Engine Barely Fires Up, But Doesn't Stay Running...
I've recently taken on a 1993 VN750 "fixer-upper" project for a family member, and I can't seem to figure out what the under-lying issue might be. I'm well versed in motors from an automotive standpoint, but a bit new to bikes. So allow me to start from the beginning (and I've been through a lot, so bare with me here):
When I got it, it wouldn't start at all with the push of the start button. However, with a short "tow" through their front yard in the country (which after looking back on, I don't recommend), we discovered it would run via a "push-start". No starter action at all could be heard when pushing the start button, only a clicking noise. I first changed out the battery. No improvement. I did some research and found that one of the most common culprits was the Starter Relay / Solenoid. I tested this theory by jumping it with a flathead screwdriver (also not recommended in hindsight) and the starter began to crank, but no firing on the motor.
I picked up a brand new Relay, and "Voila!" It worked. Now the starter would turn over, but the motor would never actually fire up and stay running. The most I could get out of it was a few sputters, and then back to the starter whining as it turned and turned. I knew the bike was a bit aged, and had sat for awhile, so I thought "It likely has dirty carbs and aged plugs" so I picked up a can of Sea Foam, some Carb Cleaner, and some brand new NGK plugs.
I proceeded to change the plugs, and then empty half of the can of Sea Foam into the gas tank (as I had read others do so many times on here) and try to shake the bike up a bit to get it mixed in. Then I proceeded to crank it up a few times to get the Sea Foam through the carbs. On top of that I sprayed in just a tad bit of Carb Cleaner into both sides of the carb.
I turned it over and (likely because of the highly volatile Cleaner) it fired up loudly, with the RPMs shooting up, and then gradually coming back down. After doing this a few times it began to start easier and easier, but if I held the throttle open to it's maximum, the RPMs still would just hover between 4,500 - 5,000, and would never climb any higher. It didn't seem to sound right, (kind of like a go-kart) so I placed my hand at different spots on the motor, and what I found was that it's only firing on the front cylinder. A friend that was well-versed in bikes said that the next thing to change out would need to be the Regulator/Rectifier which apparently is another common trouble item on VN750s. I bought a brand new one, changed it out, and it still has the same problem.
I checked the plugs, and both of the front plugs are grey/white as they should be when firing properly. Both of the back plugs are dirty and greasy. I'm fearful that there may be some internal issues, such as oil leaking into the chamber that's not allowing the air/fuel mixture to ignite.
Any suggestions as to what the next move should be?