+1.Ok, so many of you know this but worth repeating for those that are not sure. There are some plugs that can go more miles than the service schedule mileage recommended in the handbook and still be efficient. But if you are using a standard NGK (oem) or equivalent, then best practice is to change it somewhere near the recommended mileage. 7,500 is the recommended miles on the 400 399cc engine, but it's ok to go to 10k in my book without loss of efficiency. It is not the way the electrode looks that tells you how good the plug is, whether it's worn or not etc. The key point to know is the more miles a spark plug covers, the higher the firing resistance so the more voltage is needed to spark the plug. If it can't get more volts (which it cannot) it has a reduced spark performance. So less performance, less mpg, less efficiency and more likelihood of failure which in it's self is rare these days. It's such a gradual change we don't notice it, especially if our bikes are newish and still lowish miles. The reduced spark plug performance is often missed because the engine mechanicals loosen more and the engines natural power increase is still happening well on the way to 20k. The plugs lose efficiency because the internal plug insulation breaks down gradually. We tested a range of plugs a couple of years ago for our new engine which at the time was under development. We settled on a good oem plug and ascertained the correct service interval for a change. Several plug companies were helpful on this. It was a high compression engine (11.5 to 1) so the spark plug becomes less efficient sooner in an engine like that. Our 400's with the 399cc engine are 11.2 to one unless a Canadian model. We found at 20,000 miles the spark plug resistance was more than double what is was when new. That causes your coils and HT electronics to run hotter. Not good for you bike or car. The engine was producing less power too. Plug changed and an instant boost came about. If using an irridium or a brisk plug, then you can go longer.
Well a little 400 thumper is a lot different than a V-8 police car or even MC, but point well taken. The spark plugs today seem to last way longer than they used to.Went 37,500 on my 400 before changing. Once changed I saw no difference. Plugs now a days will last way longer than they use to. We went about 75k on plugs in police cars at the city I was fleet manager for.
Might show up on a real dyno but it doesn't on my seat of the pants dyno which is the one that matters. Doesn't seem to accelerate any faster with new plugs than it did with the old ones. If I can't tell the difference and mileage is the same then it's not worth worrying about. Changing them when I check the valves is convenient because I have the radiator off and it gives good access. Besides I have to pull them anyway to remove the valve cover.Buffalo, you are losing some efficiency if your plugs have covered 25k. However your ECU via the closed loop is compensating for the smaller spark so in normal riding you may not notice it much. But open the taps wide and let it rev out and you will see more of a difference as the ECU cannot keep the burn in normal parameters. Put it on a dyno and its very evident, you would see it. For what they cost new plugs are worth it in my book.