MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

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 - Low Compression on No. 3

Spurred on by the engine knocking thread by Les, I decided to finally invest in a compression tester!

That was a mistake...

Compression on 1, 2 and 4 a solid 175. But on no. 3 I only get 150.

Checked and rechecked and tested with some oil in the bore too. Always 150.

Your thoughts and advice appreciated as ever.

Cheers,
Malcolm
Malcolm Le Chevalier

Ignorance is bliss!

Not a big enough difference to worry about right now but check every now and then and be prepared to have the head off if it gets much worse.
David Smith

Inlet or exhaust valve maybe damaged and leaking and in need of lapping in or replacement.
David Billington

X2 david

id lay odds on ex valve

p
prop

If there's a problem - and I doubt that there is anything of significance with those figures, then the fact they are the same with and without oils says its not rings or bores, which must be prety good. I usually get a small difference between wet and dry. It could be the beginings of valve leak. Or it could be tappet clearances are different. Did you check them first?

I do (or did when my car was in regular use!) a periodic compression test every few months and record the figures in a notebook. But its important to be consistant in how you do them. e.g. I always adjust the tappets first. Then take all the plugs out, testing each cylinder in turn - all dry first, then each with 5cc light oil. WOT, and count 10 revolutions as it turns with the starter.
Guyw

I'm with David and Prop on the valves. Here's why...

Unlikely to be cylinder wear because why only one cylinder?

Unlikely to be bad/broken rings - the compression loss would be much greater I think.

ISTR your car is a 1500 - well known for compromised oil supply to the top end, particularly Nos 2 and 3.

Is the engine noticeably noisy (rumbly) at the bottom end? I hope not because the 1500 is noted for weak No3 big end, again because of poor design in the oil distribution. A loose bearing might account for some loss of compression, but fortunately that is easy to fix without pulling the engine.

As you don't mention any untoward noises I'm with Guy and leaning toward a leaky valve, most likely an exhaust as they run hotter.

With cylinder head stuff Peter Burgess is the go-to man.

(Oddly enough I have the same job to do tomorrow on 2 Perkins diesels).
Greybeard

Check you tappet clearances...back off number 3 cylinder valves and see if it improves. Hope fully your a little tight and there's no damage.
S

Steve,
Unless they are extremely tight, a slightly lower measurement for compression is more often related to a lose tappet adjustment. If the valve doesn't open as much as it should, the cylinder fills less efficiently and the resulting compression reading is lower.
Guyw

What Guy said too,,,
Greybeard

Hey Guy!

I've been on both ends in this situation...Old yeller my beater MGA...had excessive noisy tappets so I went threw the procedure and tried to achieve unnoisy but with in tolerance clearances. Some how after repeating three times I caused # 3 to skip.

Finally had to limp down to my buddy Mackays...and he was able to adjusted the clearances with me again and all was well. We concluded my adjustments were to tight.

The old timers always said to err on the ticky side of the adjustment to prevent burning a valve.

You are right Guy...Valve adjustment is critical for good performance.

Malc...did you have hardened seat valves put in on your rebuild?

I used to run all my MGBs on stock heads after they were rebuilt but the would always suffer from valve recession within 3000 highway miles.

Now I run hardened seat versions in the MGAs No problems.

Let us know what you find.




S

tappets were set quite meticulously about a month ago. but I will check again when I get home from a weekend away.

greybeard, curious to know why a bad big end would result it lost compression. could you enlighten me?

cheers.
Malcolm
Malcolm Le Chevalier

In theory because the piston wouldn't rise as far, but it would need to be a very loose bearing to have a noticeable effect.
Greybeard

A nice theoretical question there, Ron!
My interpretation would be that the piston stroke isn't reduced by a worn big end, in fact it could actually be increased! I think on the downwards stroke the piston will go as far as the crank will allow,fractionally further than on an unworn crank. And on the upwards stroke the momentum of the piston and conrod would carry it to the max travel as well. So I think the overall stroke is increased.

On a rough calculation, at 5,000 rpm the piston travels at an average 25 mph. So each revolution it accelerates from a standing start to well over 25mph, slows, comes to a dead stop, changes direction, and accelerates back at over 25mph in the other direction. And it does this little manoeuvre 80+ times a second. Even with lightweight components that must equate to one hell of a lot of momentum, reversing direction 160 times a second.

Eeek!
Guyw

I see what you mean there Guy. Would the piston/conrod inertia be enough to increase the stroke against the compression? And would there be a difference between the compression stroke and the exhaust? A nice conundrum - I just don't know.

Anyway I realised how daft the idea was for myself, but missed the edit window.

I did learn one thing though - it's not a good idea to write stuff like that when I've just come back from the pub!

Even so things can be misleading. The boat engines I've just come back from working on for example. The owner had been told that they were down on compression because the weren't revving like they should and the port engine was making black smoke over 1800 rpm. He was very down in the mouth about it, but they both started easily and ticked over very smoothly so I wasn't convinced there was anything wrong with the compression.
He'd also been told it couldn't be blocked airfilters because the engines didn't have any filters, which I thought was very unusual.

So I pulled off the inlet "trumpets" and found perforated metal strainers in the manifolds which were choked with gunge. The owner didn't know they were there and he's had the boat over 15 years to my knowledge.

20 minutes later they were clean and back in the manifolds and both engines run up to full power as sweet as anything with no visible smoke. Sometimes it's the simple things, which is why I like to get the easy checks done first.

Your comment about slack tappets affecting compression made good sense to me but as Malcolm says his were set carefully very recently. So unless there is heavy wear in the rocker pad which I'm sure Malcolm would know about I'm still favouring a slightly leaky valve.
Greybeard

Let me add in my own question

ive got a noisy valve im guessing in ither 3 or 4 ...

When the engine is cold there is no noise but once the engine warms up it gets real ticcy...ive left it alone on the primis that trevor mentioned long ago

noisey valves are happy valves

How should i approach this... just let it be or should i try to tighten it up...my biggest fear is over tightening it

here is what ive got head is ported with exhaust inserts. (Valves are over sized paul ivy intakes and stainless steel racing exhaust .... or maybe reversed? ) high compression valve springs 1.5 to roller tipped rocker and Alumium after market valve cover

nothing is hitting the inside of vlave cover ... i ground down all those spots

my thought is to warm the engine up then cover it up with a tarp and pull the valve cover and with the engine running at temp just find the clacking valve and just hand adjust it till it quites down

Im at that point of if it aint broke THAT bad then for god sakes leave it alone and dont fix it...haha

but yeah im just on the fence...like insaid earlier... im just gun shy at the moment with it working so well

if i do adjust it at running temp, is that a good thing ? But cold it does sound good with no clickity clackity

And yes... i do have an exhaust leak ... but thats a differant noise then this

Thanks

prop
prop

Greybeard,
I was just joining in the game of theorising! In truth I cannot see that worn big ends would make a blind bit of difference either way. A worn big end might have, what? - 10 thou of wear? Pretty small percentage of a 3.2" stroke isn't it!

I also go for the simple solutions first, partly because I am an eternal optimist and partly 'cos its cheaper that way! So I was just suggesting that a slack valve might be one easy cure solution to an apparently low compression reading on one cylinder. In a way, its a pity Malc can confirm that the clearances were done accurately so recently. It rules that explanation out, although its probably still worth checking. Failing that, then it probably needs the valves lapping, so its a head off job though if it were mine I wouldn't be rushing to do it at the height of summer!
Guyw

I also go for the simple solutions first, partly because I am an eternal optimist and partly 'cos its cheaper that way!


Im not an optimist.... im a god praying hoper that hates reality

prop
prop

Guy I like the theory game too. It's great fun and beats the s**t out of the TV any evening! And I agree that my comment was dumb, but I still think that wild theories are fun and posit that once in a blue moon they throw up something useful. Not that one though - I blame the beer haha!

Prop - you seem to have eliminated the obvious stuff. So I wonder about guide wear. It seems likely that guide clearance might change with temperature owing to the different metals involved. Easy check.

>>my thought is to warm the engine up then cover it up with a tarp and pull the valve cover and with the engine running at temp just find the clacking valve and just hand adjust it till it quites down<<

A garage owner I used to go to in Nottingham did exactly that with my Triumph 2000 Mk2 and the result was a burnt out exhaust valve in about 200 miles and my introduction to Triumph head rebuilds. I'd be cautious about it if I were you, although I have done it successfully on assorted old Fords. It seems to be to do with wear on the rocker pads giving false readings but with your roller tips that's unlikely which is why I theorise about the guides.

Another theory might be rocker shaft wear. As the oil heats up and viscosity reduces a liitle bit of wear on the underside of the shaft might manifest as a bit of a rattle. It's super-easy to check as I know to my cost as I know my rocker shaft is totally f****d.

None of which helps Malcolm.I still think he has a leaky valve and nothing much to worry about.
Greybeard

well I am a bit stumped because...

1. pretty sure tappet clearances are ok (but I will check again tomorrow)
2. when I last had the head off about three months back I checked for valve leaks by putting petrol in each of the chambers. no leaks after ten mins (although valve seat may have been damaged since)
3. rockers are ok, rocker shaft is as good as new and also had it apart fairly recently and no significant wear.

I think its maybe a slightly dud valve, I might stop putting in my lead addative and let valve seat recession fix the problem for me :-D

btw... cheers to you greybeard, if there is one thing I like as much as my MG, it is a good beer! :-) ha ha
Malcolm Le Chevalier

Cheers Malcolm. I think you're probably right. Both about the valve and the beer.

Either way - relax.........

cheers.

Rod
Greybeard

don't you worry, if there is one thing I like as much as a beer and my MG, its relaxing :-)
Malcolm Le Chevalier

Im totally vegged out today... sitting in the lazy chair in the highest setting on the AC and snoozing away the day

sadly no adult drinks today

valve guild wear would be a good guess but its a new head with less then 500 miles on the clock

and yes adjusting hot instead of cold makes me apperhinsive its alot of money if i screw that up

i guess i may try another spec test/ measure cold in the next week or so

It just occured to me... i havent retorqued the new head scence i installed it... and thats been a good 1000 miles

maybe i should do that 1st then recheck the valve lash

i dont know why i didnt know why i never thought of that before.

prop



prop

Mgs have ticky valves in my opinion. Its part of their charm!
S

Just as an aside.
My #2 is relatively low cos of the mark in the cylinder from the ring being stuck after storage. Still goes well, starts well, and done 6000 a year for 3 years since back on the road at legal road speeds.
Dave Squire

This thread was discussed between 17/06/2016 and 19/06/2016

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now