MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - I think I've burnt out a valve.

Hi all,

Whilst driving my newly acquired midget home, I had a bit of a problem, the fan belt snapped whilst we were going 65. Even though I turned the engine off straight away it still over heated. After letting it cool down it ran fine but now it will only run on 3 cylinders.

A compression test revealed 180 on 1, 2 and 3 but only 90 on number 4. What could this be ? I highly doubt the head gasket as after overheating it ran fine. And its not a piston ring as it's not burning any oil.

I'm now thinking I've burnt out a valve what do you recon ?


Cheers,

Will
W Armston-Sheret

It hardly matters... whether gasket, valve, seat, rings, or bore... then the hard comes off anyway.

So... you will certainly need a gasket. Whilst the head is off, you should inspect the gasket itself, and then the valves (and seats) for the offending cylinder - ex side more likely culprit.

If everything in order check the bore (mind you, for 90psi the bore would have to be dire!); then the piston and rings (drop the sump, undo the b/w and the piston will withdraw from the top).

If you start early enough, and have access to parts (buy a quality gasket in any case), you should be able to have the job done within the day.

A

Anthony Cutler

Well I've got a gasket spare already, so Thats good, I think I'll take the head of tonight and see whats what. What size are the nuts the hold the head on ?
W Armston-Sheret

Before you start on removing the head, please confirm the torquing sequence for the nuts, and remove them in the opposite order.

I only add this because of your question about the size of the nuts, which might indicate that you are new to this, and so I wanted to make sure. My apologies if that is not so.

A burned #4 exhaust valve might indicate that the timing was over advanced (BTDT when I had an old distributor with worn springs that was over advancing at higher RPMs, and kept burning that valve. It was a bear to find the cause of the issue).


keep us posted on what you find!

Norm
Norm Kerr

Norm
When removing the head cold the sequence of head bolts is not important.

Only when hot or of course when torquing down is it of intrest
Onno Könemann

Well it wasn't the gasket. looks like some of the valve needs re seating.
W Armston-Sheret

Will. Are you sure it's just re-seating and not burnt? 180 across the other three sounds OK. I wonder if the engine has been run with the heater tap turned off all summer, which can lead to a hot spot around number 4 cylinder, which wouldn't help. Assuming this is an A series car and not a 1500 that is.

Bernie.
b higginson

Hi Bernie,

The valve didn't seem to burnt much at all. I can't confirm about the heater tap being off, I've only just bought it.

Also the car appears to have a 12G206 head is this the right one ?

W Armston-Sheret

Will Not sure why a valve would burn out when a fan belt broke even if it had overheated the engine??

Are you sure that the bores are not scored?
Robert (Bob) Midget Turbo

Did you check valve clearance before you took it apart? Might have been no or excessive clearance on that valve.
Pieces of carbon etc on the seat can do it, as can a sticking valve - all more likely if the car has been sitting a while.
General rule is to never rely on a single compression test reading, and always check valve clearances before checking comp.

FRM
FR Millmore

12G206 head....hmmm,

What engine do you have....its not a A-series 1275

Prop
Prop

It may well have been damaged before but to be honest I'm not sure the valve looks very burnt. However they do all look a bit ropey, so they're really due for replacement anyway. I am ashamed to say I didn't check the valve clearances, but it wasn't a sticking valve.

Spec wise it has a Standard 1098 with a 12G206 head twin SU carbs with pancake filters and an LCB exhaust. I'm not sure if any of that makes a difference. I also don't know if it has the right needles. What would you expect with a spec like that ?
W Armston-Sheret

Will-
Doesn't take much "sticky" to lower a compression test. Any pics of head and valves?

12G206 is the rarer of two heads fitted to the 1098 Midgets, 12G295 is the other.

Being in UK, might be worth a call to Peter Burgess, as he can: first, fix your head correctly; second, do some mods to make some useful horses pretty economically; third, tell you what else to make it all work well. And even set it up on his spiffy RR.

FRM
FR Millmore

X2 FRM

You wont get any better then peter burguss

Did you try a vacume gauge hooked to the intake manifold yet, or athe very east a leak down test

Prop
Prop

And just to make it complete usefull heads for the 1098 are:
12G202 mini and minor head but lots of tuning potential ( more than the others) but less of a stormer than the others in std form.
12G206 early cooper and midget smallbore cracked a lot so superseeded by
12G295 later cooper and midget smallbore more reliable but still cracked.

Moral of the story get it crack tested!!!!!
And if you want ultimate power from a modded head use the cheaper 202 as a basis
Onno Könemann

Hi Will

I hope I am not being a sod but I am not sure you know what to look for when determining a burnt out valve.

All valves look burnt to the casual observer but one that is burnt out is difficult to describe.

Occassionally they are obvious when a chunk of the seat has either broken off or melted away.

However other times it is a matter of inspecting the seat. On a good valve the seat ought to be a clean "polished?" ring of something lick .1mm or a little more. A burnt out version would probably be very dull. As described by FRM this can be caused by the tappets being set too tight and subsequent valve not seating.

Do you have a photo of the valve you believe is burnt?
Bob Turbo Midget England

Hmmm, Well I guess it could've been a sticky valve then. Basically the valve that was removed was dull around the ring and not seating very well. Although I am not sure if this was enough to cause the problem, however the valves and valve guides had a lot of play in them so whilst there out I will be replacing them.

I will do a leak down test with the bores shortly to confirm it isn't a piston ring, but the engine doesn't seem to be burning any oil.

Although looking at Peter burgess I am tempted to splash out and get a lead free head.
W Armston-Sheret

Amongst the other good advice, a couple of other points:
Did you do the compression tests consistently, and all with the throttle wide open in each case?
Did you do wet and dry tests?
If the compression is low, then another possibility to check for is a worn cam lobe.
Guy

Hi Guy,

Yes, the compression test were done consistently.
I don't know what a wet test is so I presume I haven't done one of them.

I'll check the cam lobe.

Ta.
W Armston-Sheret

Sorry Will,
A wet test is when you repeat the same compression test (same number of engine revolutions, same wide open throttle) but with a few cc of oil added on top of the piston - I add 5cc when doing this. The oil helps to seal any piston leakage so if the fault is with the rings or bores the test figure will improve considerably. If the figure for the low cylinder is no better, or only marginally better, then the fault is with head gasket or valves

With the head now off, a leak down test with paraffin will show much the same thing.
Guy

Will,

When you said:
Although looking at Peter burgess I am tempted to splash out and get a lead free head.

It sounds like you were considering Peter Burgess, but that you think you can't get a lead-free head from him. I am quite confident that you can.

A lead-free head is just a standard head with different exhaust valve seats intalled.

Charley
C R Huff

This thread was discussed between 27/09/2011 and 28/09/2011

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.