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Discussion Starter · #1 ·
96 Vt600, will turn on and idle fine. Wait about a minute, and it'll shut up off as if someone hit the kill switch. No bog, no sputtering, just an immediate shut down, and the bike will fire right up. Sometimes it will idle for a few seconds, and turn off, other times it'll idle for a few minutes before it'll shut off. I have other sputtering problems while riding that I believe might be associated with this, but so far I am out of ideas on what to check.

so far I've:
Replaced spark plugs, & wires
Checked coils which both seem to be within spec according to my haynes manual
Bypassed neutral switch
Bypassed kill switch
checked all connection leading to ignition control box, all connections look good, no broken or loose wires
Tightened battery terminals
Checked fuse box for loose connections, or loose fuses
Traced wires from pulse generator grommet to connection (waiting to purchase new oil so I can remove the cover and make sure they are secure.)

When the bike shuts off, it turns on right away. Nothing out of the ordinary happens (no noise, no lights flickering, no weird engine problems), basically feels like you stalled out. Headlight stays on, oil light is on, I just hit the starter and wait for it to shut off again :(

Thanks in advance for any help!
 

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1983 Honda vt750 Shadow
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So it shuts off instantly like you turn the key off? But starts and runs good for a short while.
Maybe an electrical problem, such as kill switch contacts or ignition switch.
Can you hold the throttle up to about 2000 RPM and let it run there for a while and will it still do the same shut down ?
If it stays running at higher RPM and doesn't die it may be just very lean idle circuits. Have the mixture screw plugs been removed so it can be adjusted?
 

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Discussion Starter · #5 · (Edited)
Check your gas tank vent. Counds like it might be plugged up.
My fuel cap doesn't seal at all, so it vents pretty well haha. Its on the list of things I need to replace.

So it shuts off instantly like you turn the key off? But starts and runs good for a short while.
Maybe an electrical problem, such as kill switch contacts or ignition switch.
Can you hold the throttle up to about 2000 RPM and let it run there for a while and will it still do the same shut down ?
If it stays running at higher RPM and doesn't die it may be just very lean idle circuits. Have the mixture screw plugs been removed so it can be adjusted?
Feels exactly like I flipped the kill switch just momentarily for it to die. If I raise the idle up a lot, it will cough for a split second like it wants to die, but given since the RPM is much higher, it will only drop and ALMOST die, but will go right back up. Kill switched is bypassed, as is neutral switch. I've replaced the plugs, and they came out black.

If I'm riding in gear, it will not die. I have had sputtering problems, but if I'm rolling, and press the clutch, and coast.... the bike will most likely die after rolling for a few seconds.

After some research, someone else had the same issue with their regulator/rectifier. Basically, it would cut out for a second and kill his bike randomly without any fuel cut warning (no bogging, or sputtering. Immediate shut off, just like me). While riding, however, he experienced the same symptoms I did under heavy load, where his bike would bog, and sputter. I'm assuming the regulator could not keep up with the high demand of the coils under heavy load (which is when I misfire the most, higher RPM in my final gear)

Float valve in carburetor may not be functioning properly or misadjusted.
That what I thought, but I don't think it's a fuel issue. It dies immediately. A fuel issue would result in it slowly sputtering until the fuel ran out, and would result in a long cranking time while the carb's fill back up with fuel. It would die, and start back up almost instantly.
 

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My fuel cap doesn't seal at all, so it vents pretty well haha. Its on the list of things I need to replace.



Feels exactly like I flipped the kill switch just momentarily for it to die. If I raise the idle up a lot, it will cough for a split second like it wants to die, but given since the RPM is much higher, it will only drop and ALMOST die, but will go right back up. Kill switched is bypassed, as is neutral switch. I've replaced the plugs, and they came out black.

If I'm riding in gear, it will not die. I have had sputtering problems, but if I'm rolling, and press the clutch, and coast.... the bike will most likely die after rolling for a few seconds.

After some research, someone else had the same issue with their regulator/rectifier. Basically, it would cut out for a second and kill his bike randomly without any fuel cut warning (no bogging, or sputtering. Immediate shut off, just like me). While riding, however, he experienced the same symptoms I did under heavy load, where his bike would bog, and sputter. I'm assuming the regulator could not keep up with the high demand of the coils under heavy load (which is when I misfire the most, higher RPM in my final gear)



That what I thought, but I don't think it's a fuel issue. It dies immediately. A fuel issue would result in it slowly sputtering until the fuel ran out, and would result in a long cranking time while the carb's fill back up with fuel. It would die, and start back up almost instantly.
Not when the float bowl is out of fuel. It would shut off immediately.
 

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If the plugs are black it is running too rich. Is the air filter plugged? Try turning the idle mixture screws all the way in and back out 1/2 turn to see if leaning out the idle circuit does something to help it run.
 

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Discussion Starter · #8 ·
Last night we tested almost every electrical component, and according to the testing procedures on my Haynes manual, everything passes... Unless if it's intermittently failing.

What we found last night:
Had a timing light on, and timing is within spec. When the bike would randomly shut off during idle, the timing light would flash as the engine was turning off, so the engine is not shutting off due to loss of spark.

Battery voltage at idle is around 13.7-14.1, and rev'ed up a little, it'll stay at 13.6-13.7 ~ stator, regulator/rectifier and all connections leading to and from battery are tight, and clean.

Biggest finding:
Rear cylinder seems to fire off compared to the front cylinder. If you hold your hand behind both exhaust pipes, the front cylinder has a steady puff pulsation. The rear cylinder will be somewhat steady, with a cough, or a burp in between each 2-3 puffs. The rear cylinder is also the cylinder that produced the dirtiest spark plugs when I replaced them. They were black compared to the front cylinders whitish/orange colored plugs.

Since I've owned the bike, it has been running with no air cleaner. Previous owner installed velocity stacks on the carb's, and I've been so busy fixing other crucial parts (brakes, lights, etc) that I have not had time to instal a filtered air system. P/O said he rejetted the carb, so the A/F ratio has been messed with...

Last night while letting it idle, and waiting for it to shut off (which it actually stayed on for about 8 minutes, and never shut off... weird) our garage filled with smoke. You could literally see it pouring out the garage door. If you rev the bike, we noticed a big cloud of black smoke would pour out the exhaust. Now we know it's either 1) running too rich on the rear cylinder, or 2) something is not providing enough power to the rear coil to send enough spark to ignite all the fuel.

I've put roughly 2000 miles on the bike since I've owned it without touching the carburetors and have never had an issue with them, so if it is a A/F issue, why wouldn't it have acted up before? Maybe carb is going bad?

I will be playing with the mixture screws on it tonight and see if it clears up any.
 

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Last night we tested almost every electrical component, and according to the testing procedures on my Haynes manual, everything passes... Unless if it's intermittently failing. What we found last night: Had a timing light on, and timing is within spec. When the bike would randomly shut off during idle, the timing light would flash as the engine was turning off, so the engine is not shutting off due to loss of spark. Battery voltage at idle is around 13.7-14.1, and rev'ed up a little, it'll stay at 13.6-13.7 ~ stator, regulator/rectifier and all connections leading to and from battery are tight, and clean. Biggest finding: Rear cylinder seems to fire off compared to the front cylinder. If you hold your hand behind both exhaust pipes, the front cylinder has a steady puff pulsation. The rear cylinder will be somewhat steady, with a cough, or a burp in between each 2-3 puffs. The rear cylinder is also the cylinder that produced the dirtiest spark plugs when I replaced them. They were black compared to the front cylinders whitish/orange colored plugs. Since I've owned the bike, it has been running with no air cleaner. Previous owner installed velocity stacks on the carb's, and I've been so busy fixing other crucial parts (brakes, lights, etc) that I have not had time to instal a filtered air system. P/O said he rejetted the carb, so the A/F ratio has been messed with... Last night while letting it idle, and waiting for it to shut off (which it actually stayed on for about 8 minutes, and never shut off... weird) our garage filled with smoke. You could literally see it pouring out the garage door. If you rev the bike, we noticed a big cloud of black smoke would pour out the exhaust. Now we know it's either 1) running too rich on the rear cylinder, or 2) something is not providing enough power to the rear coil to send enough spark to ignite all the fuel. I've put roughly 2000 miles on the bike since I've owned it without touching the carburetors and have never had an issue with them, so if it is a A/F issue, why wouldn't it have acted up before? Maybe carb is going bad? I will be playing with the mixture screws on it tonight and see if it clears up any.
Did you ever figure this out? I’m having the same exact issue on my 750 ace
 
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