Leaking Carb Thoughts?

[Pro-pulsion]

I have been working on this Blaster doing multiple little things, improvements here and there. I rebuilt these carbs last year and they may have 10 rides though new kits. The carbs are Novi Max Flow 48's and have the little socket cap screws on the diaphragm side instead of the Phillips screws. We were out for a run last weekend when the Blaster started encountering some running issues. After going over everything and making a lot of corrective issues, fuel lines started to get loose, B-pipe coupler finally popped, etc. Little things, I noticed tonight after getting the pipe back together that the carbs are both leaking fuel through the diaphragm vent holes.

They were rebuilt using SBT kits, the diaphragms look like new as they should since they hardly have been used. But why the leaking through the atmosphere side of the diaphragms? What causes this on clean, new kits? I have seen it happen on kits that have seen hundreds of hours on them, they tend to wear a little hole out, but never seen it happen on new kits before. I decommissioned the ski until this gets cleared up, I don't want to see a fire happen. I tried changing the screws with the original ones from a pair of old 38's thinking that the socket head cap screws were not covering enough of the cover plate to seal, but that didn't make any difference. Would the cover plates suddenly warp and lose seal? Should they be changed too? Any thoughts on what to look for or change would be appreciated, thankya muchly.

 

[Rushford_Ripper]

Get genuine Mikuni rebuild kits, SBT ones are not great.

 

[Big Kahuna]

fuel is not suppose to pass thru the diaphragms, you got bad diaphragms. Only use OEM. Most of the time, all you have to replace is these and not doing a full rebuilt........... Ethanol in the gas eats these up.

 

[Pro-pulsion]

Thanks for the replies fellas, this was the first time I've ever installed SBT kits. He ordered them with a bunch of other parts, right now 2 of the kits are bad then. That's a rather horrible quality ratio, 50% fail rate. I've only ever used either Winderosa or WSM kits and never once seen this happen on anything new. I used to use genuine Mikuni but there must be some additive in our fuel that the genuine kit material reacts to. The o-rings and diaphragms always swell up about 10% larger on immediate contact with the fuel up here, and the diaphragms roll up into a taco lol. Check out my most recent brand new out of a new Mikuni carb diaphragm after one use, and this was about 20 minutes after it was removed. Right out it rolled up immediately. Now that it has been out for over a week it is flat again the way it should be lol.

 

[Freakrider]

SBT needle and seat will start leaking soon also, would never buy those kits. After I'm done riding for the week, I pull the fuel line off the oem single barb filter and throw a line into non ethanol fuel with oil and run it. My carbs sit most their life in non ethanol fuel.

 

[Pro-pulsion]

I'm glad I never bought these kits, he has four of them installed between 2 of his Blasters. The one has been pretty good so far. But I'll never recommend SBT kits to anyone after this.

 

[Big Kahuna]

Thanks for the replies fellas, this was the first time I've ever installed SBT kits. He ordered them with a bunch of other parts, right now 2 of the kits are bad then. That's a rather horrible quality ratio, 50% fail rate. I've only ever used either Winderosa or WSM kits and never once seen this happen on anything new. I used to use genuine Mikuni but there must be some additive in our fuel that the genuine kit material reacts to. The o-rings and diaphragms always swell up about 10% larger on immediate contact with the fuel up here, and the diaphragms roll up into a taco lol. Check out my most recent brand new out of a new Mikuni carb diaphragm after one use, and this was about 20 minutes after it was removed. Right out it rolled up immediately. Now that it has been out for over a week it is flat again the way it should be lol.

That looks ok. It is when they are all stretched looking and hold their shape (meaning all out of wack looking) - wrinkly looking.

 

[Big Kahuna]

OEM only.

 

[Pro-pulsion]

Now that I think of it, between these kits and the fuel hose we used to buy from a place called Aircraft Spruce, he had soo much fuel leakage it was scary. I mopped up about 3 blue shop towels worth laying in the hull. All my fuel hose comes from fast flow fuel lines now too, and has for the last 5 years or so. Best hose I've seen yet.

 

[Pro-pulsion]

Lads...and ladies I have removed the carbs and removed the diaphragms to discover the problem. The little metal disc in the center of them spins very loosely. Once I found this it was obvious that fuel was moving right on through that space allowing the discs to move. The other diaphragms I compared these SBT ones to are OEM, and the OEM have ZERO movement at the discs. That is how they should be, solid with the black material, glued, vulcanized, however they should be...the end word is they must be sealed and solid. I have a pair of decent shape Mikuni diaphragms that I will use and have developed a theory about why they swell and distort when in contact with fuel. It is my assumption that, the genuine Mikuni o-rings and diaphragms are designed to swell because, when they are contained within the carbs after the fuel touches them it becomes a forced seal to add assurance of no air entering where it should not. Of course I have no data to support my theory but it seems logical as every Mikuni kit I have ever installed and removed has done this. I will update on the running condition later, these fuel lines will be replaced tonight too with new high quality lines as mentioned, from fast flow.

 

[Pro-pulsion]

Success! Changed out those things with OEM and no more fuel leaks. Now it's time to try and relax for the rest of the night, thanks everyone for chiming in on this, off to be lazy now!

 

[Pro-pulsion]

I haves one more question for ya's...terrible english, it's a gun burrel full o laughs

For those of you running dual carbs that have their own fuel pump, is it safe to run both carbs from a Y adapter on one supply hose? These Novi carbs run a separate line to each, but they are 48's too, so I can see why if it is necessary, but what about for dual aftermarket 46's or smaller? Dual pulse for sure, but will a single fuel supply hose be adequate?

 

[Big Kahuna]

I always said 1 carb per line. but Tricky worked on a friends SJ with 48's and ran 1 line that split to 2 and it worked fine...........

 

[Pro-pulsion]

Did he have it set up with any filters and/or the stock water separator?

 

[Big Kahuna]

just a filter. no spearator

 

[Pro-pulsion]

This Blaster is set up that way too. I assume the separator slows down fuel flow too much which is why everyone removes them. What are your thoughts on all the numbers matching on carbs? I asked a Mikuni rep from Mikuni Australia what the numbers meant, he said there will be one grouping that will be the same if the carbs are the same...example 2 or more SBN38 carbs will have a stamped number that they all have as the same, and above them will be other numbers stamped. I asked what those numbers specifically meant and the best he said was that they are just date codes. All carbs of one model or another are manufactured identically and the only important numbers are the mentioned (the model numbers) for that size or type of carb in order to run them in multiples. I have been told this same thing by a few long term racers/builders as well. Any idea how to decode those date stamps?