82 XJ750 Maxim float needles not sealing?

Hi. Working on rebuilding the carb on my 1982 XJ750J Maxim. I have it all more or less reassembled. When doing a bench test of the fuel/float levels it seems the needle isnt sealing? I have been checking the fuel level without the jets installed yet(waiting on new ones to come in). Does that matter for the fuel level, I feel it wouldnt? Any suggestions are greatly appreciated. I am just confused right now at what I am doing wrong.

I have done the "book" check at the 17.5mm with my digital caliber, but that doesnt seem to make a difference. I have adjusted the floats WAYYY past that to try and force even a low level, it just overflows through the tube into the butterfly area.

It does seem to overflow "worse" when there is more fuel in the external tank I am using, like pressure is causing an issue.

 

There aren't a lot of causes.

Am I correct that you have this problem in all four carbs, not just one? Because the odds are that if it's all four, you're making some simple mistake consistently. If it's just one, it's more likely an actual problem with that carb.

This is going to sound super-dumb, but it is unfortunately possible to do it: are you sure the floats are installed right-side-up? The way the tang is located, if the float's in upside down (and I don't know if it's possible to put the bowl on the 750's carbs with an inverted float, but I'm pretty sure it is on my 650's carbs), no amount of bending the tang is going to close that valve, no matter how high the fuel level.

How high above the carbs is the external tank you're using for testing? Shouldn't be more than a few inches, as pressure on the needles does increase with elevation and if the tank is a foot or more above the carbs there is a chance the "head" of the fuel tank and line is exerting enough downward pressure to overcome the buoyancy of the floats. I think it's unlikely, but again not impossible.

I assume you've checked the float valve seats and the passages above them for any shreds of debris that might hold a valve open. This would be a possible cause if it's just one carb overflowing. I just had a problem with the #1 carb on my 650 overflowing, and I found this in the fuel passage above the float valve seat:



Just a little scrap of rubber, the result of a sloppy cut I made when replacing the fuel line. Found its way down into the carb and kept the valve from closing.

You may or may not know this, but expect to be told again by at least one other person: setting fuel level by measuring the float height is at best an initial approximation. The only way to set fuel level accurately is a "wet" test, using a clear hose connected to the drain on the carb bowl to read the actual fuel level. As much as we might expect Japanese manufacturing to produce identical floats, they apparently do have some small variation in weight, so two floats will have different "waterlines" when floating on the gas in the bowl. As a result, the only way to be sure of fuel level is a wet test. Still, a "dry" setting should get you close enough to avoid having the carbs overflow.

 

Thank you for the response. Its every carb. I have a feeling the kit I got isnt sealing properly.
I have the floats the correct way and checked that they float in a bowl of fuel by themselves.
I have cleaned all the carbs fully and theres no debris in there.

Not sure why it would matter how high the fuel reservoir is. Image your tank full of 5 gallons, that has alot more downward pressure than 300ml in a tube. If thats the reason then I need to troubleshoot that differently.

Yeah about the "wet" height. Right now thats not working at all, so its not going to get me anywhere right now.
Most posts are over 10 years old on info.

Just working on it today the 0-rings arent working anymore either that I replaced. So must have been a garbage kit. I have a message out to the site vendor to see about a rebuild kit.

 

Height matters. Pressure of liquid in a column is proportional to the liquid's density and the height of the column; the total volume of liquid doesn't matter. A 5 gallon tank (hah! I wish my 650 held anywhere near that much) produces no more downward pressure in the fuel line than a 300 ml tank at the same height). In normal operation the fuel level in the tank is about 5-13" above the fuel level in the carb. Simply mounting the aux tank a foot higher could double the fuel pressure.

Still, if the o-rings aren't working then it sounds like you got a bad parts kit. Have you done a bench check (carb vertical with float bowl off, support the float with your hand, hook up the fuel line and see what happens) to make sure the needles are even capable of sealing in their seats? I made the mistake of buying a cheap Chinese rebuild kit from Amazon and found its "viton" tipped needles dissolved in gasoline in a matter of minutes!

 

Gotcha that makes alot of sense. More of a distance vertically from the input vs the reservoir volume. I can use a shorter tube from the canister to reduce the height between them.
Definitely I think the kit is bad. The orings didnt seem very tight when I installed them, can see them not setting well after moving the carbs around when racking them back up. The immersion? tube wouldnt even fit into the carb body. Had to use my existing one. I am leading towards it just a really bad kit.

About the wet level, has anyone posted the dimensions of the floats - after setting the wet level. That would be good info to know as it might be closer than the 17.5mm ish dimension.

 

Well discovered something this evening. I did a test with water since it's easier to bench test. I don't think my floats are floating like they should anymore. I can close the needle and stop the flow of fuel. But when I use the float and bowl it doesn't. So I tested by taking the bowl off and placing the carb into a bowl of water. I can see the float isn't raising high enough to push the needle in. It doesn't have enough "float" to do it anymore or the internal spring it much stronger. It should be depressed and it won't move it much at all. Images are hopefully attached.

 

1. Where did you get your rebuild kits?

2. Did you replace the fuel valve AND needle as a set?

3. What dry height did you set your floats at? Where did you measure from? I’m not asking about wet-setting yet, because if it’s leaking, it’d be pretty hard to get a steady wet height.

4. floats are a solid foam. They don’t “water log” and sink. Well, that’s the FACTORY ones… if you have cheap aftermarket plastic floats, they’ll sink at the tiniest hole or crack— they’re an empty plastic bobber.

Feel free to start a private conversation with me. I’ll give you my cell/txt number too….. be a WHOLE lot faster and easier ——-tell you what…. Since you’re new here, you may not know where all the buttons are— I’ll see you a message, look for you inbox here

 

When you closed the needle, did you do it by lifting the float, or by sticking your finger against the end of the needle? When I look at the pictures, it seems to me that something looks mechanically wrong, as if something's keeping the float from going up high enough to close the valve.

As for the little spring-loaded pin in the end of the needle, it's not supposed to be compressed in normal operation; it just compresses when there's vibration or something.

 

Kits from eBay/Amazon/etc… are typically junk. I have NEVER had one work well. I dknt even bother with them, and have used the kits from Len at xj4Ever exclusively. Get the right parts the first time, and be on your way. It’s nice to have total confidence in a rebuild kit right from the get-go.

 

Lifing the float. It would only stop the flow when the spring was fully depressed. I thought when it's in the seat it stops but only when it's fully depressed.

 

Nope the spring is inside is just a cushion.

Also, look up my forum called ‘what’s inside a float’ and you’ll see the material is not something that will get ‘fuel-logged’.

https://www.xjbikes.com/forums/threads/what’s-inside-a-float.132473/#post-684072