning oil, however, is that there is a significant build up of "crud" on the underside of the valves and that this can interfere with the flow of the exhaust gasses and may, eventually, cause the valves not to seat properly. Thus my recommendation to Jacob to pull the cylinder head and inspect it. Is your experience different? If so, please let us know as, besides pissant boy who knows everything, most of us are here to share information and learn from the experience of others.

Thanks to all. Les
Les Bengtson

Les - agreed it's unlikely but I have seen two engines getting fouled plugs at least partly due to the breather. One was my own B-series, and after a while on a trailing throttle it would throw out a massive cloud of oil smoke. Admittedly the fouling would burn off quite quickly once back on normal throttle, but the plugs were a bit blacker than you would really like.

The other was an a-series which was just rubbish :o) Valve stem seals were a bit shot also though.

I also agree that it's most likely to affect all the plugs. However, as I said, it's worth eliminating it from the picture as it is free to try.

ttfn,
--
Olly
Oliver Stephenson

Very glad to have so much possitive comments.
First of all i did some investigation on the engine and found out that on the no.4 cilinder the inlet as well as the exaust valve where set must narower then the other 3 cilinders. one off the locknuts was kind of wrecked. (by the way, this was a cold engine check) so i made exactly as the other 3 cilinders. tomorrow i will take a ride and check the plug (i do not expect any improvement, but you never know). And as now I am fully prepared to do the old indian rope trick. I have got the seals in house. I can always pull the head coming winter.
I will keep you informed about the results i get.
Jacob
Jacob Halma

I don't really see the need to use rope to hold the valves in place to change springs or seals if you have an air compressor it's much easier to do as Denis suggested. Most compression gauges have a coupler in the hose. Turn the engine so that the affected cylinder is at TDC. Screw the compression gauge hose into the affected cylinder. Connect the air hose to the compression gauge hose and as Denis said it is hard to get the valve to open. If it does and the engine is at TDC the top of the piston will catch the valve. Most race engine shops use weak break-in valve springs to bed in new high lift cams and change back to heavy springs after the cam is bedded in. All engine shops I have seen make spring changes used air pressure with the piston at TDC to hold the valves in place while making the spring changes. The big problem with the MGB engine will be getting the spring compressed to install the collets as Denis mentioned. He has figured out how to do it but for a beginner to do as he suggested you need to be prepared for some flying collets, retainers and springs unless you are very lucky. It would be best to fabricate a spring compressor. Good luck, Clifton
Clifton Gordon

Some comp gauges have a coupler, but many do not. I have an air coupler brazed into a dead sparkplug body for this purpose. Be aware that when pressurized at or near TDC, the engine will want to turn with great force. It is absolutely essential to have the car in gear, with handbrake set hard, and wheels blocked solid in both directions. You can very easily put the car through the garage wall, with yourself in the middle! Not to mention getting hands caught in fan belts and such. If the engine does turn, it will lose pressure, dropping the valve into the cylinder.
FRM
http://www.usachoice.net/gofanu
FR Millmore

FRM is right about the safety factor. I should have remembered to mention that because the last time I did a leak down check I had the engine move. I guess that makes the rope idea safer. Clifton
Clifton Gordon

I surpose its how much time you have, the professionals use air pressure because its other peoples money there spending in workshop hours, where if your doing your own it doesnt matter how long you take. Most car owners own a compressor(you can buy them in oz for $99) I'v used this syston for over 50 years and I think most other people in the trade do. Having said this it is still necessary to take the precautions that F R M states, though if the car moves it will be in inches and not feet. Denis
DENIS

Denis is right that it won't move far, but many people are working in cramped conditions, often with onlookers, space heaters, and other things best not run over! Like your buddy's prized XKE, or your wife's brand new ride, or your legs.
FRM
http://www.usachoice.net/gofanu
FR Millmore

This thread was discussed between 14/09/2004 and 18/09/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.