Vacuum leak?

I believe I have singled out the problem to an ongoing issue. Was hoping I could get a little feedback. Replaced points to a Dyna S and did the coils to 3 ohm Dyna. New wires. I thought the problem of the rough and kinda hollow sound was ignition related. But all that is new and I fired the bike up and still didn't sound right. SO.. I got the torch out and when I was on the intake manifold on carb 2 it started to run smoother. Then when the torch gas was held next to the air breather the bike sounded like it was running smoother. Vacuum leak?

Yep, classic vacuum leak.

You may not see the place it's leaking, but the Propane gas is drawn into it.


(Using An UNLIT Propane Torch For Vacuum Leaks)

I'd look at any rubber pieces, the sync plugs on the carb holders, the holders them self,and the ducting from the air filter box to the carbs, along with any rubber vacuum lines.If your bike has the Kawasaki supplied "Smog Gear" (the vacuum switch,the reed valves, & the rubber lines from the head), any of those are suspect.

It's a kz 900. No vacuum lines. Pod filters.

Check the carb holders, and the propane may have been drawn in the pods,making the engine run smoother.

On all Kz900/1000's the most common vacuum leak is from the intake ducts, or what some people call carb holders. The rubber things that bolt up to the cylinder head. Idle the bike, and just spray a little carb cleaner at each intake duct, and listen for the motor note to change.

If those intake ducts are over five or six years old, I will say they need to be replaced. Get rid of those cheap phillips head screws that hold them to the head and replace them with some allen bolts too. I always sealed my intake ducts with Three Bond TB1211 since way back in the 70's. Regular silicone is not gas resistant, TB1211 is after it sets up. I never liked those stock clamps that Kawasaki used on their ducts either. Wurth has some real nice thin Greman made hose clamps that work great on the Kz900/1000 ducts.

Already have socket cap screws on the scooter everywhere. When I reinstalled the intake manifolds I used Hondabond because I knew they are aged. It's my understanding that the new rubber does not require any sort of rtv or the like. I don't like trying to over engineer a design if I can get away with it but cash is tight since I just dumped it all into the new ignition. So I'm hoping to get away with the old manifolds for a few weeks. But I can't stand a poorly tuned scooter. Ugh. So I pulled the suspect manifold off and hondaboned it again and maybe it'll work for a bit.

Matteson wrote: I believe I have singled out the problem to an ongoing issue. Was hoping I could get a little feedback. Replaced points to a Dyna S and did the coils to 3 ohm Dyna. New wires. I thought the problem of the rough and kinda hollow sound was ignition related. But all that is new and I fired the bike up and still didn't sound right. SO.. I got the torch out and when I was on the intake manifold on carb 2 it started to run smoother. Then when the torch gas was held next to the air breather the bike sounded like it was running smoother. Vacuum leak?

I would not trust a propane check with the very short distance between the intake manifold and the pod filters. I *would* maybe trust the propane check if you had a stock aspiration system, airbox entry under the seat, specifically in the stock situation where the air can only enter the engine into the 'engine silencer' in the middle-rear of the bike and far away from the intake manifolds -- that intake point is so far away from the intake manifold that yep the propane leak test might be reliable.


With pods like you have, the 'point of air intake' is moved way forward, from the mid-rear under the seat to basically the back of the carb. You have a huge air suction point right next to the intake manifolds, only the distance beteen the front and back of the carb, less distance than that actually because part of the carb's inside the intake manifold.

Think about which hole has stronger suction power in the area around the carbs/intake manifold.
A tiny hole (it must be tiny or you could see it with your eyes)?

Or the huge gaping maw on the back of the carb?


Equivalently, let's say you leave the engine off, and at the area near the intake manifolds you put a small red 'coffee stirrer straw', then near the rear of the carb you put the open end of a house vacuum (same size roughly as a carb's rear mouth opening).


You hook the straw up to the vacuum and then turn the vacuum on. Then you release propane in the area. Given that the vacuum is sucking on the tiny red 'soda straw' and the also the vacuum's nozzle, where's the propane gonna go?

Tiny red 'coffee stirring' straw





When you release gas into the area around the carbs, you can't control where the gas floats around, it will be pulled in the direction of the large opening with the greatest vacuum force, *not* the tiny hole you can't even see with your naked eye.

Only way to reliably use propane to leak-check is to eliminate that huge opening at the back of the carb to make *sure* the gas is not getting in there, and with pods it's not an option.


My point is I'd suspect
- jetting (you have both pods and an unrestrictive 4-into-1 pipe) -- may be running lean
- and/or clogged idle circuit in the carbs
- and/or your carbs are out of sync -- if your carbs are out of sync, man you cannot believe how bad your engine can sound at idle if your carb sync is off


Before shotgunning this any more and buying more parts you don't need, *check the carbs* !! It's free except your time.

QUESTIONS
- What main jet size are you running?
- pull your spark plugs -- what do they look like?

125 mains from stock 115. 17.5 idle. ran poor with 20 idles. Valves in spec. Bench synced carbs and cleaned. Ran fine until recently. I suspect manifolds are leaking.

The plugs are all black too. I took the manifolds off and a couple have splits. So, instead of spinning my wheels more on this I will get new manifolds. That way I can eliminate that possibility. If that doesn't work then could the wiring be arcing on the lines from the dyna s to the coils causing it to stumble around? I thought if it was leaking air it would cause it run lean and then the plugs should be lighter right? This is frustrating.

I use a spray bottle with water.
Spray the suspect area and you will hear
the water get sucked in the leak.

Cool. I already took them off. They are cracked on the inside. And one for sure was kinda spittin a little through a crack. Or something. I hope it's the culprit. It ran fine a couple weeks ago and then this. Everything was fine till it just started running like crap the next day. I was hoping it was something cheaper. Oh well I got a new Dyna setup through the troubleshooting process. I'm still a little green at all this.

What are the symptoms -- does is run fine after, say, 2000rpm but at lower rpms it sounds like it's not running on all cylinders all the time?

Not sure yet what the "running poorly" description is; if you can describe exactly what it sounds like and where in the RPM band it is happening, that would really help.
So far so good though.