Strange compression loss and engine lock up
Posted February 06, 2005 - 09:58 PM
I put the bike in gear and rolled it backward a bit and then tried the kick lever slowly again. It turned a bit and then brought up solid. The only thing I could think of that would lock up the engine at a certain point and cause a loss of compression was a bent valve.
I sat on the bike for a few minutes thinking about the long cold walk back to town and then I rolled the bike backwards and kicked again and it kicked through, but still no compression. I kicked slow a few more times and it turned freely which was strange since I didn't put enough pressure on the lever to push the valve away from the piston.
Then I think that I cracked another throttle plate and a bit of metal was stuck under a valve holding it open and fell out when I turned the engine backwards, but the valve wasn't sealing. I waited for a while hoping a sled would come along and now and then would give it a few kicks. Gradually the compression started coming back. I'd get a little pop but with the low compression I couldn't kick fast enough to get it to run. After an hour a few friends happened by on sleds and I got one to tow me to try a bumpstart. He dragged me for half a mile before it finally caught and was running crappy but I kept it running and aimed for home. In a few minutes the engine cleared right up and was back to normal. I took it to the shop, checked a few things, found nothing, and went back out and rode for 3 hours solid running perfectly, all the while trying to figure out what the heck happened.
Then I wonder if it is possible that an intake valve iced up and froze open.
I've run the bike longer and harder in colder weather and never had this problem. Then I think that maybe the higher lift of the new cams is pushing the valves deeper into the guides and crap buildup is causing them to stick, but why only at high speed and not high revs in lower gears? The bike is running great so I head back out onto the bay for a test and sure enough after a few minutes she starts to crap out again. I pull in the clutch and keep it running and after a minute sitting still it clears right up and works perfectly again. It was getting dark and I was half froze so I headed to a superbowl party to thaw out.
I'm gonna do some more testing but this one's got me stumped. Anybody have any ideas? Any spaceships in the area lately?
Aside from the compression virus I had a great ride. My bike has been apart for months and it felt good to hit the slopes (although it's gonna take a while to toughen up to that brick of a seat again). One nice thing about riding in the snow is you never have to wash the bike. Here's what it looks like after 4 hours solid riding. My helmet vents iced up a lot and I had to stop every half hour and chip the ice out with a key so I could breathe, and I got a touch of frostbite on my cheeks, but that's just part of the deal up here.
Posted February 06, 2005 - 10:47 PM
Posted February 07, 2005 - 06:38 AM
I had been running without the plastic rad fins for a few years. Before I got the ski I was crashing every day and my rads were so bent up the fins didn’t fit anymore. I got new rads and new fins last spring and I fixed an overheating problem. This is the first winter ride with the improved cooling system. Maybe at high speed the engine is getting overcooled allowing the intake valves to ice up. I couldn’t imagine that ice could form on a valve in a running engine but maybe if the coolant is getting too cold the frosty air could do it.
The other difference is the engine oil. I always run synthetic 0W40 but Hotcams recommends against synthetic for break in so I used the lightest oil in town, 5W20. Maybe it's thickening up with the overcooling and sticking the valves. I'll see if it improves when I switch back. I do know that the bike is a lot harder to kick over in the cold with the non synthetic.
I think I’ll invest in a temp guage and maybe a thermostat. Anybody out there have either?
Posted February 07, 2005 - 07:57 AM
Posted February 07, 2005 - 08:08 AM
Posted February 07, 2005 - 08:15 AM
but the springs forcing the valves back are mighty strong
some resonance maybe at high speeds+cold?
options are many
my lady has no problems in cold, radiators squeezed to 85%, no boiling if idling long
Posted February 07, 2005 - 08:50 AM
Posted February 07, 2005 - 09:03 AM
Posted February 07, 2005 - 11:02 AM
I didn’t remove the valves at all, just popped in the cams and checked the lash and piston to valve clearances.
The engine never got hot enough to kick coolant into the overflow.
I’m leaning more toward a cold related problem but I’m wide open for suggestions.
First I also thought that the tight valves got hot and closed up the clearance, and I actually packed snow around the engine to cool it down faster once I felt compression coming back. But, once I got it bumpstarted from the tow it didn’t run well until it warmed up. The second time when it started to loose power on the bay I was thinking that cold was the problem and I stopped and revved the engine sitting still and it cleared up after a minute or two, so I don’t think it’s heat closing the clearance. Also when it quit the first time I was just cruising at 60 on the bay, about half throttleish. The hour before that I was hitting deep snow and hills now and then which was putting the engine under a lot more load and never missed a beat.
The varnish on the stem makes good sense, but wouldn’t it also stick at high revs in a lower gear? Maybe the extra cold is gumming up the oil and combined with the varnish it sticks.
I have the new shims and am going to pop them in before the next ride. It was 2 intakes that were on the snug side so it may be that simple. I am fairly convinced that the piston was touching the valve when I tried turning the engine over. I’m surprised that it didn’t bend since there’s not much piston to valve clearance and I did coast to a stop in gear. This is one for Weird Science.