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Discussion Starter #1
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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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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Discussion Starter #3
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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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.
 

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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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Discussion Starter #6
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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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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^ X2
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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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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Discussion Starter #10
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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I would think a mechanic would readily spot a mechanical problem (that includes tensioners).
If you do a compression check, you'll know if there is a mechanical issue.
My guess is that a mechanical issue bad enough to stop engine rotation would not be intermittent.
I still lean electrical, and suspicious of pre-detonation during the compression stroke of one of the cylinders.
If you want to check for arcing, start your bike in a dark place and scan it visually for any sparking.

You might take the time to evaluate the IC ignitor as best you can. It may go through a temp range that has ignition crossing cylinders.
I know it's a reach, but it is another possibility. Too bad they (IC's) are not cheap enough to just swap out and try.
Neither the Black nor Green wires from the IC to the coils better see ground (worn insulation), or your coil will fire out of time.

Good Hunting ...

~~C8>
.
 

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Discussion Starter #12
OK...checked all connections and hoses....no sign of loose, broken, disconnected or cracked hoses or cables/electrical connections.....no signs of anything wrong.

Any other suggestions? This is driving me nuts
 

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Discussion Starter #13
I would think a mechanic would readily spot a mechanical problem (that includes tensioners).
If you do a compression check, you'll know if there is a mechanical issue.
My guess is that a mechanical issue bad enough to stop engine rotation would not be intermittent.
I still lean electrical, and suspicious of pre-detonation during the compression stroke of one of the cylinders.
If you want to check for arcing, start your bike in a dark place and scan it visually for any sparking.

You might take the time to evaluate the IC ignitor as best you can. It may go through a temp range that has ignition crossing cylinders.
I know it's a reach, but it is another possibility. Too bad they (IC's) are not cheap enough to just swap out and try.
Neither the Black nor Green wires from the IC to the coils better see ground (worn insulation), or your coil will fire out of time.

Good Hunting ...

~~C8>
.
This sounds like a solid lead to chase down. I seem to think it is a misfire somehow and agree with your thinking. There is plenty of compression and the mechanical cycle seems normal. I was thinking that the ignition timing is off, but there is no adjustment that I can find.
I will have to take a good look in the next couple of days and research a bit about the IC ignitor.

Thanks for the advice!
 

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Discussion Starter #14
Just to follow up.....in case anyone was following this.

I checked all vacuum lines and hoses...no leaks
I inspected all electrical wiring and connections and looked for arcing or leaking spark plug wires.....nothing notable!

I decided to run the fuel tank down to nearly empty and put in fresh gas with some seafoam.....no noticeable change yet.

I then decided to reset the air/fuel mix and back out the screws a bit to make the fuel mix a bit richer since I noticed the goats belly was popping and backfiring when throttle was let go at high revs. I read somewhere that it somehow helped even though that mix is for idle....couldn't hurt to try :)

I backed out the screws to achieve max RPM at idle for each carb. Right carb is now at 3 1/2 turns out while left carb is at 3 turns out. I then had to back down the idle adjustment to get it back to 1100rpm with the adjustment knob on the left side of the bike(it creeped up to about 1300rpm with new air/fuel mix setting). I noticed that the engine seemed to idle a little smoother. :)

After all this, I no longer get the pop/clunk and stall symptom....at least not yet:smiley_th.
Maybe it was just too lean to begin with (factory setting). I will run it for a bit and check the plugs to make sure they are not fouling with a rich mixture.

I do notice that the front cylinder is noisy(chain noise?) and I think the rear cylinder is making a tapping noise.

any ideas on those issues?
 

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Crap, I WAS in 5th gear.
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You need manual tensioners. Also, if you are 3-1/2 turns out, your slow (idle) jet is a tad small (or slightly clogged.) I would go ahead and run it as-is, but if you have any hesitation at cruise, pull the carbs and upsize your pilot to a #40 and add a a shim or two to the needles (assuming nothing's clogged.) Did wonders for mine.
 

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Discussion Starter #16
just to follow up on this, I reset the idle mix and cleaned out the system with seafoam. After setting the mix and idle once again, it seems to be idling really well when cold. No more "clunk" and stalling :) and the idle does not drift when it warms up, which used to be the case.

I did the Grambo trick to eliminate any cam chain noise, but I still hear a pinging or tapping in the engine. It seems to run fine and power is not an issue, but I am concerned that I may have a mechanical issue in one of the cylinders.

I can't seem to tell if it is the front or rear cylinder, maybe it is in the lower end of the engine....just not sure at this point. The noise sounds like it is not in rhythm with the cylinder travel, more of an intermittent or once every 2 to 4 cycles of the piston travel.

Any suggestions other than having the engine pulled and inspected?
 

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Don't make us nag you about getting manual tensioners! The ACCTs that came with the bike are its weakest point. The best way to resolve your noise issue is to either rule out the tensioners or implicate a stretched cam chain. In my opinion, the best way to do this is manual tensioners. Way cheaper and easier than pulling an engine. The VN750 engine is noisy anyway, but the normal noise is more of a whir. Oh, almost forgot, some guys have found a tapping sound caused by the air valve under the seat.
 

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Discussion Starter #18
Manual pensioners are next on my list :smiley_th
I haven't heard about air valves causing noise...but I will do some research and check it out.
Thanks for the input!
 

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The acct's will give intermittent ticking / clacking. Get rid of 'em. Then go from there:smiley_th . Even if a noise remains you've still upgraded and done a good thing. :) and we'll help you to chase that down if needed... but I think you'll find yourself with a much quieter lil' V twin that kicks arse.
 

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Discussion Starter #20
I did the Grambo trick a few times to make sure the ACCT were doing their job. It seems that the springs are free and have good tension on the cam chain. It seems to have quieted the chain down a bit. I have the hardware to convert the ACCT to MCCT. That will be the next thing I do.

In the mean time, I marbled and adjusted the air/fuel idle mix to try to eliminate any popping.
She idles really nice and no backfiring during deceleration. The cold starts seem better and smoother. She idles really nice when cold and stays at same idle RPM when she warms up.:smiley_th

SO......the ticking noise is still there :( The bike has very low miles and fresh oil. all hoses and electrical connections checked and tightened. The acorn nuts on the headers were a tad loose and are now tightened sufficiently to eliminate any exhaust noises that could be mistaken for "ticking" in the engine. I used a screwdriver tip with my ear against the handle to listen to the engine as it idles. Everything sounds good, and no loud clicks or ticks while listening to the engine or case...even though I hear it when standing over the bike.
I even went as far as pulling the seat and side covers to make sure nothing is hitting or rattling with vibrations. I believe I eliminated exhaust leaks, reed valve noises, backfire or pre-muffler combustion, cam chain noise, and vibrations causing something contact to frame or other parts of the bike.

Needless to say, I am still baffled and a bit concerned. :confused::confused::confused:

Any suggestions are greatly appreciated.
 
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