I think we've found the problem

[sime66]

I think we've found the problem

[sbwnik]

Soft seize? Not ideal!

[sime66]

A soft seize indeed, in the snow crossing Bodmin Moor, on the way to my mate's warm workshop to look at the rings! Happy days. New bits ordered, soon sorted. Just got a pukka used Dellorto 24/24E to play with later too.

[sbwnik]

I was going to ask if you'd changed the carb. Odds are it was running a touch lean.

[sime66]

I'm not sure the carb was the initial problem, before I 'fixed' it, but it has to be the focus of attention now; here are my thoughts after seizing:
This seize was not just due to the worn rings or air leaks; I had been tweaking:
My soft seize was due to running lean, due to too much air, due to me giving the air filter a really good clean last week and not adjusting the mixture/jetting. My 20/20 carb needs a tweak before I get back on the road with the new piston, and I will be sorting a 24/24 carb down the line.
. Should I increase main jet until then?
. Will plug selection have any affect on this?

[pxguru]

I really like to see barrels run like yours. Its had a lot of use and hasn't been apart for a long time.

IMHO while its apart; new rings, slightly bigger main jet, Lap the head and top of barrel with wet&dry on a sheet of glass to fix the leak, clean up the piston. decoke the barrel.

How long since you last changed the crank oil seals? Or checked the gear box oil? If it is a problem this is the hot starting and seizing in one!

The temperature rating of the plug only affects the operating temperature of plug itself. The engine temperature is mostly controlled by the main jet.
The only real problem in scooters is if the plug gets too hot it melts. So then you need a higher number which is a cooler plug to suit a hotter engine.....simple ;D

[sime66]

Thanks for all that. Does it really look well-run, or were you joking with me? I hadn’t been in there before and was a bit alarmed myself. I think last year, since I bought it, was the most miles it had seen for some time.
The seals are good and the oil was changed last September, but I will definitely recheck the level.
There was no hint of seizing before I played with the air filter last week, so I really think this caused it, but I really am a novice.
I’ve ordered a new piston and rings. The barrel is fine, and I will polish barrel and head before re-assembly. I do not believe that leak was present, or at least so bad, before it seized; it had leaked in the past though; would the overheating/seize cause or worsen it?
There are two little extra exhaust ports on the barrel that were a bit clogged, so will definitely decoke while the barrels off.
A bigger main jet is wise until I get the 24/24 sorted? I don’t have my scooter at present because it’s in bits after It didn’t make the return journey on Sunday, so I don’t know what main jet I’ve got; what do you think might be the optimum for my present set-up? (It’s a 177, sito+ and 20/20 carb at present)
I really do not know what I’m doing, and I’m learning as I go along, and with forum help, Haynes/web and mates’ experience, so I don’t know if I’m making sense sometimes, or asking the right questions. I’m enjoying learning, but I hope I’m not being a drag, or too much of a numpty!

[pxguru]

Seizing is no joke! My concern is that the marks on your piston are more like overheating than a normal heat seize. These kind of seizes can be caused by low compression in the crankcase. The oil doesn't get up to the top side of the barrel.

What was the gear oil level like; how much oil did it take to top up? How do you really know your crank seals are good?

[sime66]

I think I understand you: low compression in the crankcase, possibly resulting from leaks caused by knackered crank seals, can mean that oil is not reaching the top-side of the barrel, which causes overheating like that shown in my photos, which indicate a more severe problem occurred than a heat seize? Until now I had not realised that my crank seals or oil was suspect. My scooter started again about ten minutes after seizing, and I went on for about another ten miles afterwards; I took this to indicate that it wasn’t particularly severe. I’m glad you stressed the point. Those seals keep the transmission oil out and the compression up?
· The oil was changed during a service towards the end of the first year that I had the scooter, prior to my first MOT. I didn’t do the service, I was watching and learning. The oil that came out was very dark, almost black. I don’t know how old that oil was or what volume was actually drained.
· I don’t have a numerical value for the amount of oil that went in either. I do know that it was the expected amount because it was measured out first, filled with a syringe, and went in just until it flowed over. Nothing abnormal was apparent, but I did not measure or fill it myself. I admit I haven’t checked the oil level during the winter months.
· We weren’t specifically discussing my seals on Sunday when looking at my scooter, but a 1989 PX had just been in for a crankcase seal. I mentioned that mine was older and that would probably be my next issue, and it was demonstrated to me that my engine had been rebuilt at some stage recently (by the absence of any play in the crank, but I can see that a sound bearing does not mean a sound seal). We did not actually inspect my seals; I mentioned it, and was satisfied then with that conversation, but I can ask for it to be inspected thoroughly now that I realise this is important and is a real possibility as a cause. I realise it is essential to establish the cause of the overheating/seizing before rebuilding the barrel, but I was thinking the cause was most likely the jobs I’d done recently, or the symptoms I’d mentioned before that. These other jobs are new subjects and bigger than I have tackled before; I wasn’t making a joke about the subject at all, just trying to understand it, and what you meant.
I need to better understand which oil lubricates which engine parts; I will have a read-up on it. Thanks again for stressing the importance of it, I’ll learn from it.

[sbwnik]

Still looks like a lean seize to me.

Cleaning the filter will allow a leaner mix, obviously, and it's possible that you didn't tighten it up properly, compounding the issue.

Old mechanics advice to me: If you change something, then you have problems that could be attributed to it follow, then the odds are it's related to what you 'fixed' last time.

[sime66]

It’s got to be the favourite culprit since I just fixed it, but I’m definitely checking the crank oil seals too, especially as it is another possible cause of compression loss. So it’s:
New piston and rings, decoke barrel and lap barrel top and cylinder head, check transmission oil level and crank case seals, bigger main jet on carb, and check other jets for blockage. Check air filter seating – is the airbox seal important?
During running in I can keep an eye on my oil level. I need to suss out if I’ve sorted the cause of the seize (carb or seals or other), and then I’m back onto the warm starting problem, which may be sorted by the new rings, which were worn, or any cylinder leaks, which will be sorted now too. That was the aim of the work on Sunday before the seize. If that’s still not solved the next suspect is the carb. I’m also going for the bigger carb anyway, but not immediately; there’s enough new bits to keep an eye on for one ‘fix’, and I need to decide whether to overhaul and use the 2nd hand Dellorto I’ve bought, or go for a Dellorto-Spaco from Dellorto, which I believe is better than a Spaco. That is a different subject, for another time; my next project will be overhauling the 2nd hand Dellorto 24/24 carb when I get it next week. I hope I’ve taken everyone’s advice on board and have a sensible plan now, and I do appreciate it – thanks.

[kru251]

The "change one thing at a time" may be awkward here. ;D That ethos is so that if you alter something and as Nik said trouble follows then it is quite possibly that alteration that created the problem.
As an example if you change the airflow capability by cleaning the filter AND say had changed the main jet, then what caused the issue? I say this only for future reference re. the new/serviceable carb as that along with the new piston/rings will have to be set totally differently than your current set-up. If something happens again (I hope not obviously) you may not know if it was a leak from the head or the carb changes etc.
A methodical approach will work wonders and so far I'm pretty impressed with yours!

[sime66]

Thanks for that; I am going to leave the actual change of carb well out of the equation until the cylinder changes are settled down - one job at a time. But, am I right in thinking being a bit rich is good for running in? If so, increasing the main jet is good for both issues – to prevent another seize (if that was the cause) and to aid running in. Now that I’ve introduced the extra variable of the cause of the seize, it sort of makes it necessary and helpful I think. Obviously the crank oil seals and oil level still need checking before, and monitoring during, running in too.

[kru251]

Well, ask a scooterist how they like to run a scoot in and you'll get all different answers. I DO NOT like to run them rich as:- two stroke oil burns hotter than a good fuel/air/oil mix so it raises the cylinder temperature which defeats the object. b) More oil going in means less fuel so again the actual mixture is weakened with that hotter cylinder temp again.
Proper mixture with short actual road runs giving non-labouring use (as in change gears early for hills) and obviously not revving the nutz off it yet. Lots of throttle variation and up and down the gears to bed it all down helps, as constant speed/throttle opening is not good for it. You need to bed it all down first by the method outlined above. Gradually lengthening the time taken out on the road and throttle/speed increases are made so as to get a good, solid, dependable engine.I run engines in for about 450 - 500 miles ish. The ish being if it feels right to start with. No two engines are alike even when of exactly the same spec. I know others run in for longer distances (Nik) & some that do a lot less. Depends who's paying the bills I guess!

[sime66]

That's a splendid summary, which I will follow. My only concern with it is whether or not it addresses the cause of the seize and hopefully prevents it happening again, but you don't think an increase in main jet it necessary? That's the question I'm really trying to decide on now.

[joey]

Running rich has nothing to do with oil, only the fuel air ratio.
A bigger main jet during the running in process keeps the combustion temp low as not all of the fuel burns off.
As for adding extra oil during running in the difference between running 2 or 3% does weaken the overall mix but it's effects are negligible.
Let's not forget that if we were to try to achieve the IDEAL fuel air mix we'd be out every morning checking air density and moisture levels and rejetting accordingly throughout the day......... bit of a ballache.

Plenty of plugchops at various throttle openings is the best way to set a SAFE setting for your jetting.

[joey]

Incidentally looking at photo #2 it looks as though your head may be warped as there is serious blowby at the top and bottom.......... could be the cause of your seize.

[sime66]

Hi Joey,
I raised the question of the possibility of a warped head when I was investigating the problem with starting warm. I am going to check and polish/lap it before reassembly to get both surfaces good and flat again. I think the two blows are different; the top one is from an earlier incident with a stripped nut, which was replaced, but without polishing. The second is new, and I did ask for an opinion of whether it is likely that it caused the seize, or the that the seize caused it. In either case it will be rectified before I'm running in.
My theory, which I thought was good, but wasn't well received (not here I hasten to add) was this:
If the first problem caused a warp, and was then tightened, the head might possibly rock and thus open the opposite side, hence the second, later one.

[sime66]

Hi Joey (again)
I only saw your second post so didn't answer the first one:
I was only really intending to talk about the fuel/air mix (the main jet size). I knew some say to increase the fuel/oil ratio, and some don't, but that isn't was I was trying to sort out. I don't have autolube; I premix 2%. It is just the main jet size I'm querying, especially as it is prime (or at least one) suspect for the seize immediately after I cleaned the air filter. I'm only trying to decide about the initial jet size at the start of running in, to best aid that, and to prevent another seize. Maybe I've just got my knickers in a knot over it without good cause to worry - I do not know.

[joey]

If the top was the cause of a previous seize and left untreated then it would never seal properly again, it only take s a small amount of cak on the sealing face to cause grief.
I'm with you in that if it's goosed on one end it can warp at it's opposite.

Lap the head, torque tighten, start up and spray WD40 or similar around the sealing face (same as plumbers leak detection fluid,) any leakage will cause bubbles.

go for a good blast to get her up to temp and then re torque the head, check with WD40 again and if no bubbles at least you'll know you've eradicated one problem.

[sime66]

Cheers Joey; I'm glad you don't think I was talking nonsense about the head because it seemed like good logic to me.
We've ended up crossing posts because of writing two at a time; I replied to you twice before.
Did you see my other clarification about the other ongoing question to do with correcting my lean running: and that my question was about fuel/air (main jet increase) not fuel/oil mix?
It goes back to the question of whether lean running caused the seize, or the head leak caused it - or, as I'm wondering, whether it caused the head.
I'm pretty clear on what needs doing before I get back on the road and running in now - and thanks everyone for that, I'm just still wondering about that last point about the main jet after cleaning my filter and then seizing. - mainly to ensure I don't repeat the seize.

[joey]

If your filter was completely cakked up it may very well have masked the leaking head due to running rich, cleaning it then reintroduced the leaking head beast as it would then be running slightly weaker.

Personally I'd knock it up one or two sizes on the main jet, just to be on the safe side, once you know you've eradicated the leaking head. Then just do a good few plug chops which will let you know whether you're ok to downjet.

Another bit of advice is to do any running in on a hot plug to give you a true indication of engine runing....... too cold a plug will take much longer to burn off residues and give you the impression that you're running rich when in actual fact you just haven't run it long enough to burn off smeg.

Once you know you're running safe then you can try a colder plug for longer runs.

I use a B7 for running in/testing and then a B8 for longer rides and finally a B9 for long distance rallies.

[sime66]

Thanks Joey, that’s really useful; it makes good sense to me, and it fills in my gaps for the running in process:
The theory (I know it’s only a theory, but it seems very sensible to me) is that before the seize I was getting away with the leaking head because it was running rich whilst the filter was clagged. (The leaking (warped?) head might well also have been the cause of the warm-starting problem).
When I’ve got new piston etc fitted, and the head’s all flat, polished up and torqued down (checking with WD40) it’s running in and getting mix right as follows –
Increase jet size to start with to be safe, use a hot plug at first to give a true indication of rich/lean running from the plug chops, during running in regular plug chops will indicate when I can reduce the jets. Once all is well run in, along the lines of kru251’s description, and jetted right etc then go on to a colder plug – just in time for Tenby!!!
I know it won’t be as straightforward as that but I have a plan; does anyone want to tell me it’s a bad one?

[kru251]

Mar 20, 2013 8:51:57 GMT joey said:

Let's not forget that if we were to try to achieve the IDEAL fuel air mix we'd be out every morning checking air density and moisture levels and rejetting accordingly throughout the day......... bit of a ballache.

You mean you don't do this? What sort of loose regime are you running there? ;D
I'll get my coat

[pxguru]

Sounds like you have a plan You should include a pressure test in there somewhere as well.

IMHO it doesn't look like a typical lean seize. In a lean seize the big score marks on the piston are mostly near the exhaust port and small marks near the boost port.
From your photos the marks on your piston are only near the boost port. And none near the exhaust. This would appear to be an over heating seize normally due to lubrication failure.