GS450 - Tight Spinning Camshaft

[Guest]

When my GS450's exhaust camshaft holder brackets are torqued down (The ones labeled "A" & "B"), my camshaft becomes VERY difficult to turn by hand. The problem also seems localized to the exhaust camshaft holder brackets. Whichever camshaft I swap brackets "C" or "D" on turn just fine. "A" and "B" though I can barely turn by hand, even when greased. My testing is also done when the cam lobes are free of the valve buckets. https://i.imgur.com/rwxg7LG.jpg


Are the GS450 exhaust cam brackets supposed to feel this tight? If not, I'm thinking that brackets "A" and "B" are possibly bent.


Could this be completely normal, and related to how exhaust valve springs are much stronger springs than the intake springs, or do I have a problem on my hands (a possible premature head failure)?

[GregT]

Firstly, the exhaust valve springs as stock on a 450 are exactly the same poundage as the inlets.

Secondly, what do the bearing surfaces under A and B look like ? If you've had a partial seizure there that will make things tight.
I have align honed a few heads that have had cam bearing seizures, it's possible to fix them.

I'd doubt if you've got bent cam bearing caps - they're very stiff castings. You can however have a bent camshaft.
I'd check it between centers as a precaution.
Also, has the tacho drive gone tight ? That can make the exhaust feel tight too.

[Nessism]

Pull the valves and check again.

[bisontoad]

> Pull the valves and check again.

I'm referring to the short window where you can freely spin the camshaft without it actuating a valve (when the cam lobes are free of the valve buckets). I'd noticed that two bearing caps in particular ("C" & "D") allowed me to freely they turn either camshaft by hand, while bearing caps "A" & "B" didn't. The valves themselves open and close just fine as far as I can tell.

> Firstly, the exhaust valve springs as stock on a 450 are exactly the same poundage as the inlets.

Good to know.

> I'd doubt if you've got bent cam bearing caps - they're very stiff castings. You can however have a bent camshaft.

I'll look into that. What led me to believe that it was the bearing caps though, is that replacing "A" and "B" with "C" and "D" allowed the exhaust camshaft to spin just fine. Bearing caps "A" and "B" cause tightness wherever they're installed. I'm wondering, is if the gs450's exhaust cam bearing caps are able to be at least somewhat spun by hand, because unlike my intake cam bearing caps, my exhaust cam bearing caps can't be. I may ultimately end up just replacing them.

> Secondly, what do the bearing surfaces under A and B look like ? If you've had a partial seizure there that will make things tight.
I have align honed a few heads that have had cam bearing seizures, it's possible to fix them.

They seem ok. There were some dark spots, by they may be normal. I'll post pictures when I'm able to. There may have been some slight wobble when i set the bearing caps on their mating surface on the cylinder head, so I'll be checking that too.

> Also, has the tacho drive gone tight ? That can make the exhaust feel tight too.

It seems to spin fine, but I will double check this as well.

[John Park]

It is possible that the A&B caps have been sanded or filed down to tighten up the clearances. You can't buy the caps as they're machined after being installed on the head and only fit that head. You could also have ended up with caps from another head from some mixup or attempted repair.

If you remove the dowel pins and then slowly tighten the caps and the cam spins, the caps are probably orphans. Or the dowels are kooky. Also clean the rust off the dowels. The clearance is something like .001 - .002" [?] so it doesn't take much to throw the clearances or alignment off.

When final installing/timing the cams with the chain, the exhaust cam will be loaded against a bucket. When you tighten it down into place it can argue with the tach drive gear and munch the teeth on the cam sprocket. You either have to twiddle the tach cable to set the engagement as you tight the cam caps, hope to get lucky, or put the tach drive in afterward which is the most straightforward. Getting the tach drive out if it's been in there for forty years can be tough as the o-ring seal gets bonded to the head with the heat. If the head is off it's a better time to do this than in the bike with the frame in the way.

Shimming the caps up might be an option, but finding a suitable shim that thin would be difficult. If you were stuck in Guatemala and just trying to get home, I'd consider using some thin paper, but it's not a confidence inspiring fix.

[Nessism]

If the head is warped it can bind the cams in the unclamped state. After installing the head though it will flatten back out some.
I still say pull the valves and check again throughout the cams full range of rotation. You should replace the valve stem seals and lap the valves anyway.