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1990 YFM350 VIN JY43HPW05L00xxxxx

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Yes the information is all there all the componets and part numbers,which is a great help, what i was referring to is, i would like to see a drawing or photo of the bike frame in the manual of exactly where the componets are located on the frame , that would be very handy when you get one that the previous owners have yanked everything off, and thats usually what i get.

I guess you cant have everything!

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I think pay sites for manuals are generally ok. If it says ER I'd believe it.

Partzilla tell you what other models a part will fit, babbits doesn't do that.

You have the electrical component positions(hopefully correct) but you don't have a routeing diagram for the loom.. that might help. The routing diagram and the electrical component positions should solve most of it, then you start looking at wiring diagrams for pairs or triple wires of the colours you have left over to see what they might be.

If the positions or the routing don't seem right/possible then you might need a different manual, but there should be a manual that has that right. That sort of thing gets carried over from year to year for a long time.


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Ok, I don't have a video to post yet because I made a progress / question / frustration video then, in getting that out, ah freedom of speech


talking about the PO etc. The realization occured, that - if you look at my prior video you'll see it: there are two engine start/stop controls. The original Yamaha one, and one taken off a later model motorcycle or ATV, mounted on the same side of the handlebar, adjacent. It's that second one which runs underneath the tank, and did not have any matching color codes, nor was it connected to the harness. That's the one they cut into, and added crazy ROMEX to instead of proper low voltage stranded wire, then used it's START push switch, to directly electrically bridge the terminals on the starter relay (solenoid), completely bypassing the function of the solenoid, and only using it's terminals exclusively as you would use 2 isolation lugs. Like shoving a screwdriver across them every time you start the engine.

The CDI is on the forward-facing side of the battery tray, in the back, near the left-rear tire, and just underneath the CDI is where the starter solenoid is supposed to go. That's those blue and black wires from the harness, that do have a connector, that connects to a mating connector terminating in stripped wires. The new start solenoid will have it's own pigtail that will plug right into the existing harness. I'll have to look around for the retainer strap to hold the solenoid on. The tabs are there.

I completely tested, then disassembled the original START - STOP - HEADLIGHT assembly. The START button did not work. Somewhere in the wires before the harness, the two brown wires are combined. So if you run a continuity probe between the brown on the 4-pin terminal to the red w/white on the 3-pin, and push START, you should get continuity if the START button works.

Gently disassemble everything, and on the START button, use a dental pick to get underneath the bottom of the white switch body, and then you should be able to depress the START button and it will shift back then free.

Feed yourself some of the cable through the grommet. This will give you room to disassemble the START switch and clean the contacts.

Cleaning the contacts provides some conductivity, but the travel was too far and the connection was so-so.

Use a 50-watt soldering iron and silver doped solder, and non-corrosive flux, flux the button on the back tab of the START button contact. Apply a little solder to your iron, then touch the button briefly until you have a nice round bead covering the whole face of the contact.

Use a fine file and flatten the surface down some to around 1/2mm. You need to do this because too much of a nice round projection of solder, will not give enough travel for the button.

Reassemble. The START button now works every time, and travel is about half of what it was. Properly assembled, current across the START button contacts should only be enough to fire a small relay, the starter solenoid. 

While you have the switch assembly apart, add silicone grease to the light switch contacts and mechanism, and the ENGINE STOP slide switch.

I've completely removed the second START switch assembly, but may reuse it at a later time for a horn. Unless someone knows of an ATV turn signal kit they'd recommend, the switch to mount to the handlebar.


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Posted (edited)

Want to thank @Gwbarmand @Mech

Next will be carb and then oil filter/oil change, and whatever other fluids (rear diff). A battery, a new relay, getting ehe lights reconnected.

Edit: the airbox and filter is completely missing. If anyone has tips on that.

Not that I want to spend the money, but will this fit? 

Air Box Filter Box Cleaner Case With Lid Cap For YAMAHA Warrior 350 Grizzly 600 1987-2004 Wolverine 350 1995-2005 https://a.co/d/aAPHLP2

Edited by ranchhelper
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Good work! Excellent you figured it out this far, not sure if the air box will work on the Moto4, you can go to Partzilla and reference the part # to see if it will fit yours, it says its for the Wolverine in your age group and if it fits all those different models it may work. A little pricy but OEM one is discontinued.  You might look on Ebay for a used one that may come with all the hoses you need to connect it.The amazon one dosent come with the hose and believe me they are just as proud of those hoses. 

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This is kind of funny, i dont use Ebay that much so i dont know all the tricks, but i went on yesterday and looked for an air box for your bike, i think it was the first one i sent you a link to, i didnt know that the sellers were aware of everyone that looks at their parts, today i got a special offer, i have 24 hrs to decide if i want it for 28.95 instead of 29.95, i thought that was kind of comical, he offered it to me for 1.00 less. If i act now.

Its for the 225 a very good price, if it would work, it comes with the 2 hoses but missing the top. It actually looks the same as the 350 one, but there may be a size differnce, cant really tell, the hoses also look a little different, but they can be manipulated some.

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This is the air box for the 90 moto4 350



This is the one for the 225




The 350 has a indentation not on the 225 like it sits on a brace hose connections look the same but i cant find anywhere that it tells the physical size of either.

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@Gwbarm Before I spend any more money on this, I should be able to hook up a battery and get the starter relay to click. I bought a sort of standard lawnmower relay-solenoid from the local Husqvarna dealer (for the same price as the amazon yamaha copy $10 one), same terminals etc, although it has standard 1/4" spade terminals that I was able to wire the left-overs from the previous OEM solenoid harness to. And it has a nice mounting point hole/bracket, so all I have to do is remove the screw that holds the CDI on (this is mounted in front of the battery cage, just over where the original relay went), and mount it there. No fumbly straps or whatever yamaha engineers envisioned for this model.

I wired in a good battery to the ground strap on the back, with the ground strap to the transmission/motor, and that red with white strip on the harness. Turning on the key switch, I got the OIL PRESSURE light for a few seconds, then it went off. I was able to use the side-bar HLR to change it into REVERSE, and the REVERSE light lit up in red on the dash, and switch it back out of reverse and the light went off. So both of those lights are working and the ATV is getting power. 

Could not get the NEUTRAL light to come on or the relay to click, and my guess is that circuit is hooked to a start inhibitor, so unless the harness and sensors think the unit is in neutral, the start button won't do anything - will not fire the relay.

A new sensor is something like this?: https://www.amazon.com/dp/B01MQE3DYF

I'm going to look through the service manual I have that is close, and see if I can find a part number and also the partzilla catalog. My guess is the neutral switch went bad at some point, which further would make the PO decide to completely bypass the switch, the relay, and the inhibitor - essentially the entire wiring harness. I'm kind of glad he did, instead of butchering the original harness.

I have the SUPERTECH 4-STROKE ATV/UTV oil (3 qts) and 1 qt of the SUPERTECH full synthetic 75-90w, and will do an oil change before actually turning it over.

Besides that I have to inspect the carb. It's just loose like someone tried to do a cleaning and never fully remounted it.

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@Gwbarm The neutral switch is OK. Tested between the nipple and ground, and got a definitive continuity when in NEUTRAL vs in gear. Can confirm the lamp should be OK, get 6.8 ohms across it. Will test it a little further, using a battery to getting it to light up. The barrel connector that goes over the neutral switch male connector, was kind of corroded. It took a little bit to get a pair of alligator test jumpers to connect it properly to the meter to test continuity.

Started to rain so I packed-up, I have to do this outdoors. Will go back at it and see if I can get confirmation of the lamp lighting up and the neutral circuit working.

Have you ever heard of the neutral switch and lamp being OK but the light still not coming on? I'll have to look over the schematic.

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1 hour ago, Mech said:

If that wiring diagram you posted is correct then the cdi is a part of the start circuit..

While I'm coping with looking for any potentially blown fuses or additional relays (besides the main starter relay), by using the "1987-1997-yamaha-big-bear-350-service-manual", what's the best site you recommend to get the specific YFM350ER service manual for 89-90? It looks like it should cost around $10-15 for the electronic version.


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If the lawnmower soloenoid is a 3 post when its mounted it grounds, if its a 4 post it needs a ground from one of the post to operarte correctly. Did you tough the 2 big posts together to see if the starter would rotate. You might check your wiring between the neutral switch and light to make sure its good. If still nothing you might take the neutral switch out and clean it and make sure its working properly.

Good work! You have been busy, good your oil pressure and reverse lights work. Its coming together a piece at a time. I would also sqirt some oil in the spark plug hole just to lubricate rings before you turn it over and i always use a 2 stroke oil mix for first crank, just a little oil in the gas to help with lubrication.

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#48 The few manuals I've bought offline as pdf files have all been as described and good value. I don't have any particular pay site, I just browse their site checking that they don't just advertise the same manual for all models or anything like that, check the details shown in the manual looks right, then trust Visa to have checked the company is reliable.

Mostly though I manage to find free manuals.. Not for that bike though.

The electrical system is likely similar to several other models, but, because partzilla say the wiring loom only fits your year and model, it must have some odd combination of switches, relays, stator, cdi, regulator. You could mix and match manuals of different models to get the right stator specs and cdi wiring etc, but it would be quicker and safer to get the correct manual. Show us the site where you saw the manual for download and I'll check it out.

Also, in one of those earlier simple diagrams it shows two different types of relays. One relay is a normal open type, and the other relay is shown as normal closed type. It may be that the two sets of contacts are in one unit, the manual will have more details. The neutral and forward/reverse switches operation is not logical as shown. I suspect they must have connected poles if they are all in one unit. The manual will explain.

If you don't get the right manual, to narrow down which other yamaha models might have similar wiring, you can use partzilla, or check what the cdi has going into and out of it. It's a good idea to start by counting the number of wires and plug shape of the cdi, then looking through several manuals gathering those diagrams up to be cross checked against the start button wiring.. Once you have a few likely looking possible wiring arrangements as diagrams, then it's a good idea to check resistance readings of the stator and pickup coil and look through the manuals seeing what models had those specs.

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Dash lights are all tested and work. To test, remove the brown and other 3 wires, carefully, as they are 'glued' together from being connected for so many years. Every socket was pulled out and greased with silicone. Each barrel received a small amount of copper paste.

All three dash lights work, NEUTRAL too.

On the backside, above the brake light, take the first relay off the back. Tested, it works. From the wiring colors, it's the engine stop relay, referred to as the 'Starting circuit cut-off relay'. That was fine, but the contacts somewhat crusty, and the way it was bonded with the plastic indicated it had never been removed, was original. It got the same copper paste and silicone paste treatment.

The next was the Neutral relay. That had the same never-been-changed-or-removed characteristics, but the metal contacts were crustier. I didn't bother with the reverse relay for now, since that light is working fine.

These are all interchangeable, simple SPST relays. If you have the relay in your hand, and you are looking at the 4 contacts, the locking mechanism facing away from you, then the 2 right-hand contacts are the coil connections. The left-hand 2 are the switch contacts. If you apply 12v to the coil, you should audibly hear and feel the relay energize, click.

The cut-off relay sounded good, and everytime I energized it, i got continuity across the left-hand terminals. The Neutral switch relay, didn't make a noise. I tapped it briskly, and then it did. The fix is to tear the top off, where that seam is, and spray some CRC cleaner. Then take a small pick and make sure the contacts are hitting each other. I added a tiny amount of copper paste, too.

Once I could energize the relay coil and get a consistent continuity, it was reinstalled and the NEUTRAL light on the dash worked. Also, when in neutral, pressing the START button, correctly actuated the starter solenoid.

It's ready for an oil/filter change and a test start.

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31 minutes ago, Mech said:

Also, in one of those earlier simple diagrams it shows two different types of relays. One relay is a normal open type, and the other relay is shown as normal closed type. It may be that the two sets of contacts are in one unit, the manual will have more details. The neutral and forward/reverse switches operation is not logical as shown. I suspect they must have connected poles if they are all in one unit. The manual will explain.


This exactly matches what I am seeing. For the two on the left, they are SPST, or simple relays that are normally-open, and only close when power is applied to the coil. Those I removed and individually-tested.

I haven't taken the REVERSE relay off to test it. I assume it's the same. Is that what you are referring to when you say normally-closed?

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Relays 14 and 17 are shown as normally closed.

And it's really the cdi unit that's acting like a relay to operate the start solenoid. If either the brake light or the start relay contacts are closed when you push the start button, the cdi provides an earth for the start solenoid. If neither the start relay contacts nor the brake light switch contacts are closed, then the starter solenoid doesn't get it's pull in coil earthed through the cdi unit.

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10 hours ago, Mech said:

Relays 14 and 17 are shown as normally closed.

There's no '14 and 17' on mine. Can you just tell me which relays you mean?

Pulled the MIKUNI plastic cover off the carb, during removal to detach one of the two throttle cables, and it's gross in there. Will clean and get back to everyone.

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That looks like the main jet, and it just rotates.

Took the side off (the MIKUNI black plastic) rust.

Took the bowl off. Carefully punched out the float needle valve pin. Removed. Tried unscrewing what looked like the main jet. No luck. Just turns. Never seen that before on a carb. Special 'Yamaha ATV' feature? Or time for a new carb?



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