i got a 2003 z400, I have been workin a few issues with it over the past couple weeks. I have noticed a loss of power on the bottom end, compared to most other z400's I have ridden. Keep in mind, the quad runs fine.

So, like anyone, a buddy of mine and me tore it down and did some troubleshooting...

First thing we did was a compression test, the Clymer manual says at crank with the throttle full open, it should sit somewhere around 142 PSI. Well mine was at about 95 PSI. That is the first problem.

Next thing we noticed was that it didn't have rockers, just risers. We aren't too familiar with z400's and we didn't have the manual at this point so we don't know if this is norm.

Tore the carb apart completely, cleaned it thoroughly. When I bought the bike from the guy, he said it had a Dyno-Jet kit in it. What we found, was the stock brand carb. with Mikuni jets in it. I don't think that's too bad.

We didn't tear the quad down any further (cause I was going riding the next day).

I've talked to alot of guys at work about it, a couple are saying I need to replace my cylinder, piston and rings. Others are saying it runs just fine, it just doesn't have power cause it's older. A couple people said it is setup for racing, so there is less torque on the bottom end.

I am looking for suggestions, still. Expertise is welcome....

 

well if your compression is that low, obviously the engine is a little tired...... by nature, the engine will run fine, it just won't have the power of one running 140psi of compression. If it's not too slow for you, and it still pulls hard enough don't rebuild it. If you want more power, rebuild it...... it's really pretty much that simple....

 

I don't have a z400 but 95 psi seems kinda low. I've read somewhere that anything below 100 really shouldn't even be running,,, nevermind putting out decent power. My 230 put out over 200psi and a few weeks later I tore it down to find a puddle of oil on top of the piston (ie. worn rings, distorted cylinder, etc). My 220 bayou puts out 190 psi. A wolverine was a good 160 psi while it was smoking like a tar wagon. I can't imagine what kind of shape an engine would be in to only read a 95!

I think you should rerun the test with the throttle open (not just the butterfly valve) if its a cv carb. Or better yet, remove the carb from the intake. That's the best way.

 

Yes, 95 seems low. But, keep in mind that there is a compression release too. Does the manual state 140 with the compression release or 140 on a fresh engine no comp. release?? Unless a person were to take off the release, you'd never get a true reading, only a reduced number.

 

How does the compression release affect a compression test? Do z400's have an automatic compression release? Like I said, I don't have a z400.

 

How does it affect it?? A compression release holds the exhaust valves open when cranking the engine over, thus creating a low reading on a gauge from never building true pressure numbers. And yes it's automatic.

 

<div class="FTQUOTE"><begin quote>Originally posted by: theCATman
A compression release holds the exhaust valves open when cranking the engine over.</end quote></div>

Well, I knew that.... But most quads you have to reach down and press the lever before it will hold the valve open. And even then, it pops back up by itself after one revolution. So, even if you did apply it, the test would still be valid.


<div class="FTQUOTE"><begin quote> And yes it's automatic.</end quote></div>

That's what I wanted to know. When does it turn itself off??? When the engine starts or after a revolution? Come to think of it... How does it even turn itself on? A solenoid?

 

The release is an internal mechanical devise on the exhaust cam. It's spring loaded & when the engine fires the centrifugal force keeps it from engaging once the engine is running.

Obviously a compression test is done at cranking speeds & so no, the test would not be valid for true cranking compression #s. That's why my original post to Kalfin was if the 142 lbs. is for with or without compression release.

Now, seeing as how you may be doubting me, I know what your next question will be JustRandy..... "How can you get a true cranking # if the release is automatic"??.

If a person would really want to go thru the hassle, you could pull the valve cover off, pull the exhaust cam out, take off the mechanism, re-assemble & then perform a compression test. However, if the engine is in really good condition the battery & starter would have a very hard time spinning the engine over. The stock electrical system is meant to work with a compression release system. That's why you hear of fried starters & dead batteries on machines that have aftermarket cams that don't have release provisions on them.

 

JustRandy......Think of the release as a one-way bearing. At cranking RPMS it engages, but once the speed picks up over a certain RPM it freewheels. Hope this helps you get an idea on how it works.

 

I understand it now. Thanks CatMan. I also was wondering why some guys were talking about compression release mods with a new cam. Thanks for taking the time to explain it all.