Carb rebuild on a 1982 Maxim 400J. Float height setting ambiguity

[GuyS]

I’ve torn apart and cleaned my BS34 carbs. I have found a multitude of problems so far:

1. The right side carb float needle was stuck. I changed both float needles (purchased from MikeXS) in a partial kit.
2. The right main jet tube was horribly dirty, greenish, crud all over it, and the pin needle was covered in crud too. I cleaned the needle and tube, as well as ultrasonically cleaned both of the disassembled carbs.
3. The left main jet was 127.5 and the right main jet was 120. I have a Seca service manual that says the jets are supposed to be 127.5 and 117.5. So I changed them back to the Seca Service Manual spec. with the brand new main jets from MikeXS
4. The right side “carb holder” boot, was damaged at some point in the past, and repaired. But not effectively repaired, I suspect the cause of a vacuum leak. So I bought a NOS boot I fortunately found on Ebay.
5. I been having a heck of time trying to find out the correct float height for my BS34 (there are variations of the carb which make this a challenging research project. Mine were set at 24mm.

Then I ran across this post from another Maxim owner from 2013. “Greg” had been trying 32mm on the float height but found at that measurement the floats actually were hitting the bowls!

“I fiddled with the floats to bend them inward (bent too much by common wisdom, but these aren't ordinary BS34s) and then get the height adjusted (for me it's 23-24mm) until I could hear them slam open and shut reliably, and can't blow through them even once upside down.

HOORAY!​

I took it for its first test ride last night, and it was great. It revs, and slows, and goes fast, and goes slow. It doesn't backfire, it doesn't surge forward, it doesn't hesitate, it doesn't stall. It starts on the first try.

Yay, we have a working Maxim again!”

So I’ve decided to leave mine set as they were at 24mm, even though I have replaced the float needles. Once I get to the stage of running the engine I’ll post my results.

Oh and here’s another thing. The Seca service manual does not suggest the float height measurement method anywhere in the manual. They suggest the tube method of establishing fuel levels in the carbs!

 

[GuyS]

So I went with the service manual instructions and checked my float levels using tube method and found the L side was close to 3mm from the carb/float bowl joint (see the attached picture). The right side was 5mm below so I adjusted the tang to raise the float level to match to 3mm as well. When I originally measured the floats with the carb float bowl off they measured 22.5mmL and 23.5mmR. We’ll see once I get to the point of running the bike if all this works out. The XS400 SOHC bikes seem to be different in the float settings and suggest 28 or 32mm which is significantly different for my Maxim. But then the true test will be when I get the bike running.

 

[Revvy Kevvy]

What did you decide in the end? 23mm? I have mine set at 27 just now and its running terribly.

The float settings are all over the place, youtube suggests 20mm, the manual 27 for the yamaha and 32 for the haynes.

I cannot bend mine to 32, they hit a stop. So I am trying 27 but it now revs like crazy and can only start on max choke.

 

[GuyS]

What did you decide in the end? 23mm? I have mine set at 27 just now and its running terribly.

The float settings are all over the place, youtube suggests 20mm, the manual 27 for the yamaha and 32 for the haynes.

I cannot bend mine to 32, they hit a stop. So I am trying 27 but it now revs like crazy and can only start on max choke.

I ran into the same issues trying to source the correct level 400J adjustment, the recommended setting where all over the place. And the Maxim model is different from the SOHC engine. I think 27mm is probably too much. And it makes sense that since the float is lower, correspondingly that would lean up the finicky idle circuit and cause high rpms and rough idle (because the fuel goes from to being too lean to not enough fuel then trying to catch up with the fuel supply). I set mine at 23mm. Then I checked using the simple “water level type” tube outside of the carbs to verify 3mm of fuel level to carb joint measurement. I suggest you don’t miss this step as it is very definitive and will give the best setting since it relies on detecting the right float/fuel setting. After that the bike started up right away and idled fine. I posted the DYI tube method on another post on this forum.

 

[GuyS]

You take an appropriate sized clear tube and push it into the fuel drain port at the bottom of the carbs. Then open the drain screw so that fuel flows into the tube. The fuel will stop when it reaches the level of the float jet corresponding to the joint in the carbs. It should be no more than 3mm at the center of the carb. I did this with my carbs off (I tilted them roughly the way they would tilt on the bike). I knew I would have to adjust the float tab. So I did it off the bike. The best way off course is on the bike, but I found the way I did it worked.

By the way 23mm got me bang on on one carb although I did have to adjust the other carb to make it close to the 3mm.

 

[Revvy Kevvy]

I am assuming you did this with gasoline and not with water...
Mine look like this at 27 (see picture) so I guess putting them at 23 might actually solve this issue?
I will give it a shot next week and rebend them to 23, see if it runs better.

 

[GuyS]

Looks like yours is set at about 6 or 7mm (rather than 3mm max), if I’m seeing this right. Look at my picture where the joint I am describing is clearing visible, and where your the fuel level should be. Also I averaged out by measuring at the centre of the carb joint because the carbs on the 400J are tilted forward when on the bike.

A low set point could cause an over lean condition. Too high could case too much fuel to the carb.

And no not water, fuel only!

BTW, what two points of measurement did you measure the “27”?

 

[Revvy Kevvy]

Looks like yours is set at about 6 or 7mm (rather than 3mm max), if I’m seeing this right. Look at my picture where the joint I am describing is clearing visible, and where your the fuel level should be. Also I averaged out by measuring at the centre of the carb joint because the carbs on the 400J are tilted forward when on the bike.

A low set point could cause an over lean condition. Too high could case too much fuel to the carb.

And no not water, fuel only!

BTW, what two points of measurement did you measure the “27”?

Two points? I measure it at only one. I did not know I had to find two points? See attached picture.
Will readjust these to your setting next week and see if it makes a differance, at least I am getting a running bike now even if it runs terribly.
I bought this on a whim, its a barn find and paid too much for it. silly me, but now I have to get it going.

Picture below is how I set the floats to 27...trying to get to 33.

 

[GuyS]

Correct. Top of float, measured to float bowl gasket mating surface (less gasket) is typical points of measurement. I’d speculate 27mm is too much and the tube method implies that as well. I too bought a “barn” find. I’ve done EVERYTHING to it. The good news is these bikes are real beauties and very dependable once given the right care. AND you can do the work yourself! They were meant to be serviced by the owner. Not so much newer bikes. Given they’re 43 years old is a testament to their longevity. And this forum is a phenomenal resource to tap into as well. Parts are available but you must look all over the place to find some. The consumables like float jets, brakes, chains etc. are readily available. Mine handles like a dream and is surprisingly fast.

 

[GuyS]

Two points? I measure it at only one. I did not know I had to find two points? See attached picture.
Will readjust these to your setting next week and see if it makes a differance, at least I am getting a running bike now even if it runs terribly.
I bought this on a whim, its a barn find and paid too much for it. silly me, but now I have to get it going.

Picture below is how I set the floats to 27...trying to get to 33.

I’m curious. Did you get your float set right? Was that the problem or something else?

 

[Revvy Kevvy]

I got the engine running but badly and then ended up on a business trip for multiple weeks. I am back next week so will try to start the engine and turn some screws and gat back to you here if it works. I think the floats were the problem and now its just a case of twigging to get it to actually run. It was running super, and I mean super rich when I tried it.