Misfire ?

[morgan]

It gets weirder than that. These are XR range - which seem to run lower numbering.
Stock (like stock-stock-motorhome-stock) seem to be XR4s. (see post earlier about 'up to 9:1ish comp). Summitracing suggests XR5 is the next hottest - but this isnt the same as 5s on the old scale.
As I am reading it - the XR5s (which are the current ones for sale) should be the equiv of a '6'. So colder than I have now.

As you say, I think the main point is 'Do plugs stop the misfire'. If they do (even temporarily) then thats a cheap fix while I play around. My pal has a less scientific theory (which I think actually might be right). His point is that Bob had this built as a race engine and I am putt-putting around in it. In his words 'You need to give it a damn good hiding and clean it out a bit'.

[Dave999]

yes i have been unable to find an XR/standard NGK plug comparison

i guess your special heads have a different thread size and thread depth from standard hence the XR plug

it would be totally off piste for the numbering to go the other way from the rest of NGKs spark plug range but...you never know

yes if the plug is too cold it will foul up and in most circumstances that will burn off if you give it some.

mine if i drove it at full pelt, prolonged motorway Driving, with 1 eye aways in the rear view mirror with the bps 5s or 6s would misfire in 4th at 3500 rpm it would feel rough, rough enough rattle the shifter... i.e the type of driving that could be called high speed cruise, to the point where the word cruise (given noise, and the full time job involved) was not appropriate. (55-60mph, almost modern car like..... this "cruise" most definitely wasn't.

which i chased as an over rich condition for months, like i was putting the fire out on alternate cylinders. after a session of doing this it then wouldn't idle properly.i.e once i'd fanged it for 1/2 an hour plugs never recovered.

plugs had decent colour to the end of the ceramic but looked old.....nothing else to see really, compared with numerous READ YOUR PLUGS pics in books and on the net.... i believe few would have changed them based on what you could see. apart from an old geezer i got to help with the webers
who said Wrong and swapped in 7s as soon as he pulled the first lead.....

his attitude was i don't care how much power it makes or good or bad it runs if you have upped the CR considerably we should do this before we touch the carbs

moved to 7s problem has gone away and it just starts and runs better as well. high speed cruise now doesn't involve a almost perceptible hesitation and der,der,de,der,der, misfire hidden in the exhaust racket that's 2 jumps colder than standard

which was annoying as i took a box of 5s and 6s with me.

standard is advertised as 10:1 CR but most of them ended up between 8.5 and 9.5 and of course ran 1 or 2bbl carter and a cam that was ideal for a truck.. and 5ES was the modern equivalent of the aussie champion that was in there originally

all torque between 800 rpm and 2800 rpm and nothing past 4000.

its not standard now.


but i don't know where XR4s sit in a range compared to the plugs i use

[morgan]

There is SO much info out there - but this old chart looks useful . According to this though I need colder for 10.5-10.8 comp...

[Dave999]

i agree..i think you do 5670-6 or 5670-7 if you heads are iron

and one presumes you don't have standard heads
and note subtle differences for iron an ally heads as well

Dave

[morgan]

Aw fuggit. New plugs, all gapped and fitted. No difference. Misfire and splutter.

I give up. I am stumped. Its supposed to be 12v in spark out. Timings good. Maybe something got knocked when I fitted the new vac-can ? New pickup needed ? - I'm shooting in the dark now. If its 12v steady in and splutter out (and coil is fine) then what the heck else is there ?

I think it needs to go somewhere with more knowledge than I to tell me what bit isnt playing nice.

[DaveB]

Give John Sleath a ring,put it on is rolling road,will cost a few quid,but its easier sorting problems on the rollers,than trying to tune on the road.
Take the guessing out of the equation

[Matt]

x2 John Sleath is a top chap

[Dave999]

Phuket indeed

sounds like a plan unless you want to spend more time....

did you try the timing light on each lead...see if they are all firing all the time?

if there is a spark every time and it's still doing it....maybe not ignition related at all.....its still could be.. different when under load from idling revving. but...maybe worth a try


Dave

[morgan]

You are in my head. I woke up this morning thinking the same thing. I need to try the plug-light-thing on all 8 too.
Good pointers from Ian at Herts meet last night. (micro cracks in dizzy cap, Check resistance of leads individually while working along them bending - see if something breaks down. Ditto coil lead as I swapped coil but not lead).

Irritating. Can drive it, but its not happy.

Thanks for the John Sleath recommendation. Its a long way from home though.

[Adam]

Have you eliminated fueling issues? e.g. a dribbling pump squirter or power valve will cause backfires. Or just too rich?

[morgan]

Have you eliminated fueling issues? e.g. a dribbling pump squirter or power valve will cause backfires. Or just too rich?

No. I havent. Ironically the very next thing was to get it somewhere to get it setup properly - and then this. The bind I am in is that I dont want to send it in to be tuned IF I do have an ignition problem. I mean I know I have to break the cycle somewhere but...
Power valve would apparently cause an issue; but again; I cant think when that would have happened.(but then its now banging and backfiring so... ).

I think its bite the bullet time - I had been intending to ask thunderroad to do the tune - but dont know if I fancy the drive now (although its fine once rolling or at least you only notice it at low speed) so might be OK.

[MattH]

I'm sure Thunder Road can help, Cheng knows his stuff.

[morgan]

Decided to try and get it filled up today so at least I know its got some for diagnostics. Dare not take it.
Wont idle. POP ! BANG ! - Will run, but stalls and bangs on idle. But I have made some progress.

The prob seems to be in the dizzy somewhere - My logic is thus.
Battery is fine. connectivity to coil with ignition circuit open.
With car not running (so battery only) its 12v-ish as expected.
With car running coil is 14.3v +/- 0.03v - so basically rock steady. This suggests ignition circuit, alternator, volt reg all fine.

Coil has been swapped with no difference noted - so eliminated.

Timing is set to 16/34 - its fine.

Plugs brand new (not a guarantee I realise, but they made no difference basically)

BUT - I noticed the timing gun misses too - which implies the charge isnt even hitting the wire. I moved the gun to a diff wire - same result.


Conclusion - Feed into the dizzy is fine. Plugs and wires likely to be fine as problem same for all.
This leaves -
Dizzy internals (pickup etc) - feels unlikely - should work or not work
Rotor arm (looks fine but... ? )
Cap (looks fine again, no micro cracks, but...)

SO - for the want of sanity I am going to get a rotor arm and cap. There really isnt much else left. I dare not try and get it to thunder road or similar (although to be honest once the revs are up you wouldnt notice).

Logic below - (not intended to be a circuit diagram, just illustrative!)

[Dave999]

phasing of dizzy...but that is one of those things that gets mentioned but i've not had the problem and my dizzy is messed with, welded and generally not as intended, it has only 6 * of cam shaft based advance to add..... it only adds 12 degrees of advance in real terms. which means its probably well out of whack from where it started and i haven't had an issue...but.....never say never

there are solutions for mopar dizzys not sure about others multi keyway reluctors etc rick ehrenberg does them

one thing you can do if this is the case is make the contact in the dizzy more desirable to the spark.

close up the plug gaps to 28 thou or some such...make the jump from rotor to contact easier...

other thing

are you sure you have the right rotor... use a bit of plasticine pressed into one of the contacts and turn the motor by hand..

the rotor should take a slice through the plasticine nominally across the centre of the contact patch... but i can't tell you how far away is too far. minimal gap is desirable

another thing.... button in top of cap. does it scribe a nice ware pattern on the centre of the dizzy rota.... is it still nice and springy..... does it get stuck....and does the shaft in your Dizzy jump up and down....
lost destroyed shims between body and drizzy drive, worn out press in shaft bushes??? i have only seen this on gear driven dizzy's when a nylon gear was specified but a bronze gear was used. i had one what had been run with a bronze gear and it needed a total rebuild..... then again it may have been knacked before the gear was applied.... nylon is forgiving bronze is not....not an issue you have i suspect, as yours has an intermediate drive...mine runs on the cam which is bendy and full of vibration.

ps just because they all misfire doesn't discount plugs. No flash of the timing gun just illustrates where the spark energy runs away through a compromised plug and no spark jumps a gap you need a SPIKE to trigger the gun if the pulse is smooth it doesn't trigger....no spike no flash of gun. no spike when the spark energy shorts back down the outside of central electrode ceramic to the gunk in the bottom of the plug and across to its body....i.e dirty plug or the gap is massive between electrode and earth strap is massive*....if it gets better with a smaller gap....then....this might be happening...once a set of plugs is knackered, these days its really hard to clean them back to life.....mixture needs to be near.....time for the gunson colour tune


* I think the plug electrode sucks electrons from the earth strap. and the spark travels across that bridge of Ionised air in the opposite direction.
if the electrons can be sucked up the plug insulator you get no spark and no timing light. leccy takes easiest route.... i might have it back to front but that small gap needs to be the easiest route

Dave

[Adam]

Could still be plug wires - they often break down at the same time. I had misfire problems on my 66 Mustang when I first got it. When I opened the hood at night it was like a firework display!