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MG Midget and Sprite Technical - Intermittent Misfire?

Hi all,

I'm having problems at the moment on my '67 MK3 - she's developing an intermittent misfire around 5 minutes after starting, normally after opening the throttle a bit/getting hot...

I'm thinking that it's possibly a fuel vaporisation problem; the copper fuel pipe runs on the top of the chassis rail, right next to the exhaust, as per when I bought her. That's currently connected to a rubber pipe which leads up to the fuel filter before the carbs.

Alternatively, I reckon it could be something in the high tension circuit - we recently replaced the coil, dizzy cap and HT leads, and after the rotor arm chomped through two dizzy caps (because they weren't located on the distributor properly (damn you moss)), we've fettled up a new set, which I think isn't the problem.

Once the problem starts, it's a misfire that varies between being fatal (sounds like paper being hit by the fan blade, no response to throttle) and an annoying one which is like a late spark or something....


Sorry to dump this waffle on you, but I'm trying to give you all the symptoms!!

Many thanks,

Josh
Josh 'Midget Mad' Spooner

Josh: my Mk III is still as Nature intended and the fuel line runs along the right hand chassis rail i.e. remote from the exhaust. Could be getting a little too hot where yours is?
On another note would you mind sharing the chassis / car number? I am tring to place mine in sequence with others: mine is GAN4-52692.
Thanks! Cheers, Dave
David Cox

>>we've fettled up a new set, which I think isn't the problem<<

Josh have you checked that, is the cap chomped in any way now, rotor arm ok - http://www.distributordoctor.com/rotor_arms.html

Dissy Doctor’s rotor arms are highly recommended and are not expensive when you consider the hassle

intermittent problems are a pain but any problem needs solving by logical diagnostic, step by step, not moving on until the steps have been fully completed

there’s two paths, fuel or ignition, pick one and follow it through from the very basic start, check everything is clean, secure, protected, fitted and working properly

as you know don’t assume any part replaced is working properly check and test it

it certainly could be a fuel problem but if it’s a vaporisation problem why has it started now it was hotter earlier in the year

have you checked/replaced fuel filter recently

wiggle test sometimes required on wires and connectors that only go faulty when hot (did you replace coil HT lead)

Good luck
N Atkins

Josh, as Nigel says you mention changing everything but the rotor arm, so if you haven't changed it it may well be damaged. Also have you checked and or changed the points? If not they could be the problem. Also check that the LT wires inside the dizzy are not chaffed at all.

I'm also curious how you can blame Moss, or any other supplier for that matter, for a badly seated cap. Surely only the person fitting it can be blamed for that!
Trev
T Mason

9 times out of 10 troubles like this are ignition related...
Make 1000% sure this is not the case.
And remember new parts can fail....
Onno Könemann

Josh,
having seen your post on the other thread I'm now wondering if you might perhaps have a different dissy in your car

perhaps some good clear photos would help (only one photo per post on this BBS)

1275s usually have the 25D4 with side entry cap, the dissy is marked up as such some where I think

but as with the other thread I though I may have had a top entry cap on my previous Spridget but dismmissed it as my bad memory

with the dissy cap there's a locating lug on the cap and notch on dissy so they should only fit one way

if you have an electronic ignitor head fitted then sometimes the shroud locating and protecting the wires from it can be a tight fit so you have to take care when fitting the dissy cap
N Atkins

This thread was discussed between 28/07/2011 and 29/07/2011

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