Hitchcocks Forum

Hi folks. Still sorting out Rusty, my 350 Superstar. The head has been ported by BW, I'm running an 8:1 piston and it has a Mikcarb 28mm (500 Bullet size). Pretty much run in now so starting to open him up a bit. Goes well! Carburation is ok, but I think I need a different plug. I'm running the standard NGK BH7HS, and the insulator is a bit too white for my liking, and when I turn off the ignition I get some running-on, which I think is because the plug is running hot.

Which way do I go - I get confused by terms like 'hotter plug' and 'cooler plug' and don't know which grade to go for. Any help gratefully received!

Thanks

Stephen

If you go into www.ngksparkplugs.com and select VIDEOS this will tell you all you need to know about spark plugs, types problems and choosing the correct plug ect.

PO51UHD - make sure it is not running weak - that's a large carb for the 350. With unleaded fuel it is hard to get a proper colour range from the plug-chops. It actually sounds like something is becoming a glow-plug, resulting in the running on.
Does a hotter plug make the plug itself hot or a cooler plug make the engine cool? I doubt that.
Cheers, ChrisD

Sorry chaps, just spotted the deliberate mistake - that should have been B7HS. Don't know where the extra H came from...

Having looked on the NGK site, I think I now understand that a 'cooler' plug (eg B8HS or B9HS) would help. Has anyone had any experience of this that would help me choose? IE is a move from B7 to B8 slight or significant?

Thanks again

Stephen

As Chris has already said, look to the carb, it is running too lean.

As other have said - that's a big carb for a 350. A 26mm would be big enough.
OK, only 2mm less in circumference but quite a lot in area.

If you are experiencing 'running-on' after a blast, the tip of the Plug is glowing. (pre-ignition) Fit a plug 2 steps cooler: B9HS


DONT listen to the oft repeated myth that tuning by the spark plugs appearance is impossible with unleaded petrol


I have posted on this forum about the subject many times in explanation; search the old messages.




IF your Throttle slide was 'too-lean' you would be experiencing Spitting back.


IF the pilot was too-lean, you would have a Hunting idle, with a slow return to a steady rpm. Neither of which you have mentioned?




Typically too-lean a condition in any specific throttle position will initiate a Misfire on acceleration...?




Spark plug insulator a 'bit white'? Probably ok. Unleaded petrol DOES NOT colour spark plug tips! Look down into the well around the insulation for a rising ring of Carbon soot rising upwards, and a faint dusting of soot on the end of the body.




In use, the surface of Spark plugs is slowly coated with amorphous carbon, this is more so when the plug temp is below 200c, or the mixture is too rich. If it remains on the plug for periods of time, it is gradually converted into graphite; a good conductor or electricity, and also a ready absorber of water vapours. Such plugs will slowly build up increasing layers of carbon fouling, frequently wet in appearance, though not always, and are highly prone to misfiring issues. At plug temps between 200-400c, the fouling no longer accumulates, but neither does it dissipate. With out Lead in Petrol you need plug temps of between 400-500c to start to see this residue ‘burn off’ and keep the insulator clean.
The removal of Lead oxide, corrosive scavengers, and Lead from petrol has also removed the ‘self cleaning’ properties from petrol. No longer are there the coloured shades of yellow-orange-brown that once used to build up as a glaze on the central porcelain, the former generally accepted indication of ‘optimal’ air-fuel mixture. Which it never was either, it simply indicated optimal Plug operating temps!

Thanks Tim for your very through (as always!) reply. No spitting or missing, as I've already sorted the carb with a MUCH richer needle jet and main jet. So I'll take your advice and go for a B9HS.

Thanks again!.

Stephen

I still feel that if the standard plug for the engine is getting as hot as you describe then there is something causing it. Time will tell as always. good luck.

Hi, check your fuel flow through the tap, I had similar problems and it was insufficient fuel caused by the tap being to restrictive.changed it to the British type and cured it. Vince