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MG Midget and Sprite Technical - 1500 misfire

I have a misfire when cold - occasional misfire when up to temp.

1. Checked points, and other electricals... i think they're all fine

2. Haven't checked tappets yet, what would the effect of a tight clearance on the tappets be? Would that prevent the valve fully openening?

3. Carbs/fuel? Pump seems fine. Carbs are tuned properly. I am worried about the breather pipe from the rocker cover to carbs. I have temporarily put an alloy rocker cover on which has no baffle - and there is a bit of oil getting into the breather pipe - Is it possible oil is getting into the carbs and effecting the mixture?

Any ideas greatly appreciated.
C L Carter

A good starting point is to determine if this is an ignition related misfire, - or something else. Go for a test drive. When it is misfiring watch the tachometer needle. If it flickers around then the fault is with the LT ignition circuit. (Points, coil, wiring loom,or ignition switch). If the needle is relatively steady, or just falls back a bit as the revs drop, then the fault is more likely fuel or compression related.
Guy Weller

>>> what would the effect of a tight clearance on the tappets be? Would that prevent the valve fully openening? <<<

Actually, a tight tappet would prevent the valve from closing fully.

As for diagnosis, Guy's advice is right on the mark.

Gryf Ketcherside

Thanks Guy,

What would your advice be if your tacho doesn't work?

C L Carter

Could the tacho not working be related to you problem?

Or did your misfire start long after the tacho stopped working?
JB Anderson

"Actually, a tight tappet would prevent the valve from closing fully."
And then burn the valve and destroy the head.
It may make a steady miss, but not likely an intermittent one.
Always check tappets first, then compression, because otherwise you are chasing shadows.

FR Millmore

Cheap and easy to replace as a first off try.
Surprised Bill didn't suggest it ;o)
Unless you have an electronic type gizmo of course.
Bob T

I'm sure the tacho has nothing to do with it JB. Not least that the tacho had never worked for starters.

Condenser, its been running a new one since the rebuild. now done 400miles. Perhaps its just a crap one, worth trying another. Thanks Bob.
C L Carter

unless you bought the new ne from somewhere the Dissy Dr. try putting the old one back on

tappets, CB points, plugs, timing then mixture
Nigel Atkins

After driving around the country all weekend, I'm pretty sure this is carb related. It feels lack of fuel everynow and again.

I need to get new hoses for the carbs breather. Will report back when I've changed these.
C L Carter

Iím not sure your two issues are necessarily related

The oil through the breather pipes
. if the pipes are old certainly change them
. alloy rocker cover without baffle Ė there appears to be two tin types but only one alloy for the 1500 Ė Iíve no idea what the differences are but youíve mentioned a baffle and I think Iíve noticed some of the alloy type appear to come with a vented oil filler cap, so Iíd swap back to the original type of rocker cover with the original type of (unvented?) oil filler cap
. with the 1275 I remember being told that the vented chrome oil filler cap wasnít as good for engine breathing as the original plastic vented and filtered type

. if you think the new condenser might be faulty you could try putting the old one back in as this might also show if thereís been any shorting due to fitting, same as fitting of CB points order (diagram and info in some books)
. many other possibly causes of misfire including air leak at carbs/manifold, faulty coil, low tension, dissy mechanics and on and on ...
. step by step diagnostics are required as usual to find the source, from battery forward to make sure the bleeding obvious has been missed or a fault introduced by recent parts, components or work

have a look at the diagnostics pages in the Porter Service Guide :p
Nigel Atkins

Not only are they old, they are now held together with duct tape. So yes they are getting changed.
I think this is where the related problem starts being as you mentioned an air leak at the carbs.

As for the rocker cover, I will change back when i can. I had to removed the baffle from the old one to clean and replace the mesh behind it as it was full of grit and bearing material. So i'm waiting on some new oxy propane to arrive so i can braze that back together.

C L Carter

I could ask why they haven't been changed before now but of course I wont :)

doesn't always pay to rough it out rather than changing some bits, even when some bits 'look' perfectly serviceable, I understand you may not have too much spare money to spend on your midget but one thing even I sometimes forget is false economies, sometimes spending more cots you less - mend and make do, refurb recycle but remember sometimes you have to just renew some items (and before they fall apart)

at least if you do one thing at a time you'll find which actually resolves your issue (or part of)

good luck, keep going
Nigel Atkins

This thread was discussed between 20/07/2012 and 23/07/2012

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