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I've owned by 86 VN750 for about 21 years. I'm sad to say that after about 29K miles, I think Jessica is done.

Set the way-back machine to almost 7 months ago. It's early August. My wife and I are on our bikes enjoying a magnificent ride in the foothills outside Denver. We're on our way home and I notice my tach has stopped working and the engine is running really rough. I pull over and my wife says she was just about to flag me down because riding behind me she smelled gas.

Turns out the front cylinder isn't firing. I decide to try to limp it home down the hill on one cylinder and ask my wife to follow me. (While riding home, her 1993 Yamaha Virago 535 ALSO manifests an electrical problem and dies altogether, but that's a different story.) We load up both bikes on a trailer and get them to my mechanic, a local shop that specializes in older metric bikes.

And that's where they've been since then.

They've replaced the stator, the regulator, the rectifier and god-knows-how-many other electrical components, cleaned and balanced the carbs... (I don't know the complete list). Yesterday I get the good news/bad news: It's running and idles well, but he says if you accelerate hard the front cylinder cuts out; if you back off the throttle, it comes back. If you accelerate easy, both cylinders stay live. With this, the owner of the shop has pulled the plug on investing any more time or parts into it.

(Now, before I go any farther, I'm not mad at the shop. These are good guys, and I take it there because I live in an apartment without a garage or area to work on the bike. They've done well by me in the past. Plus, I realize that this is a 35 year old bike. Parts are hard to come by, they've got other bikes to work on with less nebulous problems, and I really believe they've done all they can do.)

I'll be picking up the bike next week and taking it to a friend's house to store it for about a week. Unless anyone has any magical ideas, I'll be donating it to the public radio station for a tax write-off at that time.

Any ideas? Thoughts? Last-minute calls from the governor?


Jay


P.S. -
Here's Jessica and Sweetie:
52988
 

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Not enough fuel volume flowing would be the first thing I would look for. Are the fuel hoses running uphill or the vents in the wrong location?

It sounds like the float stuck when it died on the road.

I question the diagnostic methods of the shop. Seems like they might just be tired of looking at it.
 

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If your tach stopped working completely, it's an indicator that your igniter is not sending the "fire" message to the front coil. Could be a broken or grounded wire, or a bad igniter. They may have fixed that as part of their go-over, but I'd check to see if that tach drops to zero when the front cuts out, or if it just bogs along with the RPMs as it struggles.

Even if there's no gas, or no compression, or no air, that tach signal should still be there when the engine is spinning, even if the ignition coil has been pulled off the bike.
 

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Yes, the tach runs off the ignition coils primary. If the tach is going crazy/not matching actual engine speed, then the coil is not getting a signal.

This could be a bad pickup (or bad connection to the pickup) not sensing the crankshaft lobe. It could be a bad coil shorting internally under load. It could be a bad controller (unlikely). Or a loose/bad wire. Could even be bad spark plugs and wires Definitely doesn't sound carb related.

Open throttle takes more energy to spark vs light throttle. That suggests the coil might be marginal. I once had a plug wire that worked fine at idle, but would spark to the block whenever I cracked the throttle. I also once had a corroded stator connector. Bike ran fine except WOT above 7k. System voltage was dropping, but that would affect both cylinders.

If there is a carbon track built up on your spark plugs or wires, and you only replace just the wires and not the plugs (or vice versa), the track will form again very quickly. It's best to replace both at the same time.

Lastly, I found an aftermarket ignition controller that can be purchased new under $200. Far cheaper than OEM. If it's the controller that's bad. I'm using one on mine now, and it was very quick to install.
 

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If both cylinders fire at idle then it's probably still a carb issue. If you got spark then look at the carbs again. They can be tricky sometimes but that's where I'd put my money.
 

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Yes, the reason she smelled gas was likely because it wasn’t getting burned and was just pumped through to the exhaust.
It sounds like they shop fixed the electrical problem but messed up something when they worked on the carbs. If it runs fine if you’re slow on the throttle, but seems to choke if you’re hard on it, definitely sounds like a carburetor not responding correctly. Perhaps a bad or improperly installed diaphragm or needle.
 

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A poorly seated diaphragm falls in line with inadequate fuel volume and is a good possibility since that appears to be a new symptom after the carb work. Easy to check on the bike just by pulling the tank. If they're correct that it's the front cylinder, look at the front carb diaphragm first. That's assuming the fuel lines are ok.
 

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....

Set the way-back machine to almost 7 months ago. It's early August. ....
Horrible story but very well written. Fun to read and I hope you get her going again. You got some good responses.
 

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Discussion Starter #9
53133

Well, despite the excellent advice offered, I simply didn’t have the facilities to work on the bikes and follow through on the potential repairs. They got donated to public radio, which will auction them off. Hopefully someone who has the time and a garage picks them up and puts both back on the road.

Farewell, Jessica. Farewell, Sweetie. May you both find others who will love you as much as we did.
 
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