Motorway misfire - FIXED...?

[dickie14]

Hi all

Currently driving round Spain and the van yesterday developed a misfire at high revs (anything over about 55-60mph). It would kick in
at slightly lower rpm if the engine became unloaded, eg. cresting a hill or coasting behind a truck. I figured it was heat related given the conditions, but it was back today as soon as the van got to speed.

I found the dizzy contacts on 3 of 4 HT leads had gone blue/grey, along with the corresponding sockets on the cap. Swapped the cap and leads out but it hasn't improved.

Normal driving's fine, and keeping to 50 prevents it (saves a bit of fuel too...). But it's bugging me - any ideas? Why were my leads fried?

Cheers,
Dickie

[ghost123uk]

We had this in France, all be it on a water cooled. It turned out to be the rev limiting rotor arm had gone faulty. I had a non rev limited one on board as a spare and fitting that cured it. Not sure though if your CU uses the rev limited rotor arm. Here is a pic of one (Courtesy of JK) =



And here is a standard one (Courtesy of JK) =



Might be that.


.

[dickie14]

Wow, thanks for the swift reply ghost.

I have the standard rotor arm, and I have tried changing that out too to avail unfortunately.

I forgot to mention a couple of potential clues:
1. The three leads that had tarnished (?) were only a few hundred miles old and cheap gsf ones too. (I didn't replace the last one as it was too short!)
2. Symptoms started about 3hrs into a 5hr drive at a constant 60mph in 30deg heat.
3. Having said that, oil temp was not exceptional.
4. I'm not actually sure misfire is the right term, it pops and bangs through the carbs with accompanying power loss.

Peversely I'm now daunted by going downhill, even really shallow descents as the light load seems to make it really kick in!

[CovKid]

fuel starvation?

[ghost123uk]

Popping and banging on the over-run (ie, downhill) is sometimes caused by a leak in the exhaust system (though less likely in view of you saying it does it when travelling at a constant 60mph)

[MartinR]

Casting my mind back to when I had a CU a good 10 or 15 years ago I had something that sounds very similar while batting along the M25.

At the front of the engine, near side I seem to remember, sort of tucked behind the n/s carb, are a bunch of metal pipes, from memory about 1cm diameter, joined by two soft rubber pieces, one a right angle and the second a T piece. Think is something to do with vacuum/carb balance. Anyway, they had softened with age and one had come adrift - backfiring badly particularly on the over run- just bunging them back on sorted it for a while.

Hope you get sorted

Good luck

Martin

[T25Convert]

Afternoon,

Did you ever sort your no.3 popping you had a at idle and occasional overrun and the lack of suck on the same cylinder? This sounds like a logical follow on from this!

If its popping and spluttering is sounds like it might be leaning out at higher revs, so as above ^^^ is your fuel pump keeping up?

If it is then I think that leaves blocked main jet in one or more barrels (causing it to lean out as you come off the idle circuit onto the mains at 3 - 4,000 rpm), chronic air leak or timing issue.

As a start I'd check for air leaks with a can of WD40/carb cleaner, check all your vacuum lines are attached, have a look at your points and check they are still gapped and not pitted. Might be worth popping the main jets out as well and checking they are clean and clear. If this all looks good I'd check fuel pump output and the filters for blockages....

If no joy there then its time to look at the electricals!

Good luck!

Cheers,

Alex

[dickie14]

Fuel issues were one of my first thoughts - after that long slog I could hear what sounded like the fuel boiling in the float chamber once the engine was stopped. But then the symptoms came back as soon as I was on the autopista this morning after only a minute or two of driving, so barely warmed up.

Fairly recently fitted a facet pump to replace the mech one, but surely it needs more fuel under load so would be worst then?

The slight popping at idle on #3 was almost cured by the new HT leads... Certainly very infrequent now (should 've updated that post really). This misfire is all over the place, not tied to any cylinder in particular.

Will try to have a look at the jets. I'm currently at the kind of campsite that would severely frown upon an Aircooled vw being started and revved repeatedly so air leak checks may have to wait...

Thanks for the replies, really appreciate it - keep the suggestions coming!

Cheers,
Dickie

[dickie14]

Had a better look at things today and it does seem to be just one cylinder/carb after all, no. 2. It's misfiring and popping back through it at high rpm and the carb throat is appropriately sooty.

I've taken the main jet stack out (running dells by the way), squirted carb cleaner through it and blown hard down it (didn't seem to be anything stuck in there) but it hasn't helped.

I'm confused about air leaks - wouldn't they cause most problems at idle? (it idles fine). I liberally sprayed carb cleaner around the gaskets and all the fittings on the carb and didn't detect any difference. Should I still expect a change in engine note at idle if it's a high rpm issue?

I've got another 1000 miles to go and it's difficult to drive on the autoroutes (in France now) without it happening some of the time - am I risking the engine continuing like this?

[dickie14]

Ps. Points seem fine and fuel filters (in line one and the ones on the carb banjos) are clear. The advance plate on the dizzy is pretty dirty old oily though - is that normal?

[The Bishop]

I'm not sure which dizzy you have but meantion of the advance plate reminded me. I had a dirty dizzy which seemed to be causing the advance plate to mis-behave and so cause misfire. perhaps that dirt and internal dirt is the problem. Get some electrical contact cleaner (might be easier said than done I know - try Norauto) and spray that into the dizzy to clean up the contacts. It worked for me. Might of corse mean that the dizzy is worn and if so not much you can do on the cheap.

I keep meanting to pick up a secondhand complete dizzy from an autojumble to set up with new cap, rota etc that can be swapped over if need be.

Good luck, at least you are still running so take it easy and perhaps revise your plans if over ambitious.

[dickie14]

I've found the corresponding spark plug is very sooty - does that point to a weak spark? Still can't find any air leaks and I've cleaned the dizzy up a bit but no improvement yet...

Trying to take the slower roads now but it's still hard to avoid it happening

[CovKid]

Float needle no longer doing its job? What you may find is that whilst valve may work with a weak mechanical pump, as soon as you give carb fuel under moderate pressure (Facet will do that), it may be leak past float needle, over-fuelling and causing it to run rich. Sooty plug is either over-rich or its burning oil. Suspect former. I had this issue on my DG and a gasket kit (which includes new valve), sorted it. Float valve on sooty side could be culprit.

Agree on rotor arm too, which is a cheap swap if it fixes it. Definately rule that out first.

[sarran1955]

Hello,

You are welcome to come by here...

I'll take a look...

Cordialement,

[dickie14]

Thanks for the offer Sarran, very generous! Alas we're over on the east side of France at the moment and it would be a significant detour to get to your part of the world... I'm trying to take the shortest route back!

Cheers,
Dickie