Sudden loss of power during ride; subsiquent inability to start

deadrabbit

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I was riding my 1977 xs400 and suddenly the was a small lurch and the engine died. When the electric starter is actuated, it sounds as though the starter motor is spinning without any load. The kick starter moves smoothly and oddly easily. I removed a spark plug and put a thumb over the opening while kicking the engine and felt no compression. When I put the engine in gear and then try and bump start it, the engine turns over but will not start. Would anybody hazard a guess as to what the problem may be?
 
Possibly the cam chain broke? Could also be the head gasket blown between the cylinders. I doubt it's a hole in a piston because you would easily tell that one side had compression and one side my even fire.
 
Although it seems that you may have lost compression from any of several reasons Hough mentioned, don't forget the simple things first. I had a similar incident with my xj700 and it turned out that I was simply out of gas. I had filled up earlier in the day and it just didn't occur to me that I might be of gas since I had only ridden a few miles. It turns out one of the floats got stuck open and I had a crankcase full of gasoline. The carbs were due for a good cleaning anyway.
 
Maybe a broken ring. There was no smoking or any other funny noises?
 
just take off the cam caps and see if the rockers are moving to confirm that your camchain is ok.

do a compression test, if no good

then take off head to check you haven't burnt a valve!
 
I don't think its the head gasket or the rings. I just a few days ago replaced the rings and the head and base gaskets.
 
Then I would say something came apart or was not torqued enough you might have missed something :shrug:
 
Wanted to restart this thread because the exact same thing happened to me, to a T. I'm on an '81 X400H (4R5 engine).
When i use the electric start, i hear a light ticking noise but engine doesn't do anything. Kick start has no resistance that I can feel. Battery is brand new, installed 10 days (~350 miles) ago. No oil leaks, level looks fine.

@xschris … regarding your initial question for OP – yes, for me there was smoking from the left side of the engine, which I noticed as i rolled to a stop on the side of the road.

@drewpy … I have found no other mention of cam chains breaking in this entire forum. Could this have happened? Also, I have removed the top engine case and looking at the cam and rocker (at least what I think the rockers are, based on the manual's pictures). How do i determine if they are moving or not?

If it helps, I have a 2nd spare engine which I purchased to tinker with and learn how motorcycles work.
I've been reading this forum religiously, and both of you have been a great help in learning. I am still a total rookie, but willing to learn.
 
Rockers look good after taking off head, no wear or wiggle. Took them off by following manual (pg 57).
No wear at shaft hole or cam lobe - those two sites mentioned specifically by manual.

Could it be burnt valve? What does that mean for me?
 

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If it helps – even though the engine won't start, the neutral light works and electrical seems fine. I can shift up into gears 2-5… But i cannot get into 1st gear.
 
I was planning to buy a compression gauge today to start off the diagnosis…

But will I be able to check compression if there is no response from the kickstarter, and no response from the electric starter? As I mentioned before, I do hear a 'tick tick tick' and whirring sound when i hit the electric starter button, but no feeling of it catching and turned the engine…
 
I would look at the clutch and also pull off the starter chain/clutch on the other side. When the kicker stops working it's usually the clutch being to tight.
 
Thanks @xschris ! is clutch (right side) tightness mentioned in the manual? I'll look again tonight, just wondering if you know off hand.

I have the stator removal tool (M16-1.5 tool on order and should be coming on Tuesday). Won't be able to take the chain off until then but I can examine what's going on....
 
Well, spent an hour on the bike today and It's not looking good.

I removed the left side cover, and tried to manually turn over the engine using the rotor/rotor bolt. Everything inside the stator cover looked ok (I used this thread "Stator Clutch Renovation" as a reference).

After turning the rotor nearly one full revolution, I found it hit resistance. I didn't continue because I didn't want to force it. That made me think – it could be the pistons are jammed somehow.

I chose the least invasive route, and removed the spark plugs and used a little flashlight to see what was going on. On the right side, nothing interesting was going on, other than fairly white spark plugs. But the left spark plug… well I'll just leave this picture here. And a picture of the view of the inside of the cylinder through the spark plug hole. Pretty, pretty, pretty, pretty bad.
Also, here's a little video of what the inside of the chamber looks like (sorry for the length – start near 25 seconds in).


Advice on what to do next? I haven't seen this issue in my search on this forum. I do have a second engine so I can salvage parts from it…
 

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From the pics that I can see, it look like a melted piston. I would just use the spare motor and not mess with this one.
 
@xschris – let's say best case scenario…
  1. valve cover and cylinder head on "Engine A" in the bike is fine
  2. Piston and cylinder on "Engine A" are damaged
Can I swap out the piston and cylinder from "Engine B" (my backup engine) and just pop 'em into "Engine A"? This wouldn't need rehoning or anything I don't think…
 
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