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MG MGB Technical - 72 cylinder head

I am attempting to sort out a poorly running 18v585z(overdrive) engine. I have good compression but excessive blowby through the valve guides. Here is my question: as I have begun the removal of the head #12H2923AFIH I have found dual valve springs. My research indicates the head should have single springs. Before I order parts I would like to know if the installation of the dual springs by a PO has a negative effect on the performance of the engine? In other words, could this modification contribute to the poor performance I am experiencing? Thanks, John
JR Mahone

The (Econotune) head I got from Peter Burgess had dual springs fitted. I doubt having dual springs in itself will be your problem.
Geoff E

sounds like a PO re-build. Should be no problem at all unless, and i've seen this, the springs have been put on with the single spring collars. Or you have high lift cam and the springs are becoming coil bound. The actual closing force is little different.
How are you noticing the "blow by?"
I presume you are wanting to order guides and valves. Go for bronze guides on the exhaust, these help to remove heat from the exhaust valve, as well as coping better with restricted lubrication.
Allan Reeling

The dual valve spring setup was used for most of the life of the B series engine. The single spring system was a cost cutting measure. The main difference will be noticed at higher RPMs, where the increased valve spring pressure will prevent the valves from "floating" and hitting the pistons. Installing silicon bronze valve guides will all but eliminate any "blowby" through the valve guides, and the bronze guides retain oil so that they help to lubricate the valve stems, helping to keep them cool. RAY
rjm RAY

If the engine has had a cam replaced with something not stock, it may require the dual springs to work correctly.
If the springs are correctly installed and not defective, they will not cause a performance problem, and may prevent one.

FRM
FR Millmore

Mayhap someone could be kind enough to explain to me how one gets "excessive blowby through the valve guides? I am having trouble getting my head around this concept.

I understand excessive stem to guide clearance which will produce excessive oil consumption.

I, also, understand the four stroke engine cycles:

Intake--intake valve open as cylinder fills

Compression--intake and exhaust closed as fuel/air mixture is compressed

Firing--intake and exhaust valves closed as the mixture is burning

Exhaust--exhaust valve open as the cylinder rises and forces out the burnt gasses.

It would seem to me that the only time when there is any possibility of "blowby through the valve guides" would be on a single exhaust valve, open during the exhaust portion of the cycle. And, I have some difficulty understanding why you could have any great deal of exhaust gasses, which are being pushed upwards, with the relatively flat face of the exhaust valve diverting the flowing gasses away from the stem of the exhaust valve, able to make their way up the valve stem and through the valve guide.

Should someone be able to explain this to me, I would appreciate it. I am completely unable to understand how, when most of us have to use oil seals to keep excessive oil from being sucked down into the valve guides, there could be a condition that is doing the exact opposite of the common experience.

Also, if there were some condition, such as bad valve to cylinder head seal (burned valve, worn valve/seat, or cracked valve) why would the blowby be directed up the valve guide rather than out the exhaust and/or intake manifold?

Les
Les Bengtson

Thanks for all the input. I was trying to avoid getting too deep in explaining my problems. I have been trying for two years to get a smooth running and reliable engine.Regardless of what I have tried, the engine misses at 2800 to 3100 rpm when in 4th gear. I have tried every timing setting in 1 degree increments from 10 to 25 BTDC with a professional set back timing light with built in tach. The engine becomes passable at 15 degrees at 1000 RPM. I have rebuilt HIF carbs, properly set up, and totally new ignition components. In fact I have been through 3 distributors, original, rebuilt original, and just recently a new Petronix Distributor. The compression test shows 160#+or-5# for each cylinder. The cylinder head is my last effort. With all my fiddling I observed the rocker vent was blowing air/fumes quite strongly and simply guessed exhaust was blowing past worn guides. I did not observe any positive pressure coming form the crankcase vent therefore assume my rings are good. Simply put: the cylinder head has become my last resort and a rebuild is not that expensive. If the addition of dual springs is considered an acceptable mod (assuming all the correct part were used)then I will not have to order as many parts and avoid having to convert back to single springs. I will definitely use silicon bronze guides. Thanks guys! John
JR Mahone

John -
You should have the same pressure/fumes out the rocker cover as the side cover vent - little. The rocker vent should be attached to the charcoal canister, and the side vent to the carbs via the Y connector. If this system is not connected and working properly, it throws the low throttle carb settings off, which usually gets compensated for in carb adjustment. This, in turn, upsets the mixture throughout the range.

There have been reports of the side vents being blocked up by sludge/carbon, which might explain your problem. The side cover can be washed in carb cleaner or similar. Some people have pulled the wire mesh stuff out and replaced it with stainless steel pot scrubbers.

The flow should be from the canister, INTO the rocker vent, through the engine, OUT the side vent, INTO the carbs. If the only breathing is out the rocker vent, the very small hole will cause high pressure and velocity, so it seems greater than it really is.

If you take it apart, check to determine what pistons you have, and whether there is any sign of head mods. It's possible the engine has been modified for higher compression, which could indicate the real (piston/ring) blowby is greater than you think from the compression readings.

FRM
FR Millmore

Performing a 'wet' compression test will also give you a better indication of ring/bore wear, although it sounds as though FRM may have nailed it with the blocked breather theory.
Dave O'Neill 2

John-
I have to concurr with FRM's diagnosis. Either your venting in the tappet chest cover is blocked, in which case you can clear it by washing it out with carburetor cleaner, or there is a leak in the connecting vent tube to the intake manifold.
Stephen Strange

Let us examine this conundrum. In light of the vehicle being equipped with a charcoal canister system.

The normal thoughts are that the system sucks air through the charcoal canister, down through the engine, and up through the tappet cover, into the carbs.

Is this true?

I took the oil filler cap off the rocker arm cover of my, charcoal canister equipped, 79 MGB. There was positive pressure there as evidenced by my holding a piece of thin paper over the hole in the rocker arm cover and it being pushed upwards rather than drawn downwards.

This might suggest another paradigm. Rather than the common paradigm, drawn from earlier versions of the Positive Crankcase Ventilation system, such as my 68 GT has, with its "mushroom PCV valve". Might not the later versions, which used a charcoal canister, have two sources of lower pressure? One from the front tappet cover and a second from the lower pressure area of the charcoal canister connected to the rocker arm cover through a hose and a restricted elbow in the rocker arm cover?

The charcoal canister, also, is the vent for the fuel tank. Thus, the connection to the charcoal canister is a source of negative pressure (due to the draw of the fuel tank) and would not be, under any normal circumstances, a source of "fresh air" when the engine is running and drawing fuel from the tank.

Thus, I offer a new paradigm for the charcoal canister equipped cars. The minor vacuum from the carbs are one source of lower pressure air (to keep the oil inside the engine--from the bottom end) and the vent from the rocker arm cover, with its hose to the charcoal canister, which operates at a less than atmospheric pressure, is a secondary source of low pressure.

This seems to make sense from my experience with three vehicles equipped with the charcoal canister system.

Last month, my "daily driver MG", a 79 roadster, passed the Arizona emissions control inspection. That, along with the fact that it runs very well, might indicate that my understanding of this system is correct.

Runs quite well and passes emissions--but the oil filler cap, when removed, demonstrates a slight, positive over pressure.

Now, let us address John's problem.

"Regardless of what I have tried, the engine misses at 2800 to 3100 rpm when in 4th gear."

Is this true? Why only in fourth gear? Why not in all gears? What does the tachometer do when the engine is missing, between 2800 and 3100 rpm only in fourth gear? Does the tach wander, indicating a low tension ignition system fault? Is the miss present in lower gears, but not noticed because of acceleration? Might it be some form of carbie problem? (Do not own a car with HIF carbs myself--the single one I worked on was "interesting".)

Much information on what valve guides to use, for a problem which does not seem to exist. Much information on Positive Crankcase Ventilation systems, which may not exist.

What information/thoughts as to the real problem, missing, in fourth gear, between the rpm range of 2800 and 3100 rpms?

My thought, due to Paul Hunt's teachings, over the years, is to watch the tachometer and see if it drops. If so, look to some form of a low tension ignition problem. If not, look to some form of fueling problem.

But, what do I know???

Les
Les Bengtson

Les,

When you did the crankcase pressure check with the paper, did you hold the paper near the filler neck or did you fully place the paper onto the filler neck? If the former, I am wondering if having the filler open defeated the PCV system, and so indicated the pressure you observed. If so, then placing the paper fully on the neck might result in it being held in place.

I agree that if it is rpm and load related, it should also do it in 3rd gear, but with one caveat. I have a vague recollection about part of the emission system on the newer MGBs that had some sort of vacuum ignition advance control that was only enabled (or disabled) in 4th gear. I don't think the 72 should have it, but over the years our cars often end up with a blend of parts from different models.

I do hope John has not pulled a cylinder head that is demonstrating 160 psi cranking pressure because I don't think a valve job is going to solve the problem.

Charley
C R Huff

Les-
Comments in line.

Let us examine this conundrum. In light of the vehicle being equipped with a charcoal canister system.

The normal thoughts are that the system sucks air through the charcoal canister, down through the engine, and up through the tappet cover, into the carbs.

Is this true?
>>>Yes. This is the description from the makers, and is consistent with the observed operation of the system when intact. Correct from the WSM/Bentley reprint for HIF equipped cars.

I took the oil filler cap off the rocker arm cover of my, charcoal canister equipped, 79 MGB. There was positive pressure there as evidenced by my holding a piece of thin paper over the hole in the rocker arm cover and it being pushed upwards rather than drawn downwards.
>>>Then, either your system has been disturbed, or you have more blowby than the carb vents can overcome, or there is some blockage between the tappet chest and carb(s), or the vac line from tappet chest to carbs is not attached in an appropriate way. It seems to me I recall that some or all of your cars have been converted to DD Webers?

This might suggest another paradigm. Rather than the common paradigm, drawn from earlier versions of the Positive Crankcase Ventilation system, such as my 68 GT has, with its "mushroom PCV valve". Might not the later versions, which used a charcoal canister, have two sources of lower pressure? One from the front tappet cover and a second from the lower pressure area of the charcoal canister connected to the rocker arm cover through a hose and a restricted elbow in the rocker arm cover?
>>>No. The sole source of negative pressure is the intake manifold.

The charcoal canister, also, is the vent for the fuel tank. Thus, the connection to the charcoal canister is a source of negative pressure (due to the draw of the fuel tank) and would not be, under any normal circumstances, a source of "fresh air" when the engine is running and drawing fuel from the tank.
>>>No. The fuel draw is insignificant by orders of magnitude, and in any event, the charcoal canister is open to atmosphere. This is why the rocker cover pipe has the restrictor in it. Otherwise you would have a massive air leak through the system and into the carbs.

Thus, I offer a new paradigm for the charcoal canister equipped cars. The minor vacuum from the carbs are one source of lower pressure air (to keep the oil inside the engine--from the bottom end) and the vent from the rocker arm cover, with its hose to the charcoal canister, which operates at a less than atmospheric pressure, is a secondary source of low pressure.
>>>As above, no again.

This seems to make sense from my experience with three vehicles equipped with the charcoal canister system.

Last month, my "daily driver MG", a 79 roadster, passed the Arizona emissions control inspection. That, along with the fact that it runs very well, might indicate that my understanding of this system is correct.
>>>What does your understanding have to do with the operation of the car?

Runs quite well and passes emissions--but the oil filler cap, when removed, demonstrates a slight, positive over pressure.
>>>The emissions test is just what they test, under their conditions. It says nothing about how the car is operating vis-a-vis how it is supposed to operate. The evap control system as you describe will not operate correctly, as there must be vacuum in order to draw the captured vapours from the canister into the engine when running.

Now, let us address John's problem.

"Regardless of what I have tried, the engine misses at 2800 to 3100 rpm when in 4th gear."

Is this true? Why only in fourth gear? Why not in all gears?
>>> Because the load on the engine is greatest in 4th. The problem is there in all gears but is harder to feel in the car operation. If you put it under similar loading in 3rd you can find it, but it is more subtle. If it does it at x rpm in 4th on a level road, it will do it in third at similar throttle openings. It is possible that such throttle opening will be at a different rpm in a different gear; if the advance mechanism is not right, the combination of mixture/timing could mask it. I have had this problem and found that changing timing or mixture would change the load requirements for the problem, and would move the problem up or down the rpm range, or the throttle opening range.

What does the tachometer do when the engine is missing, between 2800 and 3100 rpm only in fourth gear? Does the tach wander, indicating a low tension ignition system fault? Is the miss present in lower gears, but not noticed because of acceleration? Might it be some form of carbie problem? (Do not own a car with HIF carbs myself--the single one I worked on was "interesting".)
>>>Assuming no fault of ignition, this is called "lean surge", and is a result of carb errors, possibly a consequence of breathing system faults, as I stated earlier.

Much information on what valve guides to use, for a problem which does not seem to exist. Much information on Positive Crankcase Ventilation systems, which may not exist.

What information/thoughts as to the real problem, missing, in fourth gear, between the rpm range of 2800 and 3100 rpms?

My thought, due to Paul Hunt's teachings, over the years, is to watch the tachometer and see if it drops. If so, look to some form of a low tension ignition problem. If not, look to some form of fueling problem.

But, what do I know???
>>>Either enough to get yourself in trouble, or enough to solve the problem once you get your head straight - which might well be exactly the same thing! I think you know enough to sort it, but it is very easy for any/all of us to get twisted about, especially when we start with an idea that is almost certainly incorrect, vis "excessive blowby through the valve guides".

FRM
FR Millmore

This thread was discussed between 27/05/2011 and 07/06/2011

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