Yes if the valves are slightly open then your pressure would suck. However it probably wouldn't start at all if one or both valves were overtightened cause it would be blowing the intake air/fuel mix right back out into the intake, causing backfire as soon as it sparks. Sounds like you have it figured out. Keep your battery at full charge or you may run into some impossible starting.

 

<div class="FTQUOTE"><begin quote>I was getting about 6-7:1</end quote></div>

What he's saying is the compression gauge should give you a pressure reading, in PSI.

If it doesn't continue to backfire, you might run it for a while and see how the plug looks...

 

Yeh, I read online and it said for the pressure I was getting that it is about the ratio I stated. I seriously don't think this monster's lacking compression though [img]i/expressions/face-icon-small-smile.gif[/img]

I'm going riding this weekend and it will definitely get its break-in period or whatever it should be called [img]i/expressions/face-icon-small-smile.gif[/img].... if it doesn't just.... break =P

 

I think it's compression ratio is like 9:1. Book says 100-128 psi is normal.

Anyway, have a fun ride.

 

Well, I really got to test the 400ex out this weekend. We went about 40 miles on some trails, some pretty rough stuff, and I drove it pretty hard and it's still going so...
I guess it's all good. It WILL NOT start from cold on a low battery though, which I seem to think is normal, and I am thinking more and more that I probably have an after market ignition. I brought my charger and had jumper cables just in case though. Just wanted to leave an update on the quad and tell you all thanks again for the help and concern! Excellent forum!

 

Thanks for letting us know! Forums are nothing without willing people and riders! Have fun and post some pics up!

 

Hello everyone, I have a similar 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

 

ok my cousin has a 2000 400ex honda forwheeler and my boyfriend has been trying to fix it for him he had to put a new headgasket on it and he had to put it back in time and he added a new spark plug the starter is bad on it so he has to push start it. It worked fine b4 the headgasket messed up it has gas in it and new oil but he cant get it started it acts like its going to start but wont what could possibly be the issue hes checked almost everything on it but can figure the problem. Ps.It would start b4 when you push started b4 the gasket did what it did any help would be great.

 

Welcome to the forums Lisa.
You should start your own thread to get more help.
I'd be double checking the valve timing.

 

im having the same problem with my 2001 400ex first time ive had a 400 confuse me like this valves are in spec timeing is on point compression is holding 117 and ive got new carb new spark plug its sparking just cranks wires all look good