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MG Midget and Sprite Technical - (not so simple) misfire!

Hey guys,
still having the misfire. Bought new (BP8ES) plugs, gapped & fitted them; lovely.
Took her for a drive all good, then it started. Missing really badly although clears under load.
Popped the bonnet & the plugs are black again (sooty, not wet)
Any ideas?

spec: (if you dont know me!)

1430 stage 2 (Minispriint engine)
Lumetition optronic ignition
Head: 35x29 Msr158 025 2100
Oselli pistons
MGOC exhaust system & manifold: (C024)
256 cam

ps, I've cleaned & oiled the K&N, even tried running without it.
Mallorcaben

Here is a pic of the plugs I took out.
(Ignore fact they are mismatched as these were old plugs I had kicking around that I used to test.

By the way, I've altered nothing regarding the carb. The only thing that was done recently, is that the Dissy was out when the new starter motor was fitted.
Could bad timing cause this?
When I clean the plugs she runs like a dream until they foul up again (5 to 10 mins)




Mallorcaben

Those plugs look wet (petrol?, oil?).

A
Anthony Cutler

Put a match to them if they go woommph its petrol. It's probably petrol as i don't think you have your DCOE set up properly.
Brad (Sprite IV 1380)

I feel that it is a carburetor problem such as wrong needle. It appears to be running extremely rich in mid range and clears when you stomp on it.
Sandy
SANDY SANDERS

Possible that somebody in the carb has unscrewed his little self, so that fuel is bypassing a jet.
Or, something has clogged up an air correction thingummy. Or the float has decided to play submarine.Or the cold start works are deranged.

My pence are on a heat problem, given the temps you are reporting. I've previously posted the whole thing on all these boards I think. Hook up a fuel pressure gauge, in the line just before the carb. Must be after the pump and any pressure regulator. Drive the car and observe the fuel pressure under various conditions, especially at low speed or stopped when good and hot. Tell me what you see.

FRM
FR Millmore

FR: the A series is a stroker that's undone the carbs before, so you may be spot-on...

A
Anthony Cutler

So.......
what am I looking for. Dammed Weber is a complicated beast.
Anywhere in particular? I took out all the jets and blew through them to check for obstructions
It's odd because she was running beautifully before the new starter was fitted.
I've not changed anything on the weber, but yes my engine is stroked so it shakes & vibrates often throwing huts & bolts out.
Mallorcaben

I can't add much to previous comments other than to say that I run BP8ES plugs without any problems and my engine spec is similar but not as 'hot' as yours. For that reason I doubt your new plugs are too cold leading to fouling so I would be looking elsewhere as suggested by others above.
Chris H (1970 Midget 1275)

Fouling plugs under light loads which slears under full load means either too cold in the plugs or (as above) too much fuel or too much oil.
The plugs you show in the pic are BP6ES and N9YCC. These are hot running plugs for your state of tune (assuming CR matches other spec).
But these aren't the plugs you mention initially so I'm confused! But generally:

If all plugs are affected, that's usually fuel.
If only one plug is affected, that's usually oil.
If only no 1 plug is affected, it can be any of the above.

You seem to have more than one plug affected (but I can't be sure from the photo) so I agree with the observations above, especially the devious vibrations from the stroker motor.

Then I have another theory:

Why don't you supply the beer etc, then we can all invade and diagnose the problem in the time honoured way!

Paul Walbran

Paul,as I said, the plugs pictured were an old mixed lot that I stuck in to test.
Yes they are all fouled.
Not changed any jets, nothing changed it just suddenly started misfiring.


Mallorcaben

A very long time since I had a DCOE apart, so others should be better able to help you.
Things that would affect both barrels:
Fuel in float.
Sticky float valve.
Blocked float bowl vents.
Cold start derangement.

What exactly was touched when you changed the starter?

I will be out of touch for most of the next three weeks, so I'll not be much help probably. Will try to chreck in via random wireless connections on the way.

FRM
FR Millmore

Thinking further:
You changed the starter, then the trouble started. Given that the starter is under things ignition, it makes me wonder if one of the wires feeding the ignition is marginal or intermittent. If the plug stops sparking (due to a fault elsewhere in the ignition)it will have a wet/fould appearance even though there is nothing wrong with it or the fueling.

I've also remembered another fault which causes misfiring at low speed which clears at high engine speed: It is an unusual symptom which can occur when a coil starts to die. I have not encountered it often, and the first time it completely fooled me into thinking it was a fuel-side problem such as excess air or blocked pump jets. It had a big gasp like a racing camshaft.
Again, if the plug stops sparking then until everything clears and runs right they will be wet.
Paul Walbran

plugs not wet, they are sooty, black
Mallorcaben

On unleaded fuel, sooty is what normal running plugs get like. It's a question of how much it is sooty. It was much simpler with lead!

Looking at when it all started, I'd still check out the ignition wiring. And the coil by simply swapping to a known good one.

It's also good standard methodology - always start with the ignition before toucjing the carb(s). 9 times out of 10 you find the fault before getting to the fuel system. So I would substitute all ignition components one by one for know goog ones - in the case of the electronic bits the simple thing to do is fit points & condensor for the test.

Then if the problem is still there you can be certain it is fuel.
Paul Walbran

Still struggling.
What I dont understand is how a Weber can suddenly run rich. (if that is what is causing it)
I clean the plugs, go for a run and after 10 mins the car is misfiring. I check the plugs & they are black.
new photo attached. (Ive lightened the photo a bit )



Mallorcaben

Is choke releasing properly?

A
Anthony Cutler

looks like it from the top. I checked that.
Dunno inside the carb. Could a float be stuffed?
how do i check.
Is it worth adjusting the mixture, cant see how it could change.
Mallorcaben

Unless you have over 11 to 1 cr the 8 plug is way too cold. Try a 7 but thats not really the problem.

Make sure your ignition coil is throwing a nice white spark.

Check the igniton lead. You want 28* degrees total.

If the plugs dry out and you can rub off the black with a cloth its too rich.

Check the fuel pressure on the carb. 3 psi. If higher put a fuel regulator on.

Blip the throttle. The second blip should show no black smoke.

if the water temp has a long time coming up then you know its too fat (rich).

Do you have a good fuel filter in the fuel line. Trash gets in the DCOE and it is not happy.

Good luck.

M. Who

Uhhhh and also are you using the isolators between the carb and the manifold. ...this is a must.
M. Who

This thread was discussed between 05/08/2009 and 25/08/2009

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