XR400 blowing oil out of crankcase....HELP!!!!

[davidlomax]

Gents,

Apologies for the odd title, but this is an unusual situation. My partner and I have been planning to head south fom the UK to race in a 2 day event in Spain for nearly 6 months. My bike was already in southern France (100 miles away fom the race start after a sahara desert trip at easter, so I have just flown in...) but Chris's bike needed a rebuild and riding down from the UK to the start of the race. He has an XR 400 re-bored to a 440 with a slightly hotter cam than usual, but other than that and CRF forks the bike is standard spec. The race is due to start on Fri 6th (briefing) and we were due in northern Spain four days early to check road books etc. Two days ago after a bottom end engine rebuild to replace bearings etc he set out from northern england heading south to Dover to catch a ferry to Spain. About 30 miles into the ride his engine popped. It turns out that when he pushed more than 60 mph his engine blew oil out of the crankcase onto his back wheel. He survived that, but when the piston siezed in the bore he was pretty screwed.... I left for Spain the next day on my pre booked flight and have just sorted my DRZ, but with no partner...no race. ...Chris took his engine back to the builder and has a new piston and liner put in complete with a top end rebuild. After a mamoth effort, no sleep and 20 hours he is on the road south and waiting at the ferry terminal to head over to Mainland europe in a torrential rainstorm. Problem is that despite the rebuild we arnt much the wiser about what exactly caused the problem in the first place, and apparently its still happening. Chris is struggling south with his crank case breather attached to this filler on his top tube by a piece of hose, and every time he goes faster than 60mph it fills a little!

Even if he reaches Spain we cant race like this. I have my thoughts on possible problems and am ready for a strip and rebuild again if necessary, BUT...I dont really have the time for the job if its not necessary...So we need help!

The engine has been spun up to speed using an air gun and Chris tells me that it was pumping oil everywhere it was supposed to...I dont know much about XR;s, but am I correct in supposing the following?

1. If the engine is running correctly and lubing properly then the crankcase back pressure is likely to be due to some problem with the oil scavenging circuit, either at the pump (not getting rid of it fast enough....does it have a two stage pump?) or as some kind of restriction between the circuit to the frame and the crankcase (or round to the pump inlet), or
2. some kind of pressure increase in the crankcase that causes the normal galleries to be unable to carry the oil supplied (never heard of this before...). Possible sticking rings......(Sounds unlikely to me....)

I know diagnosis of ths stuff is nearly impossble over the web, but whatever help you can give would be appreciated. Assuming Chris makes Spain around Mon lunchtime (Spanish time) we have only 36 hours to sort the problems before heading south...............its going to be a tight one........

Many Thanks,

Dave

[Lone Rider]

This is tough.

The two oil blow problems I've dealt with have been on my 1150GS where the design doesn't contain the oil in the crankcase when running at very high speeds for extended periods (the 1200 is better in this regard, supposedly), and an oil overfill issue (our doing) on my GF's F650 due to how the engine and oil supply is designed.

I don't know the crank case ventilation system on that bike, sorry.

[Dodger]

Two thigs come to mind here:
- piston blow-by .Which could be caused by insuffucient engine break in and the rings not sealing or a partial seizure smearing the cylinder wall .

- poor scavenging due to flywheels scooping the oil past the oil collection point in the crankcase - it happens to Norton Commandos a lot .It only becomes apparent at SUSTAINED high rpm .Varying the rpm ,as in a race ,will negate this effect .
The second reason might cause cavitation in the oil pump and a resulting lack of pump efficiency ,was the pump replaced ?

Pump seals maybe ?

Sorry but I'm not familiar with this engine ,good luck .