2000 VT750C: no power, then power

[flivver]

The saga continues. No power again today... Discovered that my rear cylinder isn't running. Back to troubleshooting, I guess.

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[Scrapdog2grand]

Originally Posted by flivver

The saga continues. No power again today... Discovered that my rear cylinder isn't running. Back to troubleshooting, I guess.

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Swap the front and real coils, see if the problem moves to the front cylinder.

[flivver]

Swapped the coils... and the bike ran fine. :-/ Nothing was loose or suspicious looking when I was removing the coils, prior to the swap (i.e. everything looked snug and proper).

Dang.

So now, things are totally inconclusive.

About all I know is that there's a problem and that it's intermittent.

Advice on what to try next? Should I bite the bullet and replace the plug wires and coils? Or, is that premature, given the evidence?


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[John Hopkins]

Pretty hard to diagnose without being there..

so we need complete information about what is and isn't happening..

Do you turn the fuel off when you park the bike..

when you try to start it does the bike turn over..or is it dead..how long does it take before it starts..if it doesn't start for ten minutes you have time to do some quick fault finding..

I think you would be very wrong to just dive in and change things, if you can afford to do that just take it to a shop..

John.

[flivver]

Originally Posted by John Hopkins

Pretty hard to diagnose without being there..

so we need complete information about what is and isn't happening..

Understood... Let me see if I can summarize everything.

Originally Posted by John Hopkins

Do you turn the fuel off when you park the bike..

Nope. I tend to ride the bike at least 2-3 times a week, so I just leave the fuel on at all times.

Originally Posted by John Hopkins

when you try to start it does the bike turn over..or is it dead..how long does it take before it starts..if it doesn't start for ten minutes you have time to do some quick fault finding..

Okay, so, let me summarize here.

1. The bike always starts. It just doesn't like to stay running when the engine is cold and when it's cold outside (when it's warm outside, the bike remains running; when it's cold outside, the bike stalls a few times before I can coddle the throttle enough to get past that point, to where the bike will keep idling on its own until it warms up).

2. This thread started with a description of a problem, where the bike suddenly lost power after a little less than two miles on the road. The bike would run (and would start right up, after stalling), but would stall as soon as I'd put a load on it. During that incident, after a few minutes of waiting and a few restarts (and a switch to the reserve tank, though I no longer thing that was relevant), the bike ran fine and was able to get me back home.

3. A couple of days later, I took the bike out and it ran fine. Filled the tank, and drove home. (That was the topic of a follow-up post in this same thread.)

4. A day after that, I look the bike out again. This time, it started (as always), but sounded like a lawn mower. My neighbor heard it, and remarked that it sounded like a cylinder wasn't firing. To check, I revved the engine while he pulled plug wires off, one at a time. When he removed the front plug wires, there was a discernable change in the sound of the engine; when he removed the rear plug wires, nothing changed. This lead us to conclude that the rear cylinder wasn't firing. That day, no amount of wiggling or resetting of wires had any effect: it continued to sound like a lawn mower until I gave up. (That was the topic of my first post on page 2 of this thread.)

5. Today (a day later), I swapped the ignition coil units, per the advice given here. What I didn't do (unfortunately) is verify that the problem was still occurring prior to swapping the coils. Regardless, after swapping the coils, the bike ran fine. I ran it for about 15 minutes, alternating between letting it idle while parked, to riding it around the block and pushing the RPMs in order to give it a solid load.

So, to recap, the bike always starts... It just doesn't like to remain running when the engine is cold and the outside temps are cold (and the starting enrichment valve is of no use, per another thread of mine). Three times now (if I'm recalling all the details correctly), I've had a loss of power incident. The most recent was yesterday, when I believe I observed that the rear cylinder wasn't firing. Finally, thinking that it might be the coil, I switched the coils around, but the problem didn't recur.

At this point, I suspect an intermittent electrical malfunction of some sort, but am not sure how to narrow things down.

(Oh, and the Clymer recommends hooking up a peak voltage meter to your multimeter, to check the coil output. Unfortunately, I don't have a DVA adapter.)

Originally Posted by John Hopkins

I think you would be very wrong to just dive in and change things, if you can afford to do that just take it to a shop..

John.

Nope, can't really afford it. Just figured the coils and wires were a fraction of the cost of a shop visit (as it seems like a trip to the motorcycle shop seems to cost me 2x what a trip to the car place tends to).

Thanks for the help, folks!

[John Hopkins]

Well unlike everyone else I think you have a fuel/air fault because you say it starts every day..But I am only guessing.

John.

[flivver]

Interesting... Would a fuel/air problem cause a cylinder to not fire?

What kinds of things can I try, by way of troubleshooting?

I bought a new fuel filter on Friday, but haven't installed it because I was leaning toward it being an electrical problem. I didn't want to change multiple things at a time.

Thanks, John!

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[Scrapdog2grand]

John may very well be right!!
but you say it was the real cylinder, so by swapping the coils, next time it happens if it moves to the front cylinder, you most likely have a coil breaking down.
if it stays with the rear, you may have a fuel delivery problem, but you will be able to rule out the ignition coil as suspect.

[flivver]

That's a good point. By leaving the coils switched, I will have an additional data point if/when it happens again by virtue of noting whether it "follows" the coil that was feeding the rear.

I suppose I might as well install the new fuel filter... Seems like it can only help.

(You guys rock, by the way!)

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[rkforrest]

Coils like to Open up when Warm so the problem wont be aparrent til they do