runs better without gas tank?

[CaptainPlatypus]

Some background. I got my ninjette a month ago. She's twelve years old with only around eight thousand miles on her, which means she almost certainly spent way too much time sitting, but I bought her from a certified mechanic who promised he'd given her a complete look-over, carb clean, etc except for checking the valve clearances, and she was running fine when I got her.

So the other day, in the middle of a 20+ mile ride, my ninjette abruptly lost power at freeway speeds. Plenty of fuel in the tank, spark's working fine, but suddenly she was running really REALLY lean. I pull over and turn her off, can't find any obvious problems, turn her back on again. She'll only start with throttle, releasing clutch into first nearly stalls her even if I'm idling near redline (which takes a full twist and significant patience while the needle slowly climbs), and even once she gets going she has significantly less power available than normal. I (barely) managed to lug her home (maxed out at 50mph indicated in third gear, with a tailwind) and she's been starting only unreliably since.

Confirmed to have normal spark on both plugs, so it's not an ignition issue, which means it's almost certainly something in the fuel system.

Now here's the weird part. I took off the fuel tank to check hoses and drain the carbs. I wasn't confident in my carb-draining setup being leak-proof, so I decided to try to start the engine, just in case I could burn as much of the fuel in the carbs as possible instead of needing to drain it and maybe leak it. And here's the kicker - she started up just fine, like nothing was wrong, idled smoothly and uncomplainingly until she ran out of fuel, then died. There was some fuel coming back through the fuel filter line from the carbs to the (detached) gas tank, but I assume that was just compression-related and not the problem.

I'm pretty sure, then, that it isn't a problem with the carbs. What I'm not sure of is what the hell kind of problem it might actually be. Any suggestions or advice would be very welcome.

[alex.s]

two suggestions, check the ignition wiring loom that goes back down the center under the tank. also check your petcock... try putting the petcock on prime if it has it. sounds like it might not be letting enough fuel into the carbs. maybe you have a fuel filter and it is clogged?

[CC Cowboy]

The best suggestion I have is to get in touch with the certified mechanic that sold you the bike. He knows the bike better than anyone.

[CaptainPlatypus]

Good suggestion, Cowboy. I'm taking it over to him for him to look at it Wednesday (next day we both have off) if I haven't fixed it before then. Hoping I can get her working in the meantime; she's my commuter.

I do have a fuel filter, which looks to be completely clean. I just took apart the petcock and it's completely clean as well. Only has on/off/reserve; I'll try reserve when I have the tank back on I suppose. Can't hurt.

What do you want me to check in the ignition wiring? I don't see any obvious problems, and I know the ignition is working fine.

[DaBlue1]

How well does your fuel tank vent. Does the engine behave the same way with the gas cap open?

[CaptainPlatypus]

As much as can be expected, yeah.

[CaptainPlatypus]

Well, I have her idling more smoothly, and she's running well enough to get me to and from work for the next few days. I think I'll wait for the mechanic in question to **** around inside the carbs at all, or try to find someone who knows what they're doing to look over my shoulder at the very least.

[tc.young]

your first issue sounds like it is running on one cylinder, that would explain the lack of power when taking off. easiest way to check is to put your hand on the right and left cylinder and see if they are warm. or near the exhaust headers but be careful not to touch.

spark and compression are the easiest thing to check with fuel being last. since your confident enough to take the tank off and check for spark, you can check if it is a delivery issue by hooking an IV like setup with fuel in a bottle above the carbs and the vacuum line plugged. if the issue is still there then it's a carb, if not then it's a tank vent/petcock.

starting with the easiest checks and working to the hardest is the way to go. but if it starts up fine, just ride it around near home and see if it happens again.

[n4mwd]

Actually, based on what you are describing, it sounds a lot like fuel delivery.

You should test the petcock first because that is pretty much the simplest and will tell you the most.

Procedure:

1) Turn petcock to ON (with plenty of fuel in tank).
2) Remove big hose. Gas should NOT come out - except for maybe a little that might be in the hose.
3) Switch to reserve - should not let any gas out.
4) Remove the small hose and attach a test hose to the port about two feet long.
5) There should be absolutely no gas come out of the small hose or port - not even a drop.
6) With your mouth or syringe if you have one, suck on the small tube (again, there should be no gas in the small tube). This time, gas SHOULD come out of the big tube so be prepared to catch it in a glass jar or something.
7) Gas should flow out of the big tube in a nice thick stream when sucking on the little tube. When you stop sucking, the gas should cut off.
8) Switch to ON and repeat the test.
9) The gas that comes out should be free from any debris. If its clear, you can pour it back into the tank.
10) Lastly, switch to OFF and suck on the small tube. This time there should be no gas come out.

If the petcock does not operate as described, then you need to remove it and repair it.

Also, the test should be done when you know the bike is acting up. There may be debris in the tank clogging the intake screens and removing the tank could shake it loose causing the test to show good.

[CaptainPlatypus]

All right, so I've got news. I was on my way home from work this morning, I went over a bump - and suddenly I had full power again, for all of a second or so! I'd had the throttle cranked all the way, because that was the only way I could accelerate, and after this tiny bump suddenly the bike just surges forward (like she should with that much throttle on). It was a beautiful moment - and then it was over and she was being her new cantankerous self again, though not quite so badly. So with that in mind, some sort of blockage in the fuel tank is what sounds the most plausible to me - something that got jolted out of its blocking position by that bump, but then re-settled afterwards. I'm gonna check with @n4mwd's method and see what happens. No idea how this could be happening, because the tank looks spotless inside from all I can see. But it's the explanation that makes the most sense.

EDIT: So I tried the first half of @n4mwd's test, and got a slow but consistent drip even with the petcock on OFF, and a faster consistent drip with it set to ON or RESERVE. Will test for flow later in the day, once I've slept and once an auto store is open so I can get some hose. I want 1/4", right?

[Motofool]

Originally Posted by CaptainPlatypus

.............and after this tiny bump suddenly the bike just surges forward (like she should with that much throttle on). ................. something that got jolted out of its blocking position by that bump, but then re-settled afterwards...........

Originally Posted by CaptainPlatypus

...........It looks like what I should start with is checking the spark plug wires, but I honestly am not sure which wires I should be looking at - is there a way to do this without removing the gas tank?

The second is a quote from your previous thread.
Did you ever checked the spark plugs' cables and the connections to both coils?
Have you checked if one cylinder gets cooler than the other after a while of the engine running?

It seems to me that you have a loose connection somewhere, since bad fuel supply doesn't improve suddenly and comes back again like what just happened to you.

[Chicagobob]

It really sounds like you have an electrical problem, a loose connection or short, probably at the coils or plugs. A fueling problem wouldn't immediately give you a change in performance like that.

[n4mwd]

Originally Posted by CaptainPlatypus

So I tried the first half of @n4mwd's test, and got a slow but consistent drip even with the petcock on OFF, and a faster consistent drip with it set to ON or RESERVE. Will test for flow later in the day, once I've slept and once an auto store is open so I can get some hose. I want 1/4", right?

Until you put a vacuum on the smaller port, you should get no consistent dripping - so your petcock is bad and needs to be repaired. All that needs to be done now is attach the small hose and see if you have good flow when you apply a vacuum. This will tell you if anything is blocking the inlets.

The hardware store is probably the best bet. You should be able to get a foot of clear 3/16" hose for less than a buck. Or fish tank tubing might also work in a pinch.

Also, you said that the mechanic recently rebuilt the carburetors. Then that is also a red flag. Its possible that he didn't do it right.

Its true that electrical problems can cause similar symptoms to what you are experiencing, but what makes me shy away from that is the fact that you said it runs bad until you disconnect the tank.

Can you duplicate the good running situation when you only disconnect the big hose from the petcock and leave the tank in place? By doing that, you have eliminated the possibility that the physical presence of the tank is shorting something out.

EDIT: If you run with the petcock disconnected, make sure its switched to OFF and something is catching the gas that is dripping out of it.

[b.miller123]

My money is on a dead battery.


How old is it? (some have dates on them with a sticker or stamp)


Okay, after reading the whole thread and hearing about the bump, it's either a loose battery connection, or a loose connection at one of your coils or somewhere else in the ignition circuit, though I would check the coils first. Your symptoms in the first post sound a lot like a dead battery though. Wont idle without full throttle, hard to start, stalls as soon as you try to move in gear, etc etc.

[n4mwd]

Well the battery is simple enough to test. And a bump can cause it to intermittently fail.

Get a voltmeter out and measure the battery voltage in four situations - key off, key on, cranking, running.

Key off should be higher than 12.5V.
Key on should be higher than 12.1V.
Cranking should not go below 9V (preferably not below 10V).
Running voltage should be higher than 13.5V (tests both alternator and battery).

In most cases, a bad battery will fail at least one of the above tests.

But don't assume that because it can crank that its good. Motorcycles tend to continue to crank even when the battery is nearly completely dead and can no longer power the ignitor which requires about 8 volts minimum. But if its low voltage to the ignitor, then it generally doesn't run at all.

But having said that, I still think its fuel related. Nothing else really explains the fact that it ran well with the tank removed. My theory is that you have a stuck fuel inlet valve in one of the carb bowls causing it to run on one cylinder. Hitting a bump knocked it loose briefly and allowed gas into the bowl.

But even still, I think that after you test the petcock for flow, you should take a look at your electrical just because its easier to do than pulling out your carbs.

[CaptainPlatypus]

Both cylinders seem to get to the same temperature. I did end up pulling and checking both plugs - they both sparked fine, both wires seemed fine, and I reconnected them and the connections seem fine. I have a multimeter, so I'll check the battery and the wiring next, because, as mentioned, it's the easiest thing to do.

[CaptainPlatypus]

All right. A couple days later, some issues have been resolved (with the help of the semi-friendly mechanic mentioned earlier), and now it'll start consistently and so on, but it's been confirmed that the bike is definitely only running on one cylinder. The mechanic (and the several other mechanics he's asked for help) can't even decide what's wrong - they want to take apart the carbs to check those again, and they want to charge me for it. I'm not convinced that's the issue, and I'm not excited about paying for maintenance that doesn't make sense to me - it doesn't make sense that a carb would somehow instantly get completely clogged in the middle of the ride with the fuel filter still looking sparkling clean. But there's definitely spark in both plugs, the battery's definitely good (checked with a multimeter), fuel's definitely going to both carbs. The thing that seems most consistent with the symptoms to me (died suddenly in the middle of a ride, brief restoration of power over a bump since) is still an electrical/wiring issue. Is there any way that could be what's going on even with confirmed spark on both plugs?

[b.miller123]

Did they actually verify which cylinder isn't firing?

I'm still betting it is electrical. Most things that happen when you go over bumps are electrical.

[CaptainPlatypus]

The right isn't firing. And yeah, electrical makes sense to me, but if the plug's sparking I have no idea what electrical problem it could be.

[tc.young]

i remember something about when coils get warm after everything's running and their faulty they can spark intermittently. not sure if that's the right answer in this case.

i see that valves aren't done, i doubt that has anything to do with it but it could be worth adjusting since it's off the road anyway.

[DaBlue1]

Originally Posted by CaptainPlatypus

The right isn't firing. And yeah, electrical makes sense to me, but if the plug's sparking I have no idea what electrical problem it could be.

Have you troubleshot or swapped out the IC ignitor just to rule it out?

[CaptainPlatypus]

Originally Posted by DaBlue1

Have you troubleshot or swapped out the IC ignitor just to rule it out?

No, I haven't. Explain?

[DaBlue1]

Originally Posted by CaptainPlatypus

No, I haven't. Explain?

The IC Ignitor is responsible for the firing of the coils. If either the coils or IC ignitor is faulty, it could result in the bike possibly running poorly.

Check the Troubleshooting Guide

Poor Running at Low Speed:

Spark weak:

Battery voltage low

Spark plug dirty, broken, or maladjusted

Spark plug cap or high tension wiring trouble

Spark plug cap shorted or not in good contact

Spark plug incorrect

IC igniter trouble

Crankshaft sensor trouble

Ignition coil trouble


Poor Running or No Power at High Speed:

Firing incorrect:

Spark plug dirty, broken, or maladjusted

Spark plug cap shorted or not in good contact

Spark plug incorrect

IC igniter trouble

Crankshaft sensor trouble

Ignition coil trouble