LPG Spark plug gap.

[bimmer49]

After 2 years of running on Denso Single core copper KPR20's there developed a stutter at 1500 rpm on gas with a light throttle. Time for a change. I set the new plugs at 0.9mm. This produced even more misfire stutter but at a higher rpm. Plugs out and reset at 0.7mm....almost perfect so when I get a chance I'll drop it to 0.6mm.

[Simon Jones]

It's common practice to reduce the gap to improve quality of spark when running on gas.

[bimmer49]

I had another go and reduced the gap to 0.5mm. No stuttering at all.

[CNGBiFuel]

This has been covered before.

I can confirm plug gap and choice. My car will not run on the factory spec'ed 0.9mm unless my plugs are new and even then it gets stroppy. Maybe it did when the coils were new but not now.
Runs sweet at 0.7mm (1999 V70) If you need to run at 0.5mm you may have a coil near extinction. But 0.5mm with LPG or CNG will do no harm, and you'll get more out of a set before they or any borderline coil kicks-up.

You did know LPG is for 'ginger-beers' and Big Girl's blouses didn't you??

Run the cheapest plugs. Denso K20PR-U. Leave those snake-oil multi point and 'special' LPG things alone. I get a year or 25,000 out of mine at least and I buy a set for a £10-12 delivered on eBay. You need pay no more. Those lunatic LPG things are £20 each. 10 x the price can't make 10 x better. In fact I pretty sure that getting 25,000 out of £10-12 in plugs is as good as anyone needs. So if you don't drive much, likely you'll never touch 'em.

I changed my last set at 25,000 because I had a coil pack go, (a common fault on this engine) so to aid my faulting I did a change of plugs, but frankly i suspect they'd have gone on longer. Previously did 30,000 but I 've just never pushed it to see how far I can go because they are cheap

Go cheap. Close the gap then a plug is a plug is a plug is a plug. At roughly £2.20 each, why would you need to care? Throw them around like sweets to kids.

[classicswede]

With a healthy ignition system there is no need to close the gap.

Basic plugs are the best and specialist LPG plugs often lead to running problems and in some cases engine damage!

[CNGBiFuel]

Is that right. In which case that sorta' confirms my point. One of my coils (and one of his) is past its best and a narrower gap is covering-up an underlying issue. As I said I had to do one coll-pack recently. I might ahve another lurking in there then which explains the need for a smaller gap. Although I have to say, I've not bothered to widen it again since I did my coil. Maybe it will cope with it now, but it runs sweet enough.

Last out, if I run at 0.9mm it definitely doesn't like it. It's my understanding that the two gases, are more demanding to ignite than petrol, so are harder or shall we say more fussy about the condition of the ignition system, so you need the system to be in better condition for a given spark plug gap.

Hence driving an 'old nail', there needs to be compensations. Is this correct?

[bimmer49]

I've only ever seen one CNG station in Spain and that is in the far north and I'm not ginger either. ;-O

This car has always had a slight stutter, the previous owner spent money trying to get rid of it. Including new coil packs (3 yeas ago).

As for gap versus coil pack/ignition performance, consider also there is the condition of the engine to consider.......compression mainly and also gas quality/mixture. Mine has 200,000 on the clock but I have no knowledge as to it's history and whether this is an original engine.

I could, if I could be arsed, whip the lid off yet again and go to 0.6mm, but I can't because it's running sweet! If it burns the £2 plugs to bits in 12 months so be it...I'll buy some more...at £2 a pop!

[Simon Jones]

...as long as your cheap plugs don't end up disintegrating into the cylinder bores. I've seen an unbranded diesel glowplug fall apart and the missing bit was never found.

[CNGBiFuel]

Yeh, to clarify, cheap plugs, not sh*t plugs. Lowest, humblest spec from Denso is my choice. Will do as well as any snake-oil stuff at three-four times the price. And when you're going to see 25,000+ out of £2 a pop plugs, I'm sure you can see there's no sense in anything pricier. In fact 2-3 sets changed whenever, will be found infinitely better than one crazy priced set never refreshed over the same period, if indeed, which I doubt, they'll last as long anyway?

i couldn't be arsed to change up from 0.6mm either. Just set it narrower, I don't think it truly all that critical after that. Especially if like me, you run a 'past its best', high-miler.

[classicswede]

Originally Posted by Simon Jones

...as long as your cheap plugs don't end up disintegrating into the cylinder bores. I've seen an unbranded diesel glowplug fall apart and the missing bit was never found.

You want a good brand plug Like NGK or Denso not any random made in china plug. The reason for the plugs been cheap is that they are a basic simple plug and the basic plugs are the most reliable