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Carbs and Fuel System
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Thread: Clogged main jet? Reply to Thread
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  Topic Review (Newest First)
10-01-2019 08:41 PM
Jason Pittenger After tonight's ride, I let it idle in my driveway (to drain the fuel lines and add heat). I pulled the front coil and measured it hot. Primary measures 2.4 ohms (spec says 1.8-2.2). Secondary measures 24.2k (spec says 19-29k). Rear is 2.5 / 23.9k. It took me a bit of time to get to it. I'll measure it cold in the morning.

Symptoms are unchanged. Runs great cold or warm. When hot, runs great below 6k at all throttle positions. Runs great at half throttle to red line.

EDIT: I measured the secondary at the coil itself. Not at the plug end of the wires! I will post the cold measurements taken with and without the wires, so I have a proper comparison.

EDIT: Cold measurements.
Front: 2.2 / 21.7k no wire / 31.7k with wire.
Rear: 2.2 / 21.5k no wire / 31.3k with wire

It looks like they are on the edge when cold. Wire adding about 10k.
09-20-2019 10:44 PM
Jason Pittenger I tried running it in the dark and there is no visible spark leaking out.

I'll be out of town for about a week, so this will be on hold until I'm back.
09-19-2019 10:23 AM
Spockster Plugs don't look rich to me. One plug from each cylinder should give you a read.

Can see the soot on the wheel though, I bet that all gets hot right there.
09-19-2019 08:31 AM
Jason Pittenger
Quote:
Originally Posted by Spockster View Post
What color are the plugs?

If it's rich enough to cut out, you should be able to see black smoke (or someone following you could), dark sooty plugs, and probably find black soot at the tailpipes.
The plug closest to my fingertips is the front plug. I don't have a good socket to access the deep plugs. The rear plug looked wet.

The rear wheel is getting soot on it. Though that all happened after an hour run at 90 mph into a headwind. I'm a little rich on the top, hence I was considering going from 140 to 138 jets. On hot days, it stumbled just slightly on the top end, especially with 143s. Nothing like it is now though. 140s made it better, but I'm not going to try 138s until I figure out what's going on.

I haven't had a chance to observe it in the dark yet.
09-17-2019 08:58 PM
Spockster What color are the plugs?

If it's rich enough to cut out, you should be able to see black smoke (or someone following you could), dark sooty plugs, and probably find black soot at the tailpipes.

So, when it's cutting out, the voltage hasn't dropped any large amount? That would rule out a brownout condition in the charging system.

Wouldn't hurt to run it (fully warmed up) in pitch black lighting conditions, see if there's any arcing going on, From the spark plugs to the coil bodies. I found one of my caps arcing to the throttle cable.
09-17-2019 08:44 PM
Jason Pittenger On the ride home, I checked my battery voltage. It stayed between 13.4 and 13.6 volts, at anything above 2.5 k. I tried a pull once the temp gauge past the second line. It ran just fine. I tried again when the temp gauge reached the bottom of the thermometer symbol, and it sputtered a bit. I tried a third time when the temp gauge reach the middle of the thermometer symbol (red lights/city traffic) and it sputtered a lot.

I pulled over and open the fuel cap and tried another pull. That had no effect on it. Close the fuel cap and went on a long cruise at 60 miles an hour. The temp fell to halfway between the second line and the thermometer picture. I tried another pull and it ran flawlessly. It is definitely temperature related.

Before anyone's suggest incorrect jet size, it ran better with 143 main jet on a hotter day in the middle of the summer, than it runs now with a 140 on a cooler evening. It stumbled a bit on the high-end on the hot days, so I went down in jet size and it ran fine all the way up to redline, at all temperatures. I still have the 140 jet in today. It runs worse with heat, which would imply rich conditions. Choke has no effect.
09-17-2019 06:21 PM
Spockster
Quote:
Originally Posted by Jason Pittenger View Post
POOGS is caused by the tank not venting and forming a vacuum, correct? Wouldn't closing the petcock result in a similar lack of fuel flow?

I haven't tried running with an open cap. I'll try that tonight.

....
Yes, but if the fuel has already stopped flowing due to POOGS, the petcock won't change that, or relieve the vacuum.

High throttle/rpm also takes more air/fuel.

I think if the electrical was suffering, it would show up in the lower rpm ranges also.

Have you seen the slide diaphragms? In good shape and properly seated?

Earshaved? Got the vent lines protected from wind?
09-17-2019 05:02 PM
Jason Pittenger
Quote:
Originally Posted by Spockster View Post
Does it run better if you just open the gas cap? The petcock shouldn't have any effect on POOGS.

Could still have crud floating around. Try seafoam again? The more you ride on the main jet, the more it will get cleaned.

Clean the carbs?
POOGS is caused by the tank not venting and forming a vacuum, correct? Wouldn't closing the petcock result in a similar lack of fuel flow?

I haven't tried running with an open cap. I'll try that tonight.

Jet's are new as of spring. It hasn't gone a week without running. I've put over 5k miles on it since April. I normally fill up at the local Shell with 87 (unfortunately 10% ethanol). I suspect the main jets, but I thought those kicked in before 6k. It seems odd that both would have a problem. Most of my riding is outside of the problem area.

My coworker (general mechanic, not familiar with the VN750) suggested a weak ignition system. He said: "High throttle requires more voltage, and high rpm requires more current. The combo could cause misfiring? Stopping for fuel allows the coils to cool off, making them run well until the coils get hot again." Seems plausible...
I'll check battery voltage next time it happens.

Quote:
Originally Posted by Spockster View Post
Keep it under 6k
I've been doing that...it's just so hard sometimes!

Quote:
Originally Posted by Spockster View Post
The more you ride on the main jet, the more it will get cleaned.
So...ride faster?
09-16-2019 09:18 PM
Spockster Does it run better if you just open the gas cap? The petcock shouldn't have any effect on POOGS.

Could still have crud floating around. Try seafoam again? The more you ride on the main jet, the more it will get cleaned.

Clean the carbs?

Keep it under 6k :q
09-16-2019 09:17 PM
Knifemaker Perhaps you need to drain and clean your tank. It's possible there's just some crap in there that's floating around.
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