RC Nitro Engine Performance Tuning

[Extreme RC Mods]

This is the way that I tune engines, there are many other ways to tune but I figured this is the most simple way to do it so I will share my info. This technique is for a fully broken in engine that is ready for a full performance tune. This tuning procedure will set your needles where they need to be for optimum performance, there is no guessing, the engine will tell you.

For this to work the engine has to be at running temps (or as close to it as possible) as well as having the standard 1mm gap for the carb opening.

#1) High Speed Needle.
Pull a wot pass a couple times and listen what the engine does when you let off the throttle. As long as the rpm drops right to idle, the HSN is either good or on the rich side. At this point you can lean the HSN in 1/12 increments until the idle seems to hang a little after a wot pass, when it starts to idle high after a pass it is just on the lean side of a perfect tune. Back the HSN out 1/12 turn at a time after a wot pass until the engine drops right to idle after a pass. Now your HSN is set.

To give an explanation to what is happening is simple. If your HSN is too lean, after a wot pass when you let off the throttle the engine is still lean causing the idle to hang and idle high until the LSN has a chance to take over and meter the right amount of fuel to bring the idle down to normal running speed (given the LSN is set correctly)

Now if the opposite happens, after a wot pass when you let off the throttle, if it drops to a good idle right away and then starts to idle back up too high, this is a sign that the LSN is too lean. If it comes off wot with a good tune and will drop rpm nicely then the HSN is metering the fuel properly but once it hits idle the LSN being too lean will quickly take over causing the rpm to go back up.

#2) Low Speed Needle
Most of this setting was explained above but there are a few little tricks you can use to make sure the LSN is adjusted perfectly. We all know that you don't tune for temps but a temp gun is very handy for this part. After some wot passes with the engine good and warm, bring it in and let it sit for about 10 seconds. At this point take your temp gauge and hold it as steady as possible on the head, what you want to see is the temp dropping a degree every 4-5 seconds. You want the temp to drop very slightly at idle because when your off throttle the engine should be cooling, if not, the temps will keep pushing higher with on and off throttle running and will cause temp issues.

This is how I do it and I have tuned a lot of engines for people whether bashing or racing. I have seen across the board from beginners to veterans that when they hear the high idle after a pull, the first thing they go for is the LSN when in fact it is the HSN causing the issue. I hope it's not too hard to understand as typing it out is much harder then explaining it to someone first hand, if you have any questions feel free to ask.

Robin.

 

[Extreme RC Mods]

There are no stupid questions, nobody was born knowing RC Nitro lol.
Here is a picture I took with all the needles labeled, it should help you understand which needles to play with when tuning. As for which way to turn the needle, think of a garden hose, Counter clock wise will give you more water flow, clockwise will restrict or shut off the water, it's the same concept.

I'm not sure if I posted this here before, if not, I hope it helps to explain more about the idle gap screw.

OK, using a starting point of when you let off the throttle after a WOT pass and given the HSN is set perfectly. The carb closes and chokes the air/fuel mixture to the pre-determined idle point, the engines RPM will drop and level off to this pre-determined point as well. If the idle gap is too big, the RPM will not totally drop to where it should be, rather it hangs at the pre-determined point waiting for the LSN to take over and lower it the rest of the way to a good idle. Essentially the the "idle gap" is what you want to dictate the idle RPM, not the LSN and at this point the LSN is masking the fact that the idle gap is too big.

The idle gap should be set as small as possible along with having a consistent and reliable idle, This is easy to do at 0.5mm with most engines. You don't want to increase the idle gap and then rely on a rich LSN setting to hold the rpm down. In the picture I drew up, imagine what would happen if you went from ( B ) 1.0mm gap to ( C ) 1.5mm gap, obviously the idle RPM would increase and you would have to richen up the LSN in order to have a reasonable idle again. This is totally possible to do BUT, when coming off a WOT pass the carb can only close to 1.5mm, this is not going to be enough to choke the engine and drop the RPM very fast. It would cause it to hang and then "if" the LSN was rich enough, it would eventually take over metering and lower the idle to an acceptable level, again this is just masking the fact that the idle gap is too big. Well the same effect can happen when you go from (A) 0.5mm setting to ( B ) 1.0mm, it's just not as noticeable. Now with a 0.5mm setting, when you come off a WOT pass, the engine is choked right down and the RPM will drop more, usually right to where you want it so there is no big transition from the HSN to the LSN because the RPM has dropped right to idle immediately. This is exactly what you want especially if your a racer, a hanging idle will really have a negative effect in the air as it usually hangs the nose up high and isn't always consistent.

IMHO the 1mm gap is just like factory settings, it will get you going but some adjustment is needed for a full performance tune. With the advancement of lubrication letting us run as low as 8% oil apposed to the older fuel with much higher oil contents, and the refined carbs compared to years ago. We have to accept that a lot of info is based on old wives tales, like running different brands or % of nitro in the same engine is bad, or you need a hot plug to run in cold temps lol. Try it if you like, I hope it's not too confusing.