QT50 compression problem?

Thomas Malcolm /

Well, I got my little QT50 all cleaned up and ready to go, put a new air filter on it, and it'll crank and run like a champ...but...only if I keep my hand over the carb opening. Won't run like anything otherwise.

My first suspect was a clogged exhaust system, but once I pulled it off, it looked OK. Obligatory crud, but nothing obstructed.

Next stop: cylinder head. So I pull it off, and the inside of the cylinder has a couple of minor-looking scratches running the length of the inside cylinder wall on the bottom.

Its not terribly worn, mind you, but there are fine scratches in there.

Are a few relatively benign-looking abrasions inside the little 50cc's cylinder enough to compromise its compression? Also, if I have this thing machined, where in the world do you find over-sized piston rings for a 23 year old 2-stroke motor?

Re: QT50 compression problem?

Jake Dachel /

get a pw50 piston, they have them at www.pro-flo.com the biggest one they have will make your qt50 54cc's they also have stock sizes and sizes in between

Re: QT50 compression problem?

your hand is restricting air intake. reducing air intake makes the fuel/air mix richer. The motor needs a richer mixture than it is getting.

My guess is that something is preventing fuel from being delivered to the engine. Check your entire fuel system from the tank cap vent to the filters and lines and the carb.. all the way to the intake manifold on the engine block.. check for fuel and/or air leaks. Definately clean the carb and adjust the float..

Minor scratches on a piston or cylinder wall mean little.. and mean nothing as far as your problem goes..

Re: QT50 compression problem?

Thomas Malcolm /

Thanks. That gives me hope that I can get this little thing running again.

Re: QT50 compression problem?

the qt50 carb has a fuel/ air mixture screw on the side pointing away from the bike....mess around with that

i believe it is

-out = richer

-in = leaner

im not sure though


Re: QT50 compression problem?

Thomas Malcolm /

Awww, it was something stupid. I tightened the spark plug an eighth of a turn and it started.

Now that I got it running, it has no torque, so it won't pull me up my driveway, and bogs when I open it up.

I'm not sure, but I may have put the exhaust back on incorrectly.

I don't suppose anybody knows how to get the cylinder block off the transmission housing on one of these things? I kinda snapped off one of the engine studs, and I can't get near it with the screw extractor to get it out and replace it.

Re: QT50 compression problem?

someone or other in here pretty much knows how to do anything and everything.. but me, i can't figure out what you're asking.

Remove the cylinder by removing the head bolts. You snapped one off (is that what you mean by an "engine stud"?) so take the other three off. The cylinder will come off.

If you still don't have room to work on that broken stud, then i guess you have to take the engine out.

Re: QT50 compression problem?

Thomas Malcolm /

Yeah, Yamaha's parts catalog calls those threaded rods that run through the engine somthing like "engine stud bolt" or "engine stud screw" or something like that. I've already priced replacing all four. They're the things holding front two sections of the engine onto the big gray thing housing the transmission and crank and they poke through the aluminum head. Well one of those snapped off right at the back, behind the cast iron portion of the engine, probably flush with the surface of the large gray housing.

I probably I can't get to it any other way than removing the cast iron section. Now, on mine, that iron block is pretty nappy looking, and it won't just slide off the bolts.

I'm trying to figure out if I need to do anything special, like try to remove all the studs or drain the tranny and remove the piston, shake a chicken foot at it while dancing around it in a circle, etc., or if I can just spray WD-40 on it, smack it just right, and pull it off.

Re: QT50 compression problem?

ok.. the 'block' contains the crankshaft and transmission.. The 'cylinder' is where the piston goes up and down and the 'head' is the top.

Be careful about knocking things with a hammer. I have seen (i forget where) a cylinder that had one additional, short bolt near it's base that must be removed before the cylinder will come off. The piston can remain attached to the connecting rod.

I get the notion that you will have a few new problems further down this road unless you get some guidance. (Do you know how to compress the piston rings so you can get the cylinder back on without snappping a ring?)

Get a service manual or an owner's manual. You may find a generic small displacement engine manual that covers the QT50 at the Yamaha store.. Whatever you get will be well worth it in money saved and headaches avoided.

well.. since you are breaking head bolts i'll assume there is some corrosion down there on the block, and chances are that your cylinder/block gasket is just stuck.. Thats why it won't slide right off. A bit of gentle prying with a screwdriver should free it up.

While you are out shopping for a manual, maybe go get a torque wrench too.. A small inch-pounds wrench will do for these little bikes.

Re: QT50 compression problem?

Thomas Malcolm /

I'm more of a car guy, and you don't see too many integral bottom-end/transmission units in cars. The set up isn't terribly different, but I do know what it all does, thank you.

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