Answer me a general multi-carb question

I think a lotta my problems thinking through issues on bikes is that while I have extensive single carb experience and can rebuild with the best of 'em, I really don't have any solid ideas about how they work together in gangs.

In general...if one carb is running crazy and pulling the engine rpm up, do the others' slides try to react similarly? Slide action based on pressure differential, I wanna say they would...but I'm not sure.

I'm getting a headache trying to think it through. Someone tell me before I get a migraine

 

Think of it this way, each carb slide is only reacting to the conditions of ITS throttle valve and cylinder vacuum (ignore YICS for now).

The only thing that connects the carbs to each other is the throttle valve linkage, and the crankshaft of the cylinder.

So the throttle valves will open together at the same rate from their initial position (which may be biased through syncing).

Each cylinder is drawning a vacuum due to the momentum of the cylinder, and the momentum of other cylinders and their position relative to the Otto cycle via the crankshaft. So the force of a single piston drawing a vacuum is a function of the sum of all of the forces on that piston.

If one cylinder is running stronger than another, it can "bring along" the others by forcing another piston to draw a faster vacuum that it would if it was producing power and wasn't connected to the other cylinders via a crankshaft. So the weak cylinder is basically drawing a vacuum without contributing any power.

The slide will keep the venturi velocity "constant" with respect to that vacuum and throttle opening. But if that cylinder isn't firing (due to spark, compression, or mixture issues), then it won't contribute to the overall momentum of the other pistons on the crankshaft, thus reducing the overall power available for when the other cylinders draw a vacuum.

 

OK...I've got a runaway....motor goes to 6000 rpm and sticks.

Trying to come up with likely scenarios, and I mad at it right now and don't wanna work on it until I'm not anymore. No good would come of that.

Pulled it into the garage, normal idle speed, running ok except not starting without ether when cold.

Took the carbs off, cleaned some gunked idle jets and made sure passages were clear.

Put it back together, and it wants to run at 6000 rpm. Throttle FEELS free, seems to be dropping to base but I'll have to pull the tank to see clearly. It is NOT getting interference from the boot clamps, I can tell that much.

I forgot I did play with one thing...my #1 float for some reason was hanging much lower than the other three and not coming up as high when carb flipped upside-down. So I made it conform to the others. I'd totally forgotten I did that until now...was too tired and pissed off last night, mind not working right.

But that said, I don't see how that could cause the issue here. (?)

 

It sounds more like a vacuum leak, but you do need to verify fuel level in the bowls with the clear tube method.

 

This thing is possessed. There is just no other explanation.

Worked on engines/vehicles for 30 years and NEVER seen things happen that this thing can think up.

Here's the latest. See what you make of this.

Went out there, and just for laughs, hit the starter. It started right up. No more insane 6000 rpm nonsense.

Cracked the throttle a few times, just to see if it would hang again, but it's fine.

I don't know about you, but I'm skeptical of things that fix themselves.

So I let it run. At some point, something happened...the instrument cluster went dead, no LCD, no tach.

So I shut it off. Or should say tried to...it kept running regardless of key position.

So i start pulling fuses to finally get it to quit.

Put them back in, found the one that blew.

Now this is where it gets REALLY weird. I had bypassed the safety because the starter button didn't work, suspected a safety switch was the cause.

I pulled that wire when it wouldn't shut down, along with the fuses.

WHEN I TAKE THAT FUSE (the one I replaced) OUT, THE STARTER BUTTON WORKS...WITHOUT THE BYPASS...AND THE MOTOR WILL START, EVEN THOUGH THE KEY IS OFF...AND I MEAN START, NOT JUST CRANK...and the headlight stays on, even with the key off.

Put that fuse back in, everything returns to normal.

This thing gives me the creeps.

 

I suspect the PO did some custom wiring. I bet if you pull the TDI it won't start.

If it does, call Ripley's.

 

Interesting problem, normally for the bike to start you need power from the ignition switch to the coils and TCI on the red/white wire (from memory). Somehow it is getting power when you remove the fuse. Does a relay click when you remove the fuse?

Sounds like incorrectly modified wiring for sure.

 

Not that I noticed but I'll see tomorrow. Maybe. Kinda sick of it at the moment...

 

A cross, silver bullet, clove of garlic and a wooden stake should see you right....

 

I need to strip all the wiring out of this thing.

Then take the carbs off and disassemble.

Pull the motor out of the frame.

Take the tires and wheels off.

Then bury it in 5 different states under a full moon, and hope it never comes back.

 

How about instead you invest in a service manual and a multimeter and figure it out?

It's an inanimate electro-mechanical device, designed and built by humans. You ARE its master; but in order to exercise said mastery, you need to know how it's put together. And that's why we have service manuals.

You are currently being driven crazy by the symptoms of neglect and trying to shortcut "the process." Back up, slow down, and do it right and it won't be nearly as frustrating.

Read: http://xjbikes.com/Forums/viewtopic/t=24010.html and heed.

 

I fixed all my electrical gremlins at once - I bought a wire harness for my Seca 750 off E-Bay for $15, hope you can get that lucky.

"bury it in 5 different states under a full moon" :lol: 8O :lol: