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MG Midget and Sprite Technical - Sticking Valve Again


Hello All,

As you may remember from a few weeks ago, Mabel the Midge is now roadworthy for the first time in nearly 3 years. All is great in every way except the engine which is not enjoying itself after the 3-year lay off (the engine wasn't touched, just taken out and sat on the floor for 18 months), and I'm fairly sure it's a sticky valve. I've gone thorugh almost a full tank of petrol which was double-dosed with redex, checked the timing and fiddled with the carbs a bit - none of which have cured the problem. It's a 1275 with the twin HS2, K&Ns and a tubular exhaust, and it's an unleaded head which may possibly be an oselli but the PO wasn't sure who he'd got it from...

The problem is not constant, only happens when the engine has warmed up properly (maybe 10-15 mins), although the idle can be a bit uneven when it's cold, It will be going along just great and then start to misfire, some backfiring in the exhaust when off throttle and seems to goes a bit smokey too. Sometimes fixes itself for a few seconds then comes back. I very occasionaly had the same problem mildly before the car was taken off the road.

One other thing that is very odd is that the temperature gauge only reads above absolute minimum when the engine starts to get hot - in normal running it sits on the minimum post and doesn't budge. I replaced the gauge during the rebuild because I had exactly that symptom before and (wrongly, it appears) diagnosed that the gauge was faulty. Do I have some kind of problem with water flow around the head? - I don't know where to start with that one, and I wonder whether it is causing the head to overheat and the valve to stick?

So how do I diagnose which valve is sticking, and once I have, how do I fix it?? Or is my best bet to get another head?

Phew, long post. Thanks for reading this far, and I'd appreciate any and all advice.

Mark
MarkH1

Do you know if the head was fitted with bronze guides as part of the unleaded conversion? There have been occasional cases of bronze guides picking up on the valve stems, usually cured by reaming to a little more clearance. Easy to spot once the valve is remove, there are traces of bronze left on the valve stem.
Paul Walbran

Hi Mark

Further to Paul... some seemingly unrelated questions:

Is your temp sender in the block or the rad?

Are there any air-locks in the coolant pipes etc? (I used to fill mine up through the top heater hose - use a funnel - until the water came out of the expansion bottle; put the 'radiator' cap back on and continue filling until the water comes out of the heater outlet, and fit pipe back on.)

When did you replace the rotor arm?

Have you checked the ignition leads to make sure they are secure and have good connections wire/spade/terminals?

How full is the sump?

A
Anthony Cutler


Anthony -

The temp sender is in the block.

I don't think there are any airlocks, but I will double-check.

The rotor arm is new (Also forgot to say that it's running an Optronic ignition system)

Ignition seems OK, the connections look alright and I've checked that I'm getting a spark at each one but of course I can only do that cold, when the problem doesn't manifest...

Oil level is OK, but it is using a bit - a bit more than I was before.


Paul - I was hoping somone would say there was an easy way to check without taking the head off, but kind of guessed that wouldn't be the case!


Thanks chaps - it's so frustrating not being able to use the car now it's back in one piece!
MarkH1

I may cheer you up Mark as I am less than convinced your problem is a sticking valve.

If it were me I would sort one problem at a time.

Temp gauge? very strange and needs to be solved. The temp sensor is in the head near the thermostat is it not?. Remove the thermostat housing and the thermostat and see if you can see the temp sensor bulb. Also withdraw the bulb from the head and place in boiling water or very near boiling does it rise to 90C/200F plus? If all that is correct then you have a serious issue. Talk about that if those are the facts.

To decide on valve sticking does the engine sound extremely RATTLEY when you suspect the valve is sticking? How long after you switch off the engine can you restart and run normally?

I have run an engine with a sticking valve and I have to say I heard the problem. Also the engine refused to rev much above about 3 to 4 thousand RPM at that speed it seemed to hit a wall, due I believe to gas flow problems in the other cylinder.
Robert (Bob) Midget Turbo

Hi Paul

The bronze we use leaves no trace or damage if it grips the stem, when it cools it just lets go and all is ok till it gets hot again.

Mark

Robert is right....big clatter sound if valve nipped. We managed to nip ex valve on well tuned midget when pushed hard until we cut off the ex stem valve stem seals. Note only 4 supplied in gasket set! We do however, fit 8 seals on transverse applications as oil can puddle in the head if rockers worn then go down ex guides and smoke!

Mark again...more like crap rotor arm when warm, we now stock the red ones from ditributor doctor...buy one direct from him.

Peter
peter burgess

oops that should be distributor doctor, you can Google him.

Peter
peter burgess

Very kind of you to say so Peter. :-)
Robert (Bob) Midget Turbo


Robert, Peter,


Sounds like it's not a sticky valve, then - I'm definitely not getting rattley sound, more 'fluffy' I'd describe it as. An hour or so after switching off it's usually gone away, but seems to come back quicker than from completely cold...

I'll investigate the temp sensor, do the boiling water test and stuff, and try and figure out what's going on...seems odd that I'm experiencing it with a brand new gauge as well as the old one.
I'll also order one of the red rotor arms and see where that takes me. Would it be worth replacing the ignition leads in order to rule them out?

One final thought - the problem usually first shows itself when accelerating up hill - ie fairly heavy load, relatively wide throttle and lowish revs - I'm far from expert in this but could it be a problem with the advance mechanism? I know if it's too advanced it will pink, but what would be the symptom of the ignition being too retarded?? Is there an easy way to check the advance is working properly?

Thanks for saving me from barking up the wrong dog - I'm sure we can get to the bottom of this.

Mark
MarkH1

Fluffing sounds like it might be an HT problem...

BTW when you said RA is new... did you replace it before the problem arrived?

[You should keep at least one spare RA with the car; never replace a good working one... BTDT]

A
Anthony Cutler

I had similar problems, also getting worse and with shorter intervals.
The culprit was the coil. It was a new one, but cheapo aftermarket. It leaked its oil out and got hotter every time so more oil was pushed out until there was so little left the car was undriveable..

Easy to detect, carefully touch the coil when it happens......
Alex G Matla


I've ordered a new rotor and cap from the distributor doctor on the basis that it's quick and easy to swap them out - Anthony, yes the RA was replaced before the problem appeared. They will arrive on Friday so will get them in the car and report back.

Alex, looks like coil will be next on my list if the cap and rotor don't fix it...
MarkH1

Hey Peter B.,

You have ventured back into the old debate of "oil seal VS. NO oil seal" on the exhaust valves,,,I finally decided to go no oil seal on the exhaust valves after a lot of contemplation. my understanding is it helps keep the stainless steel valves cool and the only smoking that will occure is on start up, not during engine running

Id love to hear your take on the debate for intalling them on the exhaust.

Prop
Prop

prop
I have gone with no exhaust seal and i wish i did not.
It just does not work telling people you have rebuild your engine and than getting a puf of smoke when you start the car

so after the holiday the head will come off and the seals wil be fitted
Onno Könemann

Coil and condensor are both components which can fail in a temperature related way such as this.
Don't replace everything at once, just one at a time - then you know where to start if it ever happens again.
Paul Walbran

onno,

there is an easier way to put on oil seals then pulling the head

run the piston up to top of cly, or close, tie a string on valve stem, release valve stem holders pull off valve springs, ect (holding string) put on new seals rebuild springs...repeat for all 4, check vlave clearance,...done.

that was one off the drawl backs of leaving it off for me also, but I figured, better that then tossing a valve due to heat and binding.

Prop
Prop

Or, to remove valve springs etc, pressurize the cilinder with compressed air to hold up the valve.
My engine is eating its valve seals :( at the moment.
I'm thinking to fit Volvo Amazon valve hats, they go on top of the spring to prevent oil running down the stem, wich is told to me is the most oil wich will go trough the valve guides.

Mh, looks like where drifting of topic here, sorry Mark.
Alex G Matla

Hi Onno

Maybe you have a lot of oil escaping from your rockershaft? I can only report what we find and we have seen valves nip in hard use when they have ex stem seals in inline use, transverse can burn oil so we tend to use ex valve seals and we haven't had any nip...apart from insufficient valve/guide clearance when we were experimenting with MG Metro Turbos many moons ago, needed 8.03 guides then next year when we took the ex boss right down we had to open up to 8.05 mm to prevent nipping.

Oil burning down ex guides seems to be associated with excess oil under rocker cover due to looser clearances between shaft and bushes.

Peter
peter burgess


Thanks for the help and advice folks -

I replaced rotor, cap and leads on Saturday morning and Sat afternoon did about 40 miles with no sign of a problem so it looks as though that's solved. Needs a good tune-up as well, I'll get that done one evening this week and then start working through the niggles one by one - temp gauge, fuel gauge, rev counter.

Thanks for the contact with the Distributor Doctor, Peter. Great guy to deal with; helpful, knowledgeable, prompt delivery, reasonably priced. Gets a big thumbs-up from me.

I'm curious now about whether I have stem seals fitted - the engine is a bit smoky at times and I'm wondering what has deteriorated during the lay-off. Maybe I need to do a compression test and see if I can track it down.

It's great to be out on the road again enjoying the Midget. Thanks once again for the help - you guys great. Where else would I find such a wealth of expertise in one place and for free?

Mark
MarkH1

My 1380 is the Midget engine Peter B is talking about!

I had the head converted to unleaded, with bronze valve guides by xxxxxi and had immediate problems with a hard misfire + clatter when warming up and under load.

This tale is in the archives, but did not think of valve guides at all, so replaced the entire fuel supply system, including the tank, changed the coil, leads etc., it still occurred on occasions, sometimes accompanied by a cloud of white smoke!

The head was returned to xxxxxi after the No. 1 exhaust valve burnt out and there were traces of 'yellow' metal on the valve stem. To their credit, and this was about 2 years after the conversion, they replaced the valve FOC and checked all the clearances, which they said were correct.

After 3 years of misery, I still had problems, so went to Peter about 5 years ago, who fitted his own new bronze guides, which were much better, but still occasionally would pick up, but no clatter, under load. Spent a very enjoyable day with Peter with his rolling road. Mixture and ignition were as they should be, so he extricated the exhaust valve seals in situ (he must have been a keyhole surgeon in a previous life!) and then set the exhaust valve clearances 0.003" wider than normal to give more 'seat time' for cooling. The combination worked, and after a few thousand miles the tappet clearances were reset back to 0.019", Kent's spec for a 276 and 1.5 rockers. No problems since (oh, other than it dropped an exhaust valve head (No. 4) earlier this year, but that is completely unrelated to the 'sticking' problems previously).

Now in the process of sorting out the recently acquired Metro Turbo head etc. that is to be fitted very soon.

Mark,

I would try opening up the exhaust valve tappet clearance by 0.003" and richening the mixture slightly (cooler charge). One at a time!

What thermostat are you using, and do you have the standard water pump by-pass in place?

Richard
Richard Wale

This thread was discussed between 30/06/2009 and 06/07/2009

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