Strictly speaking, you're not supposed to use more than 20 percent nitro in the S-25 engine. 30 percent is on the high side. That said, I use 25 percent with no probs.

Without more information, it's tricky to know why your engine isn't running right.

Obviously you'll need to run your engine in before it runs good. But remember, all new engines should be thoroughly flushed through with alcohol BEFORE starting the engine for the first time. This removes particles of metal fragments/swarf left over form the manufacturing processes.

Standard tank with primer lets in air - change it. Otherwise you'll get bubbles of air in your fuel line and you won't be able to get a reliable running mixture, tickover or throttle response.

Check your S-25 back plate is held in place and is properly sealed.

Ensure your fuel tubing is free of splits or holes.

Make sure your clutch is working correctly, checking that the springs are properly located.

The airfilter should be clean, as should the carb. Don't over do it with the filter oil. If it's too wet, the oil will run into the carb and clog it up. Surplus oil should always be wiped away with a cloth.

If the engine is reluctant to speed up, spits out alot of oil and cuts out with a puff of smoke, it suggests that it's too rich. Conversely, if the engine runs unevenly, races at tickover or cuts suddenly without warning with little or no smoke, the mixture is too lean.

Leave the low end needle alone at first - you need to get the top end needle adjusted first, since altering this needle also effects fuel flow through the low end needle. At half throttle or above, ensure your truck is able to run cleanly with a visible smoke trail. NOT a plume of smoke, just a trail. If you can't see smoke, it's too lean and your engine will be damaged.

To adjust the main needle, turn it 1/8th of a turn at a time, clockwise to make it slightly leaner, anti-clockwise to make it richer. Give the engine a chance to warm up before making adjustments and also allow 30 seconds or so for any adjustments to take effect.

Once the main needle allows reliable half to full throttle runs, allow the truck to tick-over for 30 seconds. Punch open the throttle. If there is lots of smoke and slow pick up, lean the Low end needle (turn it clockwise) 1/8th of a turn until throttle response is crisp. Conversely, if the engine starts to speed up during tickover then stalls suddenly when the throttle is punched open, the low end needle needs to be richened - turn the needle anti-clockwise 1/8th of turn and try again until crisp throttling is achieved.

I use an RB no 6 plug in my S-25 with 25 percent fuel. It runs sweet as can be.

Hope this helps.