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2 Stroke Bicycle Engines & Kits 2 stroke engine kits need careful installation and setup, find out how from our professionals here!


Tips for Grubee Skyhawk GT5 Kit builders

2 stroke engine kits need careful installation and setup, find out how from our professionals here!


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Old 07-22-2011, 09:09 PM
nightcruiser nightcruiser is offline
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Default Tips for Grubee Skyhawk GT5 Kit builders

I got my first 2 Grubee Skyhawk GT5 kits this summer and boy have a learned a lot in a little time! Here's a summary of things to know when you set out on your project, things I learned along the way but wish I had known ahead of time...

First off, COUNT YOUR SPOKES! I had a standard (late model) mountain bike, Specialized RockHopper Comp circa 1990, I just assumed it was a standard wheel and the kit would just fit... LOL The sprocket kit is designed for a 36 SPOKE WHEEL, so you need to COUNT 9 PAIRS OF SPOKES on each side. Turns out I have a 32 spoke wheel (8 pairs), so I was in for extra "fun" installing the kit on my bike. If you have a choice in the matter get a bike with a 36 spoke wheel, it can be done on wheels with different numbers of spokes but might be more than you bargained for if you don't have good tools and lots of patience. When you have 9 pairs of spokes the 9 bolts on the sprocket (and rubber pieces used to mount it which is called a "rag joint") match up with the spokes and bolt down evenly resulting in a balanced sprocket without too much trouble.

Second thing, the manual is gonna be pretty useless! Some things I thought were important were not clearly defined by the manual, like order of assembly of the rag joint sprocket. (btw, order is sprocket, rubber, spokes, rubber, bracket, washer, lock washer nut!) Still read the manual thourougly front to back, but don't expect every answer to be in there for you.

When you mount the engine lots of things come to mind, wrap your frame with tape and set the engine in there and fiddle with it a bit. Fit the engine on the frame making sure there is clearance for all the parts, carb, exhaust, make sure spark plug cap can come off and on etc. (more on that later) You want to mount the engine so the carb is as level as possible, but getting a solid grab on the bike posts is important, as well as chain angle. Balance out those issues to come up with the best mounting position. Make sure you have room to slide your carb off, you're probably gonna need to do that a few times. When you mount your engine it's a good idea to wrap some rubber around your frame under the clamps, this protects your frame from scratching and also helps you get a solid no slip grip, and I would imagine it helps dampen vibration a bit. A section of an old inner tube works great, use a piece of innertube rubber under just about any part you mount to your frame.

Speaking of innertubes, if you are using an older bike you probably want to check the tires/tubes/BRAKES/cables and replace anything that is old or worn. Having an engine on the bike is gonna be rougher on the bike than the designers envisioned, so your bike should be better than up to spec to be a safe ride with the engine. So, your probably gonna be getting some new innertubes anyways, do yourself a favor and get good ones. I never had flats on my pedal bikes but got one right away with the engine, so I am going with the self sealing innertubes now, they don't cost much more than regular ones. I also added rubber rim rings over my plastic ones (cause my tube got pinched on the inside at the rim) and I added the tuffy tire liners to help keep my wheels inflated. Having a motor on your bike might get you further away from home, major bummer if you get a flat!

When you unpack your kit you will notice they give you a spare for a couple parts.... You can pretty much throw away any parts they give you two of, cause they are crap! I couldn't understand why I got two spark plug caps, two master links for the chain. First ride my spark plug cap broke, second ride my master link popped.... Now I get it! LOL The good news is this stuff is not hard nor expensive to replace with a part that is real. I had a quality master link here so I used that instead of the spare crap they gave me and it worked just fine.

The chain is crap, if you weigh over 100lbs you should replace it. A #41 roller chain (like a farm equipment chain), a wider 415 chain or just a better quality 410 chain will do you good, anything but this Chinese chain they give you. A 410 chain is 1/8" wide, the 415 chain is 1/16" wider than the 410, and the #41 is 1/16 wider than the 415. If you must use the kit chain do yourself a favor and use a better master link, or even better, when you break the chain to size it use the chain tool to mend the chain with the original rivet. A one piece chain with no master link is gonna serve you better, specially when you have a chain where every link is a weak link! LOL

The spark plug cap is just crap, very brittle, probably not going to serve you very long. After my first one broke I put on the spare and immediately went about replacing the cap and wire. I just went to the auto parts store and asked to see their single plug wires. I found a wire with rubber right angle spark plug boots on both ends for $4, I cut it in half and used one half on each engine. To replace the wire you need to twist the old wire out of the CDI unit. I used pliers, some say the wire is barely glued but both of mine were really glued in there good, so when I twisted the wire broke off inside the CDI. Don't panic! LOL just take some small tools like an exacto knife, small screwdriver, nail etc and carefully pick out the rubber from the old wire. Soon enough you will see the tip of a wood screw emerge, that's right, the plug wire just threads on to the wood screw, that's how it is connected (electrically and physically)! Once you have all the rubber from the old wire removed enough for the new wire to fit in there, put some silicone around the outside of the new wire and thread it down on the wood screw in the CDI. Get it nice an tight then smooth the silicone around the top making sure to seal it off really good. Let it dry then your good to go. If you used the plug cap that came with the kit you must have figured out that you need to screw off the cap on the spark plug to fit it, with the new wire you will need to put the cap back on the plug.

Speaking of plugs, they gave you two of those too... hmm.... better throw ém away! An NGK BP6HS plug will only cost you a couple bucks, even if you have the new improved three pronged plug in your kit a better plug is in order. I saw a nice improvement in how my engine ran after I installed the NGK plug.

The throttle leaves a lot to be desired. I haven't replaced mine yet, but that's coming... It is over all just cheap, the cables need to be adjusted all the way out to be right. It will work, so this is an optional upgrade, but I think you will see like me that you would be better off with a more solid throttle.

Depending on your frame your muffler might interfere with your cranks, mine did big time. To bend the muffler a bench vise, pipe wrench, rubber mallet and/or blowtorch are your friends! I tried the clamp and lean, and hammer, but had to bend too far. I ended up having to torch the pipe cherry red and grab ahold of the end of the muffler (with oven mitts or a welding glove etc) and lean on it (while the pipe and flange were clamped in the vise). I was afraid at first to grab the flange with the vise for fear of bending it, but in the end that is just what I had to do cause there was no other way to get the big twist I needed in the pipe. It worked out fine for me, the pipe got a little scarred up from the process but it's where it needs to be now and works fine.

The gas tank is probably gonna be just filled with crap, I freaked out how much stuff was in mine. I just used water, rags and patience to clean out the crap from the tank and then set it in front of a fan for a long time until I was sure it was dried out. If you don't do this don't expect your first ride to be long, the debris from the tank is gonna clog up the little fuel filter in the fuel valve in no time! On the subject of gas, make sure you turn off the gas after every ride. Do not let the bike sit with the gas on and engine not running. You don't want a gas leak, and the gas can actually flood through the engine into the muffler. Just get into the habit of turning the gas off, you'll remember to turn it on after it stalls out on you!

continued next post....

Last edited by nightcruiser; 07-25-2011 at 06:55 PM.
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Old 07-22-2011, 09:27 PM
nightcruiser nightcruiser is offline
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Default Re: Tips for Grubee Skyhawk GT5 Kit builders

On to the engine...

Just cause it's brand new don't assume anything is right or tight, it just might, or might NOT be. LOL I'm turning into Johnny Cochran here! If it doesn't fit, don't just quit, bend some s#it! LOL Check all of the nuts bolts and screws on your engine to be sure they are tight, and recheck them all regularly after your rides. You might even have to seat down the studs a bit (I didnt), the manual tells you to check that. Specifically, do not ignore the nut on the drive sprocket under all that grease!!! (mine spun loose and we locked up, not fun!) Wipe out all that grease from in there (its crappy gritty grease anyways) and tighten up this nut! The spark plug tool they give you will grab this nut, but do yourself a favor and get a 19mm socket for this nut, I got mine a fair bit tighter with the socket after using their tool first. If this nut is not tight your clutch will be for s#it! You should use removable lock tight on this nut and crank it on there good. Several methods to lock the sprocket while tightening can be used, a bicycle freewheel tool, or a chunk of wood wedged in there. What I found best was to wait until the engine is on the bike (with drive chain installed), clamp down the rear brake and tie it off with a wire or bungee or something and use your socket to tighten her up. When I did this I got a little more turn on the nut and my clutch action improved a whole lot. Speaking of clutch, make sure you load the clutch cable with oil, if you don't you will need a bionic hand to pull the clutch!

I didn't know what to expect on my first ride and had a tough go at getting her running, it was hit and miss. At first I was pedaling the engine, not getting any fire. I wasn't giving it the throttle it needed to fire. After I throttled her up she fired and we took off but I couldn't keep her idling. Meanwhile, I had to pull the carb to adjust the clutch cable a bit. I forgot the carb was still full of gas and flipped her over and spilled out the gas, so keep that in mind, if you have to pull the carb turn off the gas and try to burn out the rest of the fuel from the carb until the engine chokes out. When I put the carb back on for the life of me I couldn't get her started again. This was frustrating, cause she ran before. I suspected all sorts of things, I pulled the plug, it was soaked so I put in the spare plug and tried again. Still no go, pulled plug and it was soaked and I could see it has never fired (still white like new plug). It turns out the air filter got soaked in oily break-in gas when I flipped the carb and the engine wasn't breathing, and the carb was so poorly adjusted she wouldn't fire. I pulled the air cleaner off and pulled out the spark plug and rode the bike around with the gas off, engine engaged, and throttle wide open, this helped flush out the heavily flooded engine. I put back the original plug and gave it a go with the air filter off and she fired up like before. I cleaned the air filter and put it back on and we are back in business.

At this point I "wanted" to let the engine idle for the first 10 minute period, but she wouldn't idle. I ended up going to a parking lot and riding circles, starting engine, pulling in clutch and using the throttle to try and hold an idle ( with no load on the engine) Less than ideal situation really but I kept her running for 10 minutes that way. After that I let her pull me a for a few laps, seems I had to rev the engine to keep her going, she did pull me pretty good!

First couple rides went like that, no idle, throttle up and ride went ok, pull clutch or slow too much and she died. Until one day, the ride started out as usual, in a short while the engine just started running great, I could even "almost" get an idle! Then suddenly she sputtered out and died and wouldn't start up again. After a short while I realized, silly me, I forgot to turn on the fuel! So I turned on the fuel, she fired up but ran like s#it again. This got me thinking... Turned off the gas again, short while later she's running great again! This was my second clue that the engine was just swimming in gas, and the carb was way out of whack.

Don't be afraid of the float! Since the float is responsible for the fuel level in the carb, and the engine ran infinitely better after the fuel was turned off and fuel level in the carb dropped a bit, I figured the next stop should be the float. Of coarse there is always a detour, this time its the crappy plastic carb spacer on the CNS carb. I ignored this frustration and kept focus on the float, as I will do here, and will get back to the spacer later. First of all, the CNS carb isn't very difficult to work with, when you open it up parts aren't going to go popping out here and there or anything, so don't be afraid of it. First take the two screws off the top and pull out the cables with the barrel/needle and choke plunger from the carb. Make sure the fuel is off (and hopefully carb drained) and pull off the fuel line. Loosen and remove the carb. Keep in mind there's gonna be fuel in there that will come out (and stay in), I worked on my carb in a little tub to contain the fuel and any little parts that may come out. Flip the carb over, remove the four screws on the bottom that hold in the bowl and remove it, mind your gasket! While the carb is open make sure there is no crap in there, eyeball the jets to make sure they are clear. If they look clogged take a twist tie (that come with garbage bags etc), strip the plastic off and use the wire to poke through the jet. If it doesn't fit you gotta quit! (There's Johnny again! LOL) If the wire dont fit through the jet easily dont force it. On to the float... You'll see the float right there, pretty simple deal. It rotates on a pin at one end, other end floats on the fuel. Near the pin there is a little tab that pushes up on a needle valve that lets fuel in the carb, bend this tab up or down to set the fuel level in the carb higher or lower. Every bike is different, my carb is leaning a little forward, off level, so I bent the tab to lower the fuel level in the carb. Activate the float with your hand a bit, imagine it floating on gas, adjust according to your carb angle and engine needs for more or less fuel in the carb. Once I did this my engine came to life! Finally she idles! The "choke" (extra idle enrichment jet) now works as expected to adjust the idle speed. FINALLY she is running more like I expected, still needing tuning but I got idle, throttle, pull through most of the throttle range and a nice ride!

I have read all kinds of stuff about the break-in gas/oil mixture, the manual tell you to use LOTS of oil, lots of guys in the forum say to use much less. I decide to go with 20:1 for the first gallon, 24:1 for the second gallon, and 32:1 from then on. I am still on the 24:1 gallon and she is rolling better every ride, so I have decided to wait until I burn through this gallon and get on to the first gallon of regular 32:1 mix before I do any fine tuning on my carb.

So back to the CNS carb spacer that fell apart when I was adjusting my float.... Mine had broken in 2 pieces on first install but still worked with some fiddling. This time it broke into 3 pieces and it was game over! I did a lot of reading and research, everybody has gripes about these things, nobody seems to have a stock of spares. Some guys were making them from copper fittings, was the most promising avenue I found. Someone else noted their engine distributor suggest they use PVC tubing but they couldn't find a size that would fit. I was working in the evening when mine broke so the stores were closed, but I wanted to put this carb back on the bike and test the new float adjustment! So I scrounged around, found several maybe's that didnt work out, couple scrap pieces of pvc and such, none fit in the carb or around the intake close enough. Out of frustration I revisited a scrap of 3/4" PVC water pipe (the white water pipe they sell at Home Depot etc, you probably have some on your central air drain). It was larger than the intake by a good measure and wouldn't fit inside the carb, no way. But I got to thinking, the spacer is a ring with a slot, not a complete ring. So the slot can be cut and the PVC ring compressed to be smaller, small enough to fit. The thickness of the wall on this PVC matched the spacer pretty good so I cut a section to the size of the spacer, sanded smooth the edges, then cut the slot in the section with a hack saw. It was nowhere near wide enough to fit, so I took a flat file and widened the gap until when compressed it would fit into the carb throat leaving a similar amount of space as the original part. The gap ended up being pretty wide, I measured it and noted it in another thread at the time but don't recall the exact measurement. It's not overly critical, as long as the spacer fits in and secures the carb it will work. This part just holds the carb in place, the actual seal on the carb is the black thing inside the carb throat. Once I had the new spacer in the carb I found it a little to tight to fit on the intake. I used some sandpaper on the outside of the spacer to thin it down a bit until it fit on the intake and I was good to go. Took some playing around but this spacer isn't gonna break, trust me, I tried! No matter how hard I squeezed this thing just bounced back. I think I have a forever spacer now! LOL

So that's a summary of where I am at this point, lessons learned, and finally some roads traveled. I am by no means an expert on these engines or kits, just a regular guy giving an account of my experience and some advice from what I have learned along the way. Take it for what it's worth, my experience may be radically different from yours, I think there are some tips that will definitely help out first timers....

Happy trails

Last edited by nightcruiser; 07-22-2011 at 09:57 PM.
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Old 07-23-2011, 11:42 AM
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Default Re: Tips for Grubee Skyhawk GT5 Kit builders

Thank for the advice. I have a Skyhawk GT5, from kings motor bikes. I haven't got to install it on my bike yet. I was testing all my things in my kit and realized my throttle cable was bad. I called and they sent me a new one . Next i started to inspect the engine and i found that the inside of the cylinder had a chip in it. I remove the cylinder and both piston rings fell out broken in half. I now waiting on them to send me a new top end. Aint that my luck. So heads up to anyone who buys this kit. I recommend you inspect everything
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Old 07-23-2011, 02:32 PM
nightcruiser nightcruiser is offline
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Default Re: Tips for Grubee Skyhawk GT5 Kit builders

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Originally Posted by croy2007 View Post
Thank for the advice. I have a Skyhawk GT5, from kings motor bikes. I haven't got to install it on my bike yet. I was testing all my things in my kit and realized my throttle cable was bad. I called and they sent me a new one . Next i started to inspect the engine and i found that the inside of the cylinder had a chip in it. I remove the cylinder and both piston rings fell out broken in half. I now waiting on them to send me a new top end. Aint that my luck. So heads up to anyone who buys this kit. I recommend you inspect everything
I am curious what date your engine had stamped on it (and the box it came in)? Both my engines and boxes were stamped 04-20-2011, pretty fresh kits from GasBike.net.
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Old 07-23-2011, 04:23 PM
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Default Re: Tips for Grubee Skyhawk GT5 Kit builders

Well gasbike and kings motor bikes is the same company. Mine had 4-20-2011 stamped on it as well. The damage was in the cylinder where all the metal was smoot and shiny and there was a small chip of metal missing from it. I guess it was a bad casting. If you have any more questions for me please dont be afraid to ask.
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Old 07-24-2011, 02:29 AM
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Default Re: Tips for Grubee Skyhawk GT5 Kit builders

Funny I just got done with mine and had almost the exact same issues.

I had to go buy a chain because they sent the wrong one with it.
the throttle is junk and feels VERY rough and toyish.
will not idle to save its life

oh well for the very small price tag it costs you can't complain too much if this was a $3-400 kit I would be angry
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Old 07-25-2011, 01:44 AM
homejones homejones is offline
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Default Re: Tips for Grubee Skyhawk GT5 Kit builders

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Originally Posted by nightcruiser View Post
When you mount the engine lots of things come to mind, wrap your frame with tape and set the engine in there and fiddle with it a bit. Fit the engine on the frame making sure there is clearance for all the parts, carb, exhaust, make sure spark plug cap can come off and on etc. (more on that later) You want to mount the engine so the carb is as level as possible, but getting a solid grab on the bike posts is important, as well as chain angle. Balance out those issues to come up with the best mounting position. Make sure you have room to slide your carb off, you're probably gonna need to do that a few times. When you mount your engine it's a good idea to wrap some rubber around your frame under the clamps, this protects your frame from scratching and also helps you get a solid no slip grip, and I would imagine it helps dampen vibration a bit. A section of an old inner tube works great, use a piece of innertube rubber under just about any part you mount to your frame.
Wish I thought of this before I started my build - this is critical!
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Old 07-25-2011, 02:56 AM
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BarelyAWake BarelyAWake is offline
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Default Re: Tips for Grubee Skyhawk GT5 Kit builders

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Originally Posted by nightcruiser View Post
When you mount your engine it's a good idea to wrap some rubber around your frame under the clamps, this protects your frame from scratching and also helps you get a solid no slip grip, and I would imagine it helps dampen vibration a bit. A section of an old inner tube works great, use a piece of innertube rubber under just about any part you mount to your frame.
You may find that while this initially seems like a great idea, inner tube material is simply too resilient - it will prevent you from being able to torque the fasteners properly and will eventually pinch/chafe through, leaving your mounts loose and even contributing to mount failure.

Any soft material between the mounts and the bike's frame allows some amount of twist and while it may seem like it dampens vibration it actually only isolates it - instead of diffusing the vibrations throughout the frame it concentrates it to the motor's mount studs alone, which combined with the twist can lead to bent studs and/or cracks in the alloy, even stripped threads.

If the motor's mounts are seated flush to the bike's frame (which may require some minor fitting) without any "soft" materials, it's "locked" in place and cannot move, while you may feel more vibration this way - it's all being transferred and diffused by the direct & solid contact of the cast aluminum mounts, vastly reducing the sheer load on the fasteners themselves.

It's better in the long run to attempt to isolate yourself from the bike's vibrations with a large, comfortable & preferably sprung seat as well as some squishy "gel" hand grips. Interestingly, a significant amount of the felt vibration often isn't from just the engine - the stock muffler without it's lower clamp will resonate like a tuning fork, amplifying the vibrations. Another sneaky culprit is the kit supplied chain tensioner/idler as the chain links pass over it's hard plastic wheel.

Bear in mind that a new engine still in it's break in period will also vibrate more as it's still not running quite properly yet

Last edited by BarelyAWake; 07-25-2011 at 02:58 AM.
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Old 09-15-2013, 10:41 AM
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Default Re: Tips for Grubee Skyhawk GT5 Kit builders

ive been workin on these little jems for a few years now and found if i just shortened intake till it is about 2 inches that puts carb closer to jug do little adj on carb an 32:1 gas/oil mine runs like little chainsaw good luck and fyi dont do business with piston bikes.com plain rude they are and takes to long to ship an dont get what they say or you order.
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Old 10-09-2013, 09:40 PM
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Default Re: Tips for Grubee Skyhawk GT5 Kit builders

Thanks for your tips! I just ordered a GT5 from Gasbike. Should be in the mail arriving on Friday. Of course I will inspect everything. This is my third build. First one was a Flying Horse (GREAT MOTOR!). I made the mistake of buying the rubber motor mounts that Bikeberry had. BAD MISTAKE! It caused the studs to break off flush and impossible to easy out. I broke two easyouts trying to repair it to no avail. The second build was from a supplier on ebay. Just the motor for $65. Needle bearings failed and cut grooves in the cyl wall! So I used the upper parts of the flying horse to rebuild the other and it happened AGAIN! The needle bearing failed. Maybe there was still some remnants still left in the crankcase. So I am starting over with a new Grubee Gt5. Hopefully this will be a better motor with better bearings around the wristpin.
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