81 XJ750 Problems

Ok, a brief history. Acquired the bike from a from, who got it from his father. Last sticker on plate was 89!?!? Needless to say, first thing that was done was cleaning of the gas tank, and a complete overhaul of the carbs. It has new battery, and plugs. Plugs are getting wet and black... I have used the Haynes manual to test alternator, and coils... high tension leads read 23.5k ohms.. and alternator doesn't seem to be charging. I had the bike run well for 5 minutes one evening, right from cold start. My question here is, with a fully charged battery, how long can the coils sustain a good spark? Do I have a fuel issue beings the plugs are black, or is it more an electrical issue? I would like to order some parts from Chacal... and will be doing so. Any advice on this issue, I am still reading through the forums on many topics and similar troubles, just thought I should post my own thread and not steal someone elses.

 

a few thoughts/questions....
are the plugs oily wet or gas wet?
black plugs indicate rich mixture but it's not always too much fuel but lack of air flow. in other words, the carbs may be jetted right but there's an issue with air flow to the cylinders.
not enough spark will also produce black plugs due to unburned fuel.
although you didn't mention it, i'm guessing you confirmed spark to each plug by grounding the electrode while turning the engine over.
you cleaned the gas tank, how 'bout the petcock and insert from the petcock that runs up into the tank?
new fuel filter and lines?
does it make a difference whether you run the petcock in "on" or "pri" position?
have you tried starter fluid (or gas) poured directly into each spark plug hole to "prime" the fuel flow?
keep posting, there's smarter folks than me on this forum that'll chime in soon enough......

 

Well, yes there is spark on all 4 cylinders. I am unsure if it is weak though. I am testing the bike right now without the air filter in as well. I am going to reclean the tank this week, and the petcock was rebuilt and cleaned after the tank was done. I am waiting to hear from Chacal, but I am hoping to get new fuel lines and carb connections (boots) The bike does run, its not a problem as for getting fuel to cylinders.. but too much I believe. I know there is more work to be done with the carbs, setup etc.. but I want to make sure I have proper battery system going to the plugs as well seeings I know that I have fuel there. The plugs last time out were still clean, but wet with fuel. Best way I can describe it, the bike will start with throttle and go nowhere in gear.. just die, it won't rev over 2000rpm? Thursday night I had it running well for 5 minutes and then it lost power again... so am I barking up the wrong tree? Do I keep looking at the coils and alternator, or is this still just a carburetor issue?

 

sounds like you are running very rich on fuel. that or somehow not enough air is getting into the chamber. have you tuned the carbs yet? or cleaned them?

 

you're going thru almost exactly what i went thru in the last couple months as i resurrected an '82 that had been sitting for years. in my case the issue was the carbs, tank, petcock, and lines. after replacing or cleaning these items, the bike is running well. it's hard to focus on 1 part of your fuel system until you've refurbished or replaced any of the suspect parts.
my tact has always been to attack the obvious and continue to narrow my focus thru a process of elimination.
talking/purchasing with chacal will help a lot.
keep at it......

 

I don't want to be cheap, but I also don't want to be throwing parts at this bike... it still needs a fair bit of work to get it on the road.. fork seals, tires and a few other misc. items I'm sure... I haven't tuned the carbs, or synched them because I was on the understanding that I should be able to get the bike running relatively well without a synch anyway. I am picking up some cheese whey tomorrow to re-clean my tank again.. and then I am in the process of cleaning the piston chambers for the carbs etc... I really want to make sure the carbs aren't the culprit. Unlike some people on here, the bike was sitting for many many years, and it wasn't like I took apart a running bike to clean the carbs.. I really have no idea what else could be wrong. I am just hoping someone will chime in on the alternator and coil issues because its not as if its a dead short or infinite resistance.. I am getting readings, but for the coils its too coincidental to have them both read the same ohms on high tension leads... as for alternator... that's another high reading.. but usually if it doesn't work.. it should be shorted or something not?

 

I feel as if this bike might end up being a large money pit haha.. I hope its just worn parts, and not something that damaged the alternator and coils... as it seems as though they are both in need of replacement. I can't test the voltage regulator unless the bike is running.. and I need a second multimeter as well. But I tried doing a test that it said was suppose to give me less than 1.8volts, and I got 12v lol... The strange part is, I know the bike runs, as I have had it going.. my troubles are knowing whether or not there is damage in the electrical system. If I rebuild the alternator, and replace the coils... and it still doesn't run, I'm going to feel as though I wasted a tonne of money to try and fix this bike. Its too bad there isn't someone else close by with this bike that I could chat with and even check information against with my bike.

 

Just a quick update... I am going to pull the alternator and clean it, I am pretty sure its ok, not sure why I didn't get a noticeable increase in battery test when running engine.. but bike wasn't operating properly during test either. As for the coils... 23.5k ohms isn't off! Thanks to chacal, I found out that 4-6k ohms is allowable on the plug caps, and 8800-13200k ohms is allowable tollerance on the wires without caps... so I am perfect.

 

great job in sticking with it. i knew chacal could help once you talked with him. sometimes you just keep looking at a problem and it'll back down.....

 

Well I ran out of time tonight, and realized I also had a pinhole in diaphragm on #1.. could be a slight cause to bad running.. but I did polish up the piston bores with 600-1500 wet/dry sandpaper... tore apart the petcock because when I tried to use the "prime" setting to drain the tank into a can it would only run for maybe 6 seconds and then fuel would stop again? I can't see anything wrong with it, air moves through the assembly like it should... maybe the screen is still somewhat clogged... I can see through it though? I also started my fuel tank cleaning process... will see how well it works and make a new thread with some pictures for everyone to see. I also cut the heck out of my fingers cleaning my alternator up... didn't want to pull the rotor out.. so I cleaned it in the housing = ouch! lol.. Hoping to have the bike roaring good by the weekend... getting brush on electrical tape tomorrow and checking the fuel tank cleaning.. but I won't have time to finish the carbs and do a float level test. Will repost thursday evening.