Mikuni springs

[IS0LD0UT]

I bought 2 sets of springs. 4 of them produced the same result. One was right on with the manual the other a few psi low.
Are these actually different materials or is length the factor in psi?

The ones that produced different results were different lengths. Some were different colors but looked same in length and produced same pop.

I probably need to buy a few more sets.
What's your experience?

 

[Crab]

Do not assume springs are ever even close, always check pop off when setting up a carb.

 

[Quinc]

like a box of chocolate...

 

[IS0LD0UT]

Do not assume springs are ever even close, always check pop off when setting up a carb.

Underdstood. But how many sets do I have to buy to get what I paid for? Is quality control really bad on these? Is it better to buy the longest ones and then cut them to size? Or am I having an issolated issue?

 

[Little_Franny]

How far off are the pop off pressures between the springs? I have had good luck with consistency, but some minor tweaking it the arm will get you dead on.

 

[Crab]

You almost always need to mess with the arm, in some cases trim the spring. Just don't overdoo it with the arm bending per the SBN manual.

 

[IS0LD0UT]

Thanks for the replies.

So if I am understanding the spring length determines the pop? So the fact that my black, gold, and silver spring are all the same length and giving the same psi pop probably means they were colored wrong by mikuni?

Also what side of the arm do you bend?

 

[Crab]

http://www.mikuni.com/pdf/sbn_manual.pdf

page 15

 

[john zigler]

Are you using GENUINE Mikuni springs?

Yes, different lengths would make a difference. I do not recommend trimming springs. Start with setting needle arm level with carb body. You can make minor tweeks to adjust.

Other problems I have seen is the seat actually leaking at the o-ring due to corrosion in the carb. Are your needle and seats new? And are they genuine?

 

[IS0LD0UT]

I know some of this is trivial for your long time tweekers. The manual says it can be bent 1mm up but doesn't say if that will raise or lower popoff. How do you check how far it is bent, is there a trick to resetting the arm to neutral?

The manual also said the different color springs should produce different pop-offs. Clearly not in my case.

Thanks guys.

 

[IS0LD0UT]

Are you using GENUINE Mikuni springs?

Yes, different lengths would make a difference. I do not recommend trimming springs. Start with setting needle arm level with carb body. You can make minor tweeks to adjust.

Other problems I have seen is the seat actually leaking at the o-ring due to corrosion in the carb. Are your needle and seats new? And are they genuine?

Pretty sure, got them from thrust and they were all 4 colors. The needle and seat are genuine, and don't appear to be leaking all pop off crisp.

 

[Crab]

For reference, its either the 80 or 95 gram spring that has reverse wind compared to the others, I would have to check for sure which one...

 

[BruceSki]

How exactly are you testing popoff?

 

[IS0LD0UT]

How exactly are you testing popoff?

Mikuni tester. Sealed the return, pump to intake, clamp both, add wd40 to fill over needle and seat. Pump till it sprays me in the face. Record results.

 

[BruceSki]

Lol sounds right. Except you can lay the metal carb cover over the top to keep it from spraying at you.

 

[IS0LD0UT]

Sounds like I just need to get a couple more sets of springs and go from there.

 

[Vumad]

So if I am understanding the spring length determines the pop? So the fact that my black, gold, and silver spring are all the same length and giving the same psi pop probably means they were colored wrong by mikuni?

Also what side of the arm do you bend?

Spring tension determines pop.

I never go by colors. They can easily be discolored.

115g is the long spring.
95 is wound backward (it will also probably be like 1/2mm longer than 65 and 80, but it may not be)
65 and 80 are best determined by testing in the carb, but if you pay close attention and squeeze them with your finger, you can determine side by side which is stronger.

It doesn't really matter which side you bend because it will just pivot anyway, but the little side is going to bend because its weaker.

The manual says it can be bent 1mm up but doesn't say if that will raise or lower popoff. How do you check how far it is bent, is there a trick to resetting the arm to neutral?

The manual also said the different color springs should produce different pop-offs. Clearly not in my case.

Thanks guys.

The arm can be bent slightly, but I didn't have to do that with my mikuni springs. I did with my SBT springs. It would change the PSI slightly, maybe like 5 psi max. be careful bending the rocker arm. Checking your popoff is A test, not THE test. It works differently than the way the carb really works. Too much bending of the arm interferes with the nipple on the diaphragm. Try not to use bending if you don't have to.

The trick to setting the arm to a neutral position is to lay a razor blade across the carb body.

Sounds like I just need to get a couple more sets of springs and go from there.

Not sets. Just the spring you need to get the popoff you want. Put the springs you have in little baggies labeled based on the PSI they give you on the chart. Order just the spring that you need to get the missing number on the chart.

I personally like to buy from Jetmaniac. He's never let me down. No website ordering, PM direct.

Only buy genuine mikuni. Normally I buy AM for 1/2 the price, but I have found AM carb parts to be significantly inferior in quality.

 

[IS0LD0UT]

Thanks for the thourough repsonse.
Who is this @JetManiac guy? Kidding.

 

[Quinc]

 

[IS0LD0UT]

One more question.
People say you want popoff to be the same. But how close are we talking. Theoretically, At 15 psi, is within .5 good enough, or would you bend the arm to try to get that last .5 difference?