Plug#3 fouling during warm-up - LONG.

Tony 260Z

Tony 260Z

Club Member
I wonder whether anyone can help here 'cos I'm beginning to get a tad p*ssed-off with my lovely Z. She's a 1978 260 with 240carbs, lumenition ignition, K&N's, a nismo 6-2 header and 2 1/2"  system.

Anyway, I'm currently getting through a spark plug with every trip. On a hot day, I can start the car without the choke, leave her to warm up for a couple of minutes and then drive fine. However, if I need to start her on a colder day (ie almost every day the way our weather is at the moment), then I need the choke for a minute and the problems start. She will idle fine, and warm up fine too, but as soon as I start to drive and accelerate she'll stumble and misfire. If I then stop and check plug#3 I'll find it totally fouled-up with carbon. I swap in a new plug and she's right as rain. Bear in mind that I never use the choke for more than a minute, and never try to drive away with the choke on either.

I've checked the piston movement in the front SU and it seems fine. Mixture seems OK (when warm, anyway). The ignition lead is OK too. Compression is good. The timing and everything else I can think of checks out (more or less, as I couldn't get the dizzy alignment on the oil pump drive to 100% perfect - it's a degree or two out there).

Apart from this problem she drives perfectly well. As soon as I dump the fouled plug and stick in a nice new shiny one she's fine - until I need to start her cold again.

What is the problem? Any ideas? I'm getting through NGK's at such a rate that Mike Feeney must think I'm eating the blasted things. I had to stop the car in the rain last night to change the plug. Again. How annoying is that?

HELP!

Tony
 
Tony ,
You say the compression is good. How good is good?
175/180?
Also have you looked at the air gap on the distributor cap?
Lastly  is the water still connected through the inlet manifold?
 
Sounds like a tricky one, Have you checked to see if you are using any oil as it seems to be oiling up so fasr, maybe your rings are worn on that cylinder (oil control ring that is). also maybe worth checking the valve clearences on that cylinder.
And actually make sure that you have a strong spark at the plug, if you haven't got a strong spark from the cap at number 3 outlet then it will be crap at the plug.
Try these if you haven't already and keep us informed we'll get you through it.

I want NOs too
Hope this helps
Nick
 
Thanks for the replies chaps.

Well, it doesn't look like oil fouling to me - it's dull, sooty carbon. Not shiny at all. I'm pretty sure it's not oil. Also, I reckon she'd be a bit smoky if it was oil and there wouldn't be the apparent link to when I use the choke. I did the valve clearances myself and checked them very carefully. I'm sure they're OK too.

I can't remember too well what the compression was. About 150/160 I think, and it was pretty much the same for each cylinder. Water is disconnected - would it make that much difference to how quickly the manifold warms up?

Air gap. Hmmm... reaching my levels of knowledge here. I have no idea! But, like you say, the dizzzy cap could be poo. I'll see if I can swap it for another one and see if the prob goes away.

I've had my doubts about the carb itself, as #3 is of course the "single" branch element of the inlet manifold and if there was a richness problem with the carb then that's where I'd expect it to show up. However, the "drop piston listen for thunk" test works fine - it's really smooth actually - and the oil level in the damper is fine too. I wonder if there's something in the way the choke is functioning... dunno.

Anyway, I'll have a good look at the dizzy cap, find out the compression again and maybe slap a colourtune on #3 while she's warming up. Might give me a hint. You never know :)

Tony
 
It may pay to look closer at the leads too. I've had leads that test fine with an ohmmeter, but were still discovered to be faulty when replaced.

Swap the 2 & 3 leads around (on both ends naturally  <img src="http://www.zclub.net/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'> ) and see if the fault moves.

3 & 4 run the richest & 1, 2, 5, 6 run about the same (a little leaner). When tuning my 240Z (waay back when - everything's EFI now...) I set the mixtures to be good on the outer cylinders & just live with the richness of the inner 2.

As my 3.1L TT project is an almighty consumer of time & money, I'm re-building the original L24 as an interim setup. I'll have to pay attention to all this uneven mixture stuff then!
 
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