Nitro Engines – The Three Needles

Question:
Hi Doc,
I have the GS Storm Unlimited Truck 1/8th scale with the blue R25MT rear exhaust engine. I have searched high and low and can’t find the answer anywhere regarding the correct carb settings for this truck. I was wondering if you could help me with that. I’m trying to tune it on 16% Nitro fuel and can’t seem to get the mixture right. Could you possibly tell me what the starting high speed needle/HSN, the low speed needle/LSN and the idle screw should be set at? It would be alot of help if you could tell me.

Thanks
Kyle S.

Answer:
Hey Kyle.
First off, let’s get right to the settings: The HSN should be set right around 2.5 turns out, the LSN should be about .8mm in from flush. The Idle screw should be about a 1/4 turn out. All of these setting may vary depending on how the engine runs on startup. You may need to adjust the Idle screw first so that the engine will continue to run.

Now, the techie part. I’m sure you’re aware but I’m going to explain anyway for peace of mind. Here’s a quick down-and-dirty on the needles and what they do:

HSN (High Speed Needle): This is the most important needle of the bunch. This needle controls the mixture of fuel and air that enters the engine during high-RPM driving. More fuel = Rich setting, less fuel = Lean setting. A richer setting will help keep the engine cooler for two reasons: 1) the fuel has a liquid-cooling effect and 2) more fuel reduces the RPM’s. On the flip side, a leaner setting will let the engine hit higher RPM (building more power) but also cause the engine to run hotter. Just remember, to much in either direction is not good, but no one ever blew up an engine running it to rich.

LSN (Low Speed Needle): The LSN (AKA secondary needle) is used to control the air/fuel mixture on the low end, adjusting the engine’s transition from low to high RPM. Setting this needle on the rich side will reduce throttle response at low RPM as well as make it sluggish out of low-speed corners. Setting it on the lean side will have just the opposite effect, increasing throttle response at low RPM as well as perk up low-speed operation. Again, to much in either direction is not good, so keep it a bit on the rich side and you’ll be ok.

Idle Screw: Basically, this screw keeps the vehicle running at idle. You’ll want to adjust this so that there is just a small opening in the carb, enough to keep the engine running but not so much that it is engaging the clutch.

Needle Order: Finally, there’s the needle order. You’ll want to adjust the needles as I’ve listed them above, HSN – LSN – Idle.

There ya have it Kyle. I hope this info has helped and will get your GS Storm up and running with the big dogs!!! Good luck!!!

The Doc