I'm assuming you've broken in the engine?
What fuel are you using? How old is it? What percentage is it?
So a little one how glow engines work, which may help in your diagnosis. The coil of the glow plug has platinum in it; when exposed to methanol which is in nitro fuel and heat, a catalytic reaction occurs and that's what makes it glow red hot to ignite the fuel in the combustion chamber. I say this because you won't hurt the glow plug by leaving the glow ignitor attached while it's
running. If the engine is cold, it is going to be difficult for that catalytic reaction to occur, which is why you need that glow ignitor in order to start the engine. On a high compression motor, you can sometimes even restart the engine without a glow ignitor if the engine is still hot. It's a common misconception that glow engines initiate combustion with compression alone like a diesel. We know that it runs with the glow ignitor attached, so we can assume we have a problem with the fuel itself, tuning or a bad glow plug. You've stated you tried a new glow plug, so we'll focus on the other two. Make sure to use NEW good quality fuel... Traxxas Top Fuel is terriblely inconsistent and can cause a whole host of tuning issues. VP Racing fuel is much better and I'd recommend it going forward. I'd also recommend using 20% nitro fuel over 30% until you get the truck running right.
If the fuel is high quality and new (hasn't been sitting on the shelf for 3 months opened), we can continue onto the next step which would indicate a tuning issue. The first step to any issue that we suspect is tuning related is to reset the carb to its factory settings. On new motors, I like to do this regardless as sometimes they aren't correct out of the box. This should be in the manual, if not you can find the settings for the LSN and HSN for your particular engine online just about everywhere. Typically, I like to set the throttle gap to 1mm using the "idle" screw regardless of what the manual states. Don't assume it's correct if you haven't touched the "idle" screw... pull the filter and check. This should eliminate the possibility of a lean condition causing issues with combustion. Also, be sure to check O-rings on the carb and make sure none are pinched, torn or cracked which could lead to air being sucked in. Even if the engine is new, always check. People assemble the engines, they can make mistakes during assembly. Make sure the bolts that hold the cylinder head to the engine are tight as well.
Now try and start the engine, you can leave the glow ignitor attached for a couple of minutes while it warms up. These engines will load up at idle, you will have to blip the throttle on occasion to clear them out. They will not idle on their own for very long. How is it idling?
If it is idling high, this would indicate an air leak since factory settings are going to be very rich and it should be idling pretty low. Go back to the previous paragraph if that is the case.
If it is idling so low that it wants to die immediately, you can lean out the LSN 1/16 of a turn at a time until you can get the idle up high enough that it will stay running. I don't like using the "idle" screw to adjust idle as it can cause erratic tuning, I have always just used the LSN in order to set idle with good luck for years now.
Once you get the engine running and broken in, lean the HSN 1/16 of a turn at a time until you don't see a difference in performance and then richen it an 1/8 of a turn. This should take care of the "half tank lean" problem the T-Maxx's have and make it so you don't have to keep fiddling with the carb halfway through a tank of fuel. Adjust the idle with the LSN as needed. If you adjust the HSN, you will have to adjust the LSN as well, since the HSN is what controls the total amount of fuel that is flowing into the carb and it will have an effect on the LSN. Just use your ears and listen to the idle, that will tell you what you need to do to the LSN. While I wouldn't recommend this tuning method for something like a full on race engine and maximum performance, for a
TRX 3.3 it should work fine and I've never had an issue with any of the Nitro's I've owned running using that method. It is also pretty safe and should keep you away form any lean conditions that could damage the engine.
