Hello everyone, I have a an issue.

So I been helping my nephew on this 400ex. First, the original owner "rebuilt" the top end. since then, it won't run. I take a look and heard strange noise as the piston moves up and down. we take the head of and rotate the flywheel ... the whole jug moves up and down. they had left out the jug bolts.. so after get the jug installed correctly, i reassembled the top end. I made sure times was TDC as I have down many of these. I installed the factory cam with the decompression spring and pin missing. My understanding is it would still run but "tick"

so. trying to start it, it tries to fire but just just never fully fires. I checked timing, spark, compression, and fuel. I changed out carb, even changed out the cam from a working 400 ex. I set TDC and loosened all tappers so I know none of the valves were staying closed.

Still no luck, I then pulled the head again, inspected valves even let some kerosene soak over night over the valves had it on a dry towel so if it did leak I would see a wet towel and no kerosene in the morning. So valves are tight not leaking or bent.

I put everything back together, tried to start and no go. I changed cdi no luck I changed rectifyer no luck, i used jumper cables to make sure i had strong spark no weak battery, no luck, New plug, no luck,

I am at a total loss, I have Spark, Fuel, Compression

Any advise?
thanks

 

If you have spark, fuel, and compression, then it HAS to be a timing issue.

 

if she won't fire over with ether and you have a spark- then you have a timing issue.
It's either valve or ignition timing. Also make sure the piston is facing forwards-
)you mentioned you've done these before so I'm sure you would've noticed that- )

friend here "jgar" has rebuilt many of these 400ex engines- I'll forward your post to him and see what he has to say....

 

Find and install the pin-plunger and spring.
I have never installed a stock cam with out the plunger and spring. As I understand they wont start with out them, using a stock cam. If you use a hot cam there is no need for the plunger and spring.

 

i have the same problem. i can hear it fire, small amounts of exhaust. top end new. i put it all back together, it started, idled on its own but it had a slight ticking noise. I pulled the lifter cover and adjusted the valves. the left exahust valve was slightly loose. I put it all back together and now it wont start. compression is 120, timing is on, it wont even fire with small amounts of carb cleaner. the timming marks are right on.. what is this spring and plunger they were talking about?? mine is a 400ex 2001--- its yellow if that makes a differance:-)

 

We need more info, Bore ,compression, cam, carb jetting, etc.

 

its punched 20 over everything else is stock. i went back in to adjust the valves for the 3rd time and they were tight again. i couldnt get the feeler gage in. i adjusted the exhuast to .05 and the intake to .04 as i read. put it back to gether and it started right up, but has a ticking noise from the top end. right after the rebuild, it started right up but had this same ticking noise, so we went in and adjusted the valves, and it didnt start. so now the valves are adjusted again, it starts, but the ticking is back