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Heads and Cylinders All about your porting, compression, rings, cylinder and piston modifications to your bicycles engine

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  #1  
Old 07-17-2016, 08:16 PM
BikerMatt BikerMatt is offline
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Default Bad Top End?

Hello everybody.
I recently took my motorized bike, a 66cc grubee sky hawk, out for the first time since last summer, and I decided to move the kit onto a brand new bike. The 'v' in the frame of this bike was slightly smaller than my previous bike, so I was unable to fit the engine and carburetor without offsetting the carb.

I made a makeshift intake offset using some hose clamps and rubber fittings from home depot, but this may have created an air leak. The bike didn't have much power, would bog past half throttle, and it would continue to rev for a second or so after letting off the throttle. After several attempts and fixing the potential leak the bike began to run more and more rough when under load, and when revved the engine sounded particularly 'tingy', like something was rattling.

I then purchased prefabricated offset off of Spookytoothcycles, hoping this would fix any air leaks. Now the bike doesn't want to rev on its own quite as much after letting off of the throttle, but the roughness, lack of power, and rattling when revving issues are still apparent. Is it possible that my makeshift offset created an air leak, caused extremely lean conditions, and fried my piston/rings? Does it sound like I might still have an air leak with the new offset intake?

One thing to note; I rode the bike for about 2 miles today with the Spooky Tooth intake with the engine still just as rough as I described above, but I was actually able to reach some decent speeds on slight declines. The bike was very weak and rough, kind of bucking, when under a heavy load going uphill, but I hit 31 mph on a gradual decline with the stock gearing on 26 inch wheels. The engine sounded terrible and I could hear the piston slapping on every cycle, but I don't know if this rules out the possibility of the piston/rings being completely cooked.
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  #2  
Old 07-17-2016, 08:40 PM
crassius crassius is online now
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Default Re: Bad Top End?

pics might help

only way to be certain of the problem would be to put all back on bike where it ran well and see if it clears up

short of that, it may just be bad motor mounts, or cracked frame, or motor sitting at a bad angle for the carb, or a head gasket loosened during the move, or . . .

it is unlikely that the motor got damaged by the carb or intake
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Old 07-18-2016, 08:34 AM
thuan1084nd thuan1084nd is offline
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Default Re: Bad Top End?

thanks so much
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Old 07-19-2016, 07:54 PM
BikerMatt BikerMatt is offline
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Default Re: Bad Top End?

Crassius, you're right, it would definitely be best to put everything back on the old bike and see if the problems go away. Unfortunately, part of why I installed the kit on a new bike is because the old frame was no longer safe. As a result, I don't have the old bike anymore.

I've attached some pictures of my setup. I inspected the mounts and frame for cracks but there don't seem to be any. Also, the carburetor doesn't seem to be on too severe of an angle. It really sounds like the 'tingy-ness' is coming from the cylinder, though. The bike still continues to rev for a second or so after you let off of the throttle and the bike was revving high. I know this is sometimes a symptom of an air leak, but the bike idles pretty smooth and sounds fine at very low rpm's. It's only when the bike is revving high that the tingy sound and 'ghost revving' becomes noticeable.

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Old 07-19-2016, 08:03 PM
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2door 2door is offline
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Default Re: Bad Top End?

My guess? Crankshaft seal leak.

Not an associated problem but you're chain tensioner is on the wrong side of the drive chain. You want to tension the bottom run of the chain, not the top. The way you have it all the stress and force is on the tensioner wheel and bracket during pedal starts and deceleration. Not good. The tensioner bracket should be on the chain stay, below where you have it mounted.

I'd also shorten the fuel line. That big loop isn't necessary. And, I'd bet if you went to the old reliable NT carb instead of the one you have you and your engine would be happier.

Tom
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Last edited by 2door; 07-19-2016 at 08:05 PM.
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Old 07-19-2016, 08:16 PM
BikerMatt BikerMatt is offline
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Default Re: Bad Top End?

2door, how would I test for a crankshaft seal leak? What are the typical symptoms of one?

Regarding the chain tensioner, I've had several setups with the bracket in both orientations and I've actually had better luck with the one pictured below. With this frame I'm able to mount it more securely, get better alignment, and I haven't had any issues with it wanting to flex under engine load or pedal starting.
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Old 07-19-2016, 11:44 PM
crassius crassius is online now
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Default Re: Bad Top End?

seal leak will show wetness at either end of the crank under rotor and behind small bevel gear - also check for clogged muffler by loosening it a bit for a test ride

offset intakes can change the fuel mixture, so read the plug for a lean condition

rough running usually only makes noises when motor starts shaking
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