Intermittent misfire...

[JimK]

I wasn't sure whether this was a mechanical or electrical issue, but I think it might be electrical:

I've got an intermittent misfire, manifesting itself as an occasional stutter when driving and a irregularity in idling. It started last night after a 20-mile run at 40-60mph, with quite a stutter while driving at 40mph. Less so on the way home and this morning. The car idles, just not smoothly. Not noticable on acceleration, but on part throttle the stutter happens every few seconds.

It's not every revolution, so I don't think one cyclinder is completely spark-free. The coil is only just warm to the touch after 13 miles. There's clearly been arcing between the end of the rotor arm and the dizzy contacts. Points look OK (though I haven't checked the gap) and the leads are in good condition. Fuel pump is one of those blue-topped electronic ones with the fake click

Not sure where next to look - my debug skills are still developing. Any tips? I don't want it to be a symptom of something serious.

[bmcecosse]

First check would be the plugs - gaps ok - colour ok ? Then points gap - and try a different rotor arm and dizzy cap. Change one thing at a time to see what fixes the problem.

[JimK]

You have to check the spark plug gap? Blimey. Why? How does it change? It's never happened on any of my other cars.
Plug colour - checking that any deposits are browny-grey rather than soot black, right?

I need to buy a new rotor arm and dizzy cap then, as the Trav has a different type.

I relaise that changing one thing at a time is good, I just didn't know what the best thigns to change were. Thanks.

[rayofleamington]

If the idle is a bit rough and you get an occasional missfire at lower revs / part throttle, then if the electrical changes don't cure it, you might want to do a compression test

[JimK]

If the idle is a bit rough and you get an occasional missfire at lower revs / part throttle, then if the electrical changes don't cure it, you might want to do a compression test

Oh dear. What would I be dreading?

The misfire started last night as I headed across the New Forest at 60 or so, and starter quick suddenly as though I was running out of petrol. It wasn;t so bad after that, but at home this evening the idle was pretty lumpy.

I'll have to borrow a compression tester, it can't hurt to see if the electrical bits will do any good.

Might have to bring forward the engine-changing plans...

[chickenjohn]

Agree with Ray- do a compression test, low rev miss-fire (engine seems fine at higher revs) may be a very slightly burnt exhaust valve.

But first check ignition and fuelling- one of those is more likely as its an intemittent fault. Put a colour tune in one of the spark plug holes and see iff the mixture looks OK, then try those handy ignition indicators that light up to see if the spark is getting to the plugs OK.

Is there any oil in the carb dashpot?

[JimK]

Agree with Ray- do a compression test, low rev miss-fire (engine seems fine at higher revs) may be a very slightly burnt exhaust valve.

The car went from 2000 miles in 5 years to 100 miles a week, so I guess something might have given up in shock?

It's not entirely fine at speed, but less problem than at low speeds.

But first check ignition and fuelling- one of those is more likely as its an intemittent fault. Put a colour tune in one of the spark plug holes and see iff the mixture looks OK, then try those handy ignition indicators that light up to see if the spark is getting to the plugs OK.

I'll have to wander off and find one of those colourtune things. As for an ignition indicator, I assume you mean something that goes in the HT line in series? Haven't seen those.

Is there any oil in the carb dashpot?

Do you know, I have no idea. I'll have a look in the morning.

[Willie]

If it is not lack of oil in the dashpot I would check the points gap as it is failing more at low revs. 'Think Simple' before you start changing everything.

[JimK]

Thanks Willie. We had the points close on the Trav and this doesn't feel anything like that. It's still pulling OK whereas the Trav would struggle to 40.

I forgot about the dashpot since it didn't occur to me that it could be depleted.

[chickenjohn]

Check for loose connections as well, a loose feeling ignition lead for example.

[MikeNash]

Just a thought. Start it up in the dark, open the bonnet and use a torch to rev it by hand. Look at the ignition system, ie coil, distributor and leads, and if you can see any electrical discharges (like little flashs) then you've an elecrical breakdown. KEEP CLEAR OF THE FAN! Rgards, MikeN.

[JimK]

How do you rev it with a torch? I don't know, some people...

[chickenjohn]

Nudge the throttle spindle with your torch! ;-)

[Kevin]

There's clearly been arcing between the end of the rotor arm and the dizzy contacts.

If its bad this can point to the condensor and can also cause the points to faces change colour.

You have to check the spark plug gap? Blimey. Why? How does it change? It's never happened on any of my other cars.

I assume you are winding BMC up on this point

As for an ignition indicator, I assume you mean something that goes in the HT line in series? Haven't seen those.

Here you go, although use see them at autojumbles and the like for a quid and they are very useful.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... &rd=1&rd=1

[JimK]

There's clearly been arcing between the end of the rotor arm and the dizzy contacts.

If its bad this can point to the condensor and can also cause the points to faces change colour.

Thanks, that's useful. I'm going to change the electrical bits like that one at a time anyway, but nice to know what may help.

You have to check the spark plug gap? Blimey. Why? How does it change? It's never happened on any of my other cars.

I assume you are winding BMC up on this point

Me? Wind BMC up?

Actually, I'm serious. How/why does the gap change? Did I not have to do this because I changed the plugs at 20k miles?

Here you go, although use see them at autojumbles and the like for a quid and they are very useful.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... &rd=1&rd=1

Thanks. I might get one, though a colleague has a colourtune I can use and of course with that I'll be able to see if the cylinder is firing.

One reason for going to the Bristol show this weekend is to browse the jumble for stuff like that.

[Packedup]

Actually, I'm serious. How/why does the gap change?

Tiny particles are transferred across the contacts with every spark (the same as happens to the points). There's also the blast furnace effect of being inside a turbulen and extremely hot cylinder. So spark erosion + millions of explosions = Worn plugs.

An ignition system in good condition will probably fire plugs right up to the time they're due for changing, but if the system is less than perfect the widening plug gap could cause problems. Plus of course new plugs should be gapped corrently when fitted, and many aren't.

I'd suspect points or condensor myself (or dry dashpot of course), but it's certainly worth checking the plugs. The in line testers are pretty cheap - About £1.50 each at my local factors, and well worth having IMO. And a Colortune will tell you not only what sort of spark you're getting, but what the mixture is doing, so again well worth having. A compression tester is a great tool for being able to immediately discount compression problems (or immediately discover them and then despair) as being responsible for poor running, and can be picked up s/h at autojumbles etc for a fiver.

[JimK]

Thanks Packedup, that's really good to know. I've done better than £5 from a jumble and borrow one for the price of a pint

Is gapping the plugs just a question of bending the end bit? What with?

I've ordered a service kit, so I'm going to renew all the bits one at a time to see if the problem goes away. I've also borrowed a Colortune with a view to buying it from the lender.

[rayofleamington]

Is gapping the plugs just a question of bending the end bit? What with?

You bend them with 'care' ;-) and whatever you find in your toolbox, making sure not to load the internal element as this can case the ceramic to crack and you don't want bits to drop in your bores!

there are also special tools for adjusting spark plugs - never tried one of those as I've never known anyone who had one.

[Packedup]

You're supposed to gap plugs with the correct tool to keep the earth electrode parallel to the centre one. This is because if you simply bend it the only area the gap is correct is along the "front" edge, losing a hell of a lot of potential surface area at the right distance.

However, I never got on with the correct tool, and happily just tap the electrode till it's about right (using a 25 thou feeler of course), and be done with

[alex_holden]

I just lightly tap the end against a convenient hard object to close the gap, trying not to go too far because opening it up again without damaging it is a bit fiddly.