Obscure misfire (sometimes severe) problem

[stuartgb100]

Hadn't used the car for 3 weeks, then took it out. A 45 minute spin and towards the end a hesitation ..... serious, almost like a cut-off.

Worried ........ but it didn't repeat.

A few days later, the same, but this time a 15sec period of kangeroo-style bucking !!

This weekend (deciding it must be carb/electrics) I drained the tank and fuel lines and removed the carb top covers. No sign of water in the carbs and only minimal sediment. Cleaned the fuel pump filter.

No improvement, but left it ticking over.
Then it stopped. Removed both carb top covers and found plenty of fuel.

Decided to look at electrics.

Changed points and condensor. Swopped out a spare coil.
Reset the timing; balanced the carbs; reset mixture ........ all as best I could. ............. no improvement.

So after all this I've got a Sprint with a lumpy idle. which will not rev under load, and I missed the open day at Hethel.

But I think I've fixed it ........ can't be sure because it's too late to be revving away (neighbours).

Regards,
Stuart.

[miked]

Stuart,

I had some trouble with Dizzy caps after having a coil with excessive voltage. When breaking down they would wait until warm and then hesitate and miss.

Worth swapping a cap to prove. I always carry one. I now have a normal coil. Also had a rotor arm track through.


Mike

[SADLOTUS]

I concour - plus check/change plug leads - I had similar and couldn't believe that a breakdown in one of the plug leads could be so sudden.

[reb53]

Stuart,

Is it worth having a look under the bonnet on a dark night and see if you have any odd sparking going on ?
I had an old Toyota that was not so much misfiring as cross firing due to faulty plug leads.
Adjacent plugs were being fired with the result that you didn't get a mere stumble/hesitation but it felt like you'd hit something.

Ralph.

[iain.hamlton]

Stuart
I also suggest you make sure the low-tension circuit cannot flop about and short to something.

best regards, iain

[nebogipfel]

Just a thought, Could it be the security switch contacts (assuming you still have it conected)?

[martinbrowning]

Having a similar problem - convinced that it is heat related. (Onset of misfire varies, but car idles ok from cold). After problem occurs the car will start and run sweetly again after 30-60 minutes, until next time........
Interestingly, Classic and Sportscar "workshop" column in September described a similar problem with another sportscar. Turned out to be the Lucas rotor arm breaking down electrically.

May be of help.

Martin B

[stuartgb100]

Thanks for the replies.
Problem seems solved, subject to a road test.

Culprit: the rotor arm

financial cost: peanuts !
time: a few hours !
biggest loss: missing Hethel's 40th !

Brownie points to those who suggested the problem.

I actually fixed it last night, but couldn't be reasonably sure until tonight (after re-setting advance, checking dwell, carb balance and mixture, etc).
Still to road test, but it idles smoothly again, and revs willingly.

This is symptamatic, and a little worrying for me, about the quality/lack of indigenous to current simple components.

I had a condensor fail on me after 3 months ....... the weld holding the mounting bracket to the body failed. At high revs the condensor was bouncing around, acting like a random cut-off ....... very annoying !

Apparently the rotor problem is well known to the MG community, and much discussed.

Regards,
Stuart.

[miked]

Stuart, I believe it is the little brass rivet type screw thing that holds the blade on. The distance from the bottom of the hole through the body to the shaft metal is minimal.

I have taken to carrying one in the boot.

Mike

[ppnelan]

The proper (i.e. old) Lucas ones (I use the non-rev limiting type) should have the brass contact strip 'cast in' with the 'bakelite'. I had a rivetted pattern part break down once - took 30 minutes to locate the problem and 30 seconds to fix as I too always carry a spare...
I cut the old one up and could see how close the rivet was to the top of the distributor shaft...
It happened about 5 miles from home after a 150 mile journey, IMMEDIATELY after my wife & I had said how reliable the car had been! I think they can hear you, you know...

Matthew