Misfire ?

[morgan]

You see I love this car and I hate this car. There is always one more hurdle...
So , Engine in, leaks fixed, that should be IT for summer of fun. Except I have a misfire I cant chase out. All I know is it wasnt there, now it is.

Symptom - Car runs rough (I mean even rougher than cam) and periodically (like every minute or so) BANG ! Usually though the carb but sometimes for variety through the exhaust. Stinks too. Something aint burning right.

I suspect a dodgy plug. I cant see one obviously misfiring with temp-gun. I pop them out - all look about the same.
So I suspect the wires. Pop them, test them. All seems good.
So I suspect the coil. Its 12v as MSD all-in-one. Borrow a brand new blaster2 - No change. I eliminate coil.
So I suspect the vac-advance may be jumping around (not connected, but I have a diff vac-can now, so worth a look). Nothing obviously loose or moving. Sucking on the port moves the base-plate. I wind it all the way out so NOTHING should move, reblock the hole. So I dont think its the advance can.
So I reinstall and check the timing. I have an 18deg-bush on the mech advance, so set for 16init, 34 all in. Runs fine. (misfire aside)

I'm stuck.
In desperation I begin to pop the wires of dizzy one by one and put a 'spark-light-tester' (flashes bulb at spark). I pick one at random (#6) and I notice that although spark light flashes its not steady - there are significant gaps. So I try (random) #2. Same pattern.
Its tempting to suggest that IF the flashy-spark-thingy is right (and one assumes one flash per spark) then the plugs are not being fed in a steady fashion...

I'm stumped. Coil and cable OK. There is a 12v feed off the loom I suppose. Voltage reg breaking down ? Dont know what else there is down the line.

Any clue team ? Failing that, anyone wanna buy a charger ?

[morgan]

Hmmm. From the cobra club. Seems worth a try - would eliminate voltage reg.

Had this happen to me a couple years ago, and I replaced the everything. By everything, I mean the cap, rotor, finally the complete distributer, plugs, wires, coil, MSD box, and it still ran the same.

I was ready to load it on a tow truck and have someone fix it and call me when it was done.

I was talking to an old friend of mine and he mentioned that the MSD unit is very sensitive to voltage input. He suggested that I disconnect the alternator and try it. I did and it ran fine. Put a new alternator on it and it did the same thing. Come to find out it was the voltage regulator. Replace it and it ran fine.

I did however end up with great collection of spare parts.

Take the belt off of the alternator and see how it runs.

Edit - Nope. No difference.
Corroded pickup ? I took a vid. You can clearly see the intermittent breakdown. Puzzlepuzzlepuzzle....

[morgan]

Bad fuel ? Hasnt been in there that long (I mean I filled up at xmas).
The problem didnt exist before then so maybe chasing a ghost...

[Dave999]

i think you have a plug with some gunge up it, which acts as an intermittent short
Or one of them has glowing carbon on it from time to time

you can spot with timing light. good plug in good cylinder fires every time, bad plug looks like a disco where the lights don't flash in time with the music.

it fires every 2nd or 3rd and often late, and then after a bit of a rev works perfectly long enough for you to think all is OK.

are they NGKs and have you cleaned them. if so i don't think modern NGK plugs like being cleaned i've never managed to revive one even with one of those air line sand blasters.

i think you need an NGK 7 in there or may be an 8 it wasn't until i went from 6s up to 7s that i started to be able to actually tune mine without experiencing this kind of issue setting in, usually just after a drive where i congratulated myself on finally getting rid of the popping and farting

it would return at idle and never leave.... then i'd buy some more weber carb bits swap stuff about and ruin another set of plugs by repeating the exercise....

only to end up back where i started on the weber tuning front...once i got the plugs right...

its a guess....my motor has 12:1 CR. a silly cam and was built my way, can't get parts hence repurposed all sorts, pistons sat above deck by 30 thou needed bits chopped. yours is a race motor that was built by someone who knew what they were doing using parts designed for it
but one presumes yours has a higher CR and a long duration cam designed to make use of it...hence my reasoning

its already hot in there you might need a cooler plug.

then again i don'[t know what you currently have

dave

[morgan]

I'll have to pop to the garage but from memory they are NGK6.
Yes, I have given them a light clean (I mean they have only done 500 miles, but I popped and cleaned). I've got a set that have only done 100 miles, I'll swap 'em in. Hadnt considered this engine would want a different number - 6s seem to be the range of choice for just about anything. Still, its worth a go - cheap fix...

Edit : Well, whaddaya know. I am running something completely different. NGK XR4 V-Power - ref 5858 (will go look them up now. There must have been a good reason at the time).
I found an old plug of Bobs too - Its a bit painty, but looks to be an NGKR with BPR5S stamped on it ?!?

[Dave999]

and that was my problem

i ran 5s in the standard engine 9:1 CR truck like cam and 2bbl carb with no problem
i put 6s in the uprated engine and should have gone further
once i did life was easier...but as i say i made a sows ear into a Pleather man bag (i.e does everything i wanted it to do, but you can't help feeling there is still a little way to go) rather than direct to the intended silk purse....
do it once and get it right has always felt like maybe doing it twice with different, virgin, thick wall, early casting block, rather than thrashed and decked late smog model block, using the lessons learnt first time round,would be better.

Dave

[morgan]

Edit : Well, whaddaya know. I am running something completely different. NGK XR4 V-Power - ref 5858 (will go look them up now. There must have been a good reason at the time).
I found an old plug of Bobs too - Its a bit painty, but looks to be an NGKR with BPR5S stamped on it ?!?

[morgan]

Ah ! Oops. Found this on 'dodgecharger'. Needs verifying but...

The XR4 is hotter than the XR5....smaller the number, the hotter the heat range. yesnod

I wouldn't consider an XR4 unless the engine was under 9:1 Wink

[Dave999]

Well i don't know the XR plugs, the numbering could be crazy mental but you seem to have exhausted all other things...unless it is your regulator

those plugs if they follow the common or garden BP? 5 - ES type then 4 is i guess ok for a very standard motor (unless they are some very specialist range)
i think the ceramic and central electrode will be set into a really deep cavity in the plug to allow lots of hot gas around it to keep it warm enough to burn off the crap in a low CR not that hot engine....
6 or 7 will be possible to see that the gap round the ceramic goes down into the plug a bit less distance, maybe half that of the 4. plugs stays cooler as less of its ceramic and electrode is exposed to hot gas so works in an engine designed to create a lot more heat

but i went to the 7 becasue 5 was ok for standard engine and 6 was eventually after whole retail pack going funny and not cleaning up, identified as not quite right.
i also ignored what was in previously in respect to thread length (who is to say it was correct) and made damn sure i had one with exactly the right thread length by cross ref with what was quoted for the head in the manual.

The aim was to keep all the thread in the head, thread was not too exposed and not too far up the hole i.e the right length

exposed threads of either the Hole in the head or the Plug thread emerging into the chamber a mm or 2 don't help at all. damage the head thread or
magnet for black gunk and burny stuff on the plug end of things..


i could be wrong its easy to get this backwards when the manufacturers have increasing and decreasing range no.s meaning both hot and cold

i my mind i moved to a colder plug to cater for a " hotter" engine environment even though i had no indication that i needed to by looking at them i had no evidence of plug tip burning to prompt me to do this
5s and 6s just didn't work properly after a short while they didn't look like a plug that was running too hot...they just crapped out eventually.


it was infuriating as i kept buying carb parts x6 or x3 depending on what i needed (X3 dcoe 45) i now have stocks that rival webcon....

and it wasn't that, it was plugs...now just because it was plugs for me doesn't mean it will be plugs for you
buy cheap try and see, then if you have the urge get a decent funky PROPER set to last you for ever if it works

Just conscious i tend to assume everyone runs into the same issues as me..... they don't..... but it might just help, its an easily tested alternative

Dave

[Dave999]

BPR5S

presume that's a 5 heat range resistor/suppressed plug....i think think the ES is just a longer version at the thread


too cool a plug and it will foul up a little idle but taking it for a spin should burn it off..... too hot and in my experience, they look ok but just get broken, go intermittent and never quite recover.....

dave

[morgan]

Got some XR5s on the way - which seems to be the cooler XR4s more suited to high compression.
Coming from gawd-knows-where. So will let you know...

[SJH]

Rule of thumb guide, some manufacturers' heat ranges increase in number and other manufacturers decrease ????

https://www.opieoils.co.uk/t-spark-plug-heat-range.aspx

[morgan]

Thanks, yes I noticed that - helpful isnt it ?
Will on here (who lets face it, knows a thing or two about extreme-engineering) swears by (I think) Denso ones - I'll try find the thread. They are £9ea though. Will dig it out.

PS - Opie Oils. I used to know the guy who owns/runs that. Small world !

[Blue]

The only change you'll notice with a £9 sparkplug is you'll be £6 poorer than you needed to be. A regular old school plug of the right heat range is all you need, these fancy plugs are designed to use in modern engines that burn hotter and cleaner than our old dynasaurs do. Iridium scmidium...

[Dave999]

From the mouth of a horse called NGK

A good rule of thumb is, one heat range colder for every 75–100 hp added.

383 had what? about 230 proper BHP becasue you do actually have an exhaust and a air filter and alternator and a fan or at least the electrical load of a fan and its running in a car with a bonnet breathing comparatively hot air.... the one on the SAE measuring dyno that was rated at 350HP didn't have to deal with that

yours has now, what, 300 350 400 450 ? more? 627.5 BHP

cold plugs throw off heat into the head quicker than hot plugs hence they are cold range plugs

one has to expect a difference in their ability to do this if the heads are aluminium rather than Iron as well

so a cold plug in an iron head might be too cold when put in an aluminium head ally sucks the heat from the plug body and faster
and a hot plug in an iron head might need a hotter range plug in an ally head

aluminium is comparatively excellent at sucking the heat out when iron is more of an insulator..... but how this balances up against what heat gets sucked out by aluminium from the combustion chamber as well i don't know...ive talked myself round maybe it all balances out

i shouldn't be reading sparks plug stuff while stuffing down my breakfast....

You might need another set.... 5s seem like the one for the standard 383 to me....but then again as long as they are not 15 quid each....

Dave