XJ750 Midnight Maxim no power

I have a 1983 Yamaha XJ750 Midnight Maxim that tends to be sluggish on idle and has recently lost power during ride. To take off I have to rev up to at least 4rpm and let off the clutch slowly. Once running it will jerk forward for a second as I'm driving and then just back to how it was. The ride feels as if I had my brake on but I've checked them and they work perfectly fine. The bike was stopped for about 6 years when I got it from my dad and I've had to clean out the carburetors at least a dozen times to get them fully clean. I did replace the fuel injector needles and main gaskets on them. Also I've put in a fresh set of spark plugs (pilot screws set at 2.5 turns) along with a much needed oil change. New fuel filter and petcock rebuild kit, tank was vigorously cleaned out and still has the same air filter. It looks to be in good shape so I decided to leave it. I do have minor oil leak from the left side (from front of bike) but I keep up with it religiously. I have a misfiring cylinder(first from left), but the piston moves freely and I have good spark. This is my first bike and dont have much money to take it to a shop. Also it's my only transportation. I've tried going through here to find a fix, but I havent found my problem specifically. Any help would be great. Thank you in advance.

 

Have you measured the ignition coils? I had no power and bad idle, one cylinder was completely out and the reason turned out to be bad ignition coil.

 

How would I do that? I've pulled the plug out and started the bike to check for spark and there was. While I have the bike running, I've pulled out the plugs one by one and it shut off every time except for that left cylinder.

 

Based on the terminolgy you used (thrre are no fuel injection needles) I'm assuming that this may be the first motorcycle you have worked on.
Read the following link and tell us if you skipped any of the steps therein.
IN THE CHURCH OF CLEAN

 

Yes it is, I apologize for my ignorance, I'm trying to get acquainted with the names of the parts still. I was actually just messing around with the spark plugs and decided to switch them around. When they're switched, the 2 center cylinders don't fire. However when put back in their right place, only the outer left acts up. Any clues on why that is?

 

Welcome first keep it simple unthread the plug boot cut off a 1/4" of the wire and rethread on the wire...many times it will be green corrosion that keeps it from firing. If you still think it is a bad coil... Honda CB 750 coils are direct replacement and can still be bought new from Cycle Recyle part 2 they are like 39$ each the coils have replaceable plugs wires they carry the wires and the caps as well. I have ran these for 2 years with no problem . Cheers

 

Thanks for the tip. I'll be sure to try that tonight.

 

Don't. Ignorance is not a flaw, unless one chooses to be ignorant on purpose.


Buy no parts without testing what you have first.

Head back to the plugs. Swap the spark plug that won't fire with one that will. Does the fault follow the plug, or stay with the plug wire?

Keep the plug wires connected to their proper cylinders. They fire in a particular order, and swapping them around (without knowing how to do so properly) will complicate diagnosis.

Slow.
Patient.
Methodical.

 

We'll the reason for the apology is simply because proper terminology over internet can make things go a lot smoother. I hadn't thought about the timed firing on the coils. I have tried swapping the spark plugs and still the same.

 

Remove the plug caps from the non-sparker (they unscrew from the wires) and trim the wire back about 1/8", then reninstall.

 

Measure the resistance of the ignition coils, it's very simple. You just measure the resistance from the connector pins and from the plug caps or wires. You can find threads on this forum about it and videos on youtube that show how to do it.

 

K-moe asked you above to read about cleaning your carbs about

Please tell us what you did or did not do with regards to cleaning those carbs

 

Ok, just a quick update on the bike. I pulled put the carbs and started cleaning them vigorously one by one and I took every single thing out of them, then I put all of the inside pieces (pilot screws, choke levers, etc.) I'm a cup of seafoam and just let them sit there for about 5 minutes and stirring it up every now and then. All that while I cleaned the body inside and out with carb cleaner, I sont have a compressor, so I rigged up a straw to the blow end of my shop vac and it worked just fine. All in all, my problem seemed to be that the needle of my main jet had jammed itself into the opwning and came off the plunger so I was basically running on 3 carbs. Got all that cleaned and out back together and haven't really had a problem since.