Possible seizure!!!

[Minto]

Out for a fifty mile round trip today, and as I was heading up a very long hill, the plungy a10 started loosing power so knocked it down a cog, still loosing power. Thought, maybe fuel starvation?? Turned on the reserve tap as well, as I did, I lost all power and heard an odd noise. Pulled the clutch in and coasted to a stop. Lots of smoke from off the cylinder head, wafted my scarf around it to try to help cool it down. When the smoke stopped(couple of mins) I tentatively tried turning the motor over on the kicker, with the plug caps removed. Turned over ok. So left it another 10 mins and tried to start it. Nothing, nothing, nothing. Then started but revs rose to screaming point and didn’t come down til I hit the kill switch.
Turned fuel off again and tried, started first kick and ran fine. Been good as for the last 20 miles home (avoided big hills though)
Any thoughts team??

[Triton Thrasher]

Yes maybe poor fuel flow.  Easily checked.

Maybe carb needle in too weak notch.

Maybe just too long uphill in too high a gear.

[Minto]

TT, you reckon possibly seized due to fuel starvation weakening the mixture??
Last time I was out on it (2 months back) it was struggling up hills.
It was very hot when it stopped running today.

[Minto]

Guess I’ll have to tek the lid off n have a look at the bores n pistons.

[KiwiGF]

My guess is it just got too hot and conked out.....the non starting was probably due to the slide being stuck open, which also caused the over revving when it DID start.

Have you got a thermal insulator spacer between carb and head?

Obviously whenever an engine over heats there is the possibility of a piston seizing.

[Minto]

My guess is it just got too hot and conked out.....the non starting was probably due to the slide being stuck open, which also caused the over revving when it DID start.

Have you got a thermal insulator spacer between carb and head?

Obviously whenever an engine over heats there is the possibility of a piston seizing.

Aye, got one o them in there.
Something ain’t right anyhow, loss of power up hills and that kind of hot!!
Funny, ran like a Swiss watch after.

[CheeserBeezer]

Had exactly the same problem with an engine I built for a mate many years ago. Ignition was too retarded. If the points have closed up this will lead to retarded ignition. Possibly running weak as well, all leading to overheating.

[Minto]

Cheers for that, I think the ignition timing is ok, it starts really easy, runs better than it ever has since I’ve known it, though I will check it when I get a weekend off.
So, if it was a heat seize, what damage if any might it have caused?
As I said, it ran well after a few minutes to cool off, but haven’t pushed it or taken in any big hills since.

[Triton Thrasher]

TT, you reckon possibly seized due to fuel starvation weakening the mixture??
Last time I was out on it (2 months back) it was struggling up hills.
It was very hot when it stopped running today.

Yes, common causes of piston seizure are:-

Rebored with insufficient clearance;

Weak fuel mixture;

Ignition timing wrong. Can be neglect of points gap.

Labouring up long hills or into headwinds in too high a gear.

[Minto]

Cheers TT, possibly all of the above apart from the head wind.
Best get the lid off and inspect the damage.
When I picked up the cylinders from the engineers I wondered if they’d even been touched, still a ring of carbon at the top of both bores. Gave them a quick hone at home to get rid and ease them out a bit, maybe not enough.

[RDfella]

If it's running fine, no smoke or funny noises, then I'd hold off on stripping anything. Especially considering the grief involved with replacing the rocker gear! Check the items listed by TT - including flow from tank.

[terryg]

Before removing the head, would a compression and/or leak down test be any help? It’s not something I’ve done often but I’m wondering.
Comments welcome, pos or neg!

I’d also be looking carefully at possible retarded timing, as Cheeserbeezer implies.

[RDfella]

Taking the head off won't diagnose anything. As I stated earlier, if it's running OK - no smoke, no funny noises, then the only thing that a strip down would diagnose would be if the piston / bore clearance was too tight and a hone was needed. OK, you might find a piston is a bit scuffed, but so what if it's running alright? I'd check everything else first, including that the air vent in the tank is clear, as that could cause a weak mixture on a long run.

[Minto]

RDFELLA, would the fact that I can always hear a hissing from the tank cap when I pull up indicate that it’s restricted??
I think I’m going to rip it up just to check the state of the bores and pistons anyway, just for peace of mind, not like it’s a big job to take a look.
Deffo going to look into that tank venting.

[Triton Thrasher]

RDFELLA, would the fact that I can always hear a hissing from the tank cap when I pull up indicate that it’s restricted??

I think I’m going to rip it up just to check the state of the bores and pistons anyway,

You don’t seem to be in a hurry to do something obvious and simple and cheap, like see how much petrol runs out of the bottom of the carburettor in a minute.