Dropping valves

[isaac]

So this hasn't happened to me but sounds VERY impending based on head noise that's ratcheted up in the last few weeks. I've stopped riding the bike because of it. It seems to be an epidemic with these bikes considering what I've been seeing.

The evidence:

1) A few dropped valve stories on this site, even cases of the same bike doing it a second time later after a total rebuild.

2) A quick search of craigslist or ebay shows quite a few bikes that "need a piston and head" or "blown engine"

3) You just plain can't find a used cylinder head or piston for these things, but you can find used blocks, cranks, connecting rods and everything else all the time.

So what's doing it? I'm already midway into a teardown on my bike to find out. So far I'm not finding any parts that are loose and shaky like the ominous, growing rattling sound I was hearing would indicate, though at 12,500 miles parts of the exhaust cam lobe and cam journals show wear, but not exactly severe. Nothing feels loose to indicate serious wear up top. I do know this bike has had some issues where the head makes noise until the first time I ride it above 5,000rpm or so to bring oil pressure up enough to seemingly feed the head better, and that problem is getting worse.

That said, the bike doesn't burn oil, which would be indicative of serious valve guide wear or failure. It doesn't have leakdown issues from the valve guides when it's sitting, so the valve guides must be getting oil. That said, it'd have to be some serious issue to wear guides in only 12,500 miles.

So what's causing this? Has anybody really looked into it? I've heard of other engines having issues where the valve would literally stretch the stem until it just snapped at some point, sending the valve down into the cylinder and causing a total write off. But I haven't seen any discussion of it here, aside from the occasional story where somebody had it happen, everybody says "that sucks", and they either junk the bike or buy a new head and piston.

Any ideas?

[raul10141964]

the GZ come from factory tune to pas emissions, and the only way to do that with thees carburetor is set it to run lean.
a lean running engine will run hater that normal putting a lot of stress on the exhaust valves

[ovilc88]

All I know is that when it happened to me (dropped a valve). The stem broke at half point and fell in...u know the rest of the story. I feel the valve spacing gets thrown off to easily to quickly. Valve clearance can make a world of difference. Unless you decide to go for Ferrera valves made to specs for GZ250 that would be more durable, but way over budget $$$. Wouldn't really be worth it to get them. I don't really know what causes the breakage, the flaw may be more than one that contributes to this downfall of GZ250 engines, the Achilles's heal so to speak.

[isaac]

Good info so far.

Well today I finished removing the head, and everything in there looks fine, which is baffling. There is a little bit of wiggle to the one exhaust valve I've pulled in the guide, but very negligible. The stem and keepers and spring seem fine, but who knows. There's a ton of carbon on the piston at 12,500 miles, which makes me think I'm running safely rich. The cam has light wear on the journals and more wear on the exhaust lobe, but none on the intake. I may have been running these valves sliiightly tight but not by much. Like I said, this thing seemed to have a problem getting the oil pressure into the head until after I got it above ~5,000rpm the first time while riding it.

Two very odd things that make me wonder about this thing's build quality: There are 6 nuts holding the head down - 1 in each corner and one in the front center under the fins and one in the back center under the fins. The one in front was just plain missing, and the front right corner was less than finger tight. The rest came out normally. The head gasket shows no sign of why this thing was leaking oil from there, which it clearly was because the engine's pretty dark on the left front and left side.

[isaac]

Problem found: So I was looking over the head again today trying to figure out what the deal was with the rattling at certain rpm and throttle positions that had made me take this thing apart in the first place, when I figured out what it was: One of the exhaust valve seats had come completely loose and had been jingling when the exhaust valve was open. Had it continued doing this, it likely would have eventually cocked sideways in the bore and bent the valve then maybe broken the head off. at that point both the valve head and the seat would fall into the combustion chamber and carnage would occur.

This makes me wonder if guys who've had a valve allegedly fall into the engine also had valve seat damage. That's something only the guys who've had a ruined engine can testify on. So did you have seats down in the engine too? It makes me wonder if this is an issue that's pervasive but the damage is always so massive that you can't tell what fell apart first and caused what damage.

If this really is a systemic failure of valve seats dropping into the engine, then this could happen to anybody. Mine I just heard coming because of a rattling/jingling noise in the engine under varying load that had progressively gotten more noticeable, so I tore it down before it was too late. I hope I don't have to have this thing machined for oversize valve seats or something because there's almost no room for it.

[isaac]

Does anybody know where to get valve seats for these things? I've checked parts fiches on multiple sites and nowhere in the cylinder head or cam/valves pages can you find a valve seat. The closest it gets is a valve spring seat, which of course isn't the right part.

It makes it seem like I'm going to have to buy a whole head just to get valve seats, which clearly are removable from the head. If it really is gonna have to come down to that it'd probably cost me less money to swap in an engine that doesn't drop valves whenever it feels like it. My only other recourse for rebuilding this engine would be going to a machine shop, having them turn custom oversize valve seats for me, then installing them. At that point I'd take a guess and say the price would exceed the cost of a junkyard motor swap, and I can say it definitely won't be the stock engine because I'm not putting all that time into a swap only to have it drop a valve the first time I ride it.

I've already looked into swapping a DR350 engine which seems like it'd be pretty straightforward, the only problem being that they seem even more notorious for dropping valves than the GZ250, if such a thing is possible.

[jonathan180iq]

Seems like you and The Goose are going through the exact same thing right now, you guys should share stories and see what you end up coming up with.

viewtopic.php?f=7&t=6665

[Goose51683]

Hey Isaac. Check out my post above. Right now I have a torn apart engine in my basement/shop. piston is hosed, I pulled the valve out of the top of it and I can see straight thru. I'm gonna guess that makes it hard to build compression. Ha

I was able to find valves, a piston head, and the entire cyclinder from a chinese site. I think it's called aliexpress.com Either way it's for a GN250 but I was reassured on here that it is the same engine. Unfortunately, it's like a 2 week wait for the parts so I'm down for awhile.

[isaac]

Hey guys, it's good to know others are working on this problem. That definitely sucks about your issue, Goose. It's clearly a valve drop. It's just a question of whether the seat let go first.

I'm very interested in how those parts turn out that you get from China. Check the valve seats to be sure they're secure. I'm wondering if maybe we can make these engines a lot more durable just by proactively installing oversize valve seats so that it helps hold tighter than stock.

That said, I'm also shopping around for other engines to maybe finally blow through the legends of guys who've made these bikes faster. I've already taken a look at a CB350 twin engine that seems to fit shockingly well. With every engine I've based suitability of fitment off of one metric: Where the transmission output sprocket sits compared to the engine centerline. In this case the CB350 output sprocket sits only 1/4" off from the GZ250. The bizarre thing is it actually sits 1/4" inboard compared to the GZ. That requires one of two things: either I move the rear sprocket inward by somehow reversing its mount and shimming it inboard, or I offset the engine placement, which is not my preference.

For the sake of expediency I'd of course prefer to just throw good valve seats into this engine and call it a day. Incidentally the other exhaust valve seat doesn't seem loose at all.

I just know I don't want to get rid of the bike. It's my first motorcycle that I bought about 10 years ago and even with all the others in between I've never let it go. That and I really like what I've done to it.



I just can't give up on it.

[isaac]

BTW Jonathan I now realize that this thread is just an in-depth troubleshooting discussion so if you can move it to the troubleshooting subforum that's fine by me. More guys might see it there who also want to figure this out.