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Fast idle when hot, why??

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V

verylatenight

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Kyosho GXR28 Nitro. Starts up and idles perfect, runs fine. Then depending on the weather outside, after a few good passes and the engine is warm, the idle goes way high and sound like a buzz saw! I know, sounds like an air leak. Carb is sealed, new front bearing, new head gasket. Changed fuel, changed to different brand and heat range of plug. New fuel lines, checked the tank. So, we are down to a crack that I can't find or the fuel is boiling in the line. I have run the line a few different ways, but right now it is the stock setup. Everything I have done results in the same thing. Any ideas? Repeated internet searches turn up nothing.
 
If it's running okay, and it' not overheating (due to running lean - indicating too much air is getting in) then I'd look at maybe backing the low speed needle off a little and seeing if that helps.

If it's running too hot and lean, and you can't tune it out easily by running it richer then it may have an air leak.
 
Let it get warm and start to rev high, then richen the LSN up until the idle settles down enough to not hurt the engine. Then spray some WD-40 on various places of the engine. It will shut the engine down or make the idle drop.
You did not mention sealing the backplate btw.
 
The back plate has a new o-ring as well and no visible cracks. Will use the WD trick. After I installed the new front bearing I did the "blow in the fuel tube to check for air leaks" and the bearing seal showed signs of an air leak just like the bearing I replaced. I did this with another new bearing and the same thing. Should I get bubbles from the front bearing seal?
 
Some leakge is normal at the front bearing. See what the WD40 trick does to it.
 
How big is the idle gap? If it's any bigger then 1mm i suspect a rich LSN setting combined with a high idle setting, giving it a "false" idle. Also if the LSN is tuned too rich, you usually end up leaning the HSN too much to compensate.

Does it stop sounding like a buzzsaw, and how long does it take to drop to a normal idle?
 
neobart said:
How big is the idle gap? If it's any bigger then 1mm i suspect a rich LSN setting combined with a high idle setting, giving it a "false" idle. Also if the LSN is tuned too rich, you usually end up leaning the HSN too much to compensate.

Does it stop sounding like a buzzsaw, and how long does it take to drop to a normal idle?
Click to expand...



Again, when it is first started and for maybe 10 minutes of slow laps, the idle and high speed tune seems very close to ideal. The idle gap looks correct and I have slowly turned in the LSN in increments until it just will not run, knowing that it was way lean. The LSN is very close to when the car was delivered. There is some smoke from takeoff, but not excessive and no bogging from idle. And when it goes into the fast idle mode the head seems hot and it will never idle back down. I have been running with the body off. My plan is to check for air leaks with WD-40 and to also re-run the fuel line so the fuel drops into the carb instead of from below because I think maybe the fuel is boiling and creating a lean condition. This model has the fuel line running very close to the pipe and I have tried a foil insulator to reduce this probability by my experience and getting experience from others in China is limited.
 
Took a relook at everything. Moved fuel line to get away from exhaust and ran it up, then down into carb so if fuel starts to vapor lock at idle, the line is still ok at the carb. Richened LSN considerably more than what the manual states is the stock starting position. I will actually measure this to make sure as well. Slide is about 1mm or less and not changed. Took it out and great. So is it the LSN or fuel line? My guess is the fuel line as I was all over the map before on the LSN. Now I have to go back and work on which glow plug is best for the fuel since I I have been back and forth on that too.

Thanks for all's help
 
Took the original and the Kyosho replacement o-rings to work and measured them on a microscope and found they are both different sizes and not ideal for the bore and needles. I was able to find metric o-rings that are thicker and fit tighter. They are Buna rubber so I am not sure of their long-term fit, but I got 100 of each size for just over $1 (its China...) Took the car out and it runs like it should and I was able to see that my glow plug is too cool. As soon as it stops raining I will tweak the settings and get everything back on track. Appears the air leak was actually the LSN o-rings.
 

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