Top end troubles

pictures (or stories) of hideous injuries sustained by your ariel
Simon.Gardiner
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Re: Top end troubles

Post by Simon.Gardiner »

simon.holyfield wrote: Sun May 14, 2023 8:00 pm ...By the way, I have always used Super E5 fuel in my bikes.....
Ah, but are you the only one who puts petrol in? ;) :mrgreen:
....and incidentally I believe that up here in Norfolk we don't have too much ethanol inflicted on us.
I suspect that it might not be easy to nail down when the various refineries change the proportions, or exactly where they're supply what to...

Personally, I'd fit a new piston. I know all the old hands are adept at 'easing down the high spots' but personally I've never had any success with that, the pistons have always either ended up with too much clearance and slapped like blazes or just carried on seizing up. I've also inherited a couple of pistons that have been 'eased down' by other folks and they've been no better.

Maybe add 'double check mixture' to the list?

SG
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Mick D
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Re: Top end troubles

Post by Mick D »

Hi Simon

I can't help but feel you are concentrating on a possible consequence rather than the cause of your engine failure. You reported that the engine stopped firing but continued to rotate, and subsequent loss of compression. Loss of ignition will be due to either ignition or fuelling issues, hence my focus on the efficient operation of the valves. Whilst your engine shows signs of a 'nip up' would you have noticed this if you weren't investigating the shut down? The two issues may be associated but not not necessarily so.

Other thoughts:

Does your engine have a valve lifter which could have interfered with operation?
Could one of the valve seats have moved in the head and subsequently re-seated?

Regards Mick
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simon.holyfield
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Re: Top end troubles

Post by simon.holyfield »

You're right Mick. I was merrily thrumming along, and the stoppage felt like ignition or fuelling, not seizure. As you say, there seem to be two issues maybe related, maybe not.

Yes I have a valve lifter; not sure about valve seats - do iron heads have separate seats?

I am aware that the inlet valve guide is a bit loose though...
cheers

Simes

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Re: Top end troubles

Post by nevhunter »

Easing high spots was done a lot on racing 2 strokes in the early days. Seize marks on pistons often end up below the original surface so filing there will only make a mess even though it make "Look" better cleaned up a bit. It's a skilled operation that few would do well and takes quite a bit of time. Often just honing a bit more out of the bore is enough but the ring gaps will increase. Your motor may get hot(er) because the fuel flow is not adequate or the tank cap not vented properly. Giving the engine extra throttle up a hill or into a strong wind when it doesn't make it go any faster will lean the mixture a bit also, It doesn't have to happen for long to get the extra heat. Honing the bore a bit larger at the bottom may help. This is called CHOKE and means the bore when heated is more true because the top part gets hotter than the base. It's commonly done on aircraft engines. Nev
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simon.holyfield
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Re: Top end troubles

Post by simon.holyfield »

A couple of pictures after removing the head and barrel. Second ring is tight in its groove, and you can see why. The piston land is smeared into the ring groove. Nothing broken though.

More later, including measurements.
IMG_20230515_125037.jpg
IMG_20230515_125023.jpg
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG,
'30 Model A
https://ariel-square-four.blogspot.com
Simon.Gardiner
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Re: Top end troubles

Post by Simon.Gardiner »

On these engines (speaking from many experiences) you don't get everything jamming up when the piston seizes. You get a sudden total loss of power and no compression - I've never been unable to kick the engine over when this happens. (If you know the engine well you might also notice signs of it being very hot.) When things have cooled down some compression returns and you can re-start them.

There won't be any valve-seat inserts in an iron head unless someone has done an 'unleaded conversion' . It should be pretty easy to tell as the hemi of the combustion chamber won't 'flow' round to the valve heads if there are inserts.

SG
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simon.holyfield
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Re: Top end troubles

Post by simon.holyfield »

Simon.Gardiner wrote: Mon May 15, 2023 1:15 pm On these engines (speaking from many experiences) you don't get everything jamming up when the piston seizes. You get a sudden total loss of power and no compression - I've never been unable to kick the engine over when this happens. (If you know the engine well you might also notice signs of it being very hot.) When things have cooled down some compression returns and you can re-start them.
That certainly fits with my experience the other day. Would you say it feels like loss of ignition or fuel? I was cracking along and the engine suddenly died, slowing quite rapidly but not seized, at least not for very long. I fancy I remember a squeak from the back tyre and a bit of a shimmy before I whipped the clutch in.

I'll measure up later, but right now I'm thinking that I'll hone, clean the ring grooves and refit the same piston.

Must get on with tiling the bathroom!
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG,
'30 Model A
https://ariel-square-four.blogspot.com
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paul.jameson
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Re: Top end troubles

Post by paul.jameson »

It is worth checking that the petrol tap opens fully to deliver enough fuel. On my Cammy, I had a partial seizure with pistons that were incredibly old and worn after riding for about 30 miles at a good speed along the motorway. The cause was a petrol tap cork which was too long and partially blocked the petrol feed to give me too lean a mixture for sustained hard use. No doubt blockages elsewhere, such as the petrol cap vent hole, could have a similar effect.
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simon.holyfield
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Re: Top end troubles

Post by simon.holyfield »

Right, on with the saga. Bore is 2.876" all the way down, which is exactly what I asked for.

Here are the piston dimensions, top to bottom. Per Waller, there is plenty of clearance at the top and it's marginally OK at the bottom.
IMG_20230515_184652.jpg
Note that for several months after rebuild i was trouble with very poor mixture control mid range - traced to a needle that wasn't as new as had been alleged. With the critique of the honing angle I am beginning to consider that bore wash might be a factor, though that needle was changed over two years ago.

Here's the exhaust valve - it's a bit sooty, as is the valve seat in the head. The stem shows some striation close up - could this be a sign of some past seizure?
IMG_20230515_190155.jpg
The measurements for the valve & guide suggest not. The valve stem, 2500 miles old, is spot on. The guide, also 2500 miles old is to spec over most of it's length and 10 thou over near the bottom.
IMG_20230515_190748.jpg
Is this normal? Perhaps to accommodate carbon build up?
Last edited by simon.holyfield on Mon May 15, 2023 10:40 pm, edited 1 time in total.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG,
'30 Model A
https://ariel-square-four.blogspot.com
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simon.holyfield
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Re: Top end troubles

Post by simon.holyfield »

paul.jameson wrote: Mon May 15, 2023 8:50 pm It is worth checking that the petrol tap opens fully to deliver enough fuel. On my Cammy, I had a partial seizure with pistons that were incredibly old and worn after riding for about 30 miles at a good speed along the motorway. The cause was a petrol tap cork which was too long and partially blocked the petrol feed to give me too lean a mixture for sustained hard use. No doubt blockages elsewhere, such as the petrol cap vent hole, could have a similar effect.
Thanks - I'll check. I have cleaned it out recently.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG,
'30 Model A
https://ariel-square-four.blogspot.com
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