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MG Midget and Sprite Technical - Misfiring/Rough Running

1977 1500
Went down to the coast & back today - 50 mile round trip. After lunch, stopped to get some petrol - it was just on 'E' got up the road about 3 miles, there was a sudden loss of power (in 4th gear) and it felt like it was running on about 2 cylinders. I carried on and it sort of cleared itself. A few miles later on a hill accelerating you could detect that it was not running properly but not as bad. Stopped at a garden centre 5 miles from home. Got back in the car after half an hour, all seemed OK, 3 miles up the road very bad misfiring/rough running - didn't think it would get home, slight improvement as you went through the gears.
Got home, let it cool down a bit and had a look.Checked HT leads, wiring to coil, checked gap on points. Initial checks seemed OK. Took it out again first 2 odd miles no problem, just as I neared home the problem returned.
In August I replaced the fuel pump after it had failed (pump from moss)
I also replaced the points, condenser & rotor - from the Distributor Dr.
I also replaced the hoses too & from the new pump as they were pretty well perished when I disconnected them from the old pump.
Apart from the pump failure had no running issues prior to that or since the replacement pump.
Help - where do I start!!
Nigel Axtell

You should do a service Nigel - check the owners manual!

In all seriousness it sounds like you prob have a carb/fuel pump issue... I'd be surpised if it was electrical given that it is spontaneous.

what are your breather hoses like - from the rocker to carbs? If they are perished you will be getting too much air into the mixture and it will misfire... that would cause spontaneous misfiring.

When did you changed the pump? Did you choose the right one for the car? have you tried disconnecting the hose and seeing if it is working properly... try pumping fuel into a container? could be that the pump you've bought is a dud modern cheap part
C L Carter

Hi Nigel, sounds like muck or water in the fuel? maybe their tanks were low.
Any problems before your last refuel?
Have you used this fuel station before?
With the cooler weather, unlikely to be fuel vapourisation.
Regards, Tony
Tony G

Argh! Wrong Nigel! - Sorry Nigel, you can ignore my random owners manual joke.
C L Carter

Breather hoses were replaced about 4 years ago.
The pump was changed3 months ago - August from Moss

Tony your theory sort off rings alarm bells
No problems since last refuel
I haven't used this particular fuel station, it was esso, haven't even used their fuel before. When I went in, they had about 6 pumps, only one had the higher octane, the others were all locked/empty - could be muck or water.
Thanks for the info
Cheers
Nigel
Nigel Axtell

Typical of water in the jets.
Gobs of water sit in the bottom of the carb and jet tube, and only block the jet when there is moderate demand, as in acceleration and going uphill, but will idle or cruise fine.
Take the float chamber lids off and look for water in the bottom, remove same.
Take the vacuum tops & carb pistons off and blow back through the jets to get the gob of water out of the jet tubes.
Clean the vac chambers and pistons with a soft rag and Brakeleen or lacquer thinner before putting back.
This will show you what happened.

The quick and dirty fix is to take off the filters, run the engine at about 4000, and momentarily (1/2 second) choke each carb in turn by blocking it with your palm. Unchoke before the engine dies and repeat a time or two. The very high vacuum produced usually will suck any water or dirt right through the jet.

Put some isopropyl alcohol (aka DriGas) in the tank to get the remaining water out. If you have an inline filter before the carbs, it can get blocked the same way, as water does not get through them easily. The alcohol will take care of it if the car will run long enough to run some through the filter. In a pinch, you can remove the filter and shake it vigorously, with the inlet end down, which normally gets most of the water out so that you can proceed. Replacing the filter while there is still water in the system is futile.

FRM
FR Millmore

It sounds like an owners manual issue to me.... Did you read it completely. Hahaha sometimes just setting the manual on fire will fix many hidden issues temporarly, esp if you turn the radio up really loud

But there are those that live and breath by them...hahaha

Prop

And bad gas is not just a fun way of saying i farted in the elevator
Prop and the Blackhole Midget

:D

we'll see what the cause of the fault is

Nigel obviously follows good advice as he got his points, condenser and rotor from Distributor Doctor so probably reads the Driver's Handbook and carries out proper servicing :)

if you ever need new HT leads performanceleads - www.performanceleads.co.uk

Nigel Atkins

The OP said: After lunch, stopped to get some petrol - it was just on 'E' "

FRM said: "Typical of water i the jets"

The problem might not be the petrol provider but rather the grunge in the OP's petrol tank?

Richard
Richard Reeves

This thread was discussed between 14/11/2012 and 15/11/2012

MG Midget and Sprite Technical index

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