Mikuni SBN gasket question

[bored&stroked]

I was recently diaging a problem with a ski and my buddy pointed out the rubber gasket in the pump area was wrong [#5 on the chart]. I had used the large hole gasket instead of the small hole which is used on 44's. I had never know about the differences, so I pulled apart the set of 38's on my wave raider to confirm I had the right, large opening [5a] gasket according to the rebuild chart. Turns out I did, but it looks the same as it did on the 44's [with the exposed O ring], like I should be using the small hole gasket. So what gives?

 

[joeschuit]

One way or another that oring needs to be sealed by the rubber gasket. My sbn44's were also put together with the wrong gasket like yours and I battled an inconsistent fueling issue until i pulled the fuel pumps apart and found the problem. It would suck enough air through that gap at high speed to starve the fuel chamber of gas.

 

[john zigler]

Instructions are confusing.

Most ALL Mikuni SBN carbs use the smaller hole diaphragm. You only use the large hole one on early Yamaha Single SBN carbs, with the matching paper gasket, and NO o-ring.

 

[Pro-pulsion]

I've been through this too and found that most carb kits as of late from what I have experienced don't come with the small hole version, genuine Mikuni kits too, and what does has had serious issues with sealing. The material is too thin by just a touch causing pump body pressure leaks. I just rebuilt a set of dual 48's last fall and thankfully that diaphragm was in really good shape because it wasn't supplied in the Mikuni kits. On prior kits it was so thin and almost looked like a barely coated screen as you could see light through some thinner spots. I've seen that quite a few times over the last batch of kits put out by Mikuni, WSM, SBT and Winderosa which used to be a pretty good good brand. John is good, John is wise, the o-ring must be covered fully or the fuel pump body itself won't properly seal.

 

[JetManiac]

Can't remember who marked this up... High Speed Industries?

JM

 

[JetManiac]

I've been through this too and found that most carb kits as of late from what I have experienced don't come with the small hole version, genuine Mikuni kits too, and what does has had serious issues with sealing. The material is too thin by just a touch causing pump body pressure leaks. I just rebuilt a set of dual 48's last fall and thankfully that diaphragm was in really good shape because it wasn't supplied in the Mikuni kits. On prior kits it was so thin and almost looked like a barely coated screen as you could see light through some thinner spots. I've seen that quite a few times over the last batch of kits put out by Mikuni, WSM, SBT and Winderosa which used to be a pretty good good brand. John is good, John is wise, the o-ring must be covered fully or the fuel pump body itself won't properly seal.

All genuine kits have both versions included. Never seen a kit w/o or ever have a customer complaint about the small hole gasket missing. As Watcon John said, pretty much all SBNs use the small hole one except the early versions, no orings on the pump body or fuel pump, oldskool.

 

[JetManiac]

I was recently diaging a problem with a ski and my buddy pointed out the rubber gasket in the pump area was wrong [#5 on the chart]. I had used the large hole gasket instead of the small hole which is used on 44's. I had never know about the differences, so I pulled apart the set of 38's on my wave raider to confirm I had the right, large opening [5a] gasket according to the rebuild chart. Turns out I did, but it looks the same as it did on the 44's [with the exposed O ring], like I should be using the small hole gasket. So what gives?

5a is the small hole one. Sounds like your dual 38s are wrong also.

 

[Kohldanielzimmer]

Just did a single 44. It must be newer as it had the orings. When I took it apart there was two of the black gaskets with large circles ( part1) sandwiching the clear plastic film
(part 2). I put it back together that way. Should I remove the inner one (closer to the check valve)? If so I guess someone before me rebuilt it wrong, which is kinda unsurprising given the directions are quite convoluted.

 

[Kohldanielzimmer]

Figured this out ^. Early carbs have a black gasket sandwiching the clear film on each side.
Later carbs (with orings) just have the clear plastic film against the outer circular oring then a black gasket on the top.