Hmm... Bad choice on my part... Obviously was.

[jonyoon21]

Looked around for a post relating to this (since I don't really like posting) and couldn't find anything, so here goes.

So I just recently finished my break-in period on a nice 66cc grubee skyhawk. I just changed the fuel oil ratio to 4 oz per gallon after my last gallon was with 5.5 oz oil and I had 8 oz per gallon during the 2 gallon break in (more like 2 month break-in...) and had noticed a nice change in the power. I also slapped on a SBK expansion chamber and shift kit as well as a nicer air filter on the carb.

Now here is where my stupidity kicked in. I was riding to the campus with my gf in her car behind me and I was running at about 25 mph in my second gear accelerating to about 28 mph or so and got used to hearing the high pitch loud noises from the exhaust pipe. Then I felt it, a nice jerk and a sudden decrease in power. Obviously I shouldn't have rev'd it so hard but I forgot to shift up and had gotten used to the loud noise. (I usually shift once I hear the high pitch noises but recently got a muffler to cut that down, since I no longer heard the high pitch noise then I found I could push the bike throttle further with no ill effects and more power. Then I lost the muffler when i vibrated off (didn't secure it well enough). So from there I was used to pushing the engine and figured that the high noise of over revving was just the noise that used to be canceled out by the muffler.)

So long story short, I think I burned out the engine and don't really know where to go from there. A friend of mine in my class (rides a 1000cc crotch rocket - forgot what make) educated me on the fact that I could have broken a gasket or valve or something and am losing pressure.

If it's just a gasket problem I think I could fix it myself, but if it's a valve then I don't know, or worse a crack... Perhaps some JB weld on a crack would work?

Is there any way to fix this or will I have to ditch the engine and get a new one?

-One more note, I have noticed black leakage at the bottom of my engine, but I noticed that about a week ago when it dripped on the floor. I figured that it might have been from the chain as I lube it pretty good.-

Thanks

 

[foureasy]

does it still run?

 

[jonyoon21]

Yea, it pulls really well in the first gear like nothing happened, but once i reach 2nd gear going 23mph it seems to lose all power and dies back down to 17mph instantly. I'll downshift and start again.

 

[Technocyclist]

Check for air leaks... These engines are notorious for this. Check the oil seals in the crank. If it's black oil, it's mostly probable that your oil seals popped out, or damaged.

 

[a_dam]

Then I felt it, a nice jerk and a sudden decrease in power

That sounds scary, but if it still runs and pulls OK until a certain speed/rpm, then I wouldn't think something is "blown".
I don't have experience with this motor or shift kits. My motor doesn't have a float bowl. Motors that do can sometimes run fine until they get up to a certain rpm and fuel demand. Then they bog down for a bit; then rev back up and repeat. My big leaf-vac (pulled behind a riding mower) was doing this. The needle seat had a tiny rubber gasket that the gas had made swollen over time.

I hear some setups can get fuel-foaming problems from vibration at a certain rpm. Probably not your case, but something to think of.

Black leakage from a 2-stroke is not unheard of. You run those Chinese motors rich (oil-wise), so you gotta expect some oily exhaust and maybe some drips on the garage floor. Get the motor as clean as you can and try to find the source of any leaks.

 

[Venice Motor Bikes]

Maybe just a bearing?

 

[noco]

are you sure that the motor isnt 4 stroking when you hit the higher rpms...i had trouble with this till i found a thread here that got me straghtened out...the high rpms dont worry me here...these things got such a short stroke and a light crank where they can really be tacked high...does the sound of the motor change when it falls on its face???and oh yeah im sure you didnt burn a valve since you dont have any....hehehe....just messing with you

 

[jonyoon21]

Thanks for all the replies.

Now that I think about it, it did bog down and once I was around 18 mph, i reved it up again and then it repeated. Don't know if that's relevant though. I haven't been having any oil dripping from the exhaust, it was from the bottom of the engine, but my theory is that it's just the throw off from the chain, I find that black soot all over the place after I lube the chain and seeing as the engine is at an angle in the frame, I don't see why it wouldn't build up on the bottom.

By 4 stroking does that mean a change in the sound? I've heard changes in the sound when it bogs down. Kinda sounds like there are extra rotations going on or something. I guess that's what would be a four stroke.

If there is an air leak how can that be fixed? I read a blog somewhere about pressure testing and then using windex to on the engine to look for bubbles where a leak might be. But once a lead is found, how to seal it? Just some epoxy?

I actually did another stupid thing and took off the head to check out the insides myself. Now I can't get it back on since those piston rings are getting in the way.


Edit: Oh and thanks for the reassurance on the valves thing. I noticed that I didn't have any once I tore the head apart but was unsure if there were any below the cylinder head.

 

[a_dam]

Now that I think about it, it did bog down and once I was around 18 mph, i reved it up again and then it repeated

Like noco said, and Bikeguy Joe it could be 4-stroking. I'm no expert, in fact, I'm probably the last person in the world you should take any advise from on any subject. But when my motor does that, I screw in the left carb screw an hour (1-o-clock position to 2-o-clock position).

 

[jonyoon21]

Hmm, Hopefully it would be that easy.

I just spent the past 2 hours unmounting the engine and finally figured out how to put the head cylinder back on without breaking piston rings. So lets say that it is just 4 stroking, that would most likely cause my symptoms? Engine bogging down at around 23 mph then reving back up at 18ish or so mph.

So I guess I'll just reseal what I've taken apart. I'll have to wait for a new piston ring to come in the mail since I shot that one... unless it's not necessary, but I'm sure it is. I also tore a base gasket taking it out by mistake. Then I'll try that. Where is this left carb screw you're talking about a-dam? That's not the idle screw is it?

 

[noco]

i dont think that the carb screw is going to help you...but give it a try...from my understanding that thing is just an idle speed adjustment...i dont know...i like the bike shutting down and not idling...but 4 stroking definately changes the sound...there is a recent thread on it with a great video that has good audio for you...check it out...(i still cant figure out how to make links yet...i know im stone age)...and for tearing a gasket...oh well...that happens all the time when you tear stuff down...not your fault...how did you break your piston ring???if you did then you need to know that you need to compress the ring before putting it in...there are ring compressers that can be bought...i doubt any are cheap...or try compressing with the smooth side of some contact paper(one of my trany building tricks)...any way good luck and keep us posted

 

[jonyoon21]

I was being dumb and tried to put the cylinder back on and didn't realize that there was a little notch to align the ring, so I forced it on with a screwdriver (like a dumb****) and the piston ring snapped. I figured that it was just a really high tension ring or something that wouldn't bend. At least piston rings arn't pricy for the china engines.

I'll have to look into that video. Thanks for the info

 

[a_dam]

Like I said, I don't have experience with your particular motor. My motor's from a Homelite leaf blower, but certain physics are the same for any two-stroke. A too-rich condition can cause 4-stroking. I lean my mix up by turning in a mixture adjusting screw (my carb has 2). From reading other posts, it sounds like a lot of china kit motors have clip you move up or down to make the needle go farther in or out. I don't know your motor, so I'm not suggesting any specific adjustment. I just know that IF a motor is 4-stroking, it's probably too rich.

When my motor DOES 4-stroke, it doesn't rev up, then bog down. It just won't reach top rpms (and it makes the telltale sound). So I would still think your problem is with proper fuel delivery.

 

[dmar836]

Basics: Did you rejet the carb when you changed the air filter and added the pipe?
You very likely ran it lean. A gasket leak can do that but ill -applied mods are a more common way people grenade a 2-stroke. Do the sides of the piston look scored at all? How does the cylinder look? There should be no discernible scratching/scoring anywhere where they contact each other. Is the area on the underside of the piston dome caramel colored, black , shiney, scorched - looking? There is a lot to read and black stuff oozing is just one of them. Overall plug color is not a very useful tuning tool. Plug chops are but are beyond the scope of this post.
Adding performance parts without a basic understanding of how they work is asking for trouble. 2-strokes can be a tuning nightmare as it is and you didn't even know that (in a 2-stroke) valves are a function of ports in the cylinder and piston. That's okay but if you are intentional enough to be modding it, you should be intentional enough to understand what your changes can do to the air/fuel mix and power band. Look it up on the web and read about it. This goes for any mechanical object.
Another basic is to change one thing at a time and get it dialed-in(tuned) first, then make another mod and re-tune. Hitting the proper mod/tuning combo is like shooting in the dark otherwise. BTW, jet size in the carb is one of the main changes that need to be made to "tune" your modded engine(BEFORE you ride it wide open!). It's just one, but one that will keep your engine alive. Oh, and this has nothing to do with a drill bit. DO NOT go drilling the jet. People do it on here but ask any 2-stroker and he'll say there is no way to get an accurate jet size that way. There are at least 10 jet sizes for each increase in drill bit size you will have. Say you then need another larger or slightly smaller jet size - where were you compared to where you are after a drill bit and what's the next size you need - you won't know what jet size you just made? Jets are like a buck (and they don't go by drill size). Get a few larger sized than stock and learn how to swap them out. Start several sizes larger than stock and you might be back in business. I have rebuilt dozens of carbs and have NEVER bought a gasket or rebuild set(and I don't hack them). Take your time and you can swap jets over and over without needing parts.
Buy some Yamalube sealant from a motorcycle shop. Use it to seal your gaskets as you replace them. Stay inside the lines with this stuff, do a clean, detailed, and precise job with all you do. BTW, don't use the sealer on any carb parts.
Seriously, you really need to borrow a torque wrench. Just randomly tightening down the head/cylinder with a socket is unlikely to be even enough for an optimal seal. Sounds like a hassel but when a problem arises later, you will not know what shortcut step is at fault. And just because it fires up doesn't mean it will live long.
Clean everything as you go, many times over then right before you assemble a part. You should probably do this inside your dorm or house, whatever - not at the bike rack outside while the engine is still on the bike. Controlling the environment and not having to lean over will change the quality of your work.
A motor - even these Chinese things - are precise objects. Don't force anything and take your time. Get a mechanically inclined person or a shop to assist you if needed - you'll save money in the long run. You appear to be going 1 step forward and 2 steps back each time you mess with it. Just don't get it so goofed that you toss it and start over.
Mostly, don't ride it until it is sorted!
Tough advice, all true, and just my opinion.
Dave
KC

 

[BarelyAWake]

dmar836, although in many ways I agree with you - honestly, these Chinese engines really are just toys.

There's a happy balance between what you've described and wantonly banging on it with a rock. While care should be used and you're absolutely right that only one mod should be tried at a time - when it comes down to it, they're $50 engines and perfect for experimentation. I couldn't think of a simpler, easier engine to learn the basics on and part of that learning is hacking stuff up to see what happens.

While I wouldn't solder and drill a jet - it has nothing to do with precision as they make great jeweler's bits perfect for such things, I'm simply too lazy to bother lol and while you shouldn't rebuild in a mud pit of course, there's no real need for the "lab coat" approach - part of the charm of these motors is being able to do a top end rebuild on the side of the road with a 10mm and $15 of parts.

Personally, I love doing dumb*** stuff to my motor with a Dremel and file - it's part of why I bought the silly thing lol

 

[dmar836]

No, I agree with you completely but in this case it sounded like his issues were compounding the more he tried - screw drivers and all.
Trust me. I would have killed or died for a motorized bike as a kid.
I just thought he might be happier faster by going back to the stock configuration and sorting it out that way putting it together solidly.
I agree these are not Swiss watches but I just think with a little assistance from the scooter shop or other rider, he might have it back the way it was with less frustration - been there.
Yamalube would be a great thing I think and if you can beg or borrow a torque wrench you can sort of mark off a few more common problems(leaks!).
I apparently came off preachy on a few posts today (as one guy said - like the expert) - maybe I was only on my first cup of coffee. Sorry all.
I would rather help him fix it if he was local rather than dog him with info he has no need for.
I know he isn't having fun with it in pieces and this is all about fun.
Thanks for the reality check.
jonyoon21, where are you located?
Dave
KC

 

[BarelyAWake]

mmmm cawfee

It's cool man, yer helpin' and that's the important thing! When installing these motors and even their initial run-in, it's quite common to get "swamped" by wanting/needing to fix too many things simultaneously.

I'm finding that posting help here is much the same

Sometimes just stepin' back, takin' a mo to sort it out and then addressing just one thing at a time, remembering there is rarely any absolutes and just tryin' for "close enough" till everything is in it's place is the only solution.

I'm glad yer helpin' - thank you


Oh yeah, I'm boycotting Yamalube for no reason other than I live behind a Yamaha Marine dealership... and the prices are completely ridonkulous ...and I like Amsoil lol

 

[dmar836]

LOL. I guess I was in a bad mood yesterday morning cause I really came off badly - I must have been ready to rant. I'm drinking my first cup now and feel fine. Midlife crisis starting at 42 maybe?
Oh, sorry, I meant the sealant - maybe Yamabond. Great stuff and I see it recommended even for a lot of 4-stroke and auto applications.
For oil, I use Castor927 rather than Yamalube. I love the smell!!!

Dave
KC

 

[jonyoon21]

wow, i see that there has been alot of activity here since i last checked. Sorry for the delay in responding. More midterms have been kicking my ass. They never end...

dmar836, thanks for the advice. I'm not sure I completely understand everything you've said about the whole jet size thing though - I'll have to do more research.

I got the bike running again since I last posted. I checked out some of those bog problems posts - did what they suggested - float bowl and clean air filter and all - and just slapped the engine back together (after the rings and gaskets came in). It seems to ride ok, just vibrates a bit more than it used to... must not be a good sign. Maybe I could look into that whole balancing thing. I noticed there was an abrasion on one of the counterweights (it kinda dips in just a bit). The motor wasn't fourstroking anymore though.

I'm sure that you're right about the whole modding thing. I don't exactly know what the jet size thing is all about... I've seen it mentioned on the forums with people filling and redrilling, but I doubt I'd attempt that - that I know will make things worse.

The extra mods came out of need actually some want but mainly need - the stock muffler didn't fit no matter how I oriented it, so i put some (like 4) rubber gaskets in there on the studs and tightened it in position till it couldn't anymore - after running, i smelled like burnt rubber everytime and eventually the gaskets melted together, then crumbled away after break-in. I chunked that muffler and got the expansion chamber - why not upgrade while I was at it? The air filter came because of an accident and I broke the stock one when I fell off the bike at a corner of a fountain on campus - didn't see the short step... I was lucky that nothing else got hit, the carb seemed fine. The shift kit was mainly out of want since there are some hills around here but lots of flat land as well. I'll take it more slowly on the engine mods from now on though, my wallet is feeling it now... Though bike mods such as disc brakes and more gears shouldn't be a problem.

By torque wrench do you just mean a ratchet? Tis what I've been using. Also, there are abrasion marks on the pison and the cylinder. From the way you asked about them, it's probably not a good thing. Perhaps I should try to polish it?

Midlife crisis at 42 eh? I think my gf's dad had that happen too and he rushed out and got a 600cc motorcycle, lol. He keeps telling me to just get a cheap motorcycle since it'd be less work and faster, guess he doesn't understand the fun in working with these things - I wouldn't tear apart a motorcycle unless I had DEEP pockets.

I'm actually 19 and attending the University of Texas at Arlington. Funny thing is that I've been wanting a crotch rocket for a while but seeing as I live at home and my parents are against it, now look at what I have done (my dad wasn't too happy about it, he got over it though). Must be the whole teenage rebellion thing still winding down.

I've been running the engine on mixture of 4 oz per gallon (whatever that ratio is...), perhaps I should try a richer mixture. Is it ok to mix different mixtures of gas together? I still have little over half a tank left.

The motor itself isn't running at the moment though. When I put it back together, I didn't tighten the head nuts completely and had a huge loss of power, I then tightened them with a ratchet and ended up over tightening one and apparently stripped the thread and snapped the head gasket (lucky there was an extra included). So like a dumb**** again, I tried to fix it myself and epoxied (JB weld) the headstud back into the engine (I wanted to show it off at a party hosted by my friend for some odd reason - being a dumb college kid and all, that and some friends wanted to see it after I told them about it). The epoxy dried well and it rode just fine (save for the extra vibs). At the end of the night when I got home, I noticed that one of the headbolts was missing and the stud was loose again (yet I never noticed a loss in power or any air leaking unlike before when I didn't tighten it enough). Maybe some more epoxy... Maybe some more drying time... Or retapping the hole... Don't wanna go there.

So... if I make a list of issues... There is something wrong with the balancing of the crank, there are abrasions on the piston and cylinder, (fixed the issue with the kill switch not working - changed the plug), the headstud is loose, and the jet size needs to be matched up properly.

I'm also working on a better lighting system (I want to get an engine generator and a rechargable battery pack and hook that up to a lighting system, I burn though batteries pretty quickly - as I drive to work daytime and ride to night classes) and installing an analog tachometer too (so that I don't redline the engine again - got a small nos tachometer for a 1 cylinder 2 stroke engine that hooks up via the magneto). On next paycheck, Disc brakes will allow me to save my shoes (my V-brakes suck so bad that I've been braking the old-fashioned ghetto way - with my shoes) as I have a new front hub and a disc brake system lined up for installation. I just need to have it professionally installed ( I don't want to deal with respoking the wheel). Also an 8 speed cassette and new shifter (trigger shifter wasn't working on the same hand as the throttle, grip shift on left hand oughtta do it).

Thanks barelyawake for your concern as well. I'm actually enjoying these issues with the bike. Gives me something to do in my freetime AFTER studying... of course... My gf hates my bike, says it's my second lover. haha. I just hate the waiting time for parts.

 

[BarelyAWake]

Do you know of another motorized bike around? These engines all vibrate a LOT and without a comparison it's hard to tell if yer is worse than others or not. I always thought mine was a lil hinky till I got a chance to ride another - then I realized mine was actually a lot smoother than I thought lol

Torque wrenches come in two main styles, "bar" and "click" - both tell you the amount of pressure yer puttin' on a fastener. For example the headbolts on these motors usually require 10 - 12 ftlbs, this is such a small amount that you'll need be careful when shopping for a torque wrench that it'll read that low;

torque wrench - Google Product Search

JBweld is just not gonna hold a headbolt on for long. The easiest way would be to get a stud the next size up and tap the hole to take it. Another way is a Helicoil insert, in some ways this is better as you keep the original sized fastener, but it is a lil harder to do.

Honestly I would ignore the abrasions for now unless they're so huge they're causing a loss of compression.

Another solution would be to get just another motor, they usually run 60-80 bucks without the rest of the kit, I got one myself just for parts You've learned a LOT on this one and that's worth every penny