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Discussion Starter #1
Hey Guys,

So here is the deal. Took my 91 VN750 on its first ride of any distance since getting it home and getting it "road ready" and I wanted to ask you all to help me identify some issues.

1) The rear cylinder left side spark plug can be seen sparking from between the "nut" portion and what I think is ceramic part going back to the wire. It is a brand new NGK Iridium I put in with anti-seize at 12 ft/lb. Is it defective or perhaps cracked from installation? :confused:

2) After a 30 minutes / 20 mile ride in the dark, I was able to see the portion of the rear / left exhaust header (immediately off the cylinder) that is behind the chrome cover was red hot. :wow: Is this normal? When cold this pipe is totally black, I assumed that perhaps this is normal and that is why the cover is chrome, but I am no expert so I thought I would ask some, (you guys) :)

3) I am only partially through my first full tank of gas, which I did put seafoam in, approximately 1/4 can for full tank, and I am not sure if this is enough to clean the carbs, or if the carbs are actually the culprit for my "over idling." I don't think it is actually over idling because the RPMs take some time (2-5 seconds) to come back down to ~2000 rpm once I roll off the throttle when at speed, but they usually do come back down to ~2000. What I noticed today is that when I got home and put the bike in neutral so I could inspect the sparking plug, the RPM were stuck at 4000 for about 15 seconds at which time I noticed the red hot pipe so I turned the engine off. I waited 5 minutes then started her up and the idle was at 3000, still a little high, but better. I should note that it does not usually run this high when starting cold and after warming it up. Also I noticed that it took about 20 minutes of riding for it to fully warm up. I don't think that is an issue but thought I would mention it.


Ok, so that is a lot so I will end here and anxiously await some advise.


Thanks in advance :smiley_th
 

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Premium Member
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1,803 Posts
Hey Guys,

So here is the deal. Took my 91 VN750 on its first ride of any distance since getting it home and getting it "road ready" and I wanted to ask you all to help me identify some issues.

1) The rear cylinder left side spark plug can be seen sparking from between the "nut" portion and what I think is ceramic part going back to the wire. It is a brand new NGK Iridium I put in with anti-seize at 12 ft/lb. Is it defective or perhaps cracked from installation? :confused:

2) After a 30 minutes / 20 mile ride in the dark, I was able to see the portion of the rear / left exhaust header (immediately off the cylinder) that is behind the chrome cover was red hot. :wow: Is this normal? When cold this pipe is totally black, I assumed that perhaps this is normal and that is why the cover is chrome, but I am no expert so I thought I would ask some, (you guys) :)

3) I am only partially through my first full tank of gas, which I did put seafoam in, approximately 1/4 can for full tank, and I am not sure if this is enough to clean the carbs, or if the carbs are actually the culprit for my "over idling." I don't think it is actually over idling because the RPMs take some time (2-5 seconds) to come back down to ~2000 rpm once I roll off the throttle when at speed, but they usually do come back down to ~2000. What I noticed today is that when I got home and put the bike in neutral so I could inspect the sparking plug, the RPM were stuck at 4000 for about 15 seconds at which time I noticed the red hot pipe so I turned the engine off. I waited 5 minutes then started her up and the idle was at 3000, still a little high, but better. I should note that it does not usually run this high when starting cold and after warming it up. Also I noticed that it took about 20 minutes of riding for it to fully warm up. I don't think that is an issue but thought I would mention it.


Ok, so that is a lot so I will end here and anxiously await some advise.


Thanks in advance :smiley_th
Spark plug could be defective new, not uncommon, you could have a bad spark plug wire boot, or the metal clasp inside the boot is not grabbing the plug head. All would cause surging of rpms. Your pipes are too hot. I suspect a very lean carb condition. Sounds like your decelerator cable needs adjusting or lubrication. Not good if your rpms don't drop immediately. Have you tried adjusting your idle screw knob? Bike should idle around 1100 rpm. My bike takes about 10-15 minutes to warm up.
Jim
 

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Discussion Starter #3
When I get home today I plan on getting a replacement plug, then warming the bike up to full temp at idel, supposed to be 75 today so shouldn't take to long hopefully. then I'll adjust my idle screw to have 1100 RPM at rest. Also I haven't yet checked my air / fuel screws but I did notice that the caps are popped off, so maybe the PO tightened them down too much.
What type of lube should I use on the deceleration cable, I have silicon spray, white lithium grease, moly 60 (doubt I should use this there), or should I get something different?

I'll post back tonight with results.

Thanks

I'll post an update this evening. Thanks for the
 

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Premium Member
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Discussion Starter #5
Update-

Spark Plug was definitely cracked the length of the post that the wire attaches to. I noticed it the second I unplugged the wire. I remember slipping off the nut when tightening down. Now I understand when I saw someone post that the grind down the end of the socket wrench so it is flat instead of rounded.

Secondly, when I removed the throttle cables it became apparent that the deceleration cable was completely snapped in the middle, and someone had overtightened the throttle cable to compensate. Probably resulting in a portion of the high revving after rolling off the throttle. The throttle body seems to return to closed on its own now that the throttle cable is adjusted properly, but I won't be riding until I get a new cable for decel. :-(

Finally, the air mix screws were only turned out 1 turn. I suspect someone did this intentionally as the caps were already removed. I turned then to 2 1/2 turns out. Does that mean that it was running rich and that is why my pipes were red hot?

Finally, the idle was turned up WWAAAAAYYYY too high. Over 3000 RPMs at idle. Adjusted that back down to ~1100. Sounds a million times better. Choke cable is adjusted fine btw.

So thanks for the suggestions, and I can't wait for my new cable because the weather is finally getting nice.
 

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Mixture screws turned in will give it a lean mix, and cause the red hot pipes. I'm not sure if running rich will do the same thing, but lean definitely will.
The black pipe is from getting really hot when it's running, and will turn black when it's cooled down. did you notice if the front pipe was red? If not, then the rear carb could be partially plugged up as well leaning that cylinder out. After a few rides and miles, pull the plugs and see what color they are.
White is an indication of it running lean. Black is running rich, and a slight gray with a tan color is in the ballpark where ya want to be. Check both cylinders to see if they are pretty close to the same color, and it will give a better indication of how the cylinders are doing.
 

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Update-

Spark Plug was definitely cracked the length of the post that the wire attaches to. I noticed it the second I unplugged the wire. I remember slipping off the nut when tightening down. Now I understand when I saw someone post that the grind down the end of the socket wrench so it is flat instead of rounded.

Secondly, when I removed the throttle cables it became apparent that the deceleration cable was completely snapped in the middle, and someone had overtightened the throttle cable to compensate. Probably resulting in a portion of the high revving after rolling off the throttle. The throttle body seems to return to closed on its own now that the throttle cable is adjusted properly, but I won't be riding until I get a new cable for decel. :-(

Finally, the air mix screws were only turned out 1 turn. I suspect someone did this intentionally as the caps were already removed. I turned then to 2 1/2 turns out. Does that mean that it was running rich and that is why my pipes were red hot?

Finally, the idle was turned up WWAAAAAYYYY too high. Over 3000 RPMs at idle. Adjusted that back down to ~1100. Sounds a million times better. Choke cable is adjusted fine btw.

So thanks for the suggestions, and I can't wait for my new cable because the weather is finally getting nice.
With the air/fuel mixture set at one turn out your carbs would be running very lean and causing the pipes to get too hot. This should not happen with a rich mixture. Mine were also set at the factory one turn out and my plugs were white. I turned them out 2 1/2 turns as suggested on this site and the plugs are light tan,engine running great. How far out you need to go on the air/fuel screw will be determined by the color of the plugs. It has been mentioned on this site not to go over 3 1/2 turns.
 

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I let my bike warm up 5-10 minutes also.
 

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Good to hear that you were able to find and resolve your issues.
 

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With the air/fuel mixture set at one turn out your carbs would be running very lean and causing the pipes to get too hot. This should not happen with a rich mixture. Mine were also set at the factory one turn out and my plugs were white. I turned them out 2 1/2 turns as suggested on this site and the plugs are light tan,engine running great. How far out you need to go on the air/fuel screw will be determined by the color of the plugs. It has been mentioned on this site not to go over 3 1/2 turns.
It has also been mentioned to put a dab of silicone sealant on the threads of the air/fuel screw to prevent it falling out.:smiley_th
 
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