HPI K4.6 tuning issue


Old 08-22-2018, 06:50 AM
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Default HPI K4.6 tuning issue


I recently purchased an HPI Savage SS with the K4.6 and have been having some difficulty tuning it. I am new to nitro however I have been learning a lot and have made a lot of improvement with the engine so far as it would not idle, flame out constantly, no power, very inconsistent performance.

So far I have done the following:
  • Removed primer bulb as it was causing issue with leaning out and inconsistent idle, etc
  • Replaced fuel lines
  • Removed and RTV sealed mid speed needle to prevent it from moving
  • Removed and RTV sealed backplate
  • Removed piston and sleeve to test pinch, very good pinch, won't go past 1/4" from top
  • Stripped and cleaned engine internals

Symptoms I am experiencing:
  • Half tank or full tank; when it is first started, it takes 2-3mins to warm up and, it rips very well, wheelies easy, good accel, good top speed, temps around 210-240
  • I will then notice a bog happening on low end, sluggish to accel, decent top speed still, will not wheelie
  • Temps will start to rise
  • Idle will start becoming erratic it will start to idle quite high, sometimes briefly
  • Temps will start rising to 280-300+
  • I will attempt to richen HSN to lower temp and usually temps will lower, however still bog on low end and top end will start to lose power
  • I will adjust LSN however it doesn`t seem to matter which way I go it doesn`t change the bottom end
  • Spur gear nut is snugged tight

I suspected an air leak especially around the carb body so last night I filled a bowl with water, plugged all holes in the motor, submerged, and blew air through the carb fuel inlet with a piece of fuel line. I blew as hard as I could and the only place I was getting air bubbles was from the back plate where the starter shaft goes through into the OWB. I assume that is normal I can`t see how you would ever get that perfectly sealed. I also submerged the fuel tank and blew air into it and blowing as hard as I could I would only get a very small bubble of air coming from the fuel top every 4-5 seconds. I suspect this is acceptable.

I feel it is a heat related, metal expanding, causing air suck, lean out / over heating issue that I am chasing but I can`t find out where it`s at. The motor starts fine cold and hot, it doesn't fire right up but it starts reliably, it doesn't flame out, it will idle fine when cold, it sometimes idles high when hot. I don't know where to go from here and I can't find any other information online so now I'm reaching out to you guys.

Hoping someone has some additional pointers for me. I would greatly appreciate it.

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Old 08-24-2018, 05:54 PM
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Screw your LSN all the way in till tight, then back it out 2 full turns. Try to tune it from there. You mentioned that turning the LSN does nothing, which is probably where your problem lies. Get back to me on this simple request. I don't really expect any results considering that would be one of the first things to do, but you never mentioned you did that so give it a go. Make sure your gap is ~1mm. Sounds like somewhere your engine is getting air leaks and leaning it out to the point it overheats, which is very bad. If you can only rotate the piston 1/4 up the sleeve, that is probably to much pinch and you'd need more tanks to expand that sleeve more, but I could have completely misunderstood your meaning behind the pinch. Also, make sure you are shimming all the proper places on the engine. missing shims can cause airleaks and headaches.
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