Lost compression

[gtkane]

My son's '83 lost most of the compression in the front cyl. while riding at 55MPH. No death rattles or expensive noises, just a sudden loss of power and oil blown out of the crankcase breather. Only 60PSI compression on the front cyl. now. Does anyone have any guidance before we start to tear the engine apart?

[Superfly]

Welcome.

Not a very common problem for out bikes.  Luckly it was the front cylinder, because you can take the front head & cylinder off easily with the engine still in the frame.  Where was the oil coming from?  If oil is coming out of anywhere, it could be a blown head gasket (really easy fix) Take the head off and check the head gasket, and you will be able to check the valves easily with the head off.  If you don't see the problem, you can pull off the cylinder and check the rings.

For a quick test, you can drop a couple of drops of oil down the spark plug hole, and redo the compression test, if compression raises up, then it could be the rings.

[gtkane]

Thanks for the reply. He said the oil was comming out of the crankcase vent. He has done some more testing and said that fuel/air mixture blows out of the front carb when he runs the engine (it still runs on the rear cyl.). Also, when pressurizing the cyl. (at TDC) he can hear air leaking out of the exhaust. We're thinking that maybe the valve timing slipped somehow. Thanks again.

[zore]

Mine did that while riding.  It blew oil and water out the rear cyl all over my boot.  Lost power and just ran like poop.  I let her sit a little, and said, if it blows it blows.  Got on it and started riding.  It actually stopped and the power came back.  It fixed itself.  Most bizzar thing i've ever seen.

[gtkane]

My son finally got the rocker cover off the front head. The bolt that locks the cam gear to the exhaust side camshaft had backed itself out and stripped the splines or keyway on the shaft. Sounds like yours may be loose also.

[Lucky]

GTKane,
please let us know whether the engine is an 'interference' (sp) engine.  i.e., did she bend valves?
i don't think anyone's had to deal with this yet, not in recent memory anyway...

Thanks, --Lucky

[Walt_M.]

Oh boy, another thing to watch out for! How many miles are on it and do you know if it had ever had the cams removed for any reason. Some people remove the cams to change the valve shims and it may not have been properly torqued on reassembly.
It has not been a common problem, yet!

[louthepou]

I'm with Walt on this one. There's no reason for that bolt to come out on its own unless it has been badly reassemble before. Properly done, it's with red lock tite and 50 foot pounds...