Author Topic: Wots goin’ on here pls folks?  (Read 3709 times)

Offline Lobo

  • Lobo
  • SOHC Master
  • Posts: 1568
  • Lobo
    • View Profile
    • Lobo
Wots goin’ on here pls folks?
« on: February 25, 2021, 08:29:18 AM »
1/2 pistons scored down one side of a 250cc 2-stroke, which runs on 100:1 premix. The rings are seized into the grooves at this point.
Salvageable? (not too fussed about performance, but need reliability (outboard motor).
Why the damage so localised?
Little / big ends all seem good, as are crankshaft bearings...
Ta,
Simon
« Last Edit: February 25, 2021, 11:28:44 AM by Lobo »

Offline adespin

  • SOHC Expert
  • Posts: 303
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #1 on: February 25, 2021, 09:24:38 AM »

 I wouldn't be running a 2stroke on 100:1, i run my chainsaws on 40:1 even though recommend ratio is 50:1
1971 cb750 K1

Online McCabe-Thiele (Ted)

  • Grogu
  • *
  • Posts: 6120
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #2 on: February 25, 2021, 09:32:14 AM »
Looks like straight localised overheating to me certaily more 2T oil as well as possibly considering change to fully synthetic brand would not hurt.

If it's a little used motor then fuel degredation might also be a factor with degraded fuel combustion products causing raised burning temperatures. Do you drain it right down and replace with fresh fuel when its out of use?

Is the recommended 2T ratio really that low certainly an increase will help in my opinion downside of too much oil is obviously more frequent decoking and plug fouling?
Honda CB500 K1 (new pit dug out ready)
Honda CB400 four super sport (first money pit)
Link to my full restoration http://www.sohc.co.uk/index.php/topic,23291.0.html

Online K2-K6

  • Grogu
  • *
  • Posts: 5232
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #3 on: February 25, 2021, 09:38:12 AM »
Lubrication breached, obviously!  But reason is important to correct.

As others, is it really supposed to be at 100 to 1?
Presumably water cooled?
Have you pictures of combustion chamber, piston crown and spark plug to see colours.
What is the spark plug identification/grade etc.
Looks like exhaust side of piston?

Offline Lobo

  • Lobo
  • SOHC Master
  • Posts: 1568
  • Lobo
    • View Profile
    • Lobo
Re: Wots goin’ on here pls folks?
« Reply #4 on: February 25, 2021, 11:23:41 AM »
... it’s a ‘new’ motor to me; and 20yo I guess. 100:1 is the correct mix, and indeed that sticker remains on the side of the engine.
Yep, water cooled - though now apart its arteries are seen to be a wee furred up and will be corrected.
The piston crowns indicate a slightly rich burn, as do the plugs, but tbh it’ll only run on choke at the mo (or throttle) -  hence the tear-down.
Nothing looks horrible in there ....
All the ignition components / plugs now new, the carb galleries & jets cleaned through to an inch of their lives, float level ok, Reed valves look &  ‘feel’ ok, and yet it continues to die if idle drops below choke-on speed, accompanied with a lot of shuddering & the odd back fire.
I’m kinda at last resort, and will change all the crankcase gaskets and seals; the piston issue is unexpected and new to the problem.
....if (unburnt) gases are leaking past the seized piston rings would this cause the no idle & backfire?
« Last Edit: February 25, 2021, 11:31:38 AM by Lobo »

Online K2-K6

  • Grogu
  • *
  • Posts: 5232
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #5 on: February 25, 2021, 11:41:08 AM »
With rings stuck it will compromise most stages of running as it's effectively sucking (drawing in the cases) as well as holding compression. It's more or less like having leaky cases in a way.

Dilution of crankcase gases with burnt blow by effectively reduces the volatility of the next charge phase.  This is what they do on exhaust gas recirculation systems in car engines, but only at specific load demands.

You'd need the rings working properly to make realistic assessment of the running as a prerequisite.

Offline Lobo

  • Lobo
  • SOHC Master
  • Posts: 1568
  • Lobo
    • View Profile
    • Lobo
Re: Wots goin’ on here pls folks?
« Reply #6 on: February 26, 2021, 06:23:42 AM »
Thanks gents. (So far 🙄)

I started cleaning all the deposits off the cylinder block today... and found the cause. Some PO has inexplicably plugged a cooling gallery with a SS screw, and then, for good measure put putty about the whole lot... causing an interruption of water to the cooling jacket in a localised spot.

Intrigued I dremelled away all the putty, and removed the screw. So far, I can see no damage to the casting, indeed it ‘rings’ consistently everywhere I tap it with a screwdriver. But I’m bloody confused as this hole is within a boss, and is tapped (M6). It’s other end is the common cooling gallery, which feeds about 4 other such cooling inlets (varying sizes, shapes, tho’ none others with screw threads.)

Honestly I’d have sworn the head had never been removed as no markings whatever on the painted fasteners. That said, it don’t look like a factory mod!

Any ideas please? Could it be a ‘hot climate’ option.... ie to supply more water to the jackets? I’ve looked the internet for flow paths, cylinder block photos etc but keep coming up blank.

Semi planning on leaving the hole clear, and having a rebore with .25mm o’sized pistons. (though the scoring not deep so will first try a bit of 600 grit sanding.)

All thoughts & ideas welcome...  it’s an engine that won’t see much use so not too critical wrt perfection)
Ta.
« Last Edit: February 26, 2021, 06:27:18 AM by Lobo »

Online K2-K6

  • Grogu
  • *
  • Posts: 5232
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #7 on: February 26, 2021, 08:42:21 AM »
I can't think immediately of a competent reason to restrict the water way there.

Trying to determine exactly where that is in relation to the piston damage, it looks like it's the same place?

As it's been running like that, think I'd get the rings free and gently clean the "snots" off very lightly, then put it back together and run it to further assess and answer questions.  Put it on 50 to 1 ratio and see what you've got then.

If you rebore and go with new pistons,  then any initial problems with it may take those as well. I know it's not mechanically ideal to reassemble but will tell you a lot with very low risk.

As you'd expect in an engine like this,  those combustion chambers in the head are focused on complete burn with low thermal problems,  it shouldn't intrinsically be making too much heat, making the cooling jacket modification distinctly odd.

It should run pretty well with the pistons and rings cleaned,  then give the confidence to go with rebore etc if it's debugged fully.

Online K2-K6

  • Grogu
  • *
  • Posts: 5232
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #8 on: February 26, 2021, 08:53:47 AM »
Forgot to add,  yes I'd clean out the bodge in water jacket.

Reasoning that it can't run like that,  all things considered, and if there is a problem subsequently it would then show up if you run it with the components you've got there already.

Offline Lobo

  • Lobo
  • SOHC Master
  • Posts: 1568
  • Lobo
    • View Profile
    • Lobo
Re: Wots goin’ on here pls folks?
« Reply #9 on: February 26, 2021, 11:12:05 AM »
Thanks Nige...

Yep, the scoring (piston & sleeve) are exactly adjacent to where the putty / nil cooling was. I’m still at a loss why the bodge... common sense shouts ‘crack’ in the casting - except I just can’t detect any. Muddying it all is the boss (versus straight hole) and tapped thread.
(I pondered, ‘temp sensor?’ - but there’s no corresponding hole in the cylinder head)

I gingerly tried to extricate the lower / easier ring this morn, and after an hour of coaxing it had it slightly proud of the piston - but then snapped. Tbh, it was nowhere free; and even now a 10mm bit remains ‘welded’ in the groove.
And that was the easier ring...

So - I’m up for at least 1 new piston & rings. Decision time. Do I buy a standard piston and bung it all back, or rebore and go with 2x +1 size pistons?

The localised scoring is significant, ie easily felt with a finger nail. How this translates to severity / reliable idle - I don’t know. All the engine needs to do is push a 10’ tinny about the local waters for a couple of hrs a month; if the consensus is “she’ll be right” then I’ll be totally happy to just bung in a replacement piston and rings.

Water jacket and all galleries all now cleaned thoroughly of build-up; cholesterol levels down to zero.

Appreciate the help - thanks,
Simon
« Last Edit: February 26, 2021, 11:21:32 AM by Lobo »

Offline SumpMagnet

  • SOHC Pro
  • Posts: 683
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #10 on: February 26, 2021, 11:58:39 AM »
any chance of picking up used parts?

If it was mine, I would want to throw it together for minimal cash just to see if it is worth the trouble. For what you want it to do, spending money on a rebore and new pistons
is the last thing I would want....but...unless you enjoy rowing...you want it to be reliable
CB750F2 - in pieces
CB900F Hornet - the daily transport

Offline ka-ja

  • ken
  • SOHC Pro
  • Posts: 736
  • yoshi 460
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #11 on: February 26, 2021, 12:03:46 PM »
Is it possible to assemble the motor castings only and apply some water pressure to the cooling system and check for any leaks?
nice bike,nothing in the bank

Online K2-K6

  • Grogu
  • *
  • Posts: 5232
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #12 on: February 26, 2021, 12:11:38 PM »
Cracking would be my fear too, but it does show evidence of corrosion removed material around that section of the barrel compared to all the rest of both barrel circumference. Maybe that's what they responded too in trying to add material.

You can reface the top surface by double siding tape onto glass sheet some abrasive papaer,  then use paraffin to lubricate while you "sand" the surface.  Something like 180 grit for initial cut to see how it cleans up.

Piston, can you get rings for it? You can carefully clean out the ring groove with a flat needle file to get the old ring to check clearance before re-ringing to get it running and assessed.  If you haven't got a needle file,  then you can glue some abrasive paper to one side of a lolly stick to gently clean the ring lands.

Worthwhile trying if you can re ring, alloy wheel cleaner (usually contains sulphuric/hydrochloric acid or similar) to see if it'll clean the carbon tar from bore walls.

Not an easy way through it,  but with it not working now the piston is scrap anyway. If you can at least get it going competently like this, then you'll have the surety to go for a rebore longer term. 

That's a huge amount of blow by on that piston, and likely to be significant in poor running,  but most probably caused by the "modification" originally.

Online K2-K6

  • Grogu
  • *
  • Posts: 5232
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #13 on: February 26, 2021, 06:28:50 PM »
They usually suck in water and pass it through jacket, then straight out again.

After salt water they are usually purged with clean before storage,  not sure if inland water ways would have the same need to clean.

Offline Bryanj

  • Grogu
  • *
  • Posts: 10692
    • View Profile
Re: Wots goin’ on here pls folks?
« Reply #14 on: February 26, 2021, 07:04:26 PM »
Have you seen the c**p floating in inland waterways?

 

SimplePortal 2.3.5 © 2008-2012, SimplePortal