Carb needle and seat question...

[Cuban]

[emoji2] I know... I'm on the General board...

So the '77 XLT has this old Mikuni VM38 pumper carb. This year we had a problem with a rusty fuel tank, but got that cleaned out pretty damn good, using muriatic acid. The float valve is still intermittently sticking open, flooding the engine out until we tapp tapp tapp with a screwdriver handle. I've got the thing on the bench right now, and it's clean as a whistle inside. Here's a few pics of the needle and seat, but to my eye it don't look worn enough to cause this...

Whaadya think? Two little floats in there, nothing hanging or anything. Yet the damn thing will flood every once in a while...









Anyone? [emoji2]

[jeffcoslacker]

Stuff that can get past the screen is still plenty to make them get stuck open. I put an automotive inline fuel filter on them after tank issues. Transparent one, it's handy to see the fuel. You could try that....look at the wear on your float's tang and pivot pin too sometimes the problem is there and nobody looks at that.

[Herb]

Can't really see the seat, but there is a worn spot on both needles that could be causing part of the problem. If the seat is also worn, the needle can get cocked sideways and cause the issue.

[Cuban]

Stuff that can get past the screen is still plenty to make them get stuck open. I put an automotive inline fuel filter on them after tank issues. Transparent one, it's handy to see the fuel. You could try that....look at the wear on your float's tang and pivot pin too sometimes the problem is there and nobody looks at that.

[emoji2] Thanks man. I should have indicated we do run an external glass-cased filter on that thing. That, and the petcock screen have been nice and clean ever since we chased the rust out. Still good. I don't see why this little buggar is hangin'. Ima order a new set anyway though...

Can't really see the seat, but there is a worn spot on both needles that could be causing part of the problem. If the seat is also worn, the needle can get cocked sideways and cause the issue.

[emoji2] Thanks Herb. That's actually only one needle, (single carb) shot with and without the flash. I see the small ridge, but can't even feel it with a fingernail scrape, and seat looks good too. This carb is super clean inside, and I'm having trouble understanding why that needle ain't seating properly...

[jeffcoslacker]

[emoji2] Thanks man. I should have indicated we do run an external glass-cased filter on that thing. That, and the petcock screen have been nice and clean ever since we chased the rust out. Still good. I don't see why this little buggar is hangin'. Ima order a new set anyway though...

Yeah I like those too, problem is they don't filter the best. The best would be a filter made for a fuel injected car, they will strain anything that shouldn't go through an injector, which has holes almost too small to see. But you won't find one in transparent.

They seem to flow pretty good too, considering they are made to be used with a pressurized system.

I used to run big ones on my power steering return line. People said I was nuts, but it worked fine and I never had a failure...and my fluid stayed clean.

[Herb]

Stuff that can get past the screen is still plenty to make them get stuck open. I put an automotive inline fuel filter on them after tank issues. Transparent one, it's handy to see the fuel. You could try that....look at the wear on your float's tang and pivot pin too sometimes the problem is there and nobody looks at that.

[emoji2] Thanks man. I should have indicated we do run an external glass-cased filter on that thing. That, and the petcock screen have been nice and clean ever since we chased the rust out. Still good. I don't see why this little buggar is hangin'. Ima order a new set anyway though...

Can't really see the seat, but there is a worn spot on both needles that could be causing part of the problem. If the seat is also worn, the needle can get cocked sideways and cause the issue.

[emoji2] Thanks Herb. That's actually only one needle, (single carb) shot with and without the flash. I see the small ridge, but can't even feel it with a fingernail scrape, and seat looks good too. This carb is super clean inside, and I'm having trouble understanding why that needle ain't seating properly...

The needle and seat is machined to a very strict tolerance, doesn't take much to cause a problem. a lot of newer needles have a viton seal on the end that is fuel resistant. They are a lot more tolerant than the machined units.

[HARRIS]

OR THE FLOATS COULD HAVE LEAKED AND ARE NOT FLOATING ENOUGH

[Tbeck]

Cuban, sorry I didn't read this sooner.
Ok if I have this right, you believe that the float needle is causing the engine to flood out, why? A sticking or worn float needle would cause a fuel overflow or starvation issue.
I suspect that the issue is going to be your slow jet or idle screw since that controls idle to just off 1/4 throttle. Those are also the smallest passages in the carb and most frequent cause of your problem.
I would remove the slow jet and idle screw, bring some lemon juice to a boil in a pot and toss them in to remove the gunk. Blow out the passage with compressed air.
If you want to play with the float needle seat a 3/16 wooden Dow rod will smooth and polish the seat, BUT unless the bowl isn't filling, or gas is running out the overflow, you're looking at the wrong part.

[Cuban]

Cuban, sorry I didn't read this sooner...

Thanks T. I shoulda said yeah, it's dumping out of the overflow, and a lot of that is running down into the intake. Plus, it stops when you tap the bowl with a screwdriver handle. I've gone ahead an ordered a new set, so we'll see if that works. It's entirely possible this is still the original needle and seat for this carb, which has to have been built in the early to mid 70's.

Edit: Oh yeah, the two little plastic floats are still dry inside and moving freely on their pins... [emoji106]

[Tbeck]

Well if you want to check your theory use a 3/16" wooden Dow and twist it back and forth to smooth any rough spots and reassemble.
However all the stuff you are describing can be attributed to the slow jet and idle/air screw as well.
Does it start hard and die? Erratic idle? Race?

[Tbeck]

Idle also make sure the main isn't hanging because the slide needle or tube are worn or gunked.

[Cuban]

Well if you want to check your theory use a 3/16" wooden Dow and twist it back and forth to smooth any rough spots and reassemble.
However all the stuff you are describing can be attributed to the slow jet and idle/air screw as well.
Does it start hard and die? Erratic idle? Race?

[emoji2] Ever since I replaced the points with an optical trigger, the thing starts and runs like a champ. It's a very basic carb, and the engine runs great, with a stable idle. Being an Ironhead, you can idle it right down to 'potato, potato' with ease... [emoji106]

[Tbeck]

Ok, so it's just overflowing the bowl. Continuous (if you don't tap it)?

[Cuban]

Ok, so it's just overflowing the bowl. Continuous (if you don't tap it)?

Well it's intermittent as far as when it happens, but once it does start to overflow it will continue until it's tapped, or the petcock is shut off. He pulled into the driveway after one short test ride, and I could see it leaking out the overflow before he even shut it off. On a shaking Ironhead engine, that's quite a feat, for a little needle valve...

[Tbeck]

Cuban, since you have it apart, have you tried setting the needle into the seat to see if you can verify a burr? There would need to be something visible holding it off seat if what you believe is happening is in fact occurring.
I am still not convinced that is the problem. I have seen the float tab bend, but you'd leak constant.
Slow jet would affect idle which you said isn't a problem.
Main slide and needle could cause this as well, and the tapping could accommodate seating, but normally it would be accompanied by a rich condition.
Maybe I am over thinking this?

[hillsy]

Check that the movement of the float is not being interfered with by the float bowl gasket.

Not sure if that is your problem but is quite a common problem with non-genuine rebuild kits on other carbs.

Also check the float pivot pin is smooth and there is absolutely no resistance in the float movement.

You can also spin your float valve up in a drill and dress the tip surface with some fine wet and dry sandpaper to take out any imperfections.

[hillsy]

One other thing - is the spring tip on the float valve good?

[Cuban]

Check that the movement of the float is not being interfered with by the float bowl gasket. Also check the float pivot pin is smooth and there is absolutely no resistance in the float movement. You can also spin your float valve up in a drill and dress the tip surface with some fine wet and dry sandpaper to take out any imperfections. One other thing - is the spring tip on the float valve good?

[emoji2] Thanks hillsy. Yeah, there's two small plastic floats in there that slide on vertical pins, and those are both free and clear. The float lever and pivot pin are good too, as well as the needle spring. I'm going to chalk it up to minute particles of rust from the tank, in conjunction with years of wear on the needle/seat. Got a new set on the way, and I'll look around for a better inline filter.

[Tbeck]

Hillsy, actually a really bad idea to do any type of dress up on the float needle. I wouldn't even touch the needle seat with anything besides a wooden dow. Any type of sanding material will scar the plating, and/or surface material. This is why folk's generally use soft wood. Even a fine polishing compound scars the metal.
I will normally just replace any carb components that get worn.
By the way, good suggestion to check the float pivot pin.

[hillsy]

Hillsy, actually a really bad idea to do any type of dress up on the float needle.

Done it a million times in the shop on heaps of bikes and never had a problem. 1000 wet and dry, spin the needle up in the lathe and linish the surface back to a shine. You can even dress rubber tipped ones this way.

Although, ultimately, a new needle and seat is the best option.