Blaster SB44 Leak

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Jul 11, 2014
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#1
Rebuiling an SBN44 for by WB1 with a 61X single carb. Looking at the diagram provided by Mikuni (attached) I have a "late" SBN44, cause I have O-ring and not gaskets.

Here's my problem: If I install diaphram 5a and try to measure popoff, I get a massive air leak between the pump body assembly and the carb body at anything over 15 PSI. Removing 5a from the carb just using the 6a O-ring to seal it, everything works as expected.

Anybody ever see this?
 

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JetManiac

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#2
You are using the correct gasket which is thin black material, not a paper gasket.

Did you try coating parts with 2 stroke oil when installing them? Are you tightening the pump screws even and tight?
 
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#3
Yes, using the thin black material gasket. It's from an SBT kit you sold me.. ;)

I tighten the screws in an X pattern. They are tight - tight enough to where if I go more I'm going to strip the heads on them.

The failure mechanism is the same, even if I switch to a new gasket - start to pump up and around 15 psi the Mikuni pop-off pump starts to get "soggy". If I spray that joint down with soppy water, then it leaks. Going higher in pressure makes it worse. Removing the thin black gasket fixes it, as it allows the weird o-ring (6a) to seal directly to the pump body. I've double checked that I'm using the correct of the two thin black gaskets, the one that matches my pump body.

Where should I put the two-stroke oil? On the black gasket?
 

JetManiac

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#5
Yes, using the thin black material gasket. It's from an SBT kit you sold me.. ;)

I tighten the screws in an X pattern. They are tight - tight enough to where if I go more I'm going to strip the heads on them.

The failure mechanism is the same, even if I switch to a new gasket - start to pump up and around 15 psi the Mikuni pop-off pump starts to get "soggy". If I spray that joint down with soppy water, then it leaks. Going higher in pressure makes it worse. Removing the thin black gasket fixes it, as it allows the weird o-ring (6a) to seal directly to the pump body. I've double checked that I'm using the correct of the two thin black gaskets, the one that matches my pump body.

Where should I put the two-stroke oil? On the black gasket?
Yes, on both side of gasket, on orings, on carb body, we use on everything during carb assembly.
 
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#8
Using a brand new needle and seat from JetManiac...

Is a oil to allow everything to "seat", or does it seal things somehow? Won't it eventually dry out, as there's a lot less oil in the premix that what I would be adding by hand to the seals during assembly?
 

JetManiac

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#9
Using a brand new needle and seat from JetManiac...

Is a oil to allow everything to "seat", or does it seal things somehow? Won't it eventually dry out, as there's a lot less oil in the premix that what I would be adding by hand to the seals during assembly?
Learned it from Chuckie at Team Scream (later at XScream) many years ago and it has always worked for us.
 
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#12
Oiling works great..until enough WD-40 from testing popoff works its way back through the seat and mingles with the 2 stroke oil; then, that spot starts to leak. Here's hoping that real premix won't have the same effect.
 

JetManiac

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#13
Oiling works great..until enough WD-40 from testing popoff works its way back through the seat and mingles with the 2 stroke oil; then, that spot starts to leak. Here's hoping that real premix won't have the same effect.
You must have some other issue then. Carb or pump have casting flaws, out of true, etc.?
 
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#14
Not sure with SBT kits, as we sell, and ONLY recommend genuine Mikuni brand kits.

BUT In a Mikuni Kits, you get two 5(a) type diaphragms. One has a smaller hole near the fuel filter. If you use the wrong one, with the larger hole (which is for the early sbn 44mm Yamaha carbs) it will not seal, and would leak as you describe.
 

Vumad

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#15
Yes, using the thin black material gasket. It's from an SBT kit you sold me.. ;)

I tighten the screws in an X pattern. They are tight - tight enough to where if I go more I'm going to strip the heads on them.

The failure mechanism is the same, even if I switch to a new gasket - start to pump up and around 15 psi the Mikuni pop-off pump starts to get "soggy". If I spray that joint down with soppy water, then it leaks. Going higher in pressure makes it worse. Removing the thin black gasket fixes it, as it allows the weird o-ring (6a) to seal directly to the pump body. I've double checked that I'm using the correct of the two thin black gaskets, the one that matches my pump body.

Where should I put the two-stroke oil? On the black gasket?
I thought I was saving a few bucks on a SBT kit too. I also couldn't get my fuel pump to test out. I put the 8 year old O-rings back in and it passed. SBT rings are just not the same as mikuni rings.
 
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#16
I'm pretty sure I'm using the correct 5a gasket - it should match the pump body cutouts, right?

Got an new Mikuni kit from JetManiac yesterday, will try again.
 
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#17
Ok, after some really WEIRD debugging, it turned out to be the check valves (clear plastic disks).

I swapped everything pump side from SBT to Mikuni -> leak stops.
I swapped everything back, one by one, from Mikuni back to SBT except for check valves - > no leaks.

The only other thing I noticed was that 5a from Mikuni is made from a flat sheet of material, where SBT makes theirs from a roll, so it curls when you try to install. It's really curly, so as a result, it's possible that a corner curled under as I assembled. There's no crease in the gasket, so it's not a smoking gun, just something I'd be very careful with on my next SBT kit....
 
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#18
I just got some kits from jet ski plus.Com. The first rubber o-ring you put on the carb body on the pump side is too thin. Had to use the old ones.
 

JetManiac

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#19
I just got some kits from jet ski plus.Com. The first rubber o-ring you put on the carb body on the pump side is too thin. Had to use the old ones.
Sounds like you bought non oem kits so the orings are out of spec. The genuine kits seal everytime.
 
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