This is a buddy's 650,he said it will not turn over with the sparkplug in but spins fine without it & even replaced starter.I'm thinking valves?? Anyone know the valve lash,not sure of the year?

 

almost sounds like the timing is way off, did it always start a little hard, then stop spinning over with a plug in, or suddenly start doing this? if it just started all of a sudden, the timing chain could have jumped and you are trying to crank with closed valves, but it should still spin, just labor a lot and maybe pop back through the exhaust. Is he using the correct plug? if it is too long, maybe the piston actually hits the plug and stops, when the plug is pulled out,is the tip mashed down a little?

 

Good pointers,I'll check the timing as well as the tensioner.I don't think he changed the plug but I'll check to be sure,he said he drove it parked it and a few days later tried to start it but it would not turn over but you know how that goes,always more to the story.He said he put in a new battery and a new starter hoping it would solve it.I'll most likely bring it back home with me.Thanks

 

Now that you told me it sat a few days then wouldn't start, there is one more possiblity, if the carb leaks real bad, it may be possible it filled the combustion chamber with liquid gas, and liquids will not compress, so the engine hydrolocks and will not move. But if that was happening, you would get a big squirt of gas out of the sparkplug hole when it turns over without the plug, and the crankcase would likely be full of gas as well.

 

It would be interesting to measure voltage drop between the battery terminal and the starter terminal while attempting to crank with the plug out...

Could have a corroded cable up under the plastic or something.

Has he tried the pull starter?

 

Good point, Jeffin, low battery volts would make it hard to turn over with the plug in, check to see what the battery draws down to while cranking.

 

The thing is. according to him the pull start will not turn it over with the plug in

 

They are tough to pull by rope normally.

I wonder if the decompression thing is stuck... That, or hydrolocked, would be the only two things that would make sense if it really won't turn over at all with the plug in but spins fine with it out.

 

ok,I have it in my garage,it was just as described,pull the plug & it spins over fine even with the pull start"no matter how hard it is pulled" but with the plug in neither the pull start,electric start or both will turn it over.The plug is correct,I adjusted the valve lash which was not out bad but made no difference,intake & exhaust valves move free and appear to be in time with TDC & no gas in cylinder,it also has a new starter & battery,my charger on boost will not turn it over.I thought it may had jumped timing but found out the previous owner had the same issue from time to time that may had been the reason it was sold so cheap to my buddy but between the times of starting issues the previous owner drove it and it run like new,same with my friend,he drove it on a trail ride,parked it and it never started again,no symptoms,noise ect..I did have it spin over for a split second with the plug but not enough it start.My only conclusion is as mentioned by Jeff is the decompression valve ect.... I have tore down a 500 and the thing for decompression part is on the cam,which I'm guessing would be similar to the 650.If the decompression valve was bad would that cause it to not to turn over at all?

 

The decompression thing would be my guess. It is mounted on the cam on the 650.

It would be interesting to see what kind of compression reading you get when trying to cramp.