Another YZ400 Noob Needing Some Assistance...

Hello TT Community!

I am totally broke right now but want to ride with my 14 yr old son who is literally addicted to dirt biking. So recently I found a 98 YZ400 that is in very good shape but "needed some carb work" for $600. It starts after some work but won't idle. I'm pretty mechanical and have rebuilt carbs on CRF 70, 80, 150 and Suzuki DR650s. So I'm thinking this is a no brainer...pick up a bike in good shape for $600 and put in an afternoon with some Dokken and ice cold beers and start ripping. Now from hrs of reading YZ400 treads it looks like these carbs are junk and parts are hard to find and I'll need to spend some money to move to an 05> YZ450 carb. So, I am looking....

In the mean time I do have several questions for you gracious experts that are willing to help a noob out:

#1 I did take the carb apart and cleaned it up some and found out that the serial number says it's from a 98 WR400 - not YZ. Wierd. Anyway also noticed that the pilot screw assembly was put in wrong (washer then o-ring) and it was turned in 6 turns when the manual says only 1 1/4 for 98 WR400 carb. So I fix that and am stoked thinking I may have fixed it! But I can't get the freaking throttle cables to actually work with the throttle when I go to put it all back together. &%$#@!?! I've spent an hour or so putting them on and taking off. There is absolutely no snap it just hangs there. I made sure both were attached as they were before even took pictures prior to removal. Any thoughts on what I may have done wrong??

#2 I was just curious about the TPS and CDI and ran some tests on them. The TPS tested fine but when I tested the output voltage on the CDI nothing came back - very well could have done it wrong. If the CDI was bad would it still start and would I be able to ride it down the street and back and then it may die?

That's it for now. If I could get the freaking throttle cables back on I could see if I fixed something with the pilot screw. But am now also concerned that my CDI is dead.

Thanks again for any help!!

On the fuel screw, it's 1 1/4 turns OUT from seated, not in.

The bike will run without the TPS. The cruise operation MAY be affected, but heavier throttle openings will not. There is no VOM test for the CDI, so you are chasing your tail there.

Try picking up a manual from Here:

http://www.yamaha-mo...uals/index.aspx

Go all the way up to a 2002 YZ426 if you have to. Electrical tests are laid out there. If you can't get one, check back.

Thanks Gray Racer appreciate your jumping in!!

Sorry I meant out not in. I gently snugged it and then back it OUT 1 1/4 turns so than should be OK - right?

I actually got the manual from the PO and that is where I saw the test for the CDI. It has you do an output voltage check with a volt meter. "Connect the positive meter lead to the blue wire and negative lead to the black wire." Supposed to put out 4-6 volts DC but I got nothing. I also checked the spark on the plug and honestly not sure what its supposed to look like ...it was sparking but pretty small blue spark. Would it run for a bit with a bad CDI?

Otherwise I suppose the carb is dead or my valves need adjustment - I have not checked those yet. The PO did put in the hot cam upgrade.

Any suggestions on where to go next? Valves, replace carb, replace CDI?

Thanks again!

Once again, in the Yamaha owner service manual for either the '98 or '99, there is no test for the CDI, and no tests that take a voltage reading. Don't know where you're seeing that.

On the other hand, the WR400 does list a voltage test, but it's for the TPS, and it requires the bike to be running. Better review the section again.

To the question as to whether it would run a while and quit with a bad CDI, yes, that's possible, but it's much less likely than the same symptoms caused by a spark coil or one of the stator coils. The question is answerable by checking to see if it has spark right after it quits.

Yeah that is strange, reading from Clymer manual under Electrical Systems Chaper 9 page 207 there is a section for CDI Unit Output Voltage Check from CDI to the TPS that is what I was trying to check.

At this point I cant get it running at all. I kicked so much last night I nearly threw my back out.I know the wire going to the coil is pretty brittle and some of the coating has come off, I taped it up but there may be a failure point. I'll check the spark again and perhaps focus on the coil some.

But I can't get the freaking throttle cables to actually work with the throttle when I go to put it all back together. &%$#@!?! I've spent an hour or so putting them on and taking off. There is absolutely no snap it just hangs there. I made sure both were attached as they were before even took pictures prior to removal. Any thoughts on what I may have done wrong??

GR or anyone else, do you have any suggestions on what I may have done that the cables wouldn't just bolt right back up after removal and replace of carb and provide the same snap back that I had before? The throttle lever on the carb is nice an snappy but when i bolt up the cables (exactly as they were in my picture) there is NO snap back. I spent another hour last night working on it and man I'm stumped. ;(

Yeah that is strange, reading from Clymer manual under Electrical Systems Chaper 9 page 207 there is a section for CDI Unit Output Voltage Check from CDI to the TPS that is what I was trying to check.

That's not exactly right. One of the problems is that you are using a Clymer's manual. The CDI should provide an input voltage of 4-6 vdc to the TPS. The TPS then sends a modified voltage signal back to the CDI that tells the unit where the throttle is so that the timing can be mapped for part throttle operation. The function is similar to that of an old vacuum advance; more lead at lower throttle openings. That voltage will not be present unless the engine is running. Then, too, if it's running and there's no voltage at the TPS, that does mean something is wrong. Check that same lead at the CDI connector by back probing it.

On newer bikes, the adjustment to the TPS is made be measuring the return voltage, rather than the resistance of the TPS at idle, since the end result can vary based on the input voltage.

GR or anyone else, do you have any suggestions on what I may have done that the cables wouldn't just bolt right back up after removal and replace of carb and provide the same snap back that I had before? The throttle lever on the carb is nice an snappy but when i bolt up the cables (exactly as they were in my picture) there is NO snap back. I spent another hour last night working on it and man I'm stumped. ;(

The cables may simply be bad, as old as they are, or routed so as to be in a bind. Also, be sure you are using the right cable in the right place, not reversing them.

Thanks GR.

Not certain what you mean by "back probing"...can you elaborate on that some? I did put my voltmeter probes into the CDI outlet (female) that connects to the TPS - is that what you mean? My trusty Clymer didn't say if the bike needed to be running or not so was assuming one had to do it while not running as the TPS would be disconnected. So if I can't get the bike started at this point - which I can't - CAN I "back prob" with the bike off or no?

Re cables, I literally had them off the carb for 1-2 hours for carb cleaning. I marked them "T" for top and "B" for bottom with sharpie prior to removal and took several pics for reference. I put them back the exact same way they were prior and no snap on the throttle. They are old but worked fine prior to removing them from the carb. Stumper...

Re cables, I literally had them off the carb for 1-2 hours for carb cleaning. I marked them "T" for top and "B" for bottom with sharpie prior to removal and took several pics for reference. I put them back the exact same way they were prior and no snap on the throttle. They are old but worked fine prior to removing them from the carb. Stumper...

All right for anyone that cares I am a complete tool. I finally decided to get my head out of my butt and check the throttle assembly and the freaking grip was rubbing on the brake housing. Un-freaking-belivable. I think I actually may have moved the brake housing a tad bit and that was my problem. Major snap now! Yeah for me.

Now just have to get the bike running. I think I may have a bad coil having Roseville Yamaha test it tomorrow.

Edited by WrightStuff

Not certain what you mean by "back probing"...can you elaborate on that some?

Back probing is done with the harness connected, since the point is to measure the voltage of the circuit in operation. The VOM probes are inserted from the wire side, or "back" side of the connector by squeezing between the connector body and the wire until the terminal is contacted. With weather resistant connectors you may need to do the probing with a safety pin or the like, then connect the meter to that.

Gotcha, OK thank you. I'll give it a go tomorrow as well.

Thanks again for all your help Racer!

GR...The coil tested fine but the shop said that their was a gap in the wire that they fixed (no cost). I think it still would have arched when broken but it is now fixed.

 

I also purchased a 2007 YZ450 carb with cables and hot start. After a good cleaning and much cajoling I got the carb installed. without the hotstart assembly screwed in or the cable cover on, I slapped the tank on and kicked it over. After around 15 or so kicks it sputtered some. Then it started and ran for a while, when I pushed in the choke it died. 5-10 times kicking started again ran for a while revving engine and idled without choke on. I was adjusting the idle and it died. I tried to start 10 or so more kicks but couldn't get it to go with or without choke. When running it sounded as if there was some excessive noise from the valves - I still have not checked them but it did rev up and down nicely.

 

Carb was extremely dirty with black greasy, gritty junk in the vacuum and needle.  Here is what the carb looks to be jetted at:

1 1/4 turn fuel screw

160 Main

45 Pilot

Needle on 4th notch

 

Any thoughts to why the hard starts and no idle? Much appreciated.

Yes.  It runs like that because you left the hot start out and it has a huge air leak because of it.

LOL, I was thinking that may have been the reason!  I'll get it all back together here and hope for the best. Thank you sir!

Second on the hot start assembly causing your latest issue; you don't have to connect the cable to anything but the spring and valve (piston looking thing) must be in place. 

 

Also, when cleaning these carbs make sure you don't get carb cleaner anywhere near the slide assembly (either side of the main air passage or inside the top cover).  There is a seal in there that will be ruined if it contacts the carb cleaner (hope it isn't too late).  Sometimes I swear it just takes the fumes alone or the sight of the can!

Well I got the hot start back in place inside the carb (BTW do I need to get a whole new 450 clutch perch to bolt the hot start to?). I also plugged the vent coming out of the intake manifold. Same symptoms... very difficult to start then will start some but won't idle then forget it wont start. These are the same issues I had PRIOR to bolting on the 450 carb. Getting discouraged. I did hit the slide and entire carb with carb cleaner also. it was a mess when I got it. Can you show me in a diagram which seal I may have smoked?

take the cam cover off and check the cam chaine it sounds like it has been jumping when i first brought my yz400 had the same problom the old chain was 2 inches longer than a new and put the timeing out causing bad start probloms and lumpy runing

Thanks for the insight. I am actually planning on checking the valves tonight and will take a look at the chain as well.

 

Last night I put a new pilot jet in and it fired right up and ran the best it had since I got it but while I was adjusting idle (ran for 30 sec to 1 min) it died and could not get it started again. I was thinking when it warms up it may expand the valves and lose compression. Possible?

Just out of curiosity, check your flywheel. Those early models had a tendency to shear the key/keyway. The later models went to a splined crank (2001-on) due to this problem. Too late now but the original carb was workable if it wasn't physically damaged.

Pretty sure I figured out the issue...my intake valve clearance is at .075mm when it should be .15mm. I think once the bike heats up the valves are expanding creating a blow by effect and reducing compression and therefore won't run. I believe I did the math correctly and would need a shim around 1.05” however I think the smallest size available is 1.20? I also think my timing chain is past its prime and loose.

 

So the right thing to do is replace the timing chain and valves. So will for sure replace chain but since this is a budget rider wondering if anyone has tried trimming the shims? The idea would be to take a knife sharpening stone and take off 0.75” of the existing shim (1.80”) and create one that is 1.05”. I realize this is prolonging the inevitable valve replacement but this would allow me to get by now and then do a complete top end down the road.

 

Has anyone tried this?

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