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Engine locked up 10 mile after rebuild

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Just finished rebuilding my XR400.

It was a full strip down with new oversize piston 1mm.

It was running sweet for a good 20-25mins of gentle road riding then out of nowhere the lights dimmed and it just cut out.

The Kickstarter was jammed solid and I was forced to push it all the way home!

When I got back I tried turning it over with a 17mm socket, it was solid but when I turned it the other way it free'd it up and was then able to turn counter clock wise again. It seems to turn over fine now but I haven't tried to start it incase there's any damage or reason to this.

I can't understand what could have happened for it to do this. It was running fine but i did notice a strange noise when at low RPM and giving it full throttle, it was making a couple of chirping sounds then it would go, again this was only at low RPM and giving it full throttle.

Any ideas or is a tear down needed?

Absolutely gutted!

Thanks

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Hi did you run it in, ie ride for 10-15 mins, not too much throttle, let cool down fully. repeat process couple of times ?

Or did you just run 10 miles on road straight after rebuild ?

 

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When is just rebuilt it I had it idling for 5 mins, then shutdown, then did the same the next day, then the day after that I just went out for around 10 mile, this was only driving through 30mph streets though so was only going slow up and down the gears taking it easy

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'chirping sounds'

 

Did you check / set the piston ring end gaps to spec. ?

 

Edited by mlatour

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New piston, bored out cylinder, new cam chain, full strip down of everything to get rid of all the debris.

I didn't do anything with the rings as the instructions said they were just ready to fit.

Looked online and the chirping on acceleration is most likely due to head gasket / headers leaking.

I'm going to start the tear down soon.

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After boring a cyl there is no way I could ever sleep again if I didn't check the ring end gap.  When it comes to things like that I trust no one. Good luck.

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Yep. Always check the gap. Did you check crank play while you were in there? A fresh top end will put additional strain on the bottom so if it was near the end, that could push it over the edge.

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Ok just had a quick look this morning, took the rocker cover caps off and there's oil everywhere up top, so that's good.

I've turned it over around 20 times, at first it felt like there was an ever so slight grinding feel but that's cleared now, it doesn't seem to build compression as much as it used to, feels easier to turn over by hand? Also noticed that the manual decomp exhaust valve seems to have been sticking? Took a lot of turning over for it to be free when all the other 3 valves were free to wiggle, even when I'd removed the cable, and now the exhaust decomp valve has a lot more clearance than the rest. The head was given to my mechanic who re-cut the valve seats and cleaned the valves up, came back looking like new.

And yes, crank was checked over by a good known mechanic and there was slight side to side play but no up or down play.

Not sure wether to fire it back up and see or take off side case and/or strip it back to the piston...

Edited by alan0259

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Do you have access to a borescope ?

Drop it down the spark plug hole and have a look inside the cylinder.

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There is obviously a problem. It won't fix itself.

It may run but for how long?

I suggest tearing it down.

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Well I've just ordered a 7mm USB inspection camera but do you think it's just better to still take off the head and cylinder to see if piston/rings/cylinder has damage?

Also the right hand side engine casing to show the clutch and all the gearing?

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IMO that chirping sound you heard is either a load bearing surface that ran dry or, a piston ring grabbing in the cylinder wall.

It's not a 'normal' break-in noise that just goes away without issue and since it already locked up once, something is damaged.

 

If you don't want to damage anything further, (as in parts that are still re-usable now)

bite the bullet and do a COMPLETE disassembly & inspection of all components.

Edited by mlatour

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As a honda mechanic i can tell you that strange noises in a XR motor is not normal. Head and cylinder off please. Then if everything looks good its time for a case split.

Conrod sideways movement is required. The range is 0.05mm - 0.70mm

Use feeler gauges to test this.

Also. Zip tie your clutch lever in and try and turn it over to see if the grinding noise disappears. Is it does that means you have a bearing problem in the gearbox.

Good luck

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Sorry for the late reply guys, been busy lately and only had chance to open it up tonight.

 

The cylinder is scored and rough on the intake side, and the piston skirt is also really rough, nothing has broken off anywhere though.

 

This is what happened before I rebuilt it, but the piston skirt had completely ripped off and was in loads of pieces.

 

I'm assuming it's the same cause for both pistons, really gutted as it was a new piston and a freshly bored cylinder!

 

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Yes, that's why I split the cases, I removed every part to clean out everywhere, even replaced the oil pump.

 

Yes, took it to a place that does re-boring, give them the cylinder, new piston, and proper clearances as shown in the manual.

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