Almost stumped with a carb issue

[RIDE-RED 250r]

Hey guys,

I'm working on an '86 Fourtrax 4x4 350 for a friend. I know it's a quad, but this essentially has a 350x mill in it.

I'll try to make this as brief as possible while including all the pertinent information.

Friend bought this after it had been sitting a few years. He asked me to clean and rebuild the carb. I took the carb all apart and was surpised at how clean it was inside after sitting awhile. I really didn't see why it wouldn't run right. Anyway, I soaked all of the parts in a disassembled state in Napa carb soak solution. This is the stuff that comes in a 1 gallon can and has the consistency of 10w-30 motor oil.

So the carb came out beautifully and I installed the brass from the Shindy carb kit he bought. I have always had reservations about aftermarket cab brass and always buy genuine Keihin for my own stuff.

I give him the carb back and he tells me it still isn't running right.

Now after he told me he had the cam chain tensioner out and rolled the engine over I immediately suspected the cam had jumped time.

So this time he brought me the whole quad so now I can see for myself how it runs.

First thing I did was check cam timing and found it to me correct.

So now I fire the thing up and it acts like the choke is still on, running very rough and emitting black smoke, especially from just off idle to about 1/4 throttle.

I verified the choke cable was working properly, even removed the choke plunger from the cable end and held it in place in the carb while running it and it still runs extremely rich down low. If I rev it up more it seems to run OK in mid-top throttle positions.

Now I gave him back the original carb brass and told him to hang on to it. I asked for the original brass back and will be getting it tomorrow. I'm hoping that swapping the OEM pilot jet back in will cure it. But if not I am wondering if the o-ring on the choke plunger is to blame??? Choke plunger looks very good, not pitted and doesn't seem to stick at all. Would a faulty choke plunger o-ring make it run very rich down low??

Also I should note that he replaced the fuel pump and I verified the fuel pump is not overpowering the float, needle, and seat by rigging up a gravity feed fuel tank. On gravity feed it runs the same.

This one is starting to have me scratching my head because it seems like it runs OK of you rev it past the terrible rich spot in the lower RPM range. I kind of find it hard to fathom that Shindy could have the pilot jet size so wrong, but anything is possible I suppose.

[threewheelin-feelin]

did you blow out all the ports with good compressed air? ive been having alot of luck with carbs lately with just pulling them apart and blowing air thrue all ports..not even putting new parts in them.

[MIK6]

Hmmmm... did it run like your 350x at my place???
flywheel key?

[DohcBikes]

Id try the stock jet before you do anything else.

[El Camexican]

Just a question; did you check the float height? Didn't see it mentioned.

[rustyhondas]

Some times when nothing makes sense it's the blasted CDI

[RIDE-RED 250r]

Just got done swapping the OEM pilot and start jet in and replaced the choke plunger o-ring. Still runs the same.

To answer a few questions, yes I cleaned the carb thoroughly. It was soaked in Napa carb soak solution overnight then cleaned with hot soapy water followed by compressed air into all ports in the carb body verifying they are clear.

Mike: I really don't think this is a timing issue, that was the first thing I checked. With the cam in timing position, the "T" mark on the flywheel is aligned perfectly with the index mark in the sight hole. I figure if either cam or flywheel was off it would have shown with the timing marks.

I swear this thing has a stuck choke. It's crazy rich down low and won't idle. It blows black smoke and acts rich the way it runs. If I throttle it up last the low end it seems to clear up and run OK at higher RPM but it still takes some finesse on the throttle to keep it running. It acts like the choke circuit is just dumping fuel all the time. Is it possible the choke plunger bore is worn not allowing the plunger to contain the choke circuit fuel when the choke is off?? I've tried disconnecting the cable making sure the plunger is seated all the way.

I'm reluctant to tell my friend he needs another carb, but this thing has me swinging for the fences and I'm about out of ideas.

Oh and float height is not adjustable, the entire float assembly is molded plastic including the area one would bend to adjust float height. But checking it the height seems correct, float seam level with the carb body as the float tang just touches the needle plunger.

And this carb is a bit different than a 350x carb. It has a cable operated choke and the fuel inlet nipple is on the right side facing straight out where the fuel pump is. Other than that it's the same carb... I guess I could swipe the carb of the wife's 350x and try it....

Carb is synced too.

[Gearheadtom]

Had a similar issue on a 350 Forman before, idled good, but would spudder rich really bad around 1/4 throttle. Problem was the carb slide and the throttle butterfly were not synced up. Under the plastic side cover is where you can bend the tabs a little to get them synced up and both opening at the same time.

[Bren_downe]

My money is on electronics. Maybe the flywheel is rusted from sitting?

[RIDE-RED 250r]

Carb is synced. Butterfly and slide are opening simultaneously.

Like the 350x, this has a wet stator and flywheel. I'd be surprised if the flywheel and/or pickup coil were rusted being that they live in motor oil.. Flywheel looked clean as a whistle all the way around when I was checking timing. Good thought though, had it happen on a 250r (dry stator) in the past. :beer

I think I'm going to try swapping my 350x carb in next. If it runs good on that then I'll know 100% it's a carb issue, but it won't tell me what the issue is specifically. I'll just have to rig up a long fuel line to make it work

[El Camexican]

You should be able to take carb off and test the choke circuit sealing with a small hose and air (just blowing). I say this not knowing that carb but I know I have on others.

[RIDE-RED 250r]

I hadn't thought of that. Thanks for the tip. :beer

[El Camexican]

I hadn't thought of that. Thanks for the tip. :beer

Ok, but if anyone walks in and sees you sucking on a hose don't you be mentioning my name!

[RIDE-RED 250r]

Ok, but if anyone walks in and sees you sucking on a hose don't you be mentioning my name!


LMAO!!! :lol:

[RIDE-RED 250r]

OK guys, I have made some headway with this thing.

First thing I tried was swapping the known working choke plunger in from my 350x. Still ran the same. But when I swapped my complete carb in, it now runs perfect! I verified that the idle air circuit coming into the choke plunger housing is clean and clear with a small piece of wire, so I know it is getting air. For some reason, the neither choke plunger is stopping fuel from coming up through the enrichment circuit from the bowl. All I can think is that the little raised sealing surface in the bottom of the choke plunger housing has something physically going on with it preventing the needle and rubber face of the plunger from sealing off the fuel. You can see in the pics I included of the choke plunger that it is not damaged. I can't see anything obviously wrong with the base of the housing... This is weird..

But I'm selling him my stock 350x carb and I have a 400ex carb in the soak solution as we speak that will be going into my 350x. His quad will now run perfect, and I am finally doing the 400ex carb upgrade to the 350x... Win-win!

Crap, no pics.. Having trouble with the upload box......

[El Camexican]

OK guys, I have made some headway with this thing.

First thing I tried was swapping the known working choke plunger in from my 350x. Still ran the same. But when I swapped my complete carb in, it now runs perfect! I verified that the idle air circuit coming into the choke plunger housing is clean and clear with a small piece of wire, so I know it is getting air. For some reason, the neither choke plunger is stopping fuel from coming up through the enrichment circuit from the bowl. All I can think is that the little raised sealing surface in the bottom of the choke plunger housing has something physically going on with it preventing the needle and rubber face of the plunger from sealing off the fuel. You can see in the pics I included of the choke plunger that it is not damaged. I can't see anything obviously wrong with the base of the housing... This is weird..

But I'm selling him my stock 350x carb and I have a 400ex carb in the soak solution as we speak that will be going into my 350x. His quad will now run perfect, and I am finally doing the 400ex carb upgrade to the 350x... Win-win!

Crap, no pics.. Having trouble with the upload box......

You're a brave guy. I'd test that 400 carb before you commit to selling the good 350 carb. I had a bad experience like that once going from a bank of good 34mm to a bank of 36mm carbs that turned out to be trash.

[RIDE-RED 250r]

Good point, but the carb is in his Fourtrax and the deal is made. These 350 Foutrax carbs don't seem very abundant and he was talking about getting a Chinese ebay carb. I didn't want to see him go down that road and possibly get a junk carb as hit and miss as those chinese knock-offs can be... I think the 400 carb will be ok after a cleaning. It really wasn't too bad to begin with. I've brought back much worse carbs to running condition.


Wanted to note: When I was swapping his old brass into the 350x carb, I did find a few more subtle differences between the two carbs.

1: The 350x carb has a deeper bowl and taller main jet orifice/emulsion tube

2: We all know that the 350x carb uses the same main jets found in the PWK's and other popular Keihins. The Foutrax carb uses the round main jets that take a flathead screwdriver, like the start jet.


Aside from these little differences and routing a longer fuel line, the swap is pretty easy and straight forward.

I had to fatten up the pilot jet from the original #35 to a #45 for some reason. Idle was hanging. Main seemed good sticking with a 140 as found in the original carb.

The old girl runs like a watch now and for it's size it gets up and goes through the gears with surprising authority. :D Well, not too surprising, it IS a 350x mill after all. :beer

There's my pics. You can see the rubber face of the plunger where the needle is that is supposed to stop the fuel when the choke is off and seated.