Nitro mixture adjustment - tips?

[Mad Ax]

Anyone got any tips for getting the mixture set right on a nitro motor?

I got my Kyosho Sandmaster running at the weekend, but still having

trouble with it not running very smooth - seems to cough and miss when

revving up, and doesn't always hold full throttle (just seems to die at

what).  It also cuts out when idling - it'll idle for half a minute

then shut off, and also shuts off when I hit the brakes...

TBH I only had a 10-minute play with it, and might get further if I've

got time to take it somewhere distant tonight; maybe it just needs more

running to get itself sorted out...

[walawala_com]

Hi,

Idling for 30 seconds is acceptable and considered to be okay. 2 stroke engines build up unburnt fuel at idle and low speed. Up to a certain degree, the engine gets too rich in mixture and chock.

Before discussing needle setting, pls check you throttle linkage setting without starting the engine..

With servo at neutral position, carb should open up about 0.5mm to 0.75mm (idle setting) with throttle linkage spring pushing against the carb. Brakes not working. To adjust idle, check the idle screw on the side of carb. Turn CW means more and vise versa.

When at full throttle, the carb should be 100% opened without any excess travel, I mean you do not want it to open 110% as fully opened means full throttle. You want to keep the throttle moves as to the Tx trigger in direct ratio.

When taping brakes, the drive gear(s) should be locked and check if the carb returns to original idle position. Closing excessively could chock the engine instantly. If it happens, you can adjust the idle screw (on side of carb) to correct this.

What engine are you using?

Joaquim

[Mad Ax]

Hi Joaquim,

The throttle linkage is all set up correctly - I spent quite a while

getting this right.  I could open up the idle screw a little to

get it to idle a little faster; that's something I'll worry about

later.  The carb won't turn past 100% throttle; the control from

the Tx is pretty good.

The engine is a Kyosho GT size 12 that came with the Sand Master kit

back in 1998.  I've not used it much, since I was never able to

get it to run 100% even back then.

Is there a "golden rule" to getting the mixture right, i.e. if it spits

fluid it's too rich, if the smoke trail isn't visible it's too lean, or

is it just a case of turning the needle until it runs best?

[DJTheo]

With modern synthetic oils in fuels smoke isn't so visible as it used to, best way to adjust the high speed needle is by measuring the temperature at the top of the plug with a infra redtemp gauge after 20-30 seconds of full load, for the TNX the optimal temp then is 100-140°C.

Cheers

[walawala_com]

I would recommend you to take the engine apart and check if all parts are in good condiiton first. At least no rust and with compression. If you don't know how to do this, HPI USA site has a good page for this.

Regarding engine dies when you brake - if throttle linkages are set correctly, then you might want to check clutch shoes and springs. Check if they are touching the clutch bell even at idle speed. This could cause continuous clutch engage and chock the engine during braking.

The golden rule, as THEO advised, keep it under 120 degrees celcius. Needles (low, high, idle) most of the time need to adjust at the same time in order to get correct mixture at different times. Try to play with it and get a feeling. Before you got that mixture point, try to adjust a higher idle setting. Engine dieing is always one good way to upset you. Adjust one thing at a time and get a feeling and remember to check temp frequently.

I started like you from zero, no one could help by just a few words, this is true, to get nitro knowledge, you will need to start and try running it. Good luck!

Thx

Joaquim

[DJTheo]

Btw, too rich running is also not good for the engine as it runs too cold (<80°C) and engine wear again rises due to less thermal extension.

Cheers

[walawala_com]

Leaning it up, to both low and high needles, most of the time means a faster response handling and higher speed. Less fuel means more air in the mixture and more air causes stronger explosion. Less fuel also means less lubrication and could cause overheating.

But when you lean them up and pass a certain point, the engine won't get enough fuel to rev up. It will sound like "rev cutting" under acceleration and cannot reach top speed.

Too rich is like sloppy reaction and could cause engine dies easily whenever you accelerate. It sounds like BLEU BLEU BLEU during acceleration.

You should see smoke but not too much. Reving up from idle should have more smoke and once the engine passes 40% speed range, smoke should get  less and less and when it gets to top speed, it should be almost no smoke. No smoke for too long time will cause overheating. You always want to set a bit richer on the low end so that the engine gets more lubrication when you release the throttle or braking. On the high end, you want the engine to blow out any unburnt fuel inside the engine which build up from idle and low speed.

Hope you can understand what I wrote. Take care.

Joaquim.