Clutch Idle is slow when first starting, but then goes ridiculously high

[DuckmanDrake]

Hello all, this is my second motorbike build with a Grubee Skyhawk GT5 66cc, and I'm having a wee bit of trouble with idle speeds. The carb that I have is the NT Speed Carb, one that comes with a red air filter cover, and screws for both idle speed and air:fuel mixture.

As the thread title says, when I first start the bike up from a slow pedal, it will boot up just fine, and when bringing in the clutch at a slow speed (initially) the bike will idle just where I want it, which is maybe a little bit above a slow cruising speed when the clutch is engaged, for easy liftoff from a start. However, when I give the bike more throttle and it begins to spin at high rpm, when I disengage the clutch to go back to idle, it will instead idle at an incredibly high rpm, to the point where I can hear the clutch pads slip while I engage at pedal speed. The bike seems to only idle at the speed I want when it's first booted up, and bringing it back down to a slower speed doesn't decrease the idle. As well, it seems that by pulling the clutch in most of the way (not all the way), the problem seems to be alleviated to an extent, in that it will idle at a marginally lower speed.

I've tuned the carburetor to where I want it to be, yet it seems that this doesn't really help the issue out. The engine is currently in the break-in period, and despite replacing the intake gasket, it still seems to have the same problem.

Mind you, I've recently placed some scrap industrial rubber underneath the engine mounts so that it will sit better on the frame, but this seems to be causing the whole bike to vibrate a lot more. Whether or not this is exacerbating the problem is beyond me, as I simply don't have the knowledge or local support to figure this problem out.

Perhaps the heat and the vibration are causing a leak? I'm not too sure, so I thought I'd leave such judgements up to the pros before I go and muck around with the setup *too* much

Any and all help would be appreciated on this matter, thank you very much in advance!

 

[crassius]

certainly sounds lean - probably air leak, but could be your mixture setting

spraying water or something on outside of carb & intake should give more info

 

[Slogger]

What Crassius said.
I thought of some stuff to check if it isn't an air leak. If you've been working on the carb the top cap might be too tight, it is supposed to be only finger tightened and might deform and bind the slide. The slide itself may not be in right, with the little guide peg not in the slot. Continued guessing here- maybe your soft engine mounts are letting the engine dance and buzz with enough intensity to froth the gas in the float bowl, leaning your mixture and racing the engine.
I would check that slide and stuff then seal up any leaks.
The best way to mount the engine is as solidly as possible without needing overtightened clamps. Snug metal to metal without the mashing, if you know what I mean.
The whole bike becomes the damper instead of some little piece or bracket.
That's all I got.
-regards!