Timing issue

[motox92]

Well I just bought another 400ex from a good friend of mine....

He took the head off to put new exhaust studs in since they broke off on him....

Well the quad ran excellent before taking the head off and now it wont start at all....a shot of starting fluid will give me a loud back fire...

I played with the timing again tonight, on the fly wheel I have it on the T and the cam with the lobes down, put the gear back on, put the tensioner back on and still have the same thing....a loud back fire...

Am I doing something wrong? Help!

Thanks !!

[drew416ex]

Check your valve lash. I know it sounds stupid, but everytime Ive taken my head off and put it back on the valves are always out of adjustment (too tight) which is leaving them all cracked open and not having enough compression to crank.

[motox92]

Well it still has the stock cam....

upon doing some more research online..I started getting the idea that the spring and plunger might be missing...well sure enough I took the cam back out of it and their both missing....I really hope this is my problem...

I could just see the guy I bought it from turning the head upside down to look at the valves and it falling out and easily going unnoticed.

Now what are the odds my local honda dealer has these parts in stock:D

[Pimped_wart_no2]

what does that sping and plunger do? just out of couriosity. thanks.

[01boneless]

did you make sure the lines on the cam sproket are lined with the head?

[honda400ex2003]

Originally posted by Pimped_wart_no2
what does that sping and plunger do? just out of couriosity. thanks.

it is a decompressor plunger to enable use of the decomp mechanism on the stock cam.

if they are not there, that is your issue. they will not start without those two items.

steve

[Baileygunns]

Originally posted by honda400ex2003
it is a decompressor plunger to enable use of the decomp mechanism on the stock cam.

if they are not there, that is your issue. they will not start without those two items.

steve

If that's the case, how does it start with an aftermarket cam? That ditches the whole decomp mechanism all togeather...

[honda400ex2003]

The starter just turns it over without the release engaged. the hole is plugged off by the cam shaft and doesnt cause an issue like the stock cam does. the mechanism tries to keep the valve open without the plunger there to tell it not to i believe. thus this keeps the right exhaust valve open and wont let it build any compression to start up.

the numbers you need are:

spring: 14126-KV2-940

plunger: 14125-KV2-940

steve

[Baileygunns]

I just did some quick research about the decomp... Apparently it's a left over part from the xr400 kicker to keep from getting kick back (makes sence)... I've read were people have pressed the decomp off of the stock cam and the bike ran and started with no issue. It only makes sense that the decomp isn't needed if you lose it with an aftermarket cam. The decomp holds the exhaust valve open to keep compression down on start up. So taking it out will let the valve close... That kinda kills your idea about it not starting because it isn't building compression.

I believe you're fine without the spring and plunger. I would say you're off on timing still...

If you're dead set on needing the spring and plunger... Pm me your address, I just pulled the head off my wifes bike to do some work and I have them sitting on my bench. I'm dropping a hotcam II in there so I won't need them...

[honda400ex2003]

he has a stock cam, thus the decomp mechanism without the plunger is causing the valve to stay open all the time on his machine. it doesnt build compression with a valve open all the time... the plunger keeps it from opening as the rpms increase on startup letting the compression build up and let it keep running.

comp tests with the compressor mech installed were 60-80 psi less than with the mechanism off.

steve

[01boneless]

get the spring and plunger its not that expensive and your quad will run as long as you get the timing spot on cant be a tooth off

[honda400ex2003]

lol it will still pop and sputter a few times though with it off a tooth.

without the plunger and spring in there it wont even do that, there have been a few guys that have had the same luck with them not even popping after not installing the plunger when rebuilding.

poor hundies are helpless without those little parts, they need our help to save them. Please donate to the steve s fund for hundy help, together we can get every hundy healthy and running properly. pm me for funding options and where to send your money. We can do this together!

:( steve











lol :devil: :devil:

[Baileygunns]

I think you have it backwards. With out the plunger and spring the valve will close... So you would have more compression on start up. If not having the plunger kept the valve open then putting in an aftermarket cam would keep the valve open. Does that make sense or am I wrong?

The whole thing works on centrifugal force. That extra lobe on the decomp actualy moves. It sticks out on low rpm's (like kicking or starter spinning) and pulls out of the way once the bike starts. (trust me, I put my slippers on and went out in the shop to look lol). Pushing on the plunger and spring is the release.. No plunger and spring... No release...

Not arguing, just sharing...

[01boneless]

haha sthf '' save the hundy foundation''

[honda400ex2003]

Originally posted by Baileygunns
I think you have it backwards. With out the plunger and spring the valve will close... So you would have more compression on start up. If not having the plunger kept the valve open then putting in an aftermarket cam would keep the valve open. Does that make sense or am I wrong?

The whole thing works on centrifugal force. That extra lobe on the decomp actualy moves. It sticks out on low rpm's (like kicking or starter spinning) and pulls out of the way once the bike starts. (trust me, I put my slippers on and went out in the shop to look lol). Pushing on the plunger and spring is the release.. No plunger and spring... No release...

Not arguing, just sharing...

nor am i arguing, the decomp isnt talked about frequently. this is good topic for others to learn from.

it is possible that i have it backwards but i think that you do. without the plunger and spring the mechanism keeps the pressure on the valve all the time thus losing the compression instead of gaining it. the decomp is eliminated on the aftermarket cams, thus the compression is not lost. the mechanism doesnt push the valve open. the spring is on the back side of the head, the valve that opens is on the front side. the plunger disables the decomp mechanism and aides in folding the lobes in, without it, it wont fold up.

steve

[honda400ex2003]

Originally posted by 01boneless
haha sthf '' save the hundy foundation''

let it be known that i am the founder of it, just send donations to me, ill put the cash to good use restoring 400s from all over the US.

hahaha too funny
steve

[01boneless]

yeah sure you will youl have your self a fully built ex lol

[honda400ex2003]

no id have a bunch of restored and perfect 400s lol.

steve

[Baileygunns]

So I just went out again... Your right.

The spring and plunger is holding the pointed end of the lobe on the decomp down and pushing the back side out to hold the valve open. Once it spins the lobe clicks back into place and puts the pointed end back up to match the cam and matching the back sides of the lobes letting the valve close.

I guess I'm getting old, lol

Motox, I have them if you want them.

[honda400ex2003]

lol i knew i was right i just had to convince you of it. lol

shoulda been a lawyer instead of an engineer.

nah engineering is more fun i suppose. i get to build crazy stuff.

what ya think? lol ah man sorry im getting tired now... that freaking quality engineering case study i just got done reading has my head all screwed up. im off to bed.

at least we both learned a bit more about the decomp stuff. I learned a bit about it tonight while doing a bit of research myself. A+ to both of us for the night. cheers to another successful day!

steve