The NECJ is leaner than standard at smaller throttle openings, so yes this is likely to increase heat in the engine, especially with a 38 pilot. Have you tried closing the air screw and/or raising the needle?

Do you have a fan fitted?

 

Pipe bang and detonation are two different things, it's sounds like you are talking about detonation but just to be sure pipe bang happens on a closed throttle when the engine is on 'over run, , i.e the rear wheel is turning the engine over at above idle speed, you will hear the odd 'bang' in the pipe and at the same time the bike will surge forward s little bit.

 

IME, detonation is a deep metalic rattle type sound.

 

Cheers guys, thanks for explanations. Sounds more like detonation going by above, seems to happen around quarter throttle while v hot. Is there a reason it would be more noticeable the hotter the engine is?

AS is only half turn out so might try going up a clip.

Bike has a fan but seems to run hotter than my mates bikes

 

I chased a detonation issue on a 2014 300 and went through everything (and l mean everything) aside from the electrical side of things.

A friend had an intermittant issue on a bike before and that (if memory serves) was stator related as only when once really hot did expansion cause an expansion break in a wire.

That was going to be my next part to replace on my 300, then the CDI.

 

Cheers Ship. Read your post earlier.

Could actually be stator related as it’s not charging well. I had put it down to lack of use had knackered the battery but would make sense if the stator had a partto play in both issues

 

I posted about my issue at the time, thought l had sorted it (reed block air leak) ...... l didnt update thread, but l ended up rebuilding bottom end (mainly as it was almost due and to address potential for crank seals) and it was still detonating.

I didnt go further as bike was stolen shortly after, so l can only advise on what was left to fix rather than actually finding the fault with 100% certainty.

Good luck and please report back on progress as there is nothing out there to offer advice on this (aside from jetting air leaks etc)

Oh, yes, l even tried some racefuel 😂

 

It does sound like detonation and this can be down to a few things, for starters you could try a higher octane rating fuel, this might help but it may well be a lean air fuel ratio. The reason it happens more when the engine is hot is that the fuel in the combustion chamber is more 'excited' when its hotter.

Your head has been skimmed 0.3mm, that might also be the issue, 0.3mm is probably not enough to get the squish band working properly, at a guess your squish gap is probably still around 1.6mm which is too big, on top of that you will have pushed up the compression ratio, obviously the higher the CR the more likely the fuel is to detonate. Really you want the squish gap to be around 1-1.2mm and then you want the combustion chamber opening up so that the final CR is somewhere around 12.5:1. You could probably push the CR up to 13-13.5:1 as long as the squish band is working but in my opinion its not worth chasing a super high CR, the gains are not worth the hassle if you go too far. A working squish and a CR of 12.5:1 will give you a great results.

 

Why not get an NECW needle? It’s one step richer in needle diameter but the same everywhere else. The extra fuel will help cool things a touch.