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MG Midget and Sprite Technical - Idle cylinder not firing

Hi folks, first post here!
I've got a 78 midget that's been in storage for the best part of 20 years. Been over it mechanically and got it through the mot last week.
It runs ok but cylinder 2 doesn't fire at idle. I've changed points, leads, distributor cap and rotor arm, valves adjusted, compression is 130psi across the board. Haven't adjusted the carbs but assume that with no1 firing ok there's no problem there and driving it is fine. I've put a colour tune in and it starts to fire around 1500rpm.
Any ideas? Sticking valve?
S Watts

Easy things first

Check No 2 plug gap.
Swap No 2 plug out for a plug that is known to work.
Swap No 2 lead out for a lead that is known to work.

Failing that, redo the tappets on No2. Slacken off the locknut and open them well up, it may free valve up if stuck....failing that, then off with the rocker arms and a gentle tap....

Are you certain No 2 does not fire at idle? - personally I do not find Colourtune terribly accurate.
Mark O

Thanks for that, yes pretty certain it's not firing, if I pull the plug lead there's no change in rpm compared to the other cylinders. I only put the colortune in to see combustion. One thing I noticed is that the spark is yellow - shouldn't that be blue? I've ordered a new coil.
I've tried swapping plugs and leads around but no difference.

Also wondered whether it's fuel related but assume that because cylinder 1 is firing that pretty much excludes fuel? I'll try a gentle tap on the valves.
S Watts

Fuel for #1 and #2 is from front carb.

If its not been going for 20 years and the coil is working I would check the sticking valve. If you turn the engine by hand with the rocker cover off (and the plugs unseated) you can see the valves being pushed and released by the tappets.

Mine needed a good clean out around the rocker arm, springs and tappets after 10 years standing. OK now.

My spare engine has a 'sticky valve' when turned by hand as well. That's a 1500 engine that's been off the road for at least 10 years.
Dave Squire

Its NOT going to be the coil, if the engine is firing the other 3 cly then its not coil related

Id follow Mark O advice to the letter.. spot on...BUT to be more efficiant id take a 5th good spark plug and hook up the #2 plug wire to it and set it on a good metal part of the block and fire the engine up and watch the spark on the plug, if it has a rhythm and a blue spark, or no spark, or a random yellow spark ... then your 1/2 way to home plate regardless of the out come



Id add ... chase the spark plug threads in the cly head it could be just corrosion / old over zealous anti seizure build up on the threads and not allowing good spark plug to cly head contact for a good ground

8 times out of 10 on stick valves its going to be the exhaust thats sticking

If it is sticking its probably just a slow stick...you might be able to pull the valve cover and just watch it and see what it does at various rpms

If it is this, the best solution might be to add some oil additive that cleans engine enternals example seafoam and just drive it for 500 miles

Prop

Prop and the Blackhole Midget

Thanks everyone, I've traced it to a broken outer valve spring on cylinder 2 - fourth valve from the front - is that the inlet or exhaust?

Never had one break before! I only noticed when I went to check the valve clearances again and noticed that I could push the rocker down.
S Watts

Exhaust
Dave O'Neill 2

That good you found it, but sucks to replace it

Prop

Prop and the Blackhole Midget

Any suggestions re the best type of spring compressor to get for this job please? There seem to be a few different designs - wasn't planning on pulling the head just yet!

Also - I take it the whole rocker assembly needs to come off?
S Watts

Beyound the C clamp version and pulling the head I havent found one yet

Ive tried a couple of universials that mimic a 2 jaw gear puller but there not able to get up under and work on double valve springs

Maybe you can cut away the old broken spring and use a similar universal to remove the inner spring

Prop
Prop and the Blackhole Midget

You could change the spring with the rope in the cylinder trick, if you don't want to disturb the head.
Guy Weller

I might as well just pull the head and be done with it to be honest, plus I get to see what's going on in there. I'm used to building aircooled engines so I have the tools to do it.

Well, apart from a spring compressor :-)
S Watts

I'd never heard of the rope trick! Good job you mentioned it Guy because I'd have happily removed the spring and watched the valve disappear!
S Watts

Still...its how do you get the spring compressor over the double springs

Most good machine shops have a hydro spring compressor it only takes a couple of minutes to pop a valve oug and a new spring back in... probably way cheaper then buying the spring compressor

Im sure they they will want to sell you up, but that is there business...hold the line on what you want

Prop
Prop and the Blackhole Midget

<<<Still...its how do you get the spring compressor over the double springs >>>

The spring compressor presses on the spring cap, so it compresses both springs at the same time. And a basic screw down one is cheap and easy to use. Hardly a job for a machine shop which would probably involve leaving it and a "come back next week" uncertainty!
Guy Weller

A completely ordinary standard spring compressor will do the job. Here's the one I've used on lots of cars since 1980s including my Midget 1500. It should be adjusted to compress the springs the minimum required to easily remove each collet; if you try and compress the spring too much it's very hard to get the compressor to go over-centre, especially if you have smallish hands (mine are)or a grip that isn't massively strong.

Nick Nakorn

I didn't know about a screw down compressor, Guy. I can see how it would work, but can you buy them?

And, will the rocker push aside enough to get it out of the way?
Nick and Cherry Scoop

Nick, I meant something similar to t'other Nick's photo - only more basic as it doesn't have the levers and over centre bit. Basically a G clamp with a forked and that fits the spring cap.

But you can also get - or improvise, something that hooks under the rocker shaft and you just lean on the other end. Just takes a lack of imagination to enable its use without worrying about what might happen to your fingers if it slips.
Guy Weller

I like it.
I reckon I would try it, too.

But a little screw-down device that you could fix to a pedestal stud would be nice.
Destined to lie sulking on the shelf for decades, but nice.
Nick and Cherry Scoop

I have used a G clamp and a socket before now.....
Mark O

you could try with a rope and this compressor

mark 1500 on the road Preston Lancs

But a little screw-down device that you could fix to a pedestal stud would be nice.
Destined to lie sulking on the shelf for decades, but nice.


Nick....

Absolutly.... in fact id remove all my valves once year every year for life with that tool, just to make sure I got my moneys worth

Prop
Prop and the Blackhole Midget

That's very pretty, Mark.

Don't tell me the claws go into the outer spring . . . . . . ?
Nick and Cherry Scoop

Nick N and Mark O - how do you use a conventional spring compressor or G clamp with the head still on the block?
David Smith

David

You don't. Steve indicated he was going to pull the head.
Mark O

Did this job yesterday, took me about an hour without pulling the head so for anyone else thinking of doing this it's straightforward - just needs the right tool. I used the overhead spring compressor -£15 from ebay.

So - plugs out, some rope pushed into No2 cylinder (great tip that!), loosened all the tappets, off with the rocker assembly then compressed the spring enough to ease the collets out. Then just a reverse - compressed the new spring, put the collets back in, rocker repositioned with pushrods all properly aligned, torqued the rocker assembly to 32 foot lbs and adjusted the clearances.

Oh - and took the rope out :)

Idles fine now. Thanks everyone for advice!


S Watts

Nice job! The trick with the rope - which you probably learnt anyway - is to feed it in with the relevant piston about 1/3rd the way down the bore, and then turn the engine (by hand, gently!) to raise the piston and pack the rope up tight into the combustion chamber.
Guy Weller

Congrats - job done
That compresser looks a lot like the unit Mark 1500 suggested
I have a Meco brand one that is very similar and have never had a problem grabbing the inner springs with it even on our tripple spring setup on the Chev race engines Instead of using rope down the hole I have an airhose fitting for the plughole and just pressurise the cylinder I find that a bit safer than sticking foreigners in where they don't belong specialy in the pits in the dark cheers willy
William Revit

Hmmm. Bad idea if you ask me.

What I don't like about that, and thus wouldn't do it myself, is that in removing the rockers, you've also unloaded the cylinder head gasket all down one side.

I doubt it will distort the head, but it's asking for a blown head gasket.

How are you going to torque it up again? When you torque it now, you will be over compressing that side in order to reach the correct lbs/sq", whilst presumably leaving the other nuts alone.

It wasn't that much extra work to have the head off, and make sure the valve seat was still good too.

Lawrence Slater

its a 1500, Lawrence. Rocker studs don't double up as cylinder head bolts.

Besides , I have done the same job on a 1275, replacing all of the stem seal rubbers, and with no ill effects at all. The CI heads are sufficiently robust and rigid for it not to have an adverse effect on the head gasket.
Guy Weller

Nice job, S.

I also learned the rope trick from Guy.
Really useful for jamming the crank while you undo the pulley bolt - jobs like that.
Nick and Cherry Scoop

Ah, didn't look closely enough to see that it's a 1500. :(.

I Still don't think loosening off half the studs is a good idea. I had just one snap when torquing up. Hadn't even run the engine. Took out the remainder and replaced it, then drove to France. Had to change the hg in Bordeaux. Maybe I was just unlucky then, but I'd still pull the head for this job.

Lawrence Slater

Getting old Lawrence, you are too set in your ways. ;-)
Guy Weller

Well the head will be off in a couple of weeks - in the meantime I want to enjoy a few miles with what's left of our summer. I assumed that I wouldn't affect the integrity of the hg by taking the rockers off. Soon find out I guess :-)
S Watts

Not just getting old Guy, I'm already there I reckon. lol.
Lawrence Slater

It was a tease Lawrence. Some people get old ahead of their number of birthdays, whilst others seem to accumulate birthdays ahead of themselves. I would have put you in the second category!
Guy Weller

lawerance

Maybe the white shoes and white belt are sinched to tight, cutting of circulation

Prop
Prop and the Blackhole Midget

I once suggested to a friend that he try the rope in the bore trick. He called me the next day to say "I've changed the valve spring, now how do I get the rope out?"

Growler

I knew someone who tried it too. But instead of rope, an Indian came out.

Maybe I need that white stick after all Prop.

lol Guy. I've been knocking b/days OFF the count, for some years now. :).
Lawrence Slater

This thread was discussed between 11/08/2014 and 18/08/2014

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