44mm SBN mikuni carb pop-off values way out of line with chart

[96superjet]

I have a dual set of 44mm SBN mikuni carbs that I was testing pop-off values on last night. Problem was the values that the Mikuni chart says I should expect to see were WAY off. I've been running these on my JetManiac 781 big bore for like 2 years now with pretty good results. I've had the following needle seat and spring setup (actual Mikuni parts, not OEM)

Needle seat = 2.0
Spring = 95 gram

The chart says this should be about 25 PSI

I was getting 20 to 21 on both carbs repeatedly. I ran the test dozens of times with return nipple blocked off (I actually tried it a couple of times with the pulse nipple blocked off too). No difference that way.

I put new Mikuni 95 gram springs in and stiil got the same results.

So the long and short of this is that I ended up reducing my needle seats down to 1.8s. Yes they do make them and you can find the real Mikuni 1.8 needle seats online. These coupled with brand new black 80 gram springs rendered the same 20 - 21 PSI pop- off values. So I'm gonna give these a try and see how it runs.


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[96superjet]

Oh... I guess I never asked a question. What do you think would account for these much lower than expected values?



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[jaxrider]

maybe your gauge is off

 

[96superjet]

Possible... but it is a brand new Mikuni pop-off gauge.

If so, how would you test it for accuracy?

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[jaxrider]

if you have some old springs cut one and see if you pressure changes. spray you needle and seat with some wd 40.

 

[speedysprocket]

Bent spring arm...? That arm can be bent up or down up to 1mm to tweak pop off pressure.

 

[96superjet]

Bent spring arm...? That arm can be bent up or down up to 1mm to tweak pop off pressure.

yea I checked that right away. Pulled both arms out, held them out in front of me and they both looked straight as an arrow.

 

[superjetguy]

Maybe you need to bend the part where the spring is set down just a bit to get the desired pressure. If there's nothing wrong why fix it?

 

[reidfrombj]

I just bought a new mikuni gauge as well. My gauge was showing lower popoff then what the chart was saying I should have. I don't remember how much lower but it was about the same difference you are getting. I checked two of my skis and they were both lower then what the chart said. I also had new n&s and springs.

 

[96superjet]

I have a pair of 46mm SBNs that I just bought and are still in the box. I'm gonna see what this Mikuni pop-off gauge displays with them. Interested to see what happens seeing that they are brand new and never been touched.


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