Ran great.... now no compression?

[acidburn02zts]

Hey alls..   New to the boards.

A buddy of mine dropped his GY6 based 50cc scooter by the shop a month or so ago because it had been sitting for quite some time with a broken belt.  The local scooter shop was charging him 85/hr to look at it..  he took it down there and they broke off an exhaust bolt in the head... then wanted to charge him to fix it.  Long story short, I told him to drop it off and I'd look at it when I had time.

Anyways, I changed the belt for him... charged the battery and it fired right up.  Removed the broken bolt from the head... bolted the exhaust back up and test rode.  Ran good so I sent him down the road.  He ran it to and from work for a week or so and I get a call.  Says the scooter has no power...  feels like the "transmission wont shift".  He isn't mechanically inclined at all...    told him to swing by.

Belt was fine..  cvt was fine.  Scooter was a dog.. wouldn't pull itself.  Found that the carb was plugged up. I'm assuming that the fuel tank had some junk in the bottom from sitting and after running a full tank through it, it plugged things up.

Now supposedly, this scooter has a big bore kit.  No idea what size.. but can tell the cylinder and head have been off of it as there is RTV smeared all around the base of the cylinder and such.  Whom ever did it.. did a very sloppy job of putting it together.  Considering the bore upsize, we decided to upsize the carb a little.  Ordered a carb for it that came with a filter.  Scooter started fairly easy...  power was definitely up.  No flat spots..  no stumbles..  plug burning clean.  Sent him down the road.

He rode it back and forth to work for a few weeks without issue.  The other day, he shuts it off at work and after work, it wont start.  He trailered it back to my shop and started looking things over.  Wont hit on starting fluid...   pulled the plug and it's getting spark.  Noticed that when cranking, it was spinning over much faster then I remember.  With the plug out, I can hold my finger over the plug hole and compression wont push it off...   just sorta changes the exhaust note.

Now considering this thing should have ~150+ psi of compression...   I'd say its got a problem.  He says it was running great... no issues.  Just shut it off and nothing.

I haven't pulled the valve cover off yet..  but I'm thinking it is either valve or head gasket related.

Thoughts?

[scooterpimp]

Check the valves.

[ratdog]

Broken record time... shoot a pic of the plug that came out of it.

My first look would be the valve lash, but frankly I can’t think of how that would be OK driving to work, then suddenly very bad.

I’m thinking like some engine damage. If he is running a BBK and putting what ever is cheapest gas in it, and you never checked the plug to see if the mixture was right, it is very possible there is big enough damage to the piston and or head that she no longer has compression. Of course, he could have also eaten the rigs so bad that there is no compression left.

I would drain the oil and take a real good look at the drain oil. If there is engine damage, you will most likely see tiny flakes of silver in the oil. Metal flake oil is never a good thing. I really think you are going to have to pull the head. Once you have the head off, turn it upside down and fill the chamber with gas. See if any leaks out. I would pay special attention to the exaust valve.

As long as we have the head off, might as well pull the jug so you can clean up around the base and scrape it clean to install a proper base gasket.

Of course, this will let you get a real good look at 5e bore, piston, and rings. I would not be surprised to find some melted alum, but I always think the very worst when dealing with other people’s work

[acidburn02zts]

By "plug burning clean"... I meant that the plug was a light tan... not black or white meaning the mix was right. According to the owner, he only runs 93.

I'll be pulling it apart this week. I'm almost certain at this point that its an exhaust valve issue as it is blowing out the exhaust every time the piston comes up to TDC.. whether it be on compression or exhaust stroke.

Either the exhaust valve lash is out of spec.. or it burnt the valve.

Either way, I'm strongly considering a full tear down... as like you, I don't typically trust other people's work and based on how it was assembled and the little oil leaks it has... it will probably be worth it in the long run.

[GrumpyUnk]

Before taking everything apart, it would be relatively easy to remove the cam/rocker cover to observe. With the cover off, use a 12/14mm six point socket on the blower hub. Rotate slowly clockwise. Watch the dcam followers. Watch the valve springs. You should see them moving as the cam lobes push on the rockers, and the rockers push on the valve stem tip.
I suspect you may find that the exhaust valve is either loose(keepers off) or stuck.

If the former, you may be able to pull the valve up, apply pressure (air) through the plug hole, or sutff some rope in and bring the piston up to hold the valve. Then CAREFULLY install the shim, springs, washer & keepers. It can be done w/o removing the head. Adjust the valves as desired. Who know, it may work and save a lot of time and some gaskets. Inspect the keeper stuff for weirdness. It may have been installed goofy, not seated all the way, by the previous 'mechanic'.
tom

[acidburn02zts]

Absolutely... that is the plan at the moment.

I'm somewhat leaning towards a sticking valve. I'll pull the cover hopefully today or tomorrow.

[ratdog]

It’s amazing how different people think different ways. IMHO, if it’s anything other then the black ave lash being to tight, th en the head should come off. First, it will most likely take less time to fix it and second, if that valve has been hanging open, we really need to see what damage has been done inside the engine. Maybe I’m the odd one here, but once you have the shrouds off, it only takes me about ;10 min to have the head off and back on. Of course that doesn’t include what you do while the head is off. With the head off, I can have a valve out and back in in a good deal less then 10 min. I’m willing to bet you would spend a lot more time trying to compress the valve and get the keepers in correctly with the head still on the bike, and you really need to see what damage is in there. At very least, you most likely need to lap the valve, and I would pretty much expect that the valve stem is bent.

So if we fart around trying to “fix” the dropped valve with out pulling the head, then put it back together to find it’s low on compression, we still have to pull the head and all that time was wasted. Maybe it’s a by product of having to work on multiple engines in one day, but I have found time and time again that doing it the right way will be the most time efficient in the long run

[GrumpyUnk]

When I posted,I had this mental picture of a valve that had partially come out of place. As in one of the 'keepers' was out of place, preventing the valve from closing all the way. If the valve has indeed dropped, then of course it makes perfect sense to pull the head to examine for damage.
In short, look at the situation, and make a call based on what is found. So far, I have no evidence that the cam cover has even been removed, much less an inspection.
tom

[ratdog]

Tom, not ment to debate you. Just trying to point out different views. It’s like all the posts where they are screwing arond with different belt sizes. For most of my life, I helped design stuff like this. Now if , say GM had decided to make a little “peoples bike” I would have my team billing thousands of hours the make a CVT that worked perfectly with a 669-18-30 belt. We would have considered all sorts of wonderful things like side loads, bearing loads, belt wear,and min and max gear ratios,

Then someone posts on a the internet that you can use a x belt and the system
M will be way improved. And then we see posts about how someone is having problems.

Oh well, I’m just a crazy old man that went to college IN THE 60s

[GrumpyUnk]

Ratdog:Oh well, I’m just a crazy old man that went to college IN THE 60s

Yeah me too. Out in '70, into the South China sea.

tom