What am I missing? (XJ700 Won't start)

I have a 1985 XJ700 N. 31,000 kms. Ran great all last summer.
Was riding home one night and I heard one huge backfire. It's never done that before.
Next morning I go to start it and it turns over endlessly but doesn't fire. An odd small pop here and there but nothing.

I've checked..
Fuel:
-The petcock is new yamaha oem and is flowing gas.
-The hitachi carbs have been cleaned throughly, every jet has been poked, and float levels set with the clear tube method to just under the gasket surface.

Spark:
-I have around 1000 kms on the spark plugs and I've tested good looking spark at each plug when grounding outside of the head. Haven't tested if I have spark under compression. Plugs have a good color.
-I've tested both primary and secondary sides of both coils and they are within spec. The plug caps are new and have 5k ohm resistance.
-Tested pickup coil, its within spec

-I have consistent (cold) compression at ~150psi across all 4.
-Checked the cams to see if the cam chain had maybe skipped a tooth but the chain is tight and the cams are aligned properly at 1 TDC.
-Timing is non adjustable on this model.

Wondering what I could be missing? I'm confused as to what could go wrong over night to make it a complete no-start.. bump starting didn't make any difference for the record. Any ideas? Thanks in advance.

 

First thing I would do is crack the drain screw on the carb bowls and make SURE there is gas in the bowls. If you have compression and spark, then with fuel it should fire.

Did the bike run (and how well) after this one huge backfire?

 

I'd alos be looking to see if there might be a hole somewhere; like in an intake boot.
It would be a shame to haver her start up and run, only to have another problem surface and cause damage.

 

You didn't mention of you tested the battery. Your description sounds to me like you battery is low. Try jumping starting or testing the voltage. I believe the battery should stay above 10v when cranking. There is lots of info on checking the battery on this site.

 

Cracked all 4 drains and fuel flows out. I don't remember the bike riding any different after the backfire. Next I'm going to try and spray some gas straight into the cylinders and see if it fires.

Good idea, I'll check for vacuum leaks. I was hoping to get it running, even if it's not well, and then shoot cleaner around the boots and listen for an rpm change but I can't even get that far.

I've been charging the battery between tests, 13v+ when I try cranking over, and I hooked it up to a car battery for prolonged cranking. Probably worth double checking if it stays over 10v while cranking just in case.

Another thing a friend suggested was to see if the mufflers were plugged somehow. Not sure how that could happen over night unless the backfire shook something loose?

 

Don't see how it could be blocked mufflerS? If it was one, maybe, but it would still fire up.
Clean the plugs, or fit new, drain all the fuell and put some fresh in, flush it through the carbs. I honestly don't know what else to suggest - are the plugs wetting when you crank it?

 

on the pickup coil side of the tci do you have a black wire with white stripe? if so disconnect it.
to be clear every thing after the words iv'e checked was done after bike stopped starting?

how long before back fire did you fill up gas tank?

 

Check your air filter to make sure it's clear and clean.

 

OK, here's a shot from left field. What about the wiring/circuit from the run/no run switch on the handlebar? If there is something loose or a occasional short it could shut off the spark to the motor then the carbs would load up, then come back on and, BANG! Might also apply to the side safety too. Just sayin because of all the other checks that have not sorted it out yet.

 

Might have a problem with kickstand switch. Also, the red cut off switch may be causing a problem.

 

I'd also vote for cleaning the sidestand switch. I chased that for a few weeks before finding the problem was a dirty, sticky switch that would cause an intermittent no-start issue. Peel the rubber boot back, spray the plunger with contact cleaner as you move it, then follow up with a dry-film lubricant.

 

pull the sidestand safety relay to see if the sidestand switch is the problem

 

I like the idea of checking the handlebar on/off switch. But with all this switches we are missing the backfire. A bad side stand switch could stop your engine right away...no backfire needed but a bad on/off handlebar wont make you engine turn off until until next try to start the bike again (happened to me)

 

kill/run switch is fed from fuse then to tci ignitor so if that connection was lost while riding bike would shut down gas would still flow through carbs and when it reconnected bike would run and possibly back fire if it happened when running down the road . check fuse connections clean kill/run switch. also clean key switch dirty connections there could cause intermitant voltage supply but key switch failure would also shut down headlight.

 

Try unplugging this diode block in your headlight. Just to do a test. I’ve been running my 2nd xj for a while without it and it disables a few things like the kickstand switch. It could be a way to do a quick test.

 

disconnect the black/white wire from the tci unit
his motor turns over


this is a 700 trouble shooting guide

 

if ignition system becomes inoperative or misfires

 

I've made plenty of intentional backfires by flicking the killswitch. An intermittent connection would produce the same result.

 

So I'm thinking the backfire was a bit of a red herring. Checked the exhaust, no problems.
The bike fires up on starter fluid and will then run no problem. But once it cools down it will no longer start.
Then the starter stopped turning the motor over at all. Solenoid was working. So I rebuilt it using the brush kit. Then it would turn over easily again but still no start.
I've already unplugged the sidestand relay as it is dead and was causing a no-start when I first got the bike.
I checked the air filter and it was super dirty. So I replaced it with a uni-filter drop in as that's all my local moto shop said was available. Read online here only later that it flows a lot more than stock and will probably make the bike run lean.
So I tried turning the fuel mixture screws way past where I thought they should be (4.5 turns out) and the bike fired up! I had only previously tried to start it from 1.5-3.5 turns out. Seems to idle happy, rev nice. Took it for a ride though and it was surging and sputtering under mid and high throttle, I'm guessing because it's lean from the uni-filter.
So I contacted a Yamaha dealership and ordered a stock paper air filter $90 CDN later...) and hope that that returns things to normal, as there was no sputtering or surging before with my old dirty air filter.
But now the starter is making a high pitch whining sound while spinning. It also wasn't spinning enough to fire up the bike. I removed it, took it apart and inspected everything and didn't notice anything out of the ordinary. Put it back in the bike and the whine is not as loud, (but still very noticeable) and it fires up the bike again.
So now I'm trying to figure out what the whine could be in the starter. It didn't seem to have any play in the shaft. Maybe the thrust washer is worn too thin and something is rubbing? Going to take a closer look at that next. Any ideas?
And I know it's the starter making the whine as it made the same whine when I hooked a battery up to it for a second on my bench.

RE: Everyone's suggestions to check wiring. I checked the voltage at the TCI and it's barely above 10V when it gets there under cranking, starting with a strong battery at 12.8v+ I tested for voltage drop at every step in the chain and it loses the most at the ignition switch. Probably time to clean all the connections.

 

Yes, definitely clean all the connections. Sounds like you're on the right track. I could be wrong but I believe there is a thread in the diy how to section that covers a starter rebuild. May want to check it out. It may shed some light on something for you