YZ400F Project

[V8Pinto]

Help me to not sell this thing

1999 YZ400F - hard start, flat spot off idle, pops at constant throttle (1/8 opening). Decel does not pop.

I've been working on it off and on for a year or so, searched and applied all the jetting ideas for the stock FCR. Called Zip-Ty and they said they don't like to mess with the early FCR's. After many jets/screws/etc in the FCR I got it good enough to ride around for a day but I still had to throttle blip it to keep it from stalling.

I bailed on the FCR and installed an Edelbrock. Power is great but all other problems are still same. I was able to address a lot of the flat/poppy spot off idle with pumpshot but at constant throttle it still popped and was generally unhappy - again at 1/8ish throttle opening. mid/high is great (if you can get it started, which right now, I can't).

I puched a hole in my boot yesterday kicking it so it's close to finding a new home.

Since I had the same issues with both carbs, I think carb/jetting is not the problem.

I checked the valve clearances a long time ago and they were fine but I'm going to look at those again. Last night I pulled the plug and it was dry and kind of sooty with very weak spark. I swapped in a new plug and it's still a weak spark. I searched the forums and saw this is normal but cleaned up all the grounds and coil connections anyway - same spark.

I haven't verified the cam timing and I don't yet know how to check the ignition timing. Tried to find a manual download but couldn't for the 1999 YZ400F. Still looking.

If this were an old car, I would be looking at ignition timing bouncing around just off idle. but this isn't an old car. Also I was thinking of putting an 02 sensor on there just to see but I've been through both carbs SO MANY TIMES.. I'm kinda done with that at this point. Plus I'd have to kick it again so...

Anything I'm not seeing? Any fresh perspectives that see something obvious I'm missing? I don't want to sell it but my foot hurts. I keep thinking "WR....'Lectric start...mmmmmm". I would like to take the "one thing at a time" approach and get it right.

Thanks for the help and sorry for the long post.

[spc mike]

i have my bet on a tired coil or your black box is on its last leg.... anyway, dont sell it because you wont get enough money to justify the fun you get out of riding it!

[V8Pinto]

That's the thing..I don't want to sell it like it is because I won't get a good price and also, I just don't like passing a turkey to someone else. Don't want that to come back around on me.

I'll try a coil.

Also, isn't there a way to ohm out the connections of the black box and determine it's condition? Still looking for a service manual download.

[grayracer513]

Also, isn't there a way to ohm out the connections of the black box and determine it's condition? Still looking for a service manual download.

No, but here's your link:

http://www.yamahaownershandbook.com.au/?r=0

[matt4x4]

I would definitely revisit the valves to start, also could be that a valve isn't seating properly - hw's your compression when you bring it up to TDC? - can you stand on the kicker for a bit before it bleeds through or is it weak right off the get go?

[V8Pinto]

Compression is STOUT. If I rotate it to TDC and don't use the compression release, I can stand on the thing and it doesn't move.

Just downloaded the manual from the previous poster's link. Going to run through the ignition diagnosis and then the valve stuff.

Is there a spec gap on the ignition pickup?

[V8Pinto]

I stepped through the ignition troubleshooting tree from the service manual step by step and found a few things. All coils and pickups were good. Found a few bad connections but the ringer was the neutral switch. It showed 20 ohms in neutral and open in gear. I wasn't sure if it was supposed to be shorted when in neutral so I disconnected it (the "light blue wire" mod). It started right up and ran GREAT.

for about 20 minutes.

My carburetion problem is gone, throttle response is great, constant throttle good, everything good.

then it died like it was out of fuel. Came back to the garage, fuel in the bowl. Pulled the tank and the plug and had spark. Put it back together and it fired right up and ran another 10 minutes perfectly then died same as before.

Now it won't start at all. It has fuel, squirter is squirting, has spark (at least it does with the plug out of the head). Now I'm stumped.

I'm happy though - this is the best it has EVER run. The difference is night and day.

What else is on that neutral switch? Does it have sensors for the other gears too?

[grayracer513]

No. It was reading normally as tested. The function of the neutral switch is to create a ground in neutral only. A lead from the CDI connects to it, enabling the CDI to be aware of the bike being in neutral by detecting a ground at that lead. The CDI is supposed to respond to the neutral signal by lowering the rev limit, and by altering the timing and advance curve. The only two faults the neutral switch or circuit can cause are:

> Switch never closes (open in neutral). Results in a normal, "in gear" timing map when in neutral. Has no effect on performance. May be harder to start, and/or more prone to kick back.

> Switch or wiring shorts to ground when trans is in gear. Can cause lower than normal rev limit, and poor performance. May be an intermittent condition, which would cause the timing to fluctuate, and be seen as stuttering or misfire.

The bike should start and run with the light blue lead grounded or open. If it will not start with the lead grounded, either the carb tuning is off, causing it not to start as it should with normal timing, or the CDI may itself be faulty, causing an abnormal response to detecting the ground. Under ordinary circumstances the "blue wire mod" should accomplish nothing.

However, if you feel that he engine starts and runs satisfactorily overall with the lead unplugged, you can run it like that.

[V8Pinto]

Can I disconnect the whole neutral switch assembly then for troubleshooting purposes? I ohmed the rest of the gear positions out and they vary from 20-180 ohms when that gear is selected. They all go open when not in that gear. The wiring diagram in the service manual shows it to be a short to ground and my switch isn't doing that. I'm just trying to eliminate every possibility and check each component off the list.

I'm heading out to the OHV park tomorrow so my neighbors don't boycott me. The last ride tonight definitely seemed like it was running well but it quits after running for a while. I'm not sure if I'm getting junk in the carb (installed a filter) or if it is temperature related (maybe a coil going open when it gets hot).

I did find that the spark plug cap needed a little service. The resistance was varying all over the place while I moved the cap. I pulled it all apart and cleaned everything real well and payed attention to the reassembly and now the resistance is where it's supposed to be and doesn't change when I move the cable.

[mj2412]

You can disconnect the neutral switch without any problems. Dubach Racing sells a cap for the neutral switch. Maybe you switch to a modern coil setup. I installed a hayabusa spark plug in my yz426. Works great. I got it on ebay for a few bucks. When you buy it, ask for the connectors. I is important for easy installing. I had once a problem (not yamaha) with a loose spark plug cap. During riding the spark plug becomes too hot and didn't work. After cooling it worked again. Two or three cycles and the spark plug was shot. Still had a spark but didn't work. Check the cap and install a new spark plug.

[matt4x4]

the remainder of your problem DOES sound heat related - it might even be your plug - not sure if you've put a new one in or are running an old one - but a bad plug will start to misfire once hot. Otherwise - if it's dying out at idle after 10 minutes, the pilot circuit sounds suspect - possibly dirty.

[grayracer513]

Can I disconnect the whole neutral switch assembly then for troubleshooting purposes? I ohmed the rest of the gear positions out and they vary from 20-180 ohms when that gear is selected. They all go open when not in that gear. The wiring diagram in the service manual shows it to be a short to ground and my switch isn't doing that. I'm just trying to eliminate every possibility and check each component off the list.

Ignore the three leads other than the light blue. Only the light blue actually connects to the harness. As I said, the switch should show continuity from the light blue to ground in neutral, and the light blue should show open in every other position.

I have seen situations where the switch was improperly assembled. There is a contact pin and spring under it that can become dislodged if someone has had it out and not put it back right, or had the shift cam out of the case without removing the switch. Only one is used in the 426, so if you are getting any reading from the other 3 leads, it raises the possibility that the contact is out of place and moving around in the switch. If that were to happen, the timing really could bounce around, just as you suspected.

As I also said, you can leave the switch disconnected permanently if it starts and runs OK. The only thing that changes is the timing in neutral.

[V8Pinto]

You can disconnect the neutral switch without any problems. Dubach Racing sells a cap for the neutral switch. Maybe you switch to a modern coil setup. I installed a hayabusa spark plug in my yz426. Works great.

Yeah I checked my Busa plugs. They are a CR9E (400 is a CR8E). I have them in my toolbox just in case.

Good tidbit on the cap for the neutral switch. Thx!

[V8Pinto]

Grayracer513 - that's good info on the switch. Thanks very much.

Heading out to the OHV now - Hoping for the best.

When it's running, I sure like this bike!

[spc mike]

Yeah I checked my Busa plugs. They are a CR9E (400 is a CR8E). I have them in my toolbox just in case.

Good tidbit on the cap for the neutral switch. Thx!

i dont think he was talking about an actual busa spark plug, but rather a hayabusa Coil which is a coil over plug design.

like these:

I would most definitly try a fresh plug and double check your carb for debris floating in the bowl or colgged in a jet because one more time going through the carb wont hurt anybody. And i would also look into the functionality of your vent in or on your gas cap because that could lead to a starvation of fuel if there is a malfunctioning cap vent making its not able to gravity feed the fuel after a few minutes of being run. And that can be anywhere between 10-20 minutes depending on how much fuel was in the tank.

[SXP]

i dont think he was talking about an actual busa spark plug, but rather a hayabusa Coil which is a coil over plug design.

like these:

I believe TT member mike_dean has the coil-over plug kits (plug and play) put together for the 400/426s. PM him.

[V8Pinto]

IT IS FIXED!!!!!!!!!

I have had this bike for a year and a half and it has been sitting in the garage staring at me. FINALLY it is fixed. I rode all day today on it and I love it.

So...what was it you ask? I'm not sure if it was any one thing, it was likely two or three. What fixed it was stepping back and going through the manual and doing the 1,2,3 just like the manual says.

The spark plug cap was loose inside and the ~9K resistor in there was making intermittent contact. Also there were the dubious connections in the wiring harness that were covered in mud. The ground on the coil was not *great*. Also, the neutral switch is intermittent. AND I was chasing debris in the bowl without knowing it. AND it was low on water - not sure if it would heat up and shut off because of that but I fixed all this stuff and went over all the connections real well. Also, I added a fuel filter and bingo. Rode all day. I probably didn't need to switch to the Edelbrock but I'm not going back to the FCR yet. It can sit on the shelf until I get ambitious and I'll swap it back to stock.

Tremendous power. It rode great all day. It started on the 1st kick most times and only once took three kicks. I figured out that if I turn the idle up a half turn when it's cold, then turn it down a half turn when it warms up, that's all it needs.

Thanks everyone for all the help and putting up with my silly questions. Now I'll go look at that coil on plug setup - that looks interesting!

First I'm going to go ride though.

[spc mike]

Awesome! iam glad you fixed it and you are riding!!