engine seizure

[oilcooled :)]

i was working my chainsaw the other day when it seized. the saw obviously got hot, i was using my friends 50:1 mix, looked in the book and saw recommends 32;1. does the oil help keep it cold as well as lubricated. i let it cool, took out carb sprayed wd40 down port hole took out sparky, poured boiling oil in there, and got it freed up again. saw seems to run as good as ever. my q is why did it heat? cause of the mix, cause its old, cause its all made of cast magnesium which may retain heat in the engine(alu piston metal block) cause the exhaust is half shot? i bought a compression tester today of ebay 9 pound. what would be healthy a reading?

i ordered new rings exhaust carbuerettor conrod. i have a better condition piston and block than whats in it.


old homelite super xl. also petrol boils when worked, told thats a very common problem

engine tester seems cheap and handy
http://cgi.ebay.ie/ws/eBayISAPI.dll?ViewItem&item=330441767122&ssPageName=STRK:MEWNX:IT

 

[rtbcomp]

The oil reduces friction which in turn reduces heat.

I doubt the oil acts as coolant in a 2-stroke engine the same way it does in a 4-stroke.

 

[oilcooled :)]

just be nice to know why it seize, it had a good sharp chain decent bar chain no too thight, and i thought older homelites where a brute of a saw. some one will come along an say there antiques, but that saw has had it good for the last 30 years or that

 

[rtbcomp]

If there wasn't enough oil in the fuel the rings most likely stuck in the bore.

 

[shenion]

Sounds like it was running too lean. Gets real hot and will seize on low oil mictures.

I hoe that 50:1 is synthetic oil, regular does not do well at that ratio.

 

[oilcooled :)]

will a full rebuild make that saw compete with modern stihl and husky engines do you think?

 

[shenion]

New piston ring will help. These do not have a lot of compression. Carb probably just needs adjustment and cleaning and rebuild kits for them are cheap.

Check the roller bearings. I have seen them go bad when using crappy oil (they slide rather than spin until they warm up.)

Should be fine after fixup. Be sure to adjust the mixture and use good oil. I use synthetic at 40:1 and it makes a big difference: no smoke, more power and plug does not foul.

Edit: if the piston sleeve is scored or flaked , then it is scrap; most have no steel liner, one you wear through the nicklesil plating, they self destruct

 

[oilcooled :)]

thanks for the help

 

[R Kraft]

You might check to see if the exhaust ports are getting plugged up.
Robert

 

[tigler]

Lack of oil with a 50:1 ratio was more than likely the reason it seized. Generally it's only water cooled strokers that can run on 50:1 and the internals are designed to cope with that.
Most garden equipment is 25:1, some is 32:1.

 

[the snooper]

got to say i agree with tigler

but isnt it ironic as the song sings

that op "oilcooled" has seized an engine through lack of oil

 

[pedrobedro]

If it's Nikasil plated and has seized it will need replating before it's any good more than likely. If the exhaust is shot it may affect the running as two strokes need some back pressure to work efficiently. My Dynamac saw runs a 24:1 mix on normal two stroke oil and doesn't make much smoke or ash but it is only a firewood saw so it doesn't get worked hard.

 

[pidgeon chit welder]

I used to play in small plant hire ..... of the many different chainsaws I repaired most had a compression of 19 to 21 pounds when pulled over six times by hand and if you poured a dessert spoon of engine oil in the pot to do a wet test you could easily get 30 PSI.

So I'd be thinking that at anything below 17 dry tested the bore and rings are well worn.



if you have stripped it and taken off the pot /barrel wash it in a bit of pertol and polished the bore dry with a lint free cloth .. look carefully from the piston end with a bright light shining in the spark plug hole ....any dark matt grey patches pr piston travel grooves /deep scratcfhes by the ports or the top reach of the stroke or the same on the piston ..is the ring free and clean ...if so get a new barrel & piston and fit new crank case seals while your at it for they will have most likely been subject to much hotter gas than they should have been due to wear in / on the barrel /piston .

Having the greater dilution of fuel / oil will eventually affect the lub of the barrel and piston and cause overhating amd or excessive wear ( in any order ) due to the incorrect lubrication.
The damaged/ rusted exhaust box will also have an effect in the scavenging of the cylinder and affect the crankcase pressure 9 tends to make the engine " Warble " , at high no load speed ..
If the rust is not too bad you can burn out any carbon /soot & oil with a blow lamp or welding torch ..
Remove the box and away from smoke detectors heat it till it starts to glow deep red and the carbon burns off . You can then gas weld or braze on patches if needed but if it is a lace curtain effect get a new silencer and gasket .

PS.
It's usually important to use the 2 stroke oil recommended by the maker at the rate set by them rather than buy one and hope for the best.

 

[oilcooled :)]

i thought anything less than a 100 psi and an engine is ****ed ?:P

 

[pidgeon chit welder]

i thought anything less than a 100 psi and an engine is ****ed ?:P

No not qiute there are many differing compression ratios some are as low as 15 psi it all depends up in the swept cylinder volume and the effectice cc of the actual cylinder if memory serves me correct.

You can have high , low and rather low readings over a range of differeent engines when finished , for two stroke , then the range of petrol , and then diesel engines which can be in the region of 1,000 psi or more.
Wanklel engines are something different to say the least.

Try this out with different strokes and piston diameters etc.

Cylinder Diameter
(inches ex: 4.25) Cylinder Diameter (mm)
Stroke Length (inches ex: 4.25) Stroke Length (mm)
Number of cylinders (4, 5, 6, 8) Number of cylinders
Chamber in cc's (ex: 119) Chamber in cc's
Piston dome in cc's
(Dome = positive #, Flat = 0, dished = negative number)
ex: 22 Piston dome in cc's
Gasket Thickness in inches (add plus or minus amount piston is in the hole) ex: .42 Gasket Thickness in mm (add plus or minus amount piston is in the hole)

The displacement is cubic inches or liters.
You also have a Compression ratio of :1

Now I seem to think at this point you use Boyles gas law to get the actual pressure in a theoretically perfect cylinder and then lose a small percentage due to leakage . So as you can see it varies with each engine.

Boyles gas law to use is I think :-.
p1 v1 t1 over p2 v2 t2

 

[kristiaan_d]

only my 2p's worth on this but i would remove the spark plug and exhaust. look in through the exhaust port and turn the engine over slowly looking for scoring, scratches etc as this is a sign its not very happy anymore.

id also suggest taking a look at this youtube user as he does a lot with small engines and gives a lot of good advice on how to troubleshoot and repair them.

http://www.youtube.com/user/donyboy73

hope this helps.