Thoughts on high speed misfire

[jonbills]

Hi everyone -
Yesterday on the rolling road, everything was as expected upto about 6,250 rpm then misfiring stops it revving any further. Changed the emulsion tubes to richen it up and got another 500 rpm before misfiring, but AFR is around 13, so don't think it's a mixture prob.
Last year it loved to rev - 8000 rpm and still going.
What's changed? Not much - a new piston and rings and bearings and a new exhaust system (and less ignition advance!)
I'm thinking it's prob electrical - plugs, ignition leads, dizzy module, coil.
What do you think?

Bcr8es plugs, 280zx dizzy, old carbon leads (I've had them at least 10 yrs and got them 2nd hand), similarly old pertronix flamethrower coil.

 

[Rob Gaskin]

Immediate thoughts:

Yes you know the leads need changing if just to eliminate them.

Don't the rolling road people have diagnostic equipment to check for ignition faults e.g. the old Crypton tuners? Or am I just showing my age now.

The fault sounds quite fundamental to me if it won't rev beyond 6 and a quarter i.e. I would have thought it would show up on equipment.

Perhaps you need to put an old dizzy on with points.

 

[jonbills]

Thanks Rob, I'm not sure there is diagnostic equipment that'll pinpoint what causes a misfire at 400 sparks/second! I'd be happy to be corrected.
I have got spare points dizzy, even older leads, and my original lucas coil so swapping parts is indeed my next step.

I have had to remake the ends on a couple of the HT leads in the last few weeks. Maybe that is an indicator that they're the problem.

 

[Farmer42]

Are you sure you have the right plugs? I have always had NGK BPR6ES in mine which is the recommended plugs for L Series (at least those are the ones supplied by MrF when asked). If you don't have the right heat range with these, they can cause a misfire.

The higher the number the colder the running with NGK so if you have loads of air and fuel going in at higher revs, the spark could be too cold to ignite it with the new engine bits.

 

[jonbills]

Thanks Paul. I did consider that but discounted it. If the plug were too cold, I'd expect fouling and misfiring lower down maybe, but not this.
BC prefix just means its a 14mm hex socket rather than 21mm because the plugs are sunk in 5mm more.

 

[Jimbo]

when mine hit 4.5k revs and would misfire it was down to the vac pipe for the dizzy had popped off.
could it be as something as simple as a collapsed or cracked pipe somewhere?

 

[jonbills]

That's a good point - my brakes have felt heavy this year - I wonder if I've got a vacuum leak on the servo. I'll have a look at that tomorrow.

 

[jonbills]

Gold star Jimbo. Blocked the brake servo vacuum line and it revs out (AFR's gone richer too!)
I have a feeling that may have been behind the engine failure last year.
Back in July when I took the engine out, I found brake fluid in the hose when I disconnected it. So I knew the master cylinder was leaking (replaced it over the winter) but it didn't occur to me that the servo diaphragm may have rotted.
Perhaps the combination of brake fluid in the chamber and going lean caused the overheat and piston jamming.
Hooray!

 

[Rob Gaskin]

Great news, easy fix.

Jimbo

I still think a rolling road session should find that sort of thing. I get the impression that they are used just to measure power instead of tuning and improving.

You need another session now Jon.

I've only had one session but the guy had my SUs to bits, re-profiled the needles and adjusted the timing (whilst the car was running). I drove home better than on the way.

 

[jonbills]

Thanks Rob, yes got to get the mixture righter and get back on the rollers. I'm sure they would have booked me in to fix it if I'd asked, but it wasn't a booked test and tune session - it was just a spur of the moment favour and they had a proper customer coming at 1

 

[jonbills]

I spoke too soon. Now the engine is hot, it's still missing at high revs :-(
Very surprising cos there was a big vacuum leak to the servo. Hmmm

 

[johnymd]

What's your AFR's at high speed and full throttle now? 13 sounds too lean to me.

 

[Jimbo]

if its been set up on a rolling road with a big air leak now the air leak has been found and cured the tuning will be out as it will effect vaccum advance and obviously fuel mixture,
this is going to need to be set up again by a rolling road before anything else.
air intake isnt being restricted? maybe try removing air filter temporarily and see if it makes a difference?

 

[jonbills]

What's your AFR's at high speed and full throttle now? 13 sounds too lean to me.

I swapped the coil out and I *think* that fixes another part of the problem.
It starts misfiring now from about 7k, The misfire does coincide now with the AFR starting to climb quickly from low 13s to 14, so I think you're right John. Tomorrow I'll give it more fuel and see what happens.

 

[jonbills]

if its been set up on a rolling road with a big air leak now the air leak has been found and cured the tuning will be out as it will effect vaccum advance and obviously fuel mixture,
this is going to need to be set up again by a rolling road before anything else.
air intake isnt being restricted? maybe try removing air filter temporarily and see if it makes a difference?

Thanks Jimbo, it was set up by me and my AFR gauge. Perhaps I should say actually *is* being setup by me - I definitely haven't got to the optimal settings yet.
The RR was just for fun & to confirm a couple of things (mainly the accuracy of my AFR gauge).
I'm trying to avoid going to Northampton Motorsport to set it up. They'd do a great job, but it's quite spendy and it's only valid until I change something else, and I just can't leave it alone

Now I've swapped the coil, and fixed that air leak, I can see the whole lot is heading a little lean, so getting more fuel in is tomorrow's job.

 

[Wyn]

I had a problem like this yrs ago when run a L engine that would miss at v high rpm
Turned out the coil wires around the wrong way
Changed plugs leads this n that
Took weeks of trying diffo things, but was as simple as that

 

[jonbills]

I had a problem like this yrs ago when run a L engine that would miss at v high rpm
Turned out the coil wires around the wrong way
Changed plugs leads this n that
Took weeks of trying diffo things, but was as simple as that

Do you remember which wires Wyn? can't have been negative for positive surely?
I'll have to check the wiring diagram!

 

[Wyn]

I simply changed the 2 wires around on the coil, it was that nothing more !

 

[jonbills]

I know I've said this before, and then changed my tune, but I think it's sorted now
I've done a bunch of things, but mainly I gapped the spark plugs to 0.6mm. NOW it revs to 8k.