1997 Yamaha Kodiak 400 fouling plugs

[Drillbit]

Hey all,

I need some input on a problem I'm having with my Kodiak 400 that keeps fouling plugs. First off, I've done a lot of work to this thing, most of it all in good fun. Brake work, cv joints, u joints, carb kit, checked valve lash, compression good, changed all fluids, etc... My problem is it keeps fouling plugs very quickly. I can put a new (manual recommended plug) in and it starts right up, runs great, idles great, picks up good. Doesn't stutter at all. I ride it for 1/2 a mile, it cuts off and won't start back up. I let it sit overnight thinking maybe it got to hot. Try to start it the next day, nothing. Put a new plug in, fires right up, runs great for another 1/2 mile or so, dead. The plugs come out and look carbon fouled with dry black soot on them. I don't know if this is a fuel/air mixture problem or a ignition problem. A couple of things that I noticed is I never need to pull the choke out to start it. Also, when it starts it automatically idles high like an automatic choke would do, then it comes back down to a good idle. Also I noticed that the air/fuel mixture needle valve screw does nothing when I adjust it in or out. I can screw it all the way closed and the motor still runs and idles good. Any advice would be greatly appreciated. I'm almost leaning to a ignition breaking down the plugs??

Thanks! 

[Gwbarm]

First good work on getting this back up to speed. You have done a lot of work. It does sound like you are running rich with your carb, but i dont think thats the problem, no way it could fowl the plug bad enough in 1/2 mile to not start. If it was burning oil the black would be shiny. I wouls start by checking the coil, make sure its in spec, you do have a carb problem if you see no change after adjusting the mixture screw. What type carb are you running and what are your settings and jet sizes. Does it smoke at all when running, and you might also check the compression after its warm and wont start, just for curiosity sake.

[Mech]

Dry black soot sure sounds like too much fuel. The mixture adjustment doing nothing is always a sign of something wrong in a carb.

Since it's running good, no mention of missing or hard start etc I'd doubt the ignition is at fault. Check it's got a reasonably fat blue spark. Ignition systems with resistor caps or leads can have a skinny spark, but it should be blue. If it looks yellow it's a bad sign. If the soot is only on the porcelain tip it might pay to try a hotter plug. If it lost it's spark suddenly it might have a wet plug, but it wouldn't have black soot.

I'd make sure the fuel tap was good and not filling the sump with fuel. And if it's got a vacuum controlled tap check that isn't letting fuel into the vacuum line.

Then I'd start looking at the mixture. Choke's going right off, check that first because the choke only works when the throttle's shut so they run ok with a stuck on choke, till you throttle off, then air-filter, intake piping isn't crushed or blocked(mouse nest), carb breather tubes, fuel tank breather. It might be that the richness is intermittent, or caused by heat or the time run. So check the carb breathers, and tank's not presurising. Then It'd be off with the carb for another look. Check the float height(can be done before pulling the carb off) and that it isn't sinking, check the idle mixture screw tip and the seat the screw goes into for damage, check the float needle seat is sealed to the body, the breather drillings are clear, all the drillings are clear, the jets are the right size, the slide needle is the right one and attached to the slide, the slide isn't sticking. Reassemble and check the idle mixture is responsive. If it isn't then there's still something wrong in the carb.

[Drillbit]

Thanks Gwbarm and Mech for your input. I also am having a hard time believing these plugs are fouling so quickly even if rich fuel. I did a compression check initially but the motor was cold and got a 125 reading. That seems to be good but I didn't check that when motor hot. This has a Mikuni carb on it and I put a new kit on it. Yes, I've read alot about those aftermarket kits not having the right size jet holes and things but, Damn, I have no way of telling if the hole sizes are correct. I'm almost thinking about putting the old jets and seats back in it. The choke on this is just a manual push/pull knob type. It's not automatic but it sounds that way when I start it. You don't think the intake valve not seating good would cause this do you? Like I said, it runs good.

[Mech]

Start the bike and operate the choke.. You should be able to hear if it's working, or doing nothing. The choke only sucks fuel up when the throttle's closed. Soon as you open the throttle the vacuum drops in the choke circuit and it doesn't lift the fuel right to the top of the venturi  where the discharge for the choke is. So it could be on and only go rich when you throttle off hard.

It does seem unusual to have fluffy dry soot that fast. That's definitely rich though.. Oil looks wet black. You have got fresh fuel haven't you ? And a good grade ?

The compression is probably meant to be nearer 175 Lb but that's warm and with a full open throttle and cranking at the right speed. If it comes up to it's 125 in the first couple of compression strokes it's a good sign. If it has to be cranked a lot and slowly creeps up with each compression stroke it's a bad sign. You could put a teaspoon of oil down the plug hole, give it a quick spin with the starter or pull start to distribute the oil, then do a compression check. If the oil raises the compression a lot it points to rings, not valves. Having a tight or leaking valve doesn't normally cause  dry fluffy sooty spark plug. Leaking rings doesn't normally cause that sort of soot either.. Only fuel does that.

Some bikes say what size the jets are in the parts manuals. You can look that up online, or in a service manual.

[Mech]

Old worn low compression engines do get a build up on their plug, but it's generally a healthy tan or grey hard build up, not sooty. Worn rings burning oil, or leaking valve stems letting oil in to be burnt, the plug goes black and oily looking .. Black soot is always rich mixture I think..

[Mech]

A bad misfire can cause a sooty black plug, but it has to miss a lot and for a while.. I'm thinking you would notice that.

If it only goes rich occasionaly, and if it was only when you throttle off hard as would be the case if the choke was stuck on, then you wouldn't particularly notice the richness..  That could also explain why the idle mixture doesn't respond to adjustment.. -

[Gwbarm]

If its been sitting for a while the compression may come back a little the more you run it, it could also have some carbon deposites on the valves not letting them close all the way. If you have been running it regular and it just started its probably the rings getting worn.

[Drillbit]

Update. After taking the carb back off and closer inspection I noticed that the throttle plate was opening to much before the main needle jet even moved any. I have attached a picture. The linkage arm that connects the two had so much play in it that one was opening and the other wasn't doing anything. You can see that I have the idle adjustment screw all the way in and it still wouldn't idle and the choke plunger didn't have any affect when trying to start it. Anyway, By putting a carb kit on it does nothing for this linkage problem so I went ahead and bought a new cheap carb. Put it on, works great. The air/fuel screw now works and the choke and idle screw actually does something. This type of thing can be a gremlin to find. Thanks Mech and Gwbarm.

[Gwbarm]

Great you got it going, yes sometimes parts get worn that are not sold as replacement parts, so new is the way to go in that case.