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MG Midget and Sprite Technical - Misfire solved potentially ?

I've been checking the tappets this morning Valve no1 I couldn't get the feelers gauge in at all so reset that and checked all others, no 6 bit tight, do you more knowledgeable guys think this could've been the problem, once it was hot ?
Scott Coe

Scott,
whilst you wait for someone who knows what they're talking about I'll reply to get the ball rolling.

The valves wouldn't have helped.

As you've changed the tappets then you also want to carry on and check, and adjust or readjust as required, the rest of the items in the chain - setting tappets, points/plugs, timing and mixture in that order. If you adjust any item in the chain then you need to check all that follows it, so with tappets it's everything.

Doing the checks and/or the adjustments may find and resolve the misfire.

Spark plugs are often overlooked, could be faulty or passed their best or gap(s) incorrect.

HT leads have been covered in other thread, as has Guy's way of checking them and plugs to find which lead/plug/cylinder.

Also Guy would ask if the rev counter flickers at the misfire, if so LT side of ignition.

Once you've done the tappet chain checks and readjust as required, and other checks if you still have the misfire there are a few other things you can check.
Nigel Atkins

Did you check clearances hot or cold? Bottom line is did it resolve the misfiring?
If so, Bob's your avuncular relative. If it was tight enough when hot to compromise compression then yes - it would be a bit erratic. Not to mention the possibility of burning the valves and seats. Especially #1, being an exhaust valve.
Greybeard

As I understand it, one effect of excessively tight valve clerances is thet they remain on the cam too long - so open too long. With a hot engine there is less time when the valve is on its seat and this in turn stops the valve from dissipaiting its heat fast enough through the valve seat to the block. The overheated valve can then expand and begin to bind in the guide which results in a misfire, which feels like a sudden tightening of the engine as it looses power at higher revs.

I had this situation on mine after fitting a 276 cam. The solution with this cam was to open out the valve clearances to 18 thou after which it ran perfectly.

So tight valve clearance may be the root cause of your problem. Set them correctly and see what effect that has.
GuyW

I checked them cold, that's what the good book says, did about 30 miles in it yesterday no misfire, but it has been a erratic misfire, so time will tell
Scott Coe

The only way to tell is to continue using the midget.

This is a good idea anyway as using the car will also pick up other issues and wrinkles and help you to learn the car and how to drive it as it was intended.

Whilst continuing to use the Midget through the winter, when you can, and early spring into summer also carry out a rolling full 30k-mile service, or at least the checks involved, will also find and help prevent other issues, as has been proven with checking the tappets.
Nigel Atkins

Thanks Guys, will see how we go.
Nigel, how was the clutch fluid ?
Scott Coe

Scott,
don't really know yet, too soon, but it's the clearest it's been since a couple of months after the (last) new clutch and slave cylinder. More importantly the brake fluid has been changed and again clearest it's been for a good while.

More road tests required as the NEC is coming up and the last few years this has been a jinx for faults on the the car, or me, or both.
Nigel Atkins

This thread was discussed between 20/10/2019 and 22/10/2019

MG Midget and Sprite Technical index

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