Running Rich/Flooding/Choke Issues


XS650 Addict
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Newcastle, Australia.
Hey guys, I'll keep this as succinct as possible. I'll give as much information as possible in chronological order. This is my first motorbike, ever.

Two months ago I bought a 1973 TX650, with Mikuni BS38 original carbs. The bike was half-converted into a cafe racer and was never ridden and looked like crap but seemed pretty solid.

Since buying the bike I have:

- replaced the old lead acid battery with an AGM Motobatt (this solved a lot of the original issues I was having with indicators, head lamp, horn etc) and I thought I was good to go
- fitted a choke lever from MikesXS as PO had removed it, this meant I no longer had to stand revving lightly to warm the bike up so that it would idle without dying
- removed both carbs and replaced all jets, including float needle, ensured float height was 25mm, replaced o-rings on main nozzle with 1X4mm o rings from local bearing specialists
- cleaned float bowls, venturi, diaphragm with carburettor cleaner (was pretty dirty, which I expected for 42 year old carbs)
- replaced throttle cables with OEM from MikesXS

Once I'd done all this, the bike was running really well! Very powerful, fast bike.

I then removed the tracker bars that the bike had on it and put some pullbacks from japwebbike on it to restore it closer to it's original look. Ever since then, the bike is running extremely rich to the point of flooding and has lost a considerable amount of power. Starting from a deadstop in first gear the bike wants to die the entire time and you have to really ride the clutch. Once you get it going, it is SLOW!

Both spark plugs are sooting out after one 5 minute ride.

The bike will not idle with the choke on, but will idle fine without it. If i try to put the choke on, the bike will idle rough for 5 seconds and then die (presuming because it's richening to the point of flooding the carb).

I removed the carbs again this afternoon, was hoping that it would be something reeeeeally obvious like a float needle failing or something. The LHS carb was full of fuel when I removed it (concerning), but I couldn't see any problems with any of the parts. I fitted them back on and, still the same issues. Running rich and won't idle with choke on like it would 3 days ago. Why would this problem have surfaced instantly?

Any help with troubleshooting is appreciated, what do I do next?
I'm not sure, I bought them at a local bearing specialist so I assumed they would be decent. Why?

I'm from Australia so can't just order them off MikesXS, it will take a few weeks to get here.
Because if they aren't they will fail from gas. I thought I remember someone saying if they don't seal you'll run rich. Don't order mikes ones either. Made from Chinese recycled condoms I'm sure.
Also, needle and seat for the float may have an oring if they are the removable. Those can fail and leak even with the float closed.

use the clear tube method to check float height and to verify that they arent leaking past the seats.
Because if they aren't they will fail from gas. I thought I remember someone saying if they don't seal you'll run rich. Don't order mikes ones either. Made from Chinese recycled condoms I'm sure.

Haha, in that case I'm confident in my orings being the right material. The diagram for the BS38 carb doesn't mention anything about an oring on the needle or seat for the floats, and I've never seen any when i've taken it apart?

Maybe the threat title is misleading, this isn't just running slightly rich. It seems to be getting far too much fuel and is running like absolute shit to the point of nearly flooding the carbs.

Things I haven't yet done that I will do this afternoon:

- check floats in glass of hot water for bubbles
- check float plunger hole

Anything else? I'm not a carb expert by any means, and this is really starting to annoy me.
Well bs34 have removable seat with an oring for the float needle. 38s may not. Carb guide will tell you. I read if those orings on the jet nozzle fail it just fills the throat of the carbs with fuel....hence the crazy rich. Read up on the clear tube method of checking fuel level in the bowls. That will let you see the fuel level and if the floats and height are actually set properly or leaking past the seat and not shutting off the level.
No problem, so I just get the right size clear rubbing tubing, attach it to the plug on the bottom of the float bowl and turn the fuel on at the petcocks. If fuel rises in the plastic hose above the float level then I have an issue with either float valve or brass floats leaking?
I didn't do physics at high school, so I have no idea how that works, but I'll take the forums word for it and try it this afternoon. My gut tells me I have a bigger issue, the thing is running like total shit all of a sudden and adjusting the fuel mixture screw makes absolutely no difference.

I appreciate your help so far and will report back.
I can see a sudden problem occurring to one carb, but if it's happening to both, simultaneously, then that could narrow this down.

- Loose junk/sediment from the tank can disturb both float valves simutaneously.

- As CDNTX650 suggests, needle jet o-ring meltdown from fuel intolerance could happen simultaneously.

- Your carbs should have a single choke (enrichner) on the inner part of the left carb, interconnected with a hose to the right carb, which makes it a common denominator to both carbs. So, a thorough re-inspection of the choke/plunger parts and circuit may help...
Thanks for the reply mate, does your advice change if it is just one carb that is having the issues? The LHS carb was previously running nice, the plugs were tanned. Then, the very next day, sooted out plugs on the LHS and a massive drop in power and a choke that wouldn't work.

Just want to cover all bases.
...does your advice change if it is just one carb that is having the issues?

Yes, but, in summary, these are the clues that we were working with:

- fitted a choke lever from MikesXS as PO had removed it...

...the bike is running extremely rich to the point of flooding...

...Both spark plugs are sooting out after one 5 minute ride...

...The bike will not idle with the choke on, but will idle fine without it...

...the thing is running like total shit all of a sudden...

...adjusting the fuel mixture screw makes absolutely no difference...

Are you now saying that the overrich condition occurs on only one carb?
Very good point. The RHS carb always ran a little rich, sooted plugs. I'm a complete tuning novice so was just content that the bike was running nice. The LHS carb was previously good. Now, both overrich condition.
Okay, then I guess that you could look at the left carb again.

The 72-73 carbs are a bit funky in that they share the left carb's choke system, and this has led to some unusual and unique symptoms. The choke plunger on that left carb has 2 jobs, one to seal-off the fuel entry at the bottom, second is to seal-off intermix air at its sides. This intermix air comes from a passage that emerges below the slide diaphragm, the atmospheric pressure side.

If the plunger doesn't seal this intermix passage well, then manifold vacuum signals will reach the diaphragm and prevent it from lifting properly.

If one, or both, slides don't lift properly, you can get overrich condition.

If an instantaneous tear or hole occurs in a diaphragm, can also cause this overrich.

There's a 70% chance that everything I've said here has nothing at all to do with your problem...
Thanks 2M, will do the clear tube method this afternoon and report back.

The bike starts first kick every time and idles okay without choke, but god damn these carbs are just getting way too much fuel.
Okay so did some more troubleshooting just now.

- did clear tube method on both carbs. initially the fuel stopped right around the lip where the float bowl meets the body. after about 10 minute it had risen above the lip of the float bowl and was starting to come out of the air manifold on the LHS. RHS fuel stays right around the same mark, doesn't creep up.

- so straight away you think, "Problem solved, your floats are failing". to double check, I removed the float bowls and fitting the carbs without them, i turned both petcocks on and lifted the floats to check the valves were okay. Fuel shut off instantly.

- so then I thought, "okay, the floats are punctured and aren't actually floating", removed them and put them in warm water, float perfectly and no bubbles. floats aren't punctured.

- then I fit each carb individually and do the clear tube method again, this time, fuel shuts off right around the lip of the float bowl meeting the carb body. I think my floats might be working okay.

- I then check float heights (not that I even really understand what it does) and they are 26mm high when the carb is turned upside down. I subtract a mm because I didnt remove the gasket, lets say 25mm.

So now that my floats might actually be okay, what is the next step with flooding/ super over richness? I'm stumped.

Not really sure where to start with the choke mechanism? But it looks very old and dirty, spray it with some carb cleaner? Could the choke cause flooding?

PS. I have pictures of the clear tube method if anyone cares, and can provide pictures of anything you might want to see.
When measuring floats, it's best to measure each side of each float, in other words, measure each float twice. You should do this because many times the float is twisted, one side sitting higher or lower than the other. The floats are pretty delicate so this is actually quite common. It's easy enough to correct by twisting them back to level, but you won't know you even have the problem unless you check both sides. On twisted floats, the fuel level will not actually be what measuring only one side indicates.

Running rich and no effect from mix screw adjustments are both signs of incorrect float levels.