Planet Minis

theburkeman

I just restored a 1970 Honda QA50 that had been lying around in my garage for about 20 years. I replaced the rings because it had no compression. It was running excellent, and was even starting using the kick starter. Then all of the sudden it just stopped running. It lost all of it's compression again. I tore the engine back down and checked everything, cause we thought it might be a valve. Everything seemed fine so we adjusted the valves. When it was all back together the compression was back, but it was blowing compression out the intake. It would try to start running, but would not keep going. Can anyone give me some help?

Here's a video of it running, it's got a back fender now, sorry i dont know how to embed videos on here, it's different than rx8club
http://www.youtube.com/watch?v=sVQCu2_EOjo

mexicanyella

If you're sure you got the valve clearance right and it didn't creep on you when you tightened it down (I've never worked on a QA before, just so you know) the intake valve looked okay, as far as the sealing surface and seat goes, maybe it's sticking open a little. You might remove the intake manifold and take a look in the port and see if there's a bunch of crud on the valve stem...or maybe you dropped a 7mm socket in there or something.

But was it definitely wheezing compression back through the intake, or could it have been mild backfiring through the intake? Don't mean to suggest you don't know your own bike, but I've had mild intake backfires from dirty or misadjusted points ignition, and sometimes it feels sort of weird through the kickstarter, which some people might interpret as a compression problem.

theburkeman

Well I i know it's not backfires through the intake, because it's a compression leak every time. As far as the valve clearance goes, i really don't know either. I can't find the valve gaps anywhere, we kind of just winged it. So i really just dont know, is there anymore info i could give you to help you help me?

mexicanyella

What did you set the valve clearance at? If you set the valves at TDC on the compression stroke (both valves closed and valvetrain "slack") I can't see your valve adjustment causing the intake valve to leak compression. That would only happen if the valve wasn't set with any clearance, or the clearance was so small that when the motor got hot, things expanded and closed the clearance up. But that would only be while it was hot.

As a guess, I'd say you want at least .002", and maybe the OHV ones run a larger clearance, like .005" or .006" or something. Hard to say without a manual, but it should still run when set a few thousandths over like that even if it was supposed to be .002"...it might make more valve noise and eventually you might notice accelerated valve wear though.

I don't know how the QA's cam is driven, or how the valve timing is set. Maaaaybe the valve timing is jumping around from a worn timing chain or something, if they even have a timing chain. But before tearing into that I'd make sure that you have noticeable valve clearance at TDC on the compression stroke, note how much of it there is and get it adjusted down to a few thousandths if it's not already, and try to verify if the intake valve is sticking or not. As I said before, you might be able to tell by looking down the port, without having the head off.

Of course a bent intake valve could cause a compression leak, but you didn't say anything about "crunch" or "shrapnel"...

theburkeman

I like how you said something about the timing being off, cause that really seems to be the only thing that could be the problem now. There definitely aren't any bent valves. I'm really starting to lean toward the timing being off. I've tried all the other options, so i guess diving in there and checking the timing chain is the next thing.

mexicanyella

Well, I hope I haven't sent you on a wild goose chase. As I said, I've never worked on a QA. I've never actually been closer than about 50 feet from a running one. Sure, it's a two-valve pushrod single; how complicated could it be, but if you're committed to the bike I think it would be a good idea to haunt ebay or something in search of a manual.

You're definitely hearing compression blowing from the intake? Not just the intake suction noise, which is largely unsilenced (maybe even amplified) by the stock minimal air filter?

theburkeman

We took the intake manifold off before the carb and everything and plug the hole with our thumb, and there is definitely compression blowing out of it. You haven't sent me on a wild goose chase, most of the stuff you've said we've already checked. That's why I came here, it's like a mystery as to what happened. Why does a perfectly running engine just stop for no reason after a rebuild?

Resto

I have the Honda QA50 factory shop manual and will be happy to help. The manual is very short on information. I will list what I know further down in this reply.

If you have questions or need to talk through this - PM me - I will send you my phone number. Feel free to call me if you need me to look up anything for you.

Valve tappet clearance should be 0.002 in. (0.05 mm) for both intake and exhaust - with a cold engine.

As 'yella said, this is a pushrod engine.

There is a cam chain that runs from a sprocket on the crankshaft to a sprocket on the cam shaft. The cams on this camshaft actuate the pushrods that operate the valves. Since you state that the valves are OK, it could be possible that you have an issue with the cam chain, the sprocket on the crankshaft, the sprocket on the camshaft or the pushrods. It is also possible that the camshaft is worn or bent.

Here is the procedure to check camshaft and set the valve timing:

- Remove cylinder head, cylinder and push rods
- Remove right crankcase cover
- Remove clutch assembly (flatten lock washer - remove 14 mm lock nut)
- Remove 6 mm circlip and take off the change pawl shaft, spring and washer
- Remove 17 mm circlip to take off primary driven gear
- Unscrew 6 mm screw to remove the camshaft set plate
- Remove camshaft and camshaft sprocket
- Inspect camshaft for wear or damage
- Measure cam height: Intake = 21.66 mm (0.874 in) Exhaust = 21.40 mm (0.843 in)

- Reinstall camshaft and camshaft sprocket
- Turn crankshaft so that piston is at top-dead-center
- Align "O" mark on camshaft sprocket with the punch mark on the timing sprocket

- Reassemble all above-mentioned parts in the reverse order of disassembly
- Check valve tappet clearance. If necessary, adjust both intake and exhaust to 0.002"

Hope this helps !

Resto

theburkeman

Thank you so much Resto, ill be sure to do this ASAP. I'm having a hard time gettin to it right now though cause finals are comin up in school. I'll be sure to update this as soon as i get some stuff done with it.

And for entertainment purposes here's some pictures of the bike. If yall have any questions about it i could help out too.

cdoublejj

i have an lt50 i read on guy couldn't get his to run and it turned out to be a bad base gasket at the bottom of the jug.

mexicanyella

Cool QA! I wanted one of those so bad when I was a kid...looks like that one's all there, too. Get that thing running and ride it! I know a guy who has two of them and they just sit on a shelf and never get started. Me pestering him to let me ride one has not been too rewarding. In fact, I haven't heard from him in a while despite leaving messages for him...

Re: the LT50...isn't that a two-stroke? A base gasket leak in a two-stroke causes an air leak/lean condition. In a four-stroke like the QA it would just be an oil leak.

theburkeman

Yeah i love this little bike, and I restored it so I could ride it. I'm not a big fan of putting all kind of work into something and just putting it on a shelf. Surprisingly all the parts are there, including the rest of the exhaust which I just haven't put on yet. From what I hear it's pretty hard to find these bikes in this kind of condition.