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MG Midget and Sprite Technical - Cylinder 1 not firing!

Ok put back together my 1500 and cylinder 1 is not firing I have checked spark and all is ok, Compression is now 140 across the board. So thinking this is a fuel problem. I think I have found the problem, the inlet manifold is higher at the front on number 1 so the fuel isn't flowing into the cylinder? When I put it back together I noticed the roll pin on the cylinder head is missing at the front. Was going to put a new one in but moss doesn't even list it!

Is it feasible that the miss alignment of the inlet is causing fuel flow into number 1? Number 2 is firing perfectly? It is only out by 2mm max?

The car is running a weber downdraft carb not sure if that makes a difference or not.

Any thoughts would be appreciated.
C Carter

No, Its not the manifold being misaligned. That wouldn't do it.

How did you check for the spark on #1 cylinder?

Try just swapping over the spark plugs between #1 and #2 cylinders. No other changes, just swap the plugs, reconnect the plug leads as they were and retest. See if the problem remains with #1, or does it move to #2?
Guy W

I checked for spark by pulling number 1 plug out and held it against the block and it was sparking. Even swapped the plug for a brand new spare that I have, sparks away when held against the block so dont think it is a spark issue?
C Carter

OK, so if it is sparking, what test did you use to check that cylinder #1 wasn't firing?
Guy W

By pulling the HT lead of number 1 cylinder and the engine not been affected. All the other leads when pulled the engine runs worse.
C Carter

It cannot be fuel not flowing into the cylinder can it? Fuel is mixed in the carburettor with 15 parts of air to each part fuel, so what gets sucked into the cylinder is flammable substances.

That will burn if given the spark, what you need now C, is a Colourtune spark plug so you can observe any actual sparking happening. Now that fuel seems to "burn" at different colours to the days of leaded petrol I rarely use mine for setting the mixture but it makes an ideal way of seeing the spark happen inside a very high pressure area.

It is possible the leads or plug lead cap is allowing the spark to dissipate outside the engine in preference to the hard work it needs to do to spark inside the chamber.

All a little bit "fairy story" but in thirty years on breakdowns I saw this happen many times.

New leads and reset the points gap a little wider often helps. Properly working distributors can still have a wobble on one cam lobe. That might reduce the dwell at one plug.


As Bill says, it may spark when you look at the plug when held against the block, but possibly not when its in the engine under compression. You have tried a new plug, so maybe its the plug lead.

Has this only just started happening? What was the last thing you did to it? Are you sure that the other 3 cylinders are working properly? Any chance that you have had the plug leads off and now have 2 of them swapped over so the firing order is wrong?
Guy W

Plug leads have always been labeled 1,2,3,4 so there was no confusion. I will try with a new HT lead tonight and see what happens. i do have a colourtune so will also use that.
C Carter

Hi, try running the engine in the dark, you will see any sparks going astray.

Found a neighbours ignition problem in 10 seconds flat!

Tony G

These engines do NOT run equally on all cylinders at idle!!

Up the revs to 1500 /2000 and then pull the plug leads to see if the cylinder is firing or not.
Chris at Octarine Services

adding to what Tony put if you also mist spray with water it will highlight any problem areas - spray around the dissy cap too (perhaps a weakness at peg for 1)

if you need new HT leads these are excellent -

if you've got a set of new plugs is there a reason you've not installed them, plugs can continue to work way passed their optimum performance
Nigel Atkins

Ok well went home at lunch and put in the gunson colour tune and it is firing. Think Chris hit the nail on the head with the engine is not supposed to run equally on all cylinders at idle? Can anyone else confirm this on their 1500? I will try upping the revs and see if that makes a difference.

Hi Nigel in answer to your question, I have 2 midgets both 1500 so always seem to have new parts kicking around, Ht leads, spark plugs, condensers, rotor arms even oil. I service my car every year to be honest hence why the engine has 105,000 miles and is still going strong (ish)
C Carter

"""Engines are not supposed to run evenly on all clyinders at idle"""

I cant wrap my mind around that concept

...can someone please provide extra details of why and how this happens ??? or am I missing something in the translation ?

Prop and the Blackhole Midget

I must admit prop it is confusing the pants of me. The simple fact that removing the spark plug at cylinder one would mean the engine loses some of its power so I would thing the revs would at least drop but it doesn't affect it at all. At higher revs 1200 when the plug is removed the revs do drop? If some one could also try this on their 1500 and report their findings it would put my mind at rest.
C Carter

If you are removing the plug for this test, I bet it makes a noise!! The usual thing is just to pull the plug lead off. Using insulated pliers, you can then reconnect and pull it off again whilst the engine is running, and listen for the change - if any - in the engine speed. If you are removing the plug for this test you must be stopping and starting the engine and it is much harder to discern what the effect is.

The other test, with a running engine, is to use one of those remote infra-red spot temperature guns, and to check that all cylinders are at more or less the same temp. A low temp on one indicates its not pulling its weight.

Or grab hold of each plug lead in turn with the engine running, to check for leaks. (not if you have a pacemaker!)

Or run the engine with the bonnet open after dark and look for sparks. (but the moon is full at the moment so this may not work.)

You could try looking inside the dizzy cap to see if one of the electrodes is noticeably more corroded / burnt than the others.
Guy W

Sorry was rushing this morning when I typed that, I wasnt removing the plug but the HT lead to the plug. I have been doing as you stated " pull the plug lead off. Using insulated pliers, you can then reconnect and pull it off again whilst the engine is running, and listen for the change - if any - in the engine speed" There is no change in the engine at all but I know that cylinder is firing due to the accuspark. Just trying to get my head round what Prop and Chris and Octarine are saying about the engine not running equally on all cylinders at idle?
C Carter

LOL, I did wonder as it is VERY noisy running an engine with one plug removed! But as you said it twice, I thought maybe it was what you did.

Maybe they do run a bit lumpily at low idle, but I am pretty sure all the cylinders should be firing and contributing. I too, don't understand what Chris is getting at.
Guy W

The engine runs smooth all the way down at 600 rpm and idling at 800 it stays there even with number 1 removed. I know the maths on this arent as straight forward as saying at 800 rpm each cylinder is contributing 200 rpm so by removing number 1 you would see a drop in Rpm by quite a huge amount?
C Carter

Fuel, air ,spark. If it is not the first 2 then its the latter.

Thus what I would do is eliminate the easy stuff first...checking after each stage

Change the plug for a new one - preferably change all of them
Change the lead - preferably change all of them
Change the dizzy cap and rotor arm

Cost is not that great, and reliabilty and perhaps even performance will improve.

Not sure of your inital post re manifold - care to elaborate.....

Mark O

I know it seems odd but you CAN have all 4 firing and pulling one lead has no noticeable effect on the engine speed or smoothness at idle.

Sometimes this is number 1 alone sometimes it is 1 & 4.

The A & B series engines suffer from charge robbing at idle but the 1500 has individual manifold runners so I wouldn't expect that to be the issue.

It is also possible to cross number 2 and 3 plug leads and have the engine start and run apparently normally at idle, but try using the throttle and it will spit and fail to pull the skin of a rice pudding!

Can't explain the science behind this - just what I have observed over years of working with engines!

Chris at Octarine Services

I know Chris doesn't need any confirmation from me, he's been doing this for long enough to be one of the first "go to" people I would think of when there's a problem.

I did thirty years working on broken down cars at the roadside and spent long enough trying to diagnose "with a lead pulled off a plug" to confirm what Chris says.

At low revs, sparks gas and compression do work, but less efficiently than at higher revs.

Just one of those things, some days it's very hard to get a result from it. That was why I advised using the Colourtune, then you SEE what is happening inside.

Just thinking on this a bit more - the only common thing between 1500s & A/B series is the firing order so it may be that there is still charge robbing even though there are individual runners.

Essentially because of the firing order number 2 sucks immediately before number 1.

So charge in the front carb is "robbed" by 2 and 2 also sucks charge from the port of number 1, additionally since the number 1 inlet valve opens 18 degrees BTDC on the exhaust stroke and 40 degrees before the number 2 inlet valve closes there is a period of time when number 2 is actually sucking exhaust gas from number 1 into the port runner!

So when number 1 gets to suck then it gets a shot of exhaust gas followed by a depleted charge of fuel /air - no wonder it doesn't contribute much.

The same applies to cyls 3 & 4 with number 4 loosing out.

Any manifold misalignment may make this slightly worse.

The fact that this only happens at idle is probably due to the relatively low gas speeds - at higher revs the gas has some momentum.

Chris at Octarine Services

Christian have you actually driven it? I'm thinking 'perceived problem where none exists'
David Smith

I have driven it a little bit, it is having its MOT next week so cant drive it on the roads until it has one.
C Carter

very handy having two 1500s as you've already done you can swap out individually parts that are known to be working from the other car

by coincidence I was round my mate's last night and he has running problems since putting his car back together, as just a quick test after checking connections we tried the lights off and mist spray but nothing, then there was talk of swapping out parts and for a few minutes the very, very obvious was forgotten, the other car in his garage has the same engine and ignition parts - sometimes you can get so involved with an issue you can forget to step back and in this case literally look at the wider picture
Nigel Atkins

so is there a problem when driving it or not?
David Smith

Maybe a "partially slightly" sticking valve

ave you tried a putting the offending spark plug in a different plug hole and see if the problem transfers to the new cly.

What color is the spark on the offending cly, is It bright crisp blue with lots of arching pop of its spark or is it a dull flat orangy yellow thats weak and hard to see and makes almost no audible snapping sound as it sparks

Prop and the Blackhole Midget

Prop - the pressures are all the same now so that rules out sticking valves I think.

I very much doubt there is anything wrong with the engine at all.
Chris at Octarine Services

how old is the fuel?
Rob Armstrong

This thread was discussed between 21/08/2013 and 22/08/2013

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