'85 250R top end carnage

[pismorat]

Not good as you can see. This is a .020" over piston, running a 39mm flat-slide Keihin carburetor, open air box, and run at sea level in the SoCal dunes. It is obviously running lean making the piston burn through, but what will cause this? I've been told wrong fuel mix, wrong jetting, or even leaking crank seals. I run Klotz Super Techniplate at 32:1, have a 55 pilot and 150 main jet, and don't know how to inspect the crank seals. The bike has run well for three seasons now, but over the past weekend...not so much. Any help is greatly appreciated since I'm trying to go back out in a month.

[82 250r]

I've been told wrong fuel mix, wrong jetting, or even leaking crank seals.

That's about it...leaking head gasket or restrictive pipe/silencer as well.
Did someone put unmixed gas in it by mistake?

[yaegerb]

either lean or pre-detonation. What compression were you running in that motor?

Here is a good article on testing crank seals.

http://www.klemmvintage.com/airleaks.htm

[pismorat]

Thanks for the article. I never checked the compression after I rebuilt it after buying it (in pieces). It's never had straight gas in it and typically smokes out the pipe a bit...which is an FMF silencer with a massive expansion chamber. The cylinder is still good, so I think my plan is to check the seals, get a new piston and rings and then start from scratch on tuning it.

[Red Rider]

After a failure like yours, and looking at all of the debris that went into the center cases, splitting the cases to clean it all out is a definite must. Most likely the cylinder bore will need to be cleaned up as well.

[El Camexican]

Bad or old fuel perhaps. Were you on a top end run when it torched?

[82 250r]

splitting the cases to clean it all out is a definite must.

I agree...replace the lower crank bearing at a minimum... that gives you a chance to replace the main seals as well...hope it works out for you.

[threewheelin-feelin]

just a nice excuse to go with a big bore!!!

[Mosh]

150 main on a 39 mil carb is way too lean IMO.

I run 38 mils with ported engines and my mains range from 165-172 and they are a tad lean to keep the machines crisp. I dont do alot of high speed wot throttle runs.
I know everyone's stuff is different but I read all the time how guys are running 52-55 pilots in these big carbs with small mains. I have never had to run a 50 or larger pilot on a 250 based R with a 38 or larger carb. It sounds to me like guys are running lean up top an compensating rich on bottom to get rid of lean bogs coming on the main.

[barnett468]

yes, yes, yes and yes.

Here’s the deal as mentioned. All that junk went into your lower rod and crank bearings. They may or may not be damaged. Take your chances by not replacing them all.

When you switch from std gas to race gas you need to increase jet size by around 2.

Climbing a sand hill in the heat at wot will kill a bike faster than anything.

Here’s a pressure/leak down test video with the cheap leak down tool suggestions.

http://www.youtube.com/watch?v=zdq7NRcUIp8

What exactly were you doing when it happened?

Can you post photos of plug?

What is the number on your plug?

What is the elevation that you normally run your bike at?

What gas/octane exactly are you running?

Is your ignition timing stock or is it advanced?

Did you hear any pinging?

What was the outdoor temp?

Was the outdoor temp hotter than it has ever been when you rode it?

Was this the first time in the sand?

Here’s one plan of attack without having more info.



Fix what you are going to fix.

Install a 160 - 165 jet.

Run a 9 rated plug.

If ignition timing is advanced then return it to stock.

Do the crank case pressure test.

Do a compression test, if it is around 190 run like 100 motor octane gas, it its around 210 then run around 107 motor octane however if it is higher than 190 I would lower it by installing thicker head gasket or machining the head.

[pismorat]

Thanks for all the advice thus far! The fuel was fresh in the can that I added to fuel from 4 months ago in the tank. I was winding it up pretty good when it went. Answers to Barnett468's questions below:

What exactly were you doing when it happened? I was coming across the flats in 5th, about 3/4 throttle racing my wife back to camp...she won!

Can you post photos of plug? The plug is currently full of aluminum.

What is the number on your plug? Its a BR8ES.

What is the elevation that you normally run your bike at? I'm always right at sea level to 500 ft.

What gas/octane exactly are you running? I run 91.

Is your ignition timing stock or is it advanced? Stock timing.

Did you hear any pinging? No pinging that I know of.

What was the outdoor temp? It was about 70.

Was the outdoor temp hotter than it has ever been when you rode it? No.

Was this the first time in the sand? First time in deep sand with paddles, yes. But not first time in sand.

[barnett468]

ok good info

Ok good info, it looks this way to me, we’ll see what others think.

1. the only difference in the last 3 years is the addition of paddles and deeper sand, both these things put more load on the engine then you previously on flat terrain. This does not mean that it was more load then previously when you might have been going up a hill with non paddle tires so hard to say if the paddles and deeper sand did you in.

2. if it was due to an air leak then it would have idled noticeably higher and/or more erratically than before unless it happened to get the air leak while you were riding but we will likely never know so I would do to what I mentioned earlier.

Also if this is the first time you have ridden it like this for this long it is best to run a richer oil/gas mix and jet up 1 size with the richer mix.

Your prob was not caused by lack of oil.

[barnett468]

post correction

i have no edit button.

Also if this is the first time you have ridden it like this for this long it is best to run a richer oil/gas mix and jet up 1 size with the richer mix if you plan on doing this again.

[RIDE-RED 250r]

It's a little hard to tell in your pic. But usually with a burn-down due to a lean condition you will see the exhaust side of the piston crown has been eroded away.

That looks more like a failure due to detonation to my eye. However, it's odd that you didn't notice any pinging or knocking prior to the burn-down...

I have also seen similar damage due to burning excessive amounts of coolant on diesel engines. I have not yet experienced it with a 2-stroke gas engine. Any chance you can see if there are signs of a bad head gasket allowing coolant in??

I wholeheartedly agree with other advice given to replace crank bearings and seals. And at minimum have the crank assembly checked thoroughly for damage/wear.

[MTS]

You happen to pull your carb apart yet? was that the first Close to WOT run.. this time around when it happend?