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Post by Bluefront on Apr 19, 2010 11:23:32 GMT -5
This is not a how-to...there are several good ones available. But here is my own take on this, as I've never done a GY6 engine BBK before. And you needn't follow exactly how I'm doing it, as there are many ways. But I'm a practical person, and want to do it the easiest way, that will return the best results, with the least chance of failure. First....examine the kit you got. Check it for obvious damage, missing parts, ring gaps, and the wrist pin. Some kits come with a pin slightly too long. You don't have to go as far as I did with this, but do check it out good. Next.....since there's no way to know how long this will take (you may break something and have to wait for more parts), find a good place to store a partially disassembled scoot, and arrange alternate transportation. Gather up the tools....very few are needed, but you do need at least an 8 and 10mm socket, a ratchet and 6" extension. a few screwdrivers, a gasket scrapper, and a small torque wrench to re-tighten the head bolts. There's a few other things also... After removing the seat bucket, determine how you want to do this. Do you really want to work around all that plastic. I didn't, so I removed the entire rear plastic sections.....They come off easily. Remove the luggage carrier, and the entire thing pulls off in one piece. You don't have to remove any screws. (Yours may be different) To keep from forgetting which little part is which, and to remember the dis-assembly sequence, I put parts in lettered plastic bags as I go....maybe with notes. The carb does not have to be removed completely, unless you're going to work on it (like for a jet change). Just push it back, out of the way. Once you remove the plastic engine shroud, clean it off good, particularly around the base and head gaskets. A can of brake cleaner will do it, but there are alternatives. Before I ever remove cam chains, I always mark everything with paint.....so I'm certain everything goes back on exactly how it came off. I painted the TDC marks on the flywheel side, painted some new marks on the variator side (I remove the variator cover so I could turn the eng easily from the left side).....and I painted marks on the cam sprocket and even the cam chain. When I get this back together, it'll be in time. No guess work here. Up to this point everything's been easy. If the exhaust nuts are even slightly tight, you might want to hit them with some rust buster or similar....or even a little heat. Don't break the studs, or you'll be scrambling around to find replacements. That's it for now....I'll continue after lunch. Got about two hours in it so far. But I'm not in any hurry. Take your time....do it right the first time. More to come.
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Post by erictheviking666 on Apr 19, 2010 11:48:56 GMT -5
Good write-up so far, keep it coming.
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Post by rapidjim on Apr 19, 2010 12:21:25 GMT -5
Great write up and pictures.
Jim
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Post by gctkaz on Apr 19, 2010 13:50:55 GMT -5
Thanks, it's good to share your experience and practical tips. Very good point about the time and storage of a scooter in pieces.
I was working in my apartment's underground parking garage. In fact, at the apartment across the street from where I live. So basically any time I do anything, I have to do it completely to the point where I can have the scoot fully assembled by the time I leave.
What I learned from my very first day - have a snack and go to the bathroom immediately before beginning any work. Keep a lot of rags, paper towels, etc. A bucket of water and a bottle of citrus hand cleaner are good to have nearby. Also, band-aids!!! My hand slipped badly and I took a huge chunk of meat out of my finger, with nothing on hand to stop the bleeding except for a single rag which was already very much soiled with oil and gasoline. Hmm, I suppose wearing mechanics gloves could eliminate most of these problems...
Later on, if I couldn't finish a job in a single session, I just locked bits and pieces inside my car trunk and put the seat bucket back on so the scoot looks more or less assembled.
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Post by Bluefront on Apr 19, 2010 15:08:05 GMT -5
^^^^^Well I'm lucky. I'm doing it inside my garage, and I have a car to drive. And this scoot was so clean, I haven't even got my hands dirty yet. (The Roketa only has about 900 miles on it. On with the dis-assembly. Loosen the nuts holding the head a little at a time in sequence. And before you pull off the head or the cylinder, carefully break loose the gaskets on one side or the other. If you're lucky the gaskets will come off uninjured, and could be used over in a pinch. Wrap a wire around the cam chain.....you don't want it falling into the crank-case. (It could be tough to fish out). The head looks fairly normal for the mileage with respect to carbon build-up....the plug ok to me. The piston had more carbon build-up on the upper ring-land than I would have expected for 900 miles. And the markings on both thrust surfaces looked slightly excessive. The cylinder bore looked perfect. Now for the problem (?) areas. Here's a picture of a new center case gasket (from Enviromoto). After the lower crank-case is bolted together, the red area gets cut off so the cylinder can be installed. On my engine it was cut off of course, but not trimmed properly. Part of it stuck up and was smashed down by the cylinder base gasket. This could cause an oil leak at that location (the arrow). But the green section of the gasket should also have been cut off (I think). Right now you can see it hanging loose in the crank-case. It serves no purpose anymore, and could fall loose into the crank-case....possibly causing trouble. Here you can see it (the green stripe). Frankly I've never seen a gasket piece left in this position on any engine I've worked on that had a center case gasket. (not all do) Maybe this is normal for a GY6.....anybody who knows anything about this please inform me. I'm tempted to cut it out of there. The other thing....I'm going to shorten the wrist pin some more, to match the OEM clearance (maybe another .010"). More to come tomorrow.....
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Post by Bluefront on Apr 19, 2010 16:27:35 GMT -5
Something else to ponder......the increased compression with the BBK when using the stock head. Looks to be a bunch. Here's the 72cc gasket sitting on the OEM 50cc head. I'm sorely tempted to champher the head slightly to match up with the new cylinder......lowering the compression somewhat. This looks to be a problem area. Anybody have any thoughts about this issue.....
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Post by gctkaz on Apr 19, 2010 16:54:19 GMT -5
Wow, I really like the tip about wrapping the wire around the cam chain. Seems like a minor annoyance, but it's infuriating when it happens repeatedly. GRRRRR!!
While it's a good tip, I see a possible (minor) problem with the little line of paint for lining up the cam & chain, but it's not guaranteed to happen, and you'll know for sure when it does. Anyway, on my second disassembly, my cam chain somehow became derailed inside the crank case, and I don't know how since it didn't happen the first time I took apart the engine. For me, getting it back onto the crank sprocket seemed more difficult than it needed to be, but the important point is that the chain slipped all around the place so markings would mean nothing.
Anyway, really liking your documentary. Keep it up!
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Post by Bluefront on Apr 19, 2010 17:03:18 GMT -5
Yeah if the cam chain can slip teeth on the lower sprocket while you're working on it, the mark on the chain would be meaningless. I'm not sure if this could happen on a GY6, (it can't on other engines), but the other marks would tell you if it happened. You could probably move the cam chain back to the original position if it happened......but it's not really necessary.
You've got the marks on the sprocket to go by if the cam chain slipped.
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Post by wketchin on Apr 19, 2010 19:24:13 GMT -5
I had little problem with getting the timing correct. All you need to do is look at the strator line and turn it until you feel someone give away.. That's TDC. it should be lined up with T on the strator. As for the cam, i just slapped the puppy in and turned it until the big dot was on top, and the two dots lined up with the center of the case. After you affix everything, spin the engine a few times to see if everything is lined up.
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Post by Bluefront on Apr 19, 2010 22:26:23 GMT -5
I'm not worried about getting the cam timing correct....the stock marks would be fine for that, even without the paint marks. But I am worried about that compression thing. Just looking at it gives me the creeps...... I wonder if the increased compression could be the cause of bearing failure (pinging/knocking) as at least one person reported, rather than the Chinese manufacture of those bearings? I located the Dremel and the bits necessary to mod the head slightly.....still wondering about it though. This job may turn out to be more than just bolting on stuff. Thoughts on this appreciated.....
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Post by meitianman on Apr 19, 2010 23:28:26 GMT -5
if you chamfer the head which would lower the compression slightly you could always take the head to a machine ship and have them mill a few thousandths off it to compensate. you might want look into how much valve clearance there is between the valve and piston first tho, so you dont accidentally hit the valve on the piston as a result. ive been running the bbk without a modified head and had no gasket issues so either way i would not worry about it much.
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Post by Bluefront on Apr 20, 2010 2:45:08 GMT -5
When you think about it.....these GY6 50cc scoots recommend running 90 octane gas (or higher) stock. This pretty much indicates the stock compression is about as high as possible (for normal riding).
Without mods to the stock head, the compression looks like it will be upped quite a bit. I think I'm going to grind mine down till it looks better. The main problems are to avoid scratching the gasket surface, scratching the valves, or cutting too deep. I'll post some photos of the process (if I do it)......
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Post by Bluefront on Apr 20, 2010 8:25:45 GMT -5
Bump for this thread......I'm still looking for more info regarding compression ratios.
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Post by Bluefront on Apr 20, 2010 9:51:43 GMT -5
Unless I hear a convincing argument why I shouldn't lower the compression slightly, I'm going to start grinding on the head this afternoon. Here's my setup to not slip and screw up.... Using the new head gasket as a template, I made an aluminum gasket with the inner diameter a little smaller than the head gasket.....snugged it up. This protects the head surface from any unintentional scratching. I super-glued steel washers to the valve heads.....when I'm finished the washers will knock off easily. I'm familiar with porting techniques.....just haven't done it lately on a head this small. Still waiting for any info.....
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Post by tomcas on Apr 20, 2010 10:33:35 GMT -5
Bump for this thread......I'm still looking for more info regarding compression ratios. With a 47mm piston (72cc) I'd leave the head alone and just run high octane. I'm afraid cutting down the head will kill the power and it simply isn't necessary. With a 50 or 51mm piston (81 or 85cc) I share your concerns. In that case I'd have to compare the volume and displacement of the actual kit.
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Post by Bluefront on Apr 20, 2010 10:42:49 GMT -5
^^^^I'm already running 93 octane, as high as I can get. I figured with a displacement increase of almost 50%, my power increase will be substantial....without also raising compression over stock. This scoot is not used for racing or high speeds......just need a little more torque on the hills. I suppose other people just slap on the BBK and use the OEM head without mods......just doesn't seem to be the way to go for me.
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Post by tomcas on Apr 20, 2010 10:53:01 GMT -5
^^^^I'm already running 93 octane, as high as I can get. I figured with a displacement increase of almost 50%, my power increase will be substantial....without also raising compression over stock. This scoot is not used for racing or high speeds......just need a little more torque on the hills. I suppose other people just slap on the BBK and use the OEM head without mods......just doesn't seem to be the way to go for me. agree and I wouldn't suggest just slapping it on. What I suggest is you rejet so you don't run lean and just leave the head alone. A whole bunch a people are running 47mm and some even 50mm pistons with stock heads and without problems. I'm going out on a limb here to predict if you cut the head you will be very disappointed in the power and will end up getting a new head. As far as thinking you could just skim a few thousandths off the head later to get back the compression, forget it. There simply isn't that kind of room between the valves and piston. Take it from the king of worry, you don't have to worry about it.
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Post by erictheviking666 on Apr 20, 2010 12:00:48 GMT -5
You have too remember that these motors were originally 72cc stock in some parts of Asia. The Chinese do not swap out the heads too ship to the US only the piston & cylinder. So basically your original head should be just fine.
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Post by Bluefront on Apr 20, 2010 12:06:09 GMT -5
Heh...I don't consider jetting to be a "mod". I'm already running a #84, two sizes over stock. After I get this thing running again, I expect to go up a few more sizes. I've got a main jet set from Enviromoto, so I should have a jet that'll work. Oh...you're not the King of Worry, I am. The worst that can happen here.....I have to buy a new (cheap) OEM head. I'd rather have to do that, than suffer other engine damage. Thanks for the feed-back......
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Post by Bluefront on Apr 20, 2010 12:12:18 GMT -5
^^^^^^"The Chinese do not swap out the heads too ship to the US only the piston & cylinder." Where did you get that information? ? They may use the same head casting.....but the machine work in a 50cc head could be totally different from a 72cc head. The Honda 70s I've seen, do not have a tiny 50cc combustion chamber like this 50cc GY6.
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Post by erictheviking666 on Apr 20, 2010 13:18:09 GMT -5
Honda 70's are Japanese. I'm talking about Chinese the most incredibly cheap people on the planet. I know I worked at a Chinese restaurant (where I was constantly told to wash off the green chicken and use it for the egg rolls). I find it hard too believe that anybody that sells Scooters as cheap as the Chinese don't cut corners whenever possible.
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Post by Bluefront on Apr 20, 2010 16:00:36 GMT -5
Hummm....whatever, but that's speculation about the head. The Honda 70 is now a GY6. So I finished turning my 50cc head into a 72cc head. I'm sure the compression on this engine will be higher than the 50cc, but not as high as if I hadn't touched it. The aluminum gasket and the washers worked good....no scratches. I could have gone further with the polishing, but this is plenty. The intake and the exhaust ports look ok to me.....no steps at all like some people talk about. That's all for today folks.....
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Post by tomcas on Apr 20, 2010 19:10:24 GMT -5
Hummm....whatever, but that's speculation about the head. The Honda 70 is now a GY6. So I finished turning my 50cc head into a 72cc head. I'm sure the compression on this engine will be higher than the 50cc, but not as high as if I hadn't touched it. The aluminum gasket and the washers worked good....no scratches. I could have gone further with the polishing, but this is plenty. The intake and the exhaust ports look ok to me.....no steps at all like some people talk about. That's all for today folks..... Regards of the need it certainly came out pretty good.
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Post by erictheviking666 on Apr 21, 2010 0:08:47 GMT -5
Yeah it did come out looking good. Thats pretty nice work.
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Post by indy50 on Apr 21, 2010 0:48:12 GMT -5
I don't understand why you wanna lower compression My drag scoot reads 240psi, with .002 mm shaved from the deck Compression is power
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Post by Bluefront on Apr 21, 2010 5:24:53 GMT -5
I'll try to explain this, for people who might not understand the concept. Compression is based on two things (basically).....the capacity of the combustion chamber (the head/valve area) and the capacity of the cylinder (50cc to start with here).
When you install a BBK (72cc in this case) without changing the capacity of the combustion chamber, your overall compression ratio will go up. If for instance you were running a 10/1 ratio to begin with, after the BBK you might be running 15/1 (I'm guessing). More power for sure, but you may suffer consequences. Gasoline is normally available only up to 93 octane. If you use an octane too low for your compression ratio, the engine can detonate (ping). It may also over-heat, damage pistons, damage bearings, etc.
Obviously you don't want to risk that (unless you're racing)....I sure don't. So what I did here with this last head mod is to increase the capacity of the combustion chamber (by removing aluminum). This makes up for the increased cylinder capacity of the BBK......and if I removed enough aluminum, my overall compression ratio will be about what it was at the start (say 10/1). This is what I am doing here.
I'll still get the benefit of the increased cylinder capacity (72cc), but I should not expose myself to the risks of a higher compression ratio. Now if I wanted to, I could be more accurate with all this by measuring the capacity of the combustion chamber before and after the mod (that's somewhat difficult. Racers do it). But for what I'm doing with this scoot, a practiced eyeball of the new combustion chamber tells me my new compression ratio will be only slightly higher than it was stock......but the scoot will be quicker because of the increased cylinder capacity.
This may be the "dirty little secret" about these 72cc kits.....you may end up with an unsafely high compression ratio, unless you also change or modify your stock head. My 2c.....
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Post by tomcas on Apr 21, 2010 7:15:42 GMT -5
I don't understand why you wanna lower compression My drag scoot reads 240psi, with .002 mm shaved from the deck Compression is power You mean 2 thousandths of an inch (.002"), don't you? 2 thousandths of a mm is less than one tenth of a thousandth of an inch, barely measurable and almost as difficult to machine or grind.
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Post by Bluefront on Apr 21, 2010 7:32:01 GMT -5
^^^^^Heh. You've got to be careful with this mm/inch thing. I try to edit all the numbers I post, but sometimes screw up also. For sure double-check everything you read on these forums.....particularly the numbers. Oh...I may do a little more polishing on the head today. It looks a little rough in the photos, but it's mostly lighting.
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Post by indy50 on Apr 21, 2010 8:10:53 GMT -5
I don't understand why you wanna lower compression My drag scoot reads 240psi, with .002 mm shaved from the deck Compression is power You mean 2 thousandths of an inch (.002"), don't you? 2 thousandths of a mm is less than one tenth of a thousandth of an inch, barely measurable and almost as difficult to machine or grind. Yes .002" sorry Compression increased signifigantly Race only engine, not a daily driver I know of many folks that use the stock head w a bbk with no negative results
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Post by JR on Apr 21, 2010 10:44:04 GMT -5
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