Hawk 250 bent intake valve
 

So I bought a Hawk 250 a few weeks ago. My son and I assembled it. I ordered a carb and jets that came before the bike. We got it started with the old carb but it ran like crap. So we changed the carb and after a couple hours tuning it was running pretty good. It would sputter every once in a while. I put maybe five miles on ot through testing. I hadn't changed the oil yet or adjusted the valves. I was going to go for a little ride and then it got a metallic scraping sond in the motor. I gave it a little gas and it stopped. I pushed the bike out of the garage and it died. No compression. The intake valve was not closing all the way. I pulled the head and it was bent. There was little dings on the head and piston top.

I bought a new head and installed it a few days ago. It fired right up and sounded good. I also changed the oil and adjusted the valves. Today I put the rest of it back together...plastics and seat. I went for a ride. It idled great sounded great and pulled well. Less that 10 miles it died. No compression. I haven't pulled it apart but I'm guessing the valve again.

Any thoughts?

Well, ruling out a bad valve at this point seems logical. If I had to guess, I would imagine somebody put the cam in with the timing marks one tooth off. You would think it would rear it's ugly head sooner, but it could be that it just barely makes contact and the repeated hammering eventually makes it fail. That would be my guess at least.

That's a strange series of events. I will assume you corrected the timing issue when you installed the new head. To have it run then lose compression again is just bizarre. The only thing I can think of is maybe the piston was cracked from the previous valve contact and it opened up worse with some more running. But I'll keep my fingers crossed that you find something easier, like your forgot to torque a head bolt.

Seeing as the cam is in the case of the engine, below the cylinder jug, it is highly unlikely any timing issue was corrected if one does exist.

The piston theory is a good one. Only way to know for sure is the pull the head and see what is there, or isn't.

Oh my that's a great foot in mouth moment. Obviously my hawk hasn't arrived yet. I just assumed these motors were overhead cam!

Nope, they are OHV and a single cam lobe to boot. As simple and rugged as it gets. Me, personally, I wish it was an OHC motor, but the CG engines have a charm to them too.

Yeah so I'm starting to understand the whole "very simple" thing everyone keeps mentioning. But back to the OP's bike, I agree with the idea the timing is off a tooth. I definitely wouldn't throw any more parts at it without having a look there first. I hope the piston is ok!

I think it would have a hard time running anything close to smooth with one tooth timing off.
My question to folks who know these motors: Can the valves be so tight that they can be made to have contact with the piston? If so and it came from the factory with the tappets tightened all the way down, could that have bent the valve? Once the piston was maybe a crack developed loosing compression.

Does it have any compression when cold? Possible he adjusted the valves on the wrong TDC stroke?

(just thinking out loud)

Thanks for all the responses. I have been busy this week and have not had a chance to pull the bike apart. The piston looked ok before I put it together. It did have little dings on it. I thought something got in the intake. So I did not look at the timing. It was a little loud/noisy at the motor after I changed the head. But it was idling great. Could it be off a tooth and run smooth? I will check when I get into it this weekend. Anyone have a link to the timing setup?

I set the TDC to the next up after the intake opened. I think that is correct. I did roll it over after tearing it apart and seeing the little dings. I wanted to check the side walls. I went in two up increments. But there is a chance I got off??? I can't remember if I checked after I installed the head. But I am pretty sure I would have used the kick start to roll it over making sure it was all working and set before I did the final adjustments.

Ok, finally got back to the Hawk. The exhaust valve retaining ring came off. The exhaust valve is staying open. Not sure why this would happen?

The keepers for the valve spring retainer came off?

Post some pics so we know what you're talking about!

Do you check the flywheel? I have seen videos that say line it up with the F and I have seen videos that say line it up with the T.

I personally lined mine up with the T, thought it meant TDC and F meant Fire... Hope I was correct.

That is exactly what the T and F marks mean.