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LOL, I was just about to post up "buy a few main jets"...you beat me to it!

I posted up NECL, I think it was a mistake, you are right...NECJ
 
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Discussion Starter #282
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
 

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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)
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.
Not sure if I got it but he swears that he can get any two stroke to produce this sound more or less. According to him you just need to rev it high enough, then find the throttle position where it does it, normally it is enough to not further open the throttle or release it a tiny bit while the engine is still at over 80% rpm. I will test that soon with other two strokes.
In my case the rattling appears while the engine revs further and while I open the throttle. I can also get the rattle/pinging when I do what the guy said and i described above....
 

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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)

7 slide
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.
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)

Maybe I did not say it clearly. I do feel the difference between 190 and 200 as the engine produces more grunt and over rev. But there is no difference between 190 and 200 regarding the flat spot and the rattle/pinging. A horrible sound.
Frank MX has the jets only in .5 increments (190, 195, 200, 205...) and asks almost 5 quid for one.

I will try to find aftermarket jets though I normally don't use them. I am lucky that I have the slide #7. The problem is I am tight as the arse of 3000 year old mummy. Unemployed since 2014. Spendig 100 quid for jets and needles just for testing hurts a bit. LOL



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
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.

Why the hell is everyone (including me) having the needle in clip 1 or 2? I admit I hate that, while I can accept clip 2 I really don't like clip 1 ;-)
 

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Discussion Starter #285
I'll be quite honest steve I dont know exactly why it must be on clip 1 or 2 but for it to syphon break it has to be. to give you a example, I can get a NECG needle to run clean in clip 1 or 2 but I cant get a NECL (leanest needle suzuki make) to run clean in clip 3.

the length is really important although through manipulation of just fine tuning the jetting by the main jet, other clip positions arent needed.

my misunderstanding, sorry I thought you were on the stock stic Yamaha needle.

just try not to chase your tail. stick to the 7 slide, 52 pilot.
pick a needle and then work your way through those main jets till it runs clean everywhere

with the NECW, clip 1 and a 205-208 should run great.

with a 300 2 stroke itll most probably be a little rich on deceleration so you may get some pipe bang when slowing down. this is the only reason I prefer the NECJ for the 300
 

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We definitely know the engine with STIC will over-rev. Getting back to the basics, I would check some of the fundamentals engine measurements first. What is the clearance between the edge of the piston and squish chamber edge. This can be checked easily by removing the spark plug, using a length of solder insert the solder to the edge of the piston, turn the engine over to crush the solder, remove and then measure the thickness of the solder. It should be a minimum of .040." I have been building engines for +50 years and this sound has the characteristics of what they call perimeter detonation. Assuming the engine has some hours on it, and with some crank and bearing wear , you could be getting some elongation of the reciprocating mass. The sure way to checkthis is to remove the head and do a visual inspection. I have the diagrams (drawings that depict this condition). While I was a consultant for Polaris' advanced engine department, their 440cc race engine had this very problem that occurred when the engine was warm. Bottom line, remove the head and inspect the head and piston to prevent any damage. Email me at [email protected] and I will copy you of the drawings.
 

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many thanks Sand300. We will see when I get the jets/needles. Unfortunately I can't ride currently as the borders to our playground (Hungary) are closed. I will take the STIC out today. I will do a race on 12th September and I fear my engine will explode when I ride with it like this. It has already done 6 hours under these less than ideal conditions.
I have a lot of pipe bang since I put the STIC on.

George, the engine has now 38 hours of an easy life. I reduced squish from over 2mm to 1.8mm (0.07 inches) which still is on the very safe side. The head has slightly more compression as it is the GP head but I will put the original head back on soon.
 

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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?
 

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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!
 
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Discussion Starter #290
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?
 

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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?
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.

I can not legally ride my bike on public roads but as my garage is in a village I don't care too much.

Concerning my "matey", well this guy worked for Honda West Racing in 250 Moto GP class as an engine specialist. So I guess he isn't as dumb as you probably think. Today I tried two other two strokes, a 2014 KTM 250 and a 2018 GasGas 300. Guess what! ;-)

Both bikes did not have the rattle/detonation when accelerating though and it was more difficult to get them where they produced that sound...


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!
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.

All I did was installing a standard "GP" head. It comes on all 300 GP Gassers, something like a KTM Sixdays model. The head has more CR, but I do not know how much. GG says it is a tad more but not a huge difference. I measured the squish with this head and it was exactly 1.79mm. I just bolted it on I did not touch it otherwise. The engine performed a tad better and the fuel consumption was slightly better but I needed to jet richer (180 instead of 170 before). With this head my bike was more aggressive and produced noticeably more pipe bang when hot. Depending on what I will find out tomorrow with the standard jet block I will decide if I put a new standard head back on. It is only about 75 quid.


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?
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.

For tomorrow I have installed a NECH (2.725), which is richer than the NECW which has a diameter of 2.735. It may be too rich for the hot temperatures but as I am chasing that detonation problem I thought to go richer is a good idea. I also have the NECW needle in the tool box so I can swap needles if necessary. And I have a NEDW too, which has the same diameter as the NECW but is a half clip richer. It is the last possibility to get the bike jetted well for the race on 12 September. I could not adjust the idle mixture, as it is Sunday and I did not want to piss off the neighbours. I set the A/S to 1.5 turns and will see where I end tomorrow. Bike started immediately without choke so seems to be rich. But even leaner it may have started at the present temperature of 28°. I think that the rule of thumb "a bike that starts w/o choke is too rich" is not 100% correct. But hey, what do I know :cool:

Thank you all for your time and useful hints! That's indeed very nice!
 
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Discussion Starter #293
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.
 

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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.
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...

edit: you noticed that I put the standard jet block in? I need to be sure that my bike doesn't make this friggin' noise and that the STIC is creating this problem. If that will be confirmed tomorrow I will then go and poceed with testing. If not I have to find the reason for that horrible noise...
 
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Little update: I will proceed testing asap or better as soon as the borders are open again, but I will test the STIC with a mates KTM. Since I had severe engine knocking/pinging with the STIC I decided not to torture my engine to death. But I will be happy to kill my mates engine if necessary :)
 
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@sidecarbod @sand300

I installed the STIC in a stock 38 Keihin carb in a mates KTM 300/17 and started with the recommended set up (mj 190, pilot 50, JD red in 4 and a/s at 3.5 turns out.
Bike started fine but had no idle so I've turned the idle screw about 2 turns in to get it idling. The low down burble was horrible so after a few laps we lifted the needle to notch 3 which did not change much. Bike run fine till half throttle then started to lean out before it was running ok at WOT.

We tried the 200 main jet which made the lean spot better but did not cure it 100%. Although it seemed to be self-defeating I installed a 52 pilot jet and dropped the needle in position 2 and though I really do not understand it, it made the lean spot at half throttle much better. Next day the temperatures dropped from about 30 to 23 degrees and the bike run fine but had a hanging idle after a WOT run and it seemed it did not respond to air screw settings much. We then decided that it obviously makes no sense to treat the carb like we are used to and try some weird things. We decided to try your and Sand'S recommended settings and installed a 205 main, left the 52 pilot but changed the needle to a NECW. I had NEDW, NECH too but no NECK or NECJ so decided to take the NECW as it is the leanest of the three and closest to your recommended NECJ. We put it in clip 2 and did a few test laps. Holy shit, that bike rips like mad and the flat spot is history, it is totally gone.
It still has a slight burble down low but we will work on that this afternoon.
 

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LOL..."Holy shit, that bike rips"....yeah that's the normal sort of response anyone says once they get the STIC block sorted!

I don't have any details on the needles that you have tried so I can't comment on them but if you have found one that works for you then that's great, I'm doing a bit more work on my needle at the moment but I won't post anything up yet.

I've also managed to retard my ignition 5 degrees because I was a bit worried that it was still standard even though I've done a load of work on my head. It was a shit load of work to retard the ignition and I'm not sure yet as to whether it has made any difference.

By the way Frank MX has the NECK needle is stock for a reasonable price.
 
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I see, yes I was investigating if it is possible to retard the ignition on the Gasser. It is possible but it is not worth the hassle. The "soft" ignition curve feels better and I would love to have a different ignition sequence, meaning to have it retarded down low, a little less retarded in the middle and standard at top. But that seems to be very complicated without a power CDI or ECU.

However, we will fit a slide 7 (no notches) from myGasGas into my friends KTM and see if we can get rid of the slight burble it has at low throttle openings.
Yes, Frank MX is where I order all the carb parts I need. The Gasser runs best with a NECH which is a rich needle in my opinion but a lot leaner than standard N1EF. The latter has a too short straight section and together with a too rich 42 pilot jet it produces a horrible burble. It works in the middle but is also too rich at WOT.

I'll post an update soon.
 
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