Misfire - most likely culprit?

[1969Mk3Spitfire]

I went for a drive yesterday and my car has developed a misfire. In around 20 miles, it missed around 20 times, very frustrating. The miss was instant and it recovered immediately. It happened at, say, low load, constant 30 or 40 mph but also when accelerating. Seemingly no pattern.

I know that No2 cylinder has a smokey plug issue that I’ve never managed to solve. CR is pretty equal in all 4. Tappet clearances are good. In September last year, just before putting the car away for winter, I replaced plugs, points, dizzie cap and leads from the Club Shop (I didn’t replace the condenser because it looked like rubbish quality).

I haven’t touched the carbs so far this year. During my last drive, a couple of weeks ago, the engine performed flawlessly.

What are the likely culprits for a misfire, hence where should I start looking? Many thanks.

[johny]

Does sound electrical so I would replace component by component using the old items. Start with the condenser, plugs, cap, leads then if necessary move on to rotor etc

[1969Mk3Spitfire]

Thanks to you both for responding.

Plug 1 and 4 colour is good. No 2 was sooty (see above comments) and I haven’t removed No 3 because I need to use a different spanner due to poor access. They are NGK BP6ES installed 6 months ago but done probably less than 100 miles.

Points contacts and gap looked good. Rotor arm and dizzie cap contacts were more black than I would have expected after such a short period of use. I’ve cleaned using emery cloth.

Plug leads are all new. Condenser is old but I have an LRC meter and it measured 21 pF, in spec. The new one via Club Shop had a malformed bracket and wouldn’t fit so went in the bin.

I’ve checked coil connections and sprayed with contact cleaner. Not yet driven the car since the above. Anything else to check and clean? 

I agree that it doesn’t sound fuel as the misfire and recovery is instant.
 

 

[johny]

Thats the problem with an intermittent fault especially one that only shows infrequently, as appears to be your case, things can look and test ok. Other than substitution of components you can also try running it carefully in the dark and look for any tracking...

[johny]

Must admit I dont change any components on my Vitesse unless they have a problem (ok filters excepted) so it has the original dizzy cap and leads and the plugs have been in there since 1992. Alright it hasnt done a lot of miles and has electronic ignition but theres always a risk changing anything plus some new parts are of doubtful quality...

[1969Mk3Spitfire]

Thanks, again, appreciate your thoughts.

I measured the distributor shaft with a DTI last summer/autumn when I was searching for a cause of No 2 plug sooting.  Can't recall the number (although I have it written down somewhere) but it was pretty negligible. The engine has always run very well.  I only changed the plugs, leads, cap, points, rotor arm etc due to No2 sooting, albeit I've yet to resolve it. I still have the removed components so I'll start a process of substitution.

[1969Mk3Spitfire]

No thumb and finger wiggle on dizzie shaft. Advance feels positive. Engine has only done 54k miles.

To add to my woes, I’ve just started the engine, first time since cleaning and checking the ignition system. It sounds like a bag of nails and feels as though it’s about to jump off its mounts. Oh dear.

My options are to investigate what I’ve done and risk CO poisoning or wait until tomorrow when my wife will go to work so I can access the drive.

[Pete Lewis]

a hard misfire is most certainly a plug letting go 

see the post on spark plug problems 

any back fires look at condenser

see dizzy doc for condenser advice   Delco condenser: 829111,829107,1861709,1866049,18655972, 1869704,1882239, Lucas Condenser 484249, 400308, 407044, 54411935 / DCB105, 54413006, 23D4 22D 23D DM2 25D DM6 DM4 lucas distributor, Lucas condenser number 423871, GDC101. Condensor 405833 for Ford 8 and Ford 10, Lucas condensers from Distributor Doctor

cleaning with emery is the last thing to do it leaves grains of grit  in the contact face and then as you have found things get worse not better

most Repro dizzy caps fit like a bag of spanners the location and such are hopeless   stick with the old one 

i never recommend any plug with a resistive R in the suffix as that reduces the HT on out old cars 

 

 

 

[chrishawley]

Just wondering if you've made any progress with your problems? Given you've gone from occassional misfire to (presumably) permanent non-fire on one cylinder an inference would be that one plug was 'on the edge' already and any other minor factor has taken it over (or under?) the edge. Given that #2 already has unsolved issues that does seem a likely candidate.

For myself, as a diagnostic, I'd replace all four plugs with brand new ones before pursuing other matters. Either a favourable or unfavourable result will indicate whether the plugs, per se, are the diagnosis.

If plug #2 is fouling, going one grade hotter on the plugs would be possible, at least temporarily, while sorting things out. That would be Champion N12Y or NGK BP5ES as per Spitfire 1500.

As an aside; worth a check that any replacement HT leads/distributor cap make a firm connection. On mine (repro items) they were loose enough ot jump out of their own accord and needed a fix.

Hope you're making progress.

 

[1969Mk3Spitfire]

Thanks, Chris, that’s all good information, much appreciated.

Bizarrely, the last two posts of this thread appear to have vanished into the ether. My last post said that I’ve found the bag of nails issue. My plug spanner has a rubber grommet/o-ring to support the ceramic top of the plug and it remained on the plug when I removed the spanner, hence the HT cap wasn’t seated correctly. Operator incompetence.

I’ve substituted all of the ignition components. The new ones, Club Shop, were fitted in the autumn and the car has run little since then. In the garage, this didn’t seem to make much difference so I’ve put the new one back on.

It’s disappointing that everyone thinks a near-new NGK plug may be the culprit. I appreciate your recommendation re hotter plugs. The car does not have a radio and I’d prefer non-R plugs so I’ll search for BP5ES or N12Y replacements.

I haven’t driven the car since my original post, hope to tomorrow (when my wife goes back to work so the drive is clear). It’s a bit of a faff because it currently has a hardtop resting on the body as I’ve yet to bolt it down.

[Wagger]

Check that the pivot on the points is not sticking. My Bil's Minor had points that stuck open intermittently. Also, if one plug is black with all others ok, you may have a sticking valve. My Vitesse had an inlet sticking and spat back through the carbs. An exhaust valve exhibits just a missfire. In other words, it may not be electrical after all.

[1969Mk3Spitfire]

22 minutes ago, Mathew said:

Could it just need some fresh fuel?

Another valid comment. I’m planning to add fresh fuel tomorrow.

22 minutes ago, Wagger said:

Check that the pivot on the points is not sticking. My Bil's Minor had points that stuck open intermittently. Also, if one plug is black with all others ok, you may have a sticking valve. My Vitesse had an inlet sticking and spat back through the carbs. An exhaust valve exhibits just a missfire. In other words, it may not be electrical after all.

I’ve ordered a set of Sparkrite plug testers such that I can observe each spark.

 I think that the points pivot is ok but I’ll recheck.

How to diagnose a sticking valve? A CR test shows very little pressure difference between the 4 cylinders.

Help and advice is much appreciated, thank you.

[Wagger]

3 minutes ago, 1969Mk3Spitfire said:

s

I’ve ordered a set of Sparkrite plug testers such that I can observe each spark.

 I think that the points pivot is ok but I’ll recheck.

How to diagnose a sticking valve? A CR test shows very little pressure difference between the 4 cylinders.

Help and advice is much appreciated, thank you.

A compression test may not show a sticking valve. However, if you run the engine for a short while with the rocker cover off and increase the throttle opening you may see the culprit by looking at the tappet gaps.

Mine exhibited a 'Ching' sound intermittently from the front left of the engine. A good run using some upper cylinder lubricant cleared it.

If you leave an engine standing in a 'Damp' environment without running it for a while, rust occurs on the valve stems of the valves which happen to be open. A through draught up the exhaust via the cylinder to the inlet causes rust on the valve stems.

Had this on an 'A' series that I had rebuilt for a mini and it only stood for two months on M-I-L's drive.

[1969Mk3Spitfire]

1 hour ago, Wagger said:

A compression test may not show a sticking valve. However, if you run the engine for a short while with the rocker cover off and increase the throttle opening you may see the culprit by looking at the tappet gaps.

Mine exhibited a 'Ching' sound intermittently from the front left of the engine. A good run using some upper cylinder lubricant cleared it.

If you leave an engine standing in a 'Damp' environment without running it for a while, rust occurs on the valve stems of the valves which happen to be open. A through draught up the exhaust via the cylinder to the inlet causes rust on the valve stems.

Had this on an 'A' series that I had rebuilt for a mini and it only stood for two months on M-I-L's drive.

Another helpful post, thank you. Do you have a recommended brand upper cylinder lubricant?

My garage is a pretty dry. I kept an Escort in it, untouched for more than 25 years and there were no signs of rust when I finally moved the car.

[Wagger]

39 minutes ago, 1969Mk3Spitfire said:

Another helpful post, thank you. Do you have a recommended brand upper cylinder lubricant?

My garage is a pretty dry. I kept an Escort in it, untouched for more than 25 years and there were no signs of rust when I finally moved the car.

Run it low on fuel and add half a litre of two stroke oil in 5 litres of fuel. Redex used to do the job but it has been changed over the years. May still work, but I have not tried it.

[1969Mk3Spitfire]

53 minutes ago, Wagger said:

Run it low on fuel and add half a litre of two stroke oil in 5 litres of fuel. Redex used to do the job but it has been changed over the years. May still work, but I have not tried it.

Blimey, if that mixture doesn’t lubricate the top end, nothing will.

Reminds me of the early 1970s when I ran Lambrettas on 90% Redex and 10% 4-star. Happy days. 

I’ll report back…..thanks

[Pete Lewis]

does this just mis a beat and give a cough when under load or give regular hic ups 

plug lead testers wont show up whats going on under pressure  ..waste of time 

most valves are now a slack fit most unlikely to be a sticking valve which would be rythmical not random

a siezed rocker is more likley inthe valve deptartment 

but this has come about since you changed a number of components      always check the last thing you did  first 

not dive off at a tangent  into expensive unkowns 

Pete

 

 

[1969Mk3Spitfire]

Appreciate your comments, Pete.

4 plug testers cost £9 so not a major outlay. I hope to find it interesting to compare spark across the 4 cylinders. Nothing ventured, nothing gained. Every day is a school day within Triumph ownership.

There are two issues. The misfire is new. Having now cleaned plugs, points, dizzie cap and rotor arm I’m hoping to take the car for a ride tomorrow.

No2 cylinder sooting has been a problem for at least a year, maybe more. I opened a thread on the subject last year. I fitted new ignition components but shortly thereafter out the car into hibernation. There may or may not be a link with the misfire.

I wholeheartedly agree with your philosophy of avoiding tangents but sometimes helpful people make interesting comments which resonant with my experience hence warrant further investigation.

Ideally, I’d like to buy a magic wand, wave it at the engine and have the issues go away……..but where’s the fun in that.

I’ll also remove the rocker cover, start the engine and see if there is anything obvious. Any telltales for seized rocker?

The misfire was instant with instant recovery. Pretty random. Sometimes at light load, rolling at 30-40 mph, sometimes under acceleration, albeit I don’t drive it hard.

[Pete Lewis]

it still has the rings of a spark plug letting go   

ie fine till   under  pressure and it gives you a deep cough 

have fun  

a misbehaving rocker can end up with a displaced or bent push rod  but thats not going to be random  more regular

stick to the basics and simpluze   Pete

 

[Wagger]

Yup, do the simplest things first, one at a time, then you will know what you have achieved at each step. I have, probably, given you too much information without waiting for results of previous steps.

[1969Mk3Spitfire]

Thanks @Pete Lewisand @WaggerI appreciate your patience.

Taken the car for a drive this morning and the revvy little engine performed flawlessly.

The two actions of note were (1) clean sooty plug No2 and (2) clean blackened contacts on distributor cap.

I can live with having to clean dizzie cap fairly regularly but I’m still a bit perplexed by the sooty plug issue. CR between all 4 pots is good. Tappet clearances are good and there’s no appreciable play in distributor shaft.

I’m tempted by the idea of trying a hotter plug. I don’t drive the card hard, quite the opposite.

Other things to check?

[Pete Lewis]

could be oil down an inlet valve guide 

you dont happen to have an external oil feed fitted as thats a good way to oil up everything 

do check the choke is returning the jet fully , give it prod with a finger 

soot from choke use takes a good few hot miles to clear it off .

its why the dizzy cap and points neeed cleaning thats not normal 

what dizzy is fitted lucas or   delco  ???

Pete

[Pete Lewis]

and if contacts in the cap are blackend  i wonder are you running a 1.5 ballast coil on a full 12volt as this doubles the ht

its an air gap in the caps 4 HT lead contacts   but the rotor must contact the centre brush with no air gap 

too many gaps can raise the spark as can too many Volts 

Pete

 

[Pete Lewis]

and a easy check   measure the points gap on each lobe of the dizzy cam   is there any variance ???

Pete

[1969Mk3Spitfire]

Thanks, Pete,

No external oil feed.

Choke returning fully (why only 1 sooty plug).

Points were clean, only rotor arm and dizzy lugs were blackened.

Dizzy is Delco.

Coil measures 3.5 ohm across primary winding 

No measurable difference between points gap on any lobe (previously measured shaft rotation with DTI).