Putting the carb back together did not go as smoothly as my previous posts may have indicated. The carb has been a whole mess of problems since the night I got it back in. Here's a summary of the symptoms, and ultimately the causes and solutions. If one person finds this via Google and it saves them an evening, I'll feel quite good.
The Problems
Filthy, Sticky Carb
The bike's an '82 XS400 Maxim, and hasn't been treated nicely. The engine would hesitate on hills, was a huge problem to start, and would lag for seconds when pulling the throttle. It got worse over time, too, eventually stalling 40 mph.
So I did what any sensible person would do: blame the carbs. I pulled 'em, blasted 'em clean, replaced the gaskets, and put it back together. But this only uncovered more problems.
Lean Mix Syndrome
It would start okay, but would die when I hit the throttle. After 20 minutes' runtime it would rev very high: 5000 at a stop light, and wouldn't come down, which was kinda embarrassing. It would backfire. When I was at speed and let go of the throttle, it wouldn't slow down as I would expect, but would continue to surge forward.
This is typical of a lean mix: too much air getting in relative to gasoline getting in. This is often a vacuum leak, e.g. the carb throats or airbox boots not tightened properly, or a hose loose. But I gave the hoses a shot of WD40 and nothing came up. After disassembling and examining the carbs a few more times the best I can figure if that I cleaned out some gunk that was impeding airflow, uncovering some other misadjustment in the carbs. Back to the bench...
Dumping Gas!
I pulled open the carbs and cleaned everything again, checked seals, etc. Put the thing back together, and got a gas leak. When I would start the bike, gas would pour out the back of the carbs, into the airbox and onto the ground. Not a drop or two, but tablespoons at a time. Get this:
- "The blow test" is to hold the reassembled carbs up and blow through the fuel intake (use a straw if you're smart, yuck!). If the needle valves are seating properly, you can blow freely through the carbs when they're upright, but not at all when they're upside down (bowls up) because of the valve. And these passed the blow test.
- But once I got them onto the bike, and tried to start the bike, something went wrong. Once the gas started coming out the back, the needle valve had quit working. I could switch the gas cock to PRI and gas would flow out the back, switch to ON and it stops. The needle valves had quit working.
- Pull them off the bike, and they're working perfectly. Can't put air through them to save my life. Put 'em onto the bike and they're fine, until I try to start it. Then gas.
The Solutions
Blow Test
When doing the blow test to see if your needle valves are seating properly, try this: Turn them upside down and you should fail to blow through them. Now add a generous amount of WD40. On one of these two, I could hear a slight bubbly-hiss and see bubbles in the WD40. The needle valve itself (not the O ring) had a less-than-perfect seal, but only when it was wet.
This didn't account for all of the gas that was coming out, though; this was a tiny bubble, and I was seeing floods. But still another week of waiting on parts.
Sticking Floats
The big breakthrough to the leaking gas problem, came as I was reassembling them. I put the bowls on and gave a blow test... and it blew through. I shook them, and they sealed. Shake them again, one of them would reopen but only a little. Shake some more, it seals but I can hear that the other one is wide open. A clue!
I continued to carefully tilt the carbs back and forth, hearing the floats slam open, hearing them slam shut... or sometimes not shut. Fiddling with the bowls and a flashlight, I found it: The floats were sticking out too far, and were catching on the edges of the bowl, and not coming back "up" to close the valve. For the first time, I could replicate the valve failure on the bench!
Best I can figure, the floats had been straightened when I was adjusting them previously, and were sticking only after something raised them (letting in the gas) and vibrating them just wrongly (starting the bike). And I could never see the problem on the bench, because the act of removing the carbs from the bike would shake the floats loose.
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!
Other Lessons Learned
- There are Versions of the Mikuni BS34. The more common one, used on single-overhead cam (SOHC) XS400s, has a silver plug filling a hole between the carb bowl and the carb body, and the drain on the float bowl is at the bottom. The one used on dual-overhead cam (DOHC) XS400s (the Maxim and Seca) has that silver plug integral to the bowl, so the gasket for a sold for a XS400 needs you to cut a hole into it. And the bowl on the DOHC model has the drain in a different position. But otherwise, parts for a BS34 carb are pretty well matched to this one.
- The floats are not made of brass like the manual says, and are differently shaped, rendering the suggested float heights in the manual completely worthless. The float height in the manual is 32mm, which is absurd since that places the floats outside of the bowl.
- Just blow the $30 on every piece of rubber you can find for the carb, up front. Otherwise you'll end up buying it anyway, and paying for shipping repeatedly. I had to buy float bowl gaskets, an O ring and washer for the mix screw, and a set of float needle, float needle seat/valve, and its accompanying O ring. $30 in parts and $30 in shipping because I did it one piece at a time.
- This weird BS34 does NOT have a plug on the pilot hole. The manual shows a brass plug, and MikesXS sells a rubber plug. But on this model, the plug is built into the bowl and is not a complete plug. If you put a plug onto the pilot, you get instant Lean Mix Syndrome from the moment that bike starts: high revs and it dying if you open the throttle.
