I missed your edit, sorry. Unfortunately my English isn't good enough to contribute to demystify what causes the rattle. But I spoke to an Austrian who is very well knowing the two stroke engines. Unfortunately I can not explain it in English, even in Austrian I have my problems. He said, that at high engine speeds closing the throttle a bit can cause heavy detonation as the engine does not get fresh mixture and due to what he called a scavenging misconception of the two stroke the mixture leans out rapidly. A slight whack at the throttle and the problem is gone. Funny that closing the throttle completely has the same result, if I can trust him due to the fact that the engine then pulls fuel/air via the pilot circuit and the revs immediately drop.EDIT...I have actually had the same rattle as you during some of my tests, I guess a simple way to check if it is mixture related would be to stick in a rich needle just as a test. The thing that I never got my head around was that in order for an engine to detonate the cylinder pressure has to rise to a point where the mixture 'spontaneously' combusts, it happens AFTER the plug has fired but BEFORE all of the mixture is burnt by the flame front progressing outwards from the spark plug, what happens is that a second flame front is started usually at the edge of the chamber which heads inwards towards the plug, when the two flame fronts hit each other a shock wave rattles the engine. Now in order for the pressure to get high enough for this to happen the volumetric efficiency (VE) of the engine has to be high (it has to be taking in the a large amount of air and fuel so that the cylinder pressure becomes high to start with and it gets higher as the mixture starts to burn. This is the bit that still confuses me, at small throttle openings the engine is being 'throttled or choked', the VE is low which means that the initial pressure before the plug fires is not high, this should mean that the pressure after the plug fires is not all that high. Oh well one day I might work it out!
(Pre-ignition is something else, all of the mixture burns before the plug fires)
Thank you sand300, as mentioned before I don't use the N3EJ (Yamaha) needle as it is such a thick needle compared to all others I have (NECW, NEDW, NECH, N1EF, N1EG, jd red standard and stic)steveman, as sidecarbod said it sounds like pre detonation .
that Yamaha needle is awfully abrupt in the midrange as it's very lean especially if it's on clip 1 or 2. I'd put it to one side and try a NECK or NECJ
needle length is very important for the STIC to syphon break for clean low down jetting so keep to clip 1 or 2
you should really be able to tell the difference between a 190 and 200 main jet. if you cant you need a BIGGER main jet.
if you still have set up problems please follow this advice (this goes for anyone who gets one)
52 pilot. dont be tempted to try lean it. stick to 52 pilot.
NECK, NECJ needle clip 1 or 2
buy some main jets ranging from 196- 208
concentrate on the low down RPM up to 1/4 throttle. start with the smallest. you'll get a feel for it. one by one try each main until you feel the jetting go clean. this is where it 'syphon breaks' and the bike should run clean and strong from idle to flat out.
understanding this working window is really important. if you try to jet it normally you'll never get it to run 100 percent.
every needle syphon breaks at a different point. to give you a example, my cr500 uses slightly leaner setting than the husqvarna 300.
I ran some tests last week and I could get a NECG (which is a incredibly rich needle) to run clean from idle by balancing it with a 218 main jet.
Ah that sounds good! I will maybe give that a try. Another problem is, that the bloody Hungarians closed the friggin border so no riding at the moment. In Austria it is not allowed. From time to time I can ride in a friends forest (but only slow stuff) and I can compete in races, but that isn't the ideal place to test carb set up.steveman please dont be tempted to try skip a jet or 2 in the sizes.
buy one of those polini kits which cover all the sizes.
you can make the NECW needle you have work with a 205- 208 main jet with clip 1
Andrew, the bike was running fine before I installed the STIC and had no problems except a bit of pipe bang with hot engine, especially at the temperatures we had the last weeks (between 30 and 37° C). I could try the CDI of my friends bike but it is a different one as his is a GP model. But I think as the problem appeared with the STIC it should be gone now. I just installed the standard jet block and I hope that I can cross the border one last time tomorrow so that I can test if the rattle/detonation is gone.With 38hrs on the motor it will not be down to wear.
If the bike has been idle for any long periods of time, you might have an ignition side crank seal allowing air which will cause lean problems.
I am not convinced by the claims of your Austrian matey saying that he can recreate the problem on any 2T.
Try and get a CDI to test on your bike.
Can you ride your bike on the road in Austria?
Thank you for the deeper insight SCB. You may have missed that I haven't touched my OEM cylinder head, which had exactly 2.05mm of squish. I took it off and gave it to a friend who tuned it to his liking for his bike (he took 1.5 mm off the sealing surface and 1 mm off the squish band so that effectively 1.55 mm squish remained. He also laced some material out of the combustion chamber to reduce CR back to where it was. Don't ask me how he did it, but he said he did it. He sent the head back to me and asked me to test it but I did not feel like it and sent it back to him.Steveman, with regards to head work really you either need to have a working squish gap or live with the fact that you don't have a working squish and therefore keep the CR fairly low. The worst thing you can do is push the CR up but still have a non-functioning squishband. Having said that I doubt very much that taking 0.2mm off the head will have pushed the CR up enough to cause an issue. I also doubt that taking 0.2mm off the head will allow the squishband to start working. In my opinion the gap needs to be a maximum of 1.3mm if you want it to function as designed. The minimum gap depends on many things including the angle that the squishband forms with the piston, the RPM that the engine will run to, the size (and weight) of the piston, crank whip, crank case stiffness, that sort of thing. There is a certain gas velocity that if exceeded will actually promote detonation which is the opposite of what we want, the tighter the gap the higher the gas velocity will be. I settled for 1mm which seems to work for me. Anyway I doubt that you have pushed your engine into the 'danger zone' but it is something to be aware of. If you really want to get into this sort of stuff then you need a burette and some perspex so that you can work out the volume of your head. You can also use it to work out the volume that the dome on your piston occupies. You can get cheap plastic burettes off ebay, they are good enough for our needs, I had a glass one for about 10 years but gradually bits broke off it until it could no longer be used!
First I run the JD-red needle, but not the one that came with the STIC. As I wasn't so happy with spooge and fuel consumption I put a NECW in clip #3 with a smaller pilot jet (40 instead of 42) and a 172 main jet and the bike run very well but quite aggressive. But it had no burble or stutter or hesitation anywhere. It just pulled very quick and aggressive. No sand300, I did not try either of the two needles with the STIC.your engine will be fine steveman.
what needle were you using before fitting the stic?
have you tried using the same needle in conjunction with the stic?
No the standard slide is a 7 without notch, at least on the 2018 and 2019 models. I will continue testing if the noise is gone with the standard jet block. But due to this darn Covid virus the borders will be closed tomorrow at midnight. So no more testing for a long time...doesn't the gasgas come with a 8 slide as stock?
a NECH will syphon break around 210, so go armed with a 208, 210, 212, 215 main jet. one of those will make it run clean. most probably 210 or 212 will be the correct one.