What Can Cause Overheating?

[buchanan84]

Hi

i've got a 2003 sherco 290

its used mainly for put putting about off road with the occasional blast to try and clear it out,

but a few times i've given it a hard run up the hill

on the last time giving it a fairly hard run (top gear up hill) it cut out, bump started it and it started to rattle a bit. so stopped, engine looked hot (steam coming from wet mud and stuff) but checked it had water and fan was working.

so carried on (oops)

it then cut out a few times (really should have stopped) before it stopped completely with a cloud of steam between my legs (looked like the top of the head) bike had no compression so pushed it home (luckily mostly downhill)

on a previous hard ride, i had noticed the exhaust was slightly creased and was getting hot (the plastic heatshield was melting onto my boots) but bike was working fine

since it stopped, i've stripped the top end off it and the piston and barrel are badly scored, the piston is as bad that the piston metal has gone over the rings holding them in, so no compression. all the gaskets and seals seamed ok

when it overheated there wasnt any steam or water from the radiator cap so guessing that doesnt work and how it got pressured enough to blow past the head seals

so getting cylinder replated and new piston, should run fine

but why did it get so hot???

had water

fan was working

pump looks ok

running the same fuel as my mate in his gasgas

would a creased exhaust cause that much heat build up in the engine?

sorry for the essay but thought i'd put in the background

any ideas???

[1oldbanjo]

Well it sounds to me like your cooling system is fine - but your bike may be running too lean. With too much air and not enough fuel - a 2 stroke engine runs too hot and does not have adequate lubrication. Your carb may have been dirty, your fuel system may not have been flowing enough fuel to keep the float bowl full during the heavy throttle work going up a hill, your jetting may be wrong, your fuel tank vent may not be working right.

First you need to get the engine repaired. Hopefully the cylinder can be salvaged, and you will need to get any aluminim transfer from the piston off the cylinder bore. Then fit a new piston and rings and get the engine back together. Then clean the carb and confirm the jetting is what others are using in your area, then confirm the fuel flow from the fuel tank by letting the fuel hose run into a container and check that the flow is regular and does not slow down.

When you get the bike running again be careful to be a little gentle while it breaks in......and do some plug readings to make sure the bike is not running too lean. Fuel/oil mix of 80:1 is normal - and use a good fuel and a good synthetic oil.

Edited July 20, 2010 by 1oldbanjo

[dan williams]

Carburation too lean (blocked jet, sticking float, air leak), ignition too advanced (set wrong, bad ignition, sheared woodruff key), blocked exhaust, no air flow through radiator, no coolant flow through radiator (low coolant level, bad impeller/water pump), Bad fan or thermo switch, air leaks in intake tract, pinhole in coolant system causing loss of pressure, bad radiator cap, leaking head gasket/o-ring forcing combustion gases into coolant system, the list goes on.

Yeah you really should have stopped and baby'd the poor bike back to the garage.

Once you get it back together, and the act of reassembly gives you a golden opportunity for an in-depth visual inspection, check each system off one at a time.

Fluid level in radiator correct, filled with proper coolant so it has a higher boiling point then water. Hint: While filling pinch the hoses to chase out air bubbles.

No visible leaks. Radiator cap on correctly.

Exhaust system not gunked up.

Timing checked with a timing light.

Carburation - bike doesn't ping and runs clean. If it does ping or run lean put on choke to see if it helps. If it does carb off for cleaing and look for air leaks.

Radiator clean, fins aren't packed with mud or debris.

Fan comes on after a few minutes of running.

Remember the cooling system has to be pressurized to work. That allows the coolant to be at a higher temperature without boiling. If the pressurization fails then the coolant boils and undergoes a

[copemech]

This is why I like OLD people! You see, they have a much better understang of just how things work, and a lot more time to explain it than I do!

Dan and Jon are full of it! Knowledge, that is, and to be honest it would take me quite a while to type all that out whilst accmulating my random thoughts amongst other tasks at hand during the day.

One must have due respect for their elders! Although I do kinda think most the good ones are already dead, however most of the modern world was built with nothing but sliderules! And I am talking Aircraft, Spacecraft, Ships and Subs! One should take a good look into the history of engineering to gain a real appreation of just what all these old F--ksticks really did!

All credit given, now I must think deep about just what this self admitting fellow has indeed done to himself?

And being much a fundamentalist, I shall start off with a short question. Buchanon, did you grab the correct tin out of the shed? You know, the one that is not marked for the mower? The real one that ahs a proper amoun of 2T oil in it?

Edited July 21, 2010 by copemech

[buchanan84]

well the fuel should be alright, its the same mix that my mates gasgas is running and theres definately mix as its coloured, and had just filled up.

the bikes are stored in a mates workshop so you never use a random can, but we keep 3 5L cans mixed up for the trials bikes, though its probably nearer 50:1 as thats what the sticker says on the bike (noticed most people on here run 80:1)

from the rest of the suggestions,

the exhaust is slightly creased so could be a restriction, probably quite gummed up to as its mostly slow put putting about that the bike does

so will clean it and try and straighten it or drill an extra hole to make sure theres no restriction (new exhaust will have to wait)

ignition, i dont trust the stator, the bike can be a real pain to start when cold (fine warm) and ive always thought weak spark

so will probably get this rewound while the bikes in bits (not 100% sure if its needed but i dont trust it)

fuel starvation (running lean), will give the carb a good clean out and once over

whats the easiest way to set the carb, ie how do you tell if its rich or lean?

banjo - what do you mean by plug readings??

in general the plug has been pretty black, but only had it off to clean it when the bikes been reluctant to start (though the day it went bang it started really easy)

thanks for the input guys, the cylinders away to get replated so will be a couple of weeks untill i can get it back together

[dan williams]

This is why I like OLD people!

Ouch.

Jon and Billy T have forgotten more then I'll ever know.

[1oldbanjo]

If the spark plug porcelain around the electrode is white.....the bike is running too lean and the spark plug is too hot. If the mixture is correct and the bike is running well, the spark plug insulator should be a nice chocolate brown color. If the bike is running too rich the spark plug insulator will be black.

Spark plug reading are best taken under full throttle - so it may be a bit hard to tell what is going on in a trials bike when you are just puttering around. The high speed blast up the hill you did would have been perfect for taking a plug reading - but you have to stop the bike almost immediately and take the plug reading before you do any additional idling or slow speed riding. I have a lot of experience with plug readings on motocross bikes and chainsaws......but my Sherco 290 ran fine and I never took the plug out and did any readings while I owned it. I now have a new 3.2 and it runs just fine....I have never taken the plug out yet.

Here is a link to some photos - but a lot of this does not apply to your bike. I think the normal/tan plug is a bit light colored for a 2 cycle engine. Look at how the overheated spark plug has melted the sharp corners off the electrodes and the porcelan is white. http://www.classic-car-magazine.co.uk/arti...uide_chart.html

Here is a page from a site that shows a better color of tan for a normal running 2 stroke. If you are putting around at slow speed you may not be able to keep a nice tan color - but after high speed runs it should have a nice tan color. http://www.nh-scooters.com/article/articleview/55/1/2/

[dabdab]

I have an 03, which had a factory gliche from new

The pickup tube on the petrol tap was too long, cutting of fuel after a burst of throttle.

Theres a fix on Wayne Thai's site

Probably not causing your problem, but check it while you are at it