My bike's engine turns over, but it doesn't start.

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DuckmanDrake

New Member
Aug 21, 2015
2
0
0
Brantford, ON
Hi there all, I have a 66cc GruBee Skyhawk attached to a P.O.S. mid 90's Yakota Mountain Bike.

While riding the other night, the bike lost all power and slowed to a stop. I put it away for a few days, only to find that when I walk the bike around with the clutch out, the motor spins freely without any resistance.

I took the clutch and drive chain housing off and found nothing abnormal or otherwise off-looking. Adjusting the clutch flower nut doesn't add or remove resistance. When the clutch is pressed in, the plate spins freely as usual, and when not pressed in, it spins with the gears as normal, but has no resistance. It makes that puffing sound while I walk it still, which suggests that everything is working fine when it comes to the piston.

Now, as for pedaling it to try and get it to start, the engine spins and there's the same puffing sound coming from the muffler, but louder. Pulling the clutch in at this point works fine as well.

I'm at a loss as to what it could be. Any suggestions?

Thanks in advance!
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
first guess would be loose head or spark plug
I concure. The cylinder head fasteners need to be torqued (tightened) but also the head gasket checked.

120 to 140 inch pounds. They need to be checked and re-torqued after several heat/cool cycles. If you're using the kit supplied chrome acorn nuts, throw them in the scrap pile and replace them with standard or flanged hex nuts.

But check that gasket!

Tom
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
I also concur... sounds like you lost compression either because the head bolts came loose or it could be the spark plug, worst case scenario would be the rings not sealing but you would notice that because it would slowly lose power before finally not having enough to start the engine. Another issue that could kill the compression would be if the plating in the cylinder peeled in a big enough area.... good thing tho is if the rings are the culprit it's not that expensive to fix... about $20 or so for a cylinder and $5 to $7 for a new set of rings. It will require a break in period again but it'll run like new again.
Try re torquing the head and check that the spark plug isn't loose, then see if the compression is restored or not, if it's better then it's fixed, but if not you'll need a new jug and ring set. You can also re hone a jug if all the plating is still intact but a flex hone can cost as much as the cylinder.
 

DuckmanDrake

New Member
Aug 21, 2015
2
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Brantford, ON
I definitely will be taking the advice of those who have posted thus far!

I was thinking it might have been a compression issue, but didn't know exactly what avenue I should go about fixing it. Thanks very much for the advice, and I'll reply to this thread with results (or lack thereof) soon!
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
Hopefully you can find the problem and get it back on the road soon.... luckily these things are crazy simple and easy to work on as well as very easy on the wallet....
 

Jumpa

New Member
Aug 12, 2011
607
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Cape Cod
Drake ,
First off ,these guys answering you are all professionals with very good advise.especially 2door

I have a question Drake, when your bike "lost all power" and stopped was the motor still running fine?? You say it lost power ..

I only ask because I had a guy describe to me the exact same symptoms on his bike last week with a brand new motor.I put on for him.
However, he mistakenly thought the sound of the clutch spinning freely was the sound of the motor turning over when it was not, ...

somehow this guy's's woodruff key fell out on the bevel gear "small gear on the clutch side. I'm not at all saying this is whats going on with your motor just pointing something out and bringing it to light in case other folks are having issues with new motors as well !

Ive seen it happen twice on brand new motors


the Woodruff key "or keyway if you will" keeps this gear from spinning freely on the crankshaft .."pictured"
 
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Davezilla

New Member
Mar 15, 2014
2,705
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San Antonio Texas
that's another possibility, I've had mine do the same thing while riding, the bike worked just fine until I slowed down, then when I cracked the throttle to take off again, nothing... the engine free reved but the bike stood still.
Luckily I was right in front of the shop when it happened so I just pushed it into the shop and took off the clutch cover, the screw that holds the bevel gear down was backed out and the woodruff key was shoved back out of the way of the bevel gear.

The fix was easy, pull the bevel gear, put the woodruff key back in and put it back together. I used a drop of blue locktite on the screw this time and it hadn't come out since.
 

Slogger

Member
Sep 8, 2014
544
4
18
nohio
I found that drive gear screw looser than I left it once, now it is checked and torqued every time I grease the gears in there. It seems to stay tight ever since, but it doesn't cost anything to check it.
I keep a wedge of pure copper in the toolbox, I've always used it to jam gears to hold them still when cranking a bolt. The copper won't damage the gear's teeth.
 

Legwon

Member
Mar 2, 2013
248
0
16
Van bc Canada
from the sounds of what OP is sayin... the bevel gear is tight and in place, otherwise he wouldnt get the piston action/sound while riding.
so that leaves 1 of 2 things...
1 as mentioned above... loose head/jug ... tighten them up.
or
2, badly blown gasket somewhere, youll need to look for it and replace that gasket

but as always, these are just china dolls. find your problem, then go over everything, to ensure you dont have another one sneaking up on you.
 

crassius

Well-Known Member
Sep 30, 2012
4,032
158
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USA
had one come in yesterday - either a california kit or best bikes kit - guy put piston in backwards, ran 2 minutes, broken rings and big chunks everywhere : (
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
There are a lot of things that can go wrong and cause a compression loss... The most common obviously is the head nuts working loose, this is also the easiest to fix and requires no parts to be purchased... other than some regular nuts to get rid of the stock acorn nuts if that's the problem.
The spark plug can come loose and bleed off compression but it would have to be really loose to bleed off enough to prevent a start.. like ready to fall out loose, most the time it'll start and blow the plug out shortly after, this also leaves tell tale signs of brown to black residue on the head around the plug.

The cylinder wall can get scratched or the plating can peel off in a section which means the only fix is a new jug and set of rings... and any necessary gaskets that got disturbed in the process of removal/installation such as the head, base, intake, and exhaust. No sense trying to reuse a paper gasket, but the head gasket can usually be reused.

Head or jug warpage, usually comes this way from the factory and the engine won't start brand new, or from overheating, or a combination of not having a true surface to start with then an overheat warped it just enough to bleed off the compression. Lapping the head usually fixes this problem but it never hurts to lap the cylinder as well. I had one cylinder brand new that needed almost .040" taken off before it trued up on the lathe.

Other things can also go wrong like a piston pin clip that wasn't installed peoperly at the factory, piston installed backward, ring damage, etc that'll ruin a top end.

I always recommend the easiest fixes first like re torquing the head and checking that the plug is torqued, this'll usually revive an engine that was running but lost compression, but if it doesn't, it's time to pulll the head and look down the cylinder with the piston all the way down to see if you can see any scratches or missing plating. If this looks good and the inside cylinder walls are nice and shiny it's the rings that failed to seat before the cylinder wall glazed. Re honing the cylinder will usually breathe new life into it if you have a 48mm flex hone with the nikasil compatible stones, but can be done with a small engine hone and fine stones. The cross hatch pattern on some new cylinders is almost non existant so these can fail this way. Honing will put enough of a cross hatch pattern on the cylinder wall to finish seating the rings if they failed to seat or new rings can be used as cheap insurance... here are a few examples of these cylinders brand new, 6 months old, and the same cylinder after re honing...

This is a brand new cylinder, never installed or run... notice there's no real cross hatch pattern for the rings to seat against... just a light un polished but smooth texture...


Here is a mildly ported cylinder that's been run in one of my bikes for about 6 months of daily use... Notice it's mirror like shine (I put my finger down in the bore to show the reflection)... this is what it should look like after the rings have seated, but if new rings are installed in this one without honing, the new rings wont seat in this cylinder but it would most likely work, it just wouldn't make full compression especially at lower rpm so torque would suffer.


And here's the same cylinder after honing it for about 60 seconds with a 48mm flex hone made for nikasil coated bores. The hone needs to be slightly larger than the bore in order to cut properly, but not too big... the 48mm flex is what they recommended for the 47mm bore and it also has fine stones for finishing, you definitely want the fine stones for something like this since you don't want to remove very much metal, just enough to break the glaze and that's all. This is all it takes to properly seat the rings and it will develope a glazed finish once the engine is broken in with maybe just a hint of visible cross hatch after break in which is the desired result as what's left after break in will be just enough to properly lube the rings and help them last a lot longer.


This may or may not be the issue here, but if someone has a persistant problem with losing compression at the rings, this could be why...

Relevant to why compression was lost here or not, hopefully it helps someone.
 

Agreen

Member
Feb 10, 2013
792
11
18
Southeastern GA
When you replace the rings in a car engine, you generally have to file the ring gap to get the proper clearance. I say "generally" because tolerances today are so awesome, I haven't had to break out the file kit in years. Now, I've only ever had 2 china engine kits. I've also always just assumed that they are properly clearanced out of the box, and I guess I've been pretty lucky so far. But does that differ when you buy rings separately? I would definitely check the gap at the very least, but they have a little locator pin that could interfere with setting it up properly. Or am I just splitting hairs on a subject that doesn't matter?

As a side note, it's interesting to see how different opinions are on honing these. When I put my first kit together, I didn't know how full of shavings they would be, and the cylinder bore got scored pretty bad. I was told to not hone it, as the inner wall was so super thin that even light honing with a fine stone would flake the "chrome" coating. The bike was shortly stolen thereafter, so I never got to experiment with it. I now have a 48cc version which I tore apart and cleaned with nuclear precision. The cylinder wall is polished to a mirror shine like the one you posted, and it has reached 44.8 mph with only a poopoo pipe, matched transfers, and a properly jetted carb. It's amazing what a little care and patience can accomplish!
 

Davezilla

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Mar 15, 2014
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San Antonio Texas
In my personal experience, all the ring gaps I've seen are too wide to start with so there's really no need to file on them other than maybe to polish the corners so they don't scratch the bore... I hadn't seen any rings yet that have a proper gap or even one that could be filed to proper since these gaps are usually too wide to begin with... too bad we can't add metal to them...

For the honing, I've found that the nikasil plating is .010" thick inside the bore when I was surfacing a jug on my lathe and the others were pretty consistant at the same thickness for the plating, this is good because when done right it's durable, but bad because if it dis bonds it leaves a huge gap between the ring and the wall if a chip is missing, and it's plenty big enough to almost completely lose compression, so if the plating does flake off on a cylinder, there's no real fixing it.
When I hone a cylinder I'm just breaking the glaze and not really removing metal, I can measure one before and after and not notice any difference since it removes about .00001" ... or at least enough less than a ten thousandth of an inch that it doesn't show a difference on a bore gauge. Now you wouldn't want to sit there and hone on it for like 5 minutes, but just doing a quick 30 to 60 seconds at about 300rpm on the drill using 320 grit stones the amount of metal actually removed is neglidgeable. I would recommend checking the condition of the plating before honing, using lots of lube during the honing (I do the honing with the jug submerged in mineral spirits, set the drill speed to about 300 rpm, and push in and out about twice per second to get the cross hatch pattern). The idea is to do a little, check it, and do a little until the glaze is gone then stop, no need to go any further or it could take off too much.
So far all the ones I've honed before assembly or when reusing a cylinder they've all come out good, now if there was a scratch in the plating or any plating missing, or even any signs of the plating delaminating would be a reason to reject the cylinder for further use, otherwise, as long as you just break the glaze and use really fine stones it works out really well.
 

Davezilla

New Member
Mar 15, 2014
2,705
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San Antonio Texas
I've noticed that in the last batch of cylinders I've bought, the plating on them is nice and thick, there's little to no casting flash to deal with and the plating around the ports is a lot neater and chamferred which used to be a big issue about a year or so ago.
This batch I had to get them in black but the finish on them is a lot better and it looks like they actualy used some quality paint instead of the typical 10 year old with a krylon can looking finish I'm used to seeing and there's absolutely zero overspray inside the bore or in the ports, and the exhaust & transfer ports are nice and wide on these, even wider than the first cylinder I ported.