Bad Top End?

BikerMatt

New Member
Sep 10, 2011
18
0
0
Connecticut
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.
 

crassius

Well-Known Member
Sep 30, 2012
4,032
156
63
USA
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
 

BikerMatt

New Member
Sep 10, 2011
18
0
0
Connecticut
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.

IMG_1390.JPG

IMG_1389.JPG

IMG_1388.JPG
 

2door

Moderator
Staff member
Sep 15, 2008
16,326
125
63
Littleton, Colorado
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
 
Last edited:

BikerMatt

New Member
Sep 10, 2011
18
0
0
Connecticut
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.
 

crassius

Well-Known Member
Sep 30, 2012
4,032
156
63
USA
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
 

Dirty Patches

New Member
Jan 18, 2017
13
0
0
USA
I'm having some of the same issues with me 48cc Sky Hawk kit (minus the offset carb). Was stuck in full throttle, cleaned up parts and got it idling normal and revving good. In gear it is gutless, only give it about 1/4 throttle before bogs down. Only going about 10 mph on flat, won't get above that.
Sitting still it has some steam/ smoke coming off the top but not sure its just burning off gas that spilled on it.
 
Feb 2, 2017
155
6
18
California
Dude..... with how that fuel line is set up, how does it get fuel? You'd need a pump-around fuel pump to suck fuel into that silly straw set up of yours! Shorten fuel line and see if that helps!
 
Jan 21, 2015
610
21
18
Portland, Oregon
I ****ing hate those carbs. NT carbs are much easier to tune and have far less issues. I have actually never seen a carb like the one you have where the bike actually worked correctly. I'm sure they exist, and with hours and hours of tuning you could probably make them work, but I hate them and whenever someone brings me a bike with a carb issue with that carb, I get rid of it and put an NT carb on it (with a better air filter and a smaller jet) and have it running within an hour or two most of the time.
 

Slogger

Member
Sep 8, 2014
545
4
18
nohio
Dittoes on the carb and head gasket. Was the head loose?
If you take the head off you can have a good look at the top of the cylinder walls, check the gasket and retorque it.