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post #1 of 42 (permalink) Old 10-19-2011, 09:39 AM Thread Starter
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Question need help with stalling engine

I have an 06 VN750 which is mostly stock other than a windshield and saddle bags. I have an issue that I can't seem to grasp.

When the bike is started cold and is warmed up just enough to run without the choke, It idles at about 1100rpm. There is no smoke or smell from the exhaust and all seems just fine. What happens is odd to me. I will hear the cylinders cycling fine, yet randomly it happens...I hear a clunk and the bike just stops dead. I can re-start it without any problems, but it sometimes does this and other times it is just fine. It only happens when cold started. Once the bike is warmed up and ridden, I don't experience it at all.

Keep in mind that I am no mechanic, but it sounds to me like a timing issue where one cylinder is fighting the other and stops the engine rotation dead in it's tracks. Maybe a cam tension-er issue??

I checked for loose hoses or clogged air cleaner. I pulled the plugs and they are fine...no signs of pre-ignition or fouling at all. I do get the occasional backfire after stalls (goats belly still in tact, and not marbled) which is inherent for these bikes. The only mod I really did was the pickup coil gap which is now set to .020" rather than factory setting of .035". This was done recently hoping it would fix this issue as a stab in the dark. It did help with hot starts, so I am glad I did it.

Still stumped on this one and so is my mechanic apparently. Has anyone else heard of this type of symptom?

Any suggestions are greatly appreciated!
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post #2 of 42 (permalink) Old 10-19-2011, 10:41 AM
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Does the "clunk" happen after you put it in gear and try to take off or is it without touching the bike?

If it is when you are putting it in gear and letting go of the clutch- maybe your little plunger for the kickstand safety switch that prevents you from driving off with the think down. The cold weather may make the rubber sticky.
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post #3 of 42 (permalink) Old 10-19-2011, 11:16 AM Thread Starter
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Originally Posted by Papa Smurf View Post
Does the "clunk" happen after you put it in gear and try to take off or is it without touching the bike?

If it is when you are putting it in gear and letting go of the clutch- maybe your little plunger for the kickstand safety switch that prevents you from driving off with the think down. The cold weather may make the rubber sticky.
The "cold Start" I mentioned is after the bike has been sitting overnight during any temperature including summer days over 90 degrees.

The bike is just sitting there at idle without being in gear or touching the throttle.

It sometimes makes a Clunk noise but keeps idling...other times it stops dead when it clunks. It has no rhythm or pattern related to when it happens during idle...it seems erratic at best.
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post #4 of 42 (permalink) Old 10-19-2011, 11:38 AM
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That is indeed a puzzle. It is very common for an engine to stall, under pretty much any condition. It is usually a carburetor problem, but can be an intermittent electrical issue. The oddity in your case is the clunk. It is not a timing issue, as the engine runs fine otherwise. It is also not a cam chain tensioner, if you had that problem (and you will) the engine will make a loud noise while running, especially when cold. It seems to get better when hot, though I have non idea why. It never goes away completely

The clunk sounds like the noise most bikes make when put into gear from neutral, but that only happens when the gears engage, so that is not it. A clunk almost always means something mechanical, which pretty much rules out both the carbs and ignition, and the fact that it otherwise runs ok, and without any strange noise mostly rules out cam chain tension, ignition and valve timing.

The fact that it does not happen when the bike is fully warmed up would suggest a temperature connection, but that seems unlikely (but not impossible) it is more likely just because the bike has been running longer.

I will put some thought into this, I'm not going to start making wild guesses. If it is happening, there is a reason. I think the clunk is the key symptom here.

I am a motorcyclist, NOT a biker.


1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
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2001 Yamaha XT225, heavily modified
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post #5 of 42 (permalink) Old 10-19-2011, 11:50 AM
Crap, I WAS in 5th gear.
 
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Wow, this is an oddball problem. It could be that the clunk is a pre-ignition, which does implicate electrical and carbs. It might be something that you need to wait on so it gets bad enough to diagnose. Could be a sticky float, causing it to run lean occasionally, but I would exect you'd see some evidence on the spark plugs, perhaps not-- and the idle speed would probably change before it died. I could see a petcock diaphragm not opening all the way, causing it to be lean at idle and pre-igniting. Put some clear tubes on the carb drains and fasten them next to the idle enrichener cover-watch the float level while it idles. If it's electrical all you can do is check all your connections- it's too intermittent to really get a bead on. If it's really bugging you you're going to have to start checking things out, the easy stuff first. Float levels are pretty easy to check.
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post #6 of 42 (permalink) Old 10-19-2011, 12:08 PM Thread Starter
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Wow, this is an oddball problem. It could be that the clunk is a pre-ignition, which does implicate electrical and carbs. It might be something that you need to wait on so it gets bad enough to diagnose. Could be a sticky float, causing it to run lean occasionally, but I would exect you'd see some evidence on the spark plugs, perhaps not-- and the idle speed would probably change before it died. I could see a petcock diaphragm not opening all the way, causing it to be lean at idle and pre-igniting. Put some clear tubes on the carb drains and fasten them next to the idle enrichener cover-watch the float level while it idles. If it's electrical all you can do is check all your connections- it's too intermittent to really get a bead on. If it's really bugging you you're going to have to start checking things out, the easy stuff first. Float levels are pretty easy to check.
I suppose it is possible that what I hear is more like a POP than a clunk when it doesn't stall the bike at idle. The clunk sound might be attributed to the engine suddenly stopping. I am wondering if the POPing during idle I hear is a slight backfire or back pressure issue which forces the rotation to stop. I do feel the engine almost stop rotating at times when it hakes the noise, but it is hard to tell which cylinder it is coming from.
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post #7 of 42 (permalink) Old 10-19-2011, 12:44 PM
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If you do think it is a pop and the clunk could just be the engine stopping- my bike makes a pop every time I switch the bike from run to off. Happens on my friend's bikes too. It could just be that you are noticing it in this instance since it is happening when it shouldn't.

If so I would lean towards electrical. Maybe some insulation is heating up a touch and expanding a crack. Cheapo me would probably undo the ground and refasten it as a first check then start looking at important wires.
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post #8 of 42 (permalink) Old 10-19-2011, 01:48 PM
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While you are checking the wiring (including ground points), be sure to check the condition of the wiring to the neutral switch.

~~C8>
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post #9 of 42 (permalink) Old 10-19-2011, 02:00 PM
Crap, I WAS in 5th gear.
 
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Had another thought regarding backfiring. True backfires (into the carb) can be caused by a vacuum leak-- check your vacuum lines.
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post #10 of 42 (permalink) Old 10-19-2011, 04:00 PM Thread Starter
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The bike is in great condition and all electrical lines are in tact and not worn or arcing. I will double check all the connections as well as all the vacuum lines and hoses one more time just in case.
I keep thinking it is mechanical and not electrical due to the sound and the fact that the engine jolts when I hear that pop even if it continues to idle.
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