Diagnosing a Carb Leak - 1981 XJ650 Maxim

Hello everyone!

I’ve noticed some fuel residue on the engine of my ‘81 XJ650, underneath the carbs. See the pictures attached for the residue on both sides, as well as some pics of the carbs themselves.

Seems like an open and shut case, but here’s the problem: I can’t see anywhere on the carbs that’s obviously leaking fuel. I’ve never seen any drips, and it seems to only occur while I’m riding.

Any idea where I might start troubleshooting this? I should mention, I didn’t notice any leakage until I washed the bike a few weeks ago. Could I have loosened something or knocked something loose?

 

turn your petcock to Prime setting and see if fuel starts leaking.
go the extra mile and pop off the rear intake boots that go between the carbs and the air box. You don't necessarily need to take them all the way off, just pull away from the car so that if fuel comes out the back of one of the carbs, you know which one it is.
You may have a stuck float.
Clean up the resedue firsts, so you have a clean slate to look at. Use a flashlight to spot the shine of the gas.
if you get despirate, you can splash on baby powder to coat and see the leak location.
When it comes time to pull the carbs, remember that heating up the rubber boots with a heat gun or hair dryer (hot to the touch but don't burn the rubber) will make pulling the carbs really easy.
Since learning to use heat, I don't need any tricks with wooden dowels or wratchet straps or anything like that, just my hands and some UMPH (for the carbs to cylinder head intake boots)

Do your testing on a level surface and on the center stand so you can tell better where its coming from and not just on side stand where it will all flow to #1

Good Luck

 

Just to add.
Wow. That is what one clean engine.
Pictures are excellent.
I see staining on cylinder 1 carb bowl at the top but the rest of the bowls look clean. Just underneath look stained.
Cylinder 4 carb shows staining at the bottom rack mount which looks like it ran down over it.

Yes. As melnic said. Everything level and check.
But just maybe the highest stained component is the culprit.

I wish my gear looked that clean!

 

Argh. Just realised that the staining I see is probably the lighting.
Bah. Internet detective I ain't.

 

Thanks guys, this sounds like a great place to start.

Like an idiot, I actually went for a ride today, not thinking about the fuel leak until I was underway. It did give me a chance to observe the carbs when I was sitting at a red light. Noticed a very slow drip from the bottom front of the rack, near the #4 carb. Could be running down there from either of the right-hand carbs, but the left side seemed dry.

I suspect it’s reaching the left side only after I set it on the side stand, which would explain why the staining is heaviest on the right. I’ll update once I’ve done more troubleshooting.

 

Update: didn’t take the carb boots off, but I’m now fairly convinced it’s carb #3. I tried switching to prime, and immediately a slow dribble of gas started collecting on #3. I tried the “hack” methods of unsticking the float (mainly, hitting the carb body with the handle of a screwdriver), with no success. Guess I’ll have to pull the carbs off and open them up.

 

Float needle valves on these older bikes often need to be replaced and then a wet set

https://www.xj4ever.com/setting fuel levels.pdf

 

Update 2: took the carbs off the bike today and opened the bowls. First time doing this, and boy.. it took some doing. I do not have a rebuild kit handy, so I did what cleaning I could without disassembling the rack. Didn’t seem to be too much in there that could be causing a stuck float, so I suspect the float needles are contributing.

Put everything back together, and it doesn’t seem to be leaking anymore. It’s a slow leak, so it’s possible I won’t find out it’s leaking again until later. But hey, it’s a start.

Also, bonus pic. One of the carbs had some insane epoxy in it, or something. What the heck did someone do to this carb?

 

They broke the float post, probably while trying to remove the float pin. It must have been a clean break (snap!) and then they've epoxied it back in place. That may work as a temporary fix, but most epoxies are not fuel resistant, and eventually it may break loose.

Is this the carb that was leaking? If so, I'll bet the 2 issues are related..........

 

verify float levels on all 4, check both sides to make sure one is not higher than the other.
Then check the wet level on them and tweek if necessary.

 

It’s not the leaky carb. The epoxy seems really solid for now — when I first saw it, I assumed it was gunk from something that got disintegrated and mixed into fuel. But then I touched it, and it was like hard plastic.

The previous owner claimed to have rebuilt the carbs. I’m guessing he’s the culprit…

 

They always are.............

 

Hello again. Updating this thread since there have been some developments.

The bike had no issues for a while, and I was routinely checking for the smell of fresh fuel and drips from the carbs. Alas, about a week ago, the problem started up again. I took the carbs off the bike and checked out the internals, and it looks like there's a slight wear ring around some of the float needles. Doing a quick cleaning and putting the carbs back together did not fix the issue this time, though the leakage has slowed somewhat.

I think at this point, I need to replace the float needles and wet set the fuel level. I'm going to just order the full rebuild kit from @chacal , but I hope I can put-off the complete rebuild until winter time. If replacing the needles doesn't fix the leak on its own, then it's time to take the carbs to church.

 

Committing some threadomancy to give this a final update, in case someone stumbles on this in the future looking for the solution. Got the rebuild kit and commenced replacing the needles and seats. I made a carb rack out of PVC pipe and connected an auxiliary fuel tank to wet set the floats, and…

…fuel INSTANTLY started pouring out of the fuel stub connecting carb #3 and #4. The higher static head pressure from the aux tank (hung a few feet in the air) made all the difference.

So, I commenced with a full rebuild. Broke the rack, and all the o-rings on the fuel stubs were rock solid. Only one had cracked, but the others were bound to fail pretty soon. I couldn’t even get them off their seats without them falling apart. I will confess, I didn’t take my carbs fully to church. The insides were spotless, and I used a flashlight to confirm that my diaphragms were intact and that all jets had through-holes. So, I did not use carb dip + ultrasonic, but I did clean things up with carb cleaner spray and a wire or plastic brush (depending on the metal).

Put everything back together, colortuned and synced the carbs, and she runs great with not even a hint of leaking fuel! Guides on this forum were a huge help, and many thanks to Len for the rebuild kit and tools. Here is a pic of the fuel stubs: