'83 XJ900 doesn't want to start

I finally got through "un-botching" the carbs on the XJ900. Everything nice and clean with all the correct parts now. Adjustments should be reasonably close. I did 2 turns on the mixture screw and bench set the butterflies to be reasonably in sync. I've also checked valve clearance and verified spark. Everything looks good. But she still doesn't want to start.

It cranks fine, and once in a while it will kinda fire a little like it wants to start, but then nothing. If I keep trying, it will occasionally backfire, which is scary as heck. Sometimes the backfire will happen several seconds after I stop cranking while it's just sitting there.

When I bought it, I was able to get it to somewhat start, it just ran like crap due to the carb issues. Now I'd be happy just to get back to that point.

I'm running out of ideas.

The only other thing I can think of is valve timing? Maybe it jumped a tooth while I was checking the valves? Is that even possible on the 900 without crashing the valves? I didn't notice any nasty noises or resistance like a valve at hit at any point.

Open to suggestions if anyone has any ideas.

 

If the valve timing was right it cannot jump a tooth when checking valves l think. What voltage are you getting with the multimeter set to 20 volts dc with the probes on the battery terminals when cranking the engine. Must not fall below 10 volts or it won't start.

 

Though I haven't checked with a meter, battery voltage should be good. Battery is fairly new, and it has plenty of energy for cranking. Can't imagine the voltage would be low.

 

I would check it to be sure. Does your bike have the original fuse box? If so fit a blade fuse box oil Yamaha ones cause electrical issues. Did you clean the tiny cold start jets in the float bowls?

 

Not that I've done this. Okay I have.
Check the coils are connected the correct way.
I reversed the coils when I did my first rebuild.
That sucked.

 

Original fuses, but in good shape. No corrosion or anything. This bike seems to have had a pretty easy life.

Bowl jets clean. Verified I see light through the tube when a small flashlight is shown through the hole in the base.

 

Coils haven't been removed since it last ran, so no opportunity to swap anything.

 

What voltage are you getting at the connectors from the TCI to the coils, having stated that unlikely they are both affected. The ignition switch body grounds to the handlebars check that in case there is corrosion or paint between the body and handlebar. Is the ground to the rear of the ignition coil mounting bolt connector to frame clean?

 

Have you tried starting it from an auxilliary tank? Or using starter fluid?

 

I put a voltmeter on the battery. Bottoms out at around 10.6v after a few seconds of cranking.

Pulled a plug to check spark again and tried to take some video to show spark health. Not the fattest spark I've seen, but should be sufficient. Note that due to the frame rate it appears like it's missing firings. That's not accurate. In real life it doesn't look like that - it fires on every revolution like it should.
https://www.youtube.com/shorts/tvzuSgCgpsk

 

Yes. All tests have been off an IV bottle. Tried some starting fluid. Didn't get me anywhere, though it's hard to get it pointed in the right direction through the air box so I may not have gotten enough in there.

 

The fact that I am getting spark implies that everything is connected and making contact as it should. Doesn't seem to be a spark issue.

This is a real head scratcher. The one thing I haven't tested yet is compression, though it seems to be fine. (when I pull a plug for spark test it cranks faster). I'd find it hard to believe all 4 cylinders lack enough compression to fire...unless the cam timing is somehow off. What's the easiest way to verify the timing?

 

It will crank faster one less cylinder under compression with plug removed.

 

T mark should align with centre of top pickup on the 891cc engine when holes in camshaft are Visible through right hand side camshaft caps.

 

A couple more ideas.
After you crank with fuel enrichment on are the plugs wet and smell like fuel.

Try opening the throttle all the way on cranking to see if the air filter is blocking flow if not.

Open the bowl screws to make sure the bowls are filling.

 

Thanks Franz. It's the 853, but I assume the same process. I'll check it next time I get back out to the garage.

 

There are some vapors coming out of the pipes while cranking. Don't particularly smell fuel, but my smeller's not that great these days. The backfires tell me at least some fuel is getting somewhere. Not sure if it's really a true "backfire". Sounds more like detonation in the exhaust. Like unburnt fuel is getting in the pipe then getting ignited somehow.

Plugs seemed wet, but that got me thinking...I was going to get some new plugs anyway and I was reading about the resistor vs non resistor plugs. If I understand correctly, the US model should take a BP8ES, or maybe a BPR8ES if it doesn't have resistor caps. However, when I checked I found BPR7ES plugs installed. Doesn't seem like something that should prevent starting completely, but yet another thing that the PO got wrong. Guess I need to order some plugs. I need to test the resistance in the wires to verify which to get though.

Was initially trying with air filter installed, later removed to try starter fluid, so definitely no blockage. Plus, it was a brand-new filter. The rear boots aren't even connected at the moment. Tried cranking at all throttle positions, with and without choke.

Just put in new float valves and adjusted float levels so I know those are good.

 

A couple more for you.
Vacuum nipples on intake boots all capped.
Intake boots tight on head.
Double check the plug leads are on the right plugs.

After all that. Ouch painful and down the electrical rabbit hole or timing.

 

It's different on the 853 l check my manual when I get home.

 

Capped.
Tight.
Correct.

Rabbit hole indeed.

Now we can begin to understand why I'm here for ideas instead of out riding my new bike.

Never had a bike that seemed as correct as this one that still wouldn't even fire. It's gotta be something stupid simple.

 

Check the manual you need here.

https://www.xjcd.org/index.php/service-manuals

https://www.xjcd.org/index.php/node/1333

https://www.xjcd.org/

 

If you own a colourtune you can use it to see if the plugs are sparking in the cylinders.

 

Thanks. I'll look. Probably will be a couple days before I can get back to the garage though.

 

I do have one. I was thinking the same thing.

 

Worth a try.

 

Need to take a breath, overlooking something easy here. If the bike is not starting up right away on spay and running until it burns it off, something is not right. I would put the bike on a jump pack with another battery, car, truck something other than just the battery you have in the bike. This will really make a difference.

You mentioned looking at the bowl jet with a flashlight in the above read. The choke jet in the bottom of the bowl you would not be able to do that with. That jet is very small and you need compressed air and carb cleaner to check to see if it is open.

This is the jet I am talking about, you really need to make sure these are clean.
carbs_floatbowl_choke by Tim Brown, on Flickr

 

Yes l would try another battery, even new batteries have been faulty. He could also try and bump start the bike so most of the electrical power is going to the ignition system.

 

Actually that jet can be checked with a flashlight directed at the hole at the base. You should see a nice, round hole at the bottom of the well. if it is irregular or too small, cleaning is required.