New guy with a non-runner
 

So, I just picked up a little dirt bike, free, since it doesn't run. And from what I can tell, this is the best place to turn for advice on them!

I snapped a few pics, just to show what I'm workin with...

http://img.photobucket.com/albums/v3...019_120451.jpg

http://img.photobucket.com/albums/v3...019_120532.jpg

http://img.photobucket.com/albums/v3...019_120731.jpg

http://img.photobucket.com/albums/v3...019_120750.jpg

From what I've researched, I guess it's a 90cc 4 stroke. But I really don't know much about them at all. Most of my experience is with cars, so I've got a steep learning curve ahead of me to get this thing running.

From initial checks, it seems to be getting fuel and spark, but won't actually fire. Any standard, more in depth checks I can do to trace down the issue? Talk to me like a child and I'll figure it out! :D

Oh yeah, and abuse/neglect may be an issue with it as well. It's got quite a few cracks and bends. Pretty sure it just got beat on when the kid got a newer, bigger bike.

Thanks for any help!

Ian

Welcome Ian!

That bike will need a bit of massaging to optimize it, but it'll be worth it. :D It's a Honda-cloned 90cc engine in a Yamaha PW80-cloned frame. It's a good mix, except that the Yamaha was designed to have a forward facing air cleaner, and this engine has the air cleaner facing the rear.

My son had a very similar bike, and I bought a 45 degree intake tube and modified the frame to clear it. I then used a pod filter, and it was able to breathe better and not pack the airbox with mud from the back tire. This tube is like the one I used: http://www.ebay.com/itm/24mm-CARBURE...da047d&vxp=mtr You could also just try turning the intake tube around and see if you can fit a small filter on it and clear the front fender.

The exhaust is a major restriction. It's a close copy of the Yamaha exhaust, but the Yamaha is a two stroke; the pipe is totally wrong for a four stroke. Remove the right side panel and you'll see what I mean. BTW, please show us pics of the right side. This exhaust would do the trick: http://www.ebay.com/itm/Exhaust-Pipe...4002a7&vxp=mtr

There are four bolt holes under the engine that are tapped, and they'll mount a complete footpeg / kickstand mount like this:
http://www.ebay.com/itm/PIT-DIRT-BIK...item20cb056431 You can cut all of that low clearance junk off.

You can replace that awful shifter with a spring-loaded piece like this:
http://www.ebay.com/itm/Motocross-Go...1082ab&vxp=mtr

The bike won't run because of dirt in the jets. Remove the carb, remove the float bowl and unscrew the slow jet and main jet. Blast 'em clean with aerosol carb cleaner from Walmart.

Have fun! Parts are cheap as chips, and these bikes really boogie once they're set up.

Awesome, thanks for all that! I'll see if I can get the thing running this weekend, and if it all works out, I'll see about breaking the bank on all those high dollar parts you pointed out hahaha.

It could use the pegs and shifter anyways, as they're all mangled or missing right now.

And as for the right side pic, here ya go!

http://i104.photobucket.com/albums/m...0-19232829.jpg

You will probably want to adjust the valves too.

That's exactly like the one we had. See how the exhaust pipe comes out of the head and then transitions into an expansion chamber? From there, it necks down to a small pipe and then feeds the muffler. That design is just like the Yamaha two-stroke.

I agree with Kato; the valves will need some love.

Start here:

http://www.dansmc.com/troubleshooting2.htm

Spark, mixture, compression.

That's the next step.

Remove the spark plug and put your finger over the hole. Push as hard as you can while kicking/cranking the motor. If it can't smartly blow your finger off the hole you have a compression problem.

If that's the case it may be as simple as a tight valve so check those regardless. If that's not the case it may need cylinder/piston or valve work.

IIRC new motors for these are like $200 so no way you can lose money on this deal. :D

Well, I dug into it a little bit yesterday. Gave the carb a good cleaning, though it was pretty spotless inside. I started into the valves, but my feeler gauges have gone missing, so I won't be able to properly check them until I grab a new set. Though there was no visible slop in them. The air filter/box was pretty caked with dirt, but still no fire without it on.

Compression was the 3rd thing on my list to check as well. Though I didn't have the correct adapter for my tester to fit the tiny plug hole. I've been trying to track one down, but got put off for the weekend. I'll see if I can order one today.

These things are definitely a fun way to learn how to work on small engines though! Basically the same as "brand name", but parts are pennies on the dollar :P


Oh, a quick add on. The last rider (a 15 yr old), said it was blowing a lot of smoke before it died. His buddy was actually riding it at the time. Knowing him, I almost suspect that trying to kill it was a fun game, after he got his new bike. Hearing that, kinda leads me more towards piston/rings. But I'm having a bit of fun with this, and it'll be awhile before my son is big enough for it (he's 5), so I'll continue my education and troubleshoot until I find out exactly what's up! Thanks for all the help thus far!!

My favourite aspect of these bikes is the ridiculously low parts prices. :D To expand your search criteria on eBay, you gotta think like a person who doesn't speak English well, and who might not understand bikes. Search for items like DIRT BIKE MUFFLER, for example. Sometimes I laugh out loud.

I expect that your valves are too tight, and setting them will likely solve the problem. It'll be interesting to hear your observations about how it runs with the stock pipe and airbox, vs any mods that you make. Please let us know.

I'll be sure to keep you up to date on anything I do with it. And yes, I have noticed some of the adventurous English used in some of the Chinese auctions haha. Certainly makes shopping fun! :lol:

btw, I edited my last post. I did wonder about the valves being too tight, as just for fun, I tried to slide some tin foil between them, and couldn't. But at the same time, I have a hard time believing anyone ever tried to maintain this thing well enough to screw it up!!! hahaha

Though there was no visible slop in them.

Too tight.

That may explain the non-running due to lack of compression.

Not the smoke though. You may be looking at rings and a cylinder honing.

Might be another $30! :lol:

LOL!

Hahaha, awesome! It's all good experience. As long as I get it running in the end, I'll be happy.

30 bucks.... hahahaha

Here's a twist: if you need a cylinder and piston, you can buy a 110 bore. The 110 shares a crank with the 90.

Well, no luck with the valve adjustment. The intake was locked right down tight. The exhaust had some clearance, but very very little. I wasn't sure where to set them, so went with .003".
I guess the last thing on the list is that compression check. Just waiting for a phone call from acklands grainger for when that 10mm/14mm adapter comes in

That's a safe guess.

To be maybe more percise I'd go by what Honda uses in their little 70 engines this one was cloned from.

It does not appear that was the problem though.

I tried to base my adjustment on the Honda, but found a lot of conflicting info online. Anywhere from .002 to .006. So I figured .003 would be on the tighter side of safe, just for troubleshooting.
I'm hoping that adapter will be in tomorrow, so I can check compression with and without a squirt of oil to check the rings

I think you are on the right track. Sounds like it is ready for a top end.

Quite likely. BTW, the top end wear could have been accelerated by a plugged or kinked vent hose. IIRC, the vent hose on that model is stuffed between the swingarm and the frame.

Hm, it must be well hidden, or I must be blind, because I can't seem to find any vent at all on it...

Oh, and out of curiosity. Is there any recommended source for parts, or is ebay the best bet? Since it's starting to look like I might be needing some gaskets and stuff. And hey, if bumping up to 110cc is actually that cheap, can I afford NOT to? :P

Here's one: http://www.ebay.ca/itm/110cc-ATV-Top...item1c241a8894 I'd use eBay.

Check on the left side of the engine, just behind the top motor mount bolt. Should be a black hose there that routes behind the engine.

Awesome! If you didn't tell me exactly where to look, I wouldn't have seen it. So covered in grime that it was basically hidden! I think this thing needs a good wash before I pull anything else off it hahaha.

Just as an idea, what should the compression be? I'm STILL waiting on that adapter, but considering it was supposed to be in yesterday, it shouldn't be too much longer. I hope!

If it tries to blow your thumb off of the spark plug hole, it has compression. :lol:

Seriously, I don't have a clue, but I'd guess around 140psi. If you intend to install a new cylinder and piston, it'll be optimized. That said, I'd be sure to hone the new cylinder and clean it thoroughly before installing it.

Did you find that the vent hose was trapped between the engine and swingarm? :?:

The vent hose appeared to be free and in good shape, other than being coated with mud and grease.

I did some checking online and found that while compression should be in the range you mentioned, it would be impossible to get a good reading with a 4 stroke.
I do have a leakdown tester in the toolbox as well though, so it might be a better indication of what's going on. Still need that pesky little adapter though... hahaha

Leakdown against a standardized 80 PSI is how we check airplane engines at annual inspection.

What's the issue with a regular compression test?

Check out this short article:

http://www.dansmc.com/compression_test.htm

I saw a few theories against it.
One being that the combustion chamber on these little bikes is so small that the hose on the tester can actually have a larger volume of air in it and absorb alot of the pressure, leading to a low reading.
Another said that most 4 strokes have auto decompression to make them easier to kick over. So it would vent alot of the pressure out.
And ANOTHER person said that it would be virtually impossible to get a good reading with a kickstart as you can't kick it all the way through a full compression stroke.

I'm not sure if any of these apply or don't apply here, but it seemed like an awful lot of reasons for a low reading

Thumb test always worked for me. I don't think that these bikes have a compression release and I would just kick it a few times in a row.

Good points.

One has to disable the compression release if the bike has one that's automatic.

I can see how the hose volume would matter.

Thanks for the thoughts. :idea:

Yeah, I have no idea if any of them are valid or not haha. I'll try to check compression either way and see what it says. Check leak down as well, since it makes it easy to pinpoint an issue before tearing it down, which I'm sure it'll probably need at this point

Relic, you sound very experienced with such things. There is no decompressor on this motor, and I agree with Kato on kicking it several times. If the gauge has a check valve in it, it will record the highest pressure achieved. Nevertheless, you'll get a sense of how it's doing.

I wonder if that vent hose is at least partially clogged with goo. On another note, these engines aren't equipped with an oil screen, so oil must be changed regularly.

I've got a fair bit of experience with cars, but none whatsoever with bikes.
i will give it a shot with the compression tester in the morning, since my fitting finally arrived! I'll be interested to see what #'s I come up with, and compare it to the leak down. Might be good for reference for someone else one day :)

I'll check the vent hose more closely too once I get the motor cleaned up a bit. Thanks for all the help so far!

Compression test is 50psi on my "good" tester. About 40psi on my cheap one

That there's your problem. :cry:

Time to shell out $30 for parts. :lol:

Leakdown gives me 90% leakage. I could hear it blowing into the crankcase, so went to unscrew the cap to verify, and it blew it into my hand when it got to the last thread. Me thinks it's got some piston/ring damage! hahaha

$45 later you're good to go! :P

I'd probably inspect and lap the valves while it's apart.

I'd also be very careful to make sure the air filter is clean and effective to help prevent the new unit from wearing prematurely.

That second ring looks a little stuck. :lol:

For a few bucks more, you can buy a complete head, assembled. You can also get bare castings, but why would you?

Now that's a diagnosis if there ever was one. :wink:

Haha yeah the problem is pretty evident now, eh? The piston skirts and cylinder wall are pretty scored up too. The valves and piston are both just caked in carbon as well. What's a complete head go for? Are they reliable, or do they need a rebuild, right outta the box? Haha
i think I'll order that 110cc cylinder and piston kit unless there's a cheaper way to get it complete with head?

Dunno which way is cheaper, but here's a head: http://www.ebay.ca/itm/110cc-Complet...item231f36ab97 Knowing that you get a new cam with it is a good thing. How's your cam?

Shipping is a killer from that vendor. $40+!

I hadn't thought of that, since I go for the free US shipping and cross the border to get it.