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MG Midget and Sprite Technical - Misfire under load

Hi all, Happy New Year.
I'm ashamed to admit my Sprite hasn't seen the light of day for many months this year for many reasons and now its bitten me in the backside by not running properly.
It starts and idles no problem, when you apply throttle without load it revs fine, if you give it WOT under no load there is a misfire but not hugely noticeable.
As soon as you move away in 1st gear there is a very noticeable misfire and although it will drive through all gears, the misfire is very noticeable and power is very much down across the entire rev range in all gears. I took a very short drive and allowed it to idle up to temp with no issues, didn't overheat (although I didn't leave it idling for that long) and oil temp rose OK too.
The car hasn't been run for probably 8 or more months, just the battery kept charged.
I've checked the plug leads and pulled the plugs, 2 looked a little oily on the threads but all had a similar black sooty appearance. The car is a modified 1275 with a single 45DCOE, does very low mileage and most components are in excellent condition. It has electronic points and a non-vacuum Aldon Dizzy.
My initial thought was an electrical problem but the steady idle has put me off this. I then started to think about the carb getting clogged with lack of use, the carb is in perfect condition but has sat unused for many months. I pulled the top cover and the 4 (?) jets, blew them out, they looked perfectly clean anyway but this has done nothing.
I've never tried stripping the carb any further than this, I'm not familiar with DCOE's so don't know what else could cause this?
I also wonder if the head gasket has failed due to lack of use? My compression tester is broken so I need to buy a new one to check the compression but again, wouldn't this cause idle problems as well?
So first step is a compression test, if this comes back fine I'm looking for advice on where to go next?

Thanks
Mark
Mark M

Hi Mark,

If the plugs have a black sooty appearance, that would suggest the mixture is too rich and there is not enough air for the amount of fuel.

This will cause the engine to misfire as you describe. I had what sounds like the same thing happen to me. I cleaned up the plugs turned down the fuel mixture and that sorted the misfire.

I hope that is a fairly quick and easy one to try out before looking at head gasket!

Good luck

James
J Paul

Hi Mark,

If the plugs have a black sooty appearance, that would suggest the mixture is too rich and there is not enough air for the amount of fuel.

This will cause the engine to misfire as you describe. I had what sounds the same thing happen to me. I cleaned up the plugs turned down the fuel mixture and that sorted the misfire.

I hope that is a fairly quick and easy one to try out before looking at head gasket!

Good luck

James
J Paul


Mark.

If it was running Ok before it's holiday I can't see why it should have blown a head gasket.

There's been quite a bit of chat on the BBS about fuel deteriorating over short periods of time. How about trying some new fuel?
Gavin Rowles

Thanks James/Gavin.

The car has always been a bit rich at low revs I believe due to the setup (if that makes sense, its always idled roughly but produces great power at higher revs) It was setup on an RR years ago and the carb has never been touched so I don't understand how a layup could cause the fuelling to change?
Bad fuel is something I was wondering, hence my comments about cleaning the carb up but realistically, how quickly does modern fuel deteriorate?
I did have to replace the fuel pump earlier this year as it failed, so its quite possible the carb has been left 'dry' for some time. I will try fresh fuel as its an easy fix, the only problem is I have half a tank full!

Mark
Mark M


Mark - I am thinking weeks rather than months.

Not sure about Webber but on SU's there is small reservoir on each carb you can fill to test new fuel.

Can you fix a temporary gravity feed with a helper?
Gavin Rowles

Hi Gavin, Yes, I can give this a go, its worth a try. Thinking about it more, the misfire is very consistent, not a spluttery, bad fuel type of misfire which is making me think more about it being a single cylinder, but I'll try this anyway, its usually the most simple thing causing the problem!

Mark
Mark M

May be the balance weights sticking in the distributor. this can happen after standing and could cause your symptoms. Often clears after a bit of use.
N McGurk

Hi Mark,

Have you tried cleaning the spark plugs and restarting to see if the misfiring improves?

How long does it take for the plugs to soot up once cleaned.

If the last time it was run was for a reasonable amount of time then build up on the plugs would have occurred during that journey, you then turn the engine off for a few months. The sooty build up will still be there causing you the misfires this time round.

Good luck

James
J Paul

Hi Mark,

This could help you identify the causes:

Soft, black, sooty deposits easily identify this plug condition. This is most often caused by an over-rich, air/fuel mixture.
Check for a sticking choke, clogged air cleaner, or a carburetor problem - float level high, defective needle or seat, etc.
This may also be attributed to weak ignition voltage, an inoperative preheating system (carburetor intake air), or extremely low cylinder compression.
J Paul

Most definitely the fuel must be chief suspect and first to get refreshed.
No point in fiddling with anything else until you have "new" fuel.
JB Anderson

if the car has been left standing for 8 months it could one or a combination of small things

if it ran well before and was RR set up for that carb then do not alter the carb settings

trying the direct feed suggested with new high octane fuel (I'd try 99 octane) would eliminate fuel and fuel delivery problems to the carb

I'd also add a very little clean engine oil to the dissy body for the weights (part of servicing anyway) and clean up and check the spark plugs, wipe out and check the inside of the dissy cap, check the rotor arm and that it's fully seated, wipe and check the outside of the dissy cap, check the fit and conditions of all the HT leads

then go for at least a 50 mile drive

the doing very low mileage is doing the whole car (brakes, tyres, suspension, electrics) as well as the carb and engine no favours at all
Nigel Atkins

Thanks all for your suggestions. I will try new fuel as well as checking over the ignition system.
As I mentioned in my last post, the misfire comes across as a single cylinder is not firing, it is a consistent, repetitive misfire, not a random, intermittent one that I would associate with Fuelling.
It feels like an ignition problem on a single cylinder, leads, plugs, cap etc all need to be checked.
I can't go for a 50 mile run as its too bad for that, I don't know what sort of damage I'm potentially doing by running the engine like this.

Mark
Mark M

Check your rotor arm and if you have a spare, fit it. They only short out under load.
Robin Cohen

"but realistically, how quickly does modern fuel deteriorate?"

Far quicker than it used to! Lead was a great stabiliser, modern octane boosters don't seem to last as well, and Ethanol tends to absorb moisture. However what you've described sounds more like an ignition fault, degraded fuel might just be making it show up more clearly.

A spark finds it hardest to jump in a rich mixture- a consistent misfire when accelerating suggests that one cylinder has a weak spark. You just need to figure out which cylinder, and where the spark is leaking from.

I would clean all the leads, cap, and rotor, and set the gap on all 4 spark plugs, looking for damage or deterioration in any of those components. The cap may have a crack or tracking from one of the lugs, one of the leads may be greasy, dirty or worn where it rubs on something, one of the spark plugs may have a cracked insulator.

Ideally I'd just replace the whole lot- but where's the fun in that?
Growler

The obvious thing would be to run the engine in the dark and look for any tell-tale blue tracking sparks. But if it only misfires under load this perhaps isn't possible. But they may still show up if you rev the engine.

Also, look for flickering of the tachometer needle which would indicate it as a low tension electrical fault somewhere.
Guy W

I think 2 things

1. expired fuel, 8 month old ethiniol fuel is never good, esp in a rainy wet part of planet earth

2. your fuel pump points...im assuming you have an SU fuel pump orginal, it has points just like the dissy, if its not used very offten, the points devolopes a film...you just need to sand the film off and reset the points and check your fuel pump ground for good clean contact

but... id lose the old fuel 1st thing for new fuel, itz going to make a huge differance... use the old.fuel in.the lawn mower

prop
Prop and the Blackhole Midget

okay...if its clearly on only one cly. then id look at a sticking exhaust valve, and if you know which one...which is easy to test for by removing the leads one at a time while its running

prop
Prop and the Blackhole Midget

Mark,
the 50 mile run is after you've checked things

if it sounds like one cylinder then yes check by testing each plug in turn to find which then check that lead both ends then cap

any chance you could have mixed up HT leads
Nigel Atkins

I'm not familiar with the Aldon dizzy but does it have a vac advance? because I'm thinking that the main difference between revving freely unloaded and misfire under load is the vacuum condition.
M J Chapman

Thanks again, amazing to get so many responses in such a short time. Even more amazing to see the same names posting on here after 15 or so years!

Anyway, a little more info. I won't get the chance to look at anything until the weekend but to answer a couple of comments. The Aldon dizzy is vacuum-less so yes, its weights could be sticky, but would this cause lack of power instead of a misfire?
The Fuel pump is a Facet that was replaced earlier this year after the last one failed and is working fine.
Putting old petrol into an Electric Flymo lawn mower sounds a bit dangerous to me, how do I do that Prop? ;o)
I might be able to try running it tonight to check for errand sparks, good idea.
Leads/plugs/cap etc haven't been touched so they couldn't have been messed up. It has to be a deterioration problem, will try to run at night, then try new fuel and see what occurs.

Thanks to all once again.
Mark M

Mark,
Another thought, and easy to eliminate...does it have an air filter and if so, is it clear? Restricted air supply to the carb would cause an over-rich mixture. Maybe you have a mouse nest in there somewhere!

And don't forget to check on the rev counter behaviour when it is misfiring. If the needle jumps around erratically as the misfire occurs, then its a low tension ignition problem.
Guy W

Hi Guy, Air filter is fine, its a foam ITG one and there are no mice fitted!
The Tacho was fine to my memory but I have just "reconditioned" the battery with some fancy charger my brother in law lent me. I noticed the fluid levels were low so I've toppped up and used this charger that I think drains the battery and then does something to clean up the plates with pulses of high amperage charging, or something along those lines. It tells me it now has 14.1 volts so I assume its all good.

Mark
Mark M

Mark,
No mice is good!
The point about the tacho is that being in series with the LT ignition circuit the needle will flicker violently if and when there is a misfire caused by that part of the system. Its the first tell-tale to look for as it begins to misfire. If it doesn't do this, then the misfire is not related to the LT circuit items.
Guy W

x2 Guy

but I have to agree with you mark...its got to be deterriation.some where....its nothing that fell out of adjustment

im guesing it will somethjng simple stupid and ungodly time consuming to find

prop
Prop and the Blackhole Midget

Problem solved :o)

I reconnected the battery last night after it had been "reconditioned" and started the car, gave it a minute or two so it would idle (The Weber choke doesn't seem to help it idle) switched off the lights in the garage and revved to see if I could spot any errant sparks. It seemed to rev OK, without the misfire I was getting before. The only proof was to go around the block, got some warm clothes on and drove off and all seemed well. Came back, picked up the wife and we went off for a 45 drive all around the countryside without a glitch, very enjoyable and reminded us both how lovely it is to drive in cold night air with almost no traffic on the roads.

The only thing I wasn't sure about was the Ammeter was showing a pretty constant +10 to 15A current all the time which I thought was high and made me wonder if the Battery could be knackered, but the Alternator is providing enough output to mask this whilst driving?
I had Dipped beam on, and for at least half of the journey full beam as well as the Heater on, nothing else. Is this an expected reading, I can't remember it being so high in the past, especially as I had literally just disconnected the battery from the charger before starting up.

Mark
Mark M

This thread was discussed between 02/01/2013 and 04/01/2013

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