weak needle valves causing leak?

[Guest]

I just wanted to check with you guys before I buy a set of needle valves. I recently did a complete carb build with stage 3 dyno kit. Reinstalled on the bike and carbs 3 and 4 started leaking. Checked float height and its per spec, but I did notice one needle that when the weight of the float was resting on it it was pressing the spring down when the others weren't. I moved the needle to carb 1 to see if the problem followed and it did and now carb 1 is leaking.

So is replacing the needle valves my next option? Thanks

[JEEPRUSTY]

If the spring are weak yes replace.

[claygs750e]

I don't understand how the float operation works because I thought if the float rises and the spring is weak it would be easier to push and seal quicker causing maybe the opposite problem. Do I have this backwards?

[JEEPRUSTY]

I think the spring is used because the floats never really fall to the bottom of travel in normal running. The valves are always metering fuel into the bowls and the floats never move much. I imagine the spring is there to even out the motion.

There is a good article on this somewhere on the interwebs. I could not find it though.

[chuck hahn]

The spring is there so when the floats are all the way up and the bowls are at the proper level there is a little added pressure on them to seat and hold them firmly into the seat holes.

[JTGS850GL]

The spring loaded rod on the float needle is like a shock absorber to reduce damage to the needle and seat when the float bounces up and down. Many carbs don't even have the spring under the pin and work just fine.

The problems that cause the float level to very is when the pin sticks down or the spring collapses and allows the float height to change. If the needle sticks the fuel level will very all over the place depending on when it's stuck and when not. Some times the fuel level will be correct and some times it will be very high and over flow the carbs. If the spring gets weak or collapses completely the float height will gradually or suddenly get higher and stay that way.

When you measure float height the pin should not compress. If it does, then the spring is weak and the float valve should be replaced. There is some debate on weather to just replace the valve or the complete seat and valve assembly. Personally I think it depends on the condition of the seat. If the seat is in good shape then you should be able to just replace the valve. If the seat is bad then replace both.

[tkent02]

The spring is there so when the floats are all the way up and the bowls are at the proper level there is a little added pressure on them to seat and hold them firmly into the seat holes.

No, the bouyancy of the floats provides the pressure on the valve, it is the same pressure no matter if there's a spring or not. I'm thinking it is there to smooh out the operation somehow, like not letting the valve bounce open when the fuel sloshes around or something.

Maybe it has to do with engine vibration.

Not all bikes have the springs, the ones without them seem to run just fine.

[claygs750e]

That makes more sense, but I would think that my gas leak would be cause more of a stuck valve than a weak spring. Is it because there's just not enough pressure on the needle to seat tight?

[tkent02]

NO, the float provides the pressure, no the spring. What JTGS850GL said.

[chuck hahn]

Well then lets examine the evidence as provided by the original poster. barb X leaked and he noticed the spring thing and swapped them around and the problem followed to the other carb he put the weak spring in.

So what I would infer from this is the following..

1) Bike wasnt running ( far as he has said ) so there wasnt any vibration or floats moving from fuel levels changing....but it leaked.

2) Bike wasnt rolling down the road and bouncing, so there wasnt any need for any shock absorbing action...and it leaked.

Based on it still leaking, I would rule out both of the above functions..because it still leaked.. HOWEVER, the one with a spring strong enough to hold the needle up into the fully seated position DID NOT leak. I rest my case. Change the weak spring needle out and move on.

[tkent02]

Once again Chuck, your grasp of physics is lacking. Do you weigh any less wearing springy shoes? No you do not. Can you jump any higher? Yes you can. It's exactly the same only different.

At least we can agree he needs new valves.

[Agemax]

The spring minimizes the movement of the float during severe service, and maintains a more constant fuel level in the bowl. ie. it acts as a mini shock absorber to stop the seat from getting a pounding (as mentioned before)

[chuck hahn]

No Tom I understand perfectly that the float ht will raise the float only a certain amount as the float in the fuel.

I also understand that the tang DOES NOT press the needle all the way up..it only compresses the spring about 1mm OF ITS ENTIRE LENGTH. this is how the spring comes in to effect to hold the needle up and seated. You can read whatever you wish into whatever i say but I know full FULUCKING WELL how they work.

If the float were responsible for ALL the holding pressure the springs would even be needed in the first friggin place!!!!!!

[tkent02]

Again, a lot of them have no such springs, they work just fine.

[chuck hahn]

And I agree on the mini shock absorption too. The OP moved the needle to another carb..presumably with the same float hts set but the leak followed.

The needle with the good spring didnt leak and the bad one did. Why didnt the one with the good one leak even when moved around? WHAT one SINGLE FACTOR followed from carb to carb and made it leak..the spring. Why the spring? Because it wasnt applying upward pressure like the ones with good springs are doing.
Say whatever you want, but that spring DOES INDEED assert pressure on the needle to keep it seated upward.

Dont move a single thing in any of the other carbs that werent leaking and break one of the springs from holding against the needle and youll get the same thing again. The only factor that has changed in my scenario is the PRESSURE the spring was asserting before it was broke...but you dont want to hear the truth.