'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.

 

Yes. Exactly. Nice round hole on mine.

Really the carbs were in pretty good shape when I got it, so it wasn't hard to get them clean. A little time in the ultrasonic bathtub and blowing it out with some carb cleaner has them sparkling clean. The biggest issue was the PO broke the #1 body and replaced it with the wrong one, then put in the wrong pilot jets. Took a while to sort out that mess. Most of the time I spent on them was replacing the #!;$# throttle shaft seals.

This weekend I'll see if I can get time to verify valve timing, check compression, and stick in a colortune. Maybe try and get a better angle with the magic spray too. Not convinced I was getting enough in there through the airbox.

 

Haven't gotten a whole lot of time this weekend, but I took a few minutes to check the coils today just for grins...
Instructions from here: https://www.xjbikes.com/forums/threads/how-to-test-ignition-coils-and-plug-caps.130696/
Specs from here: https://xj4ever.com/catalog/d-6-ignition.html

Here's what I measure:

Primary (spec: 2.43 - 2.97 ohms)
1/4: 2.8 ohms
2/3: 2.9 ohms

Primaries look ok.

Secondary (spec: 18,560 - 27,800 ohms total)
1/4: 31,320 ohms (coil 12,540, caps 8,730 & 10,200)
2/3: 20,920 ohms

I started by measuring cap to cap for the secondary, so doing the math for an RK model, it should be 13,200 + 5,000 + 5,000 or 23,200 +/- 20%. (coil + wires + 2 caps)

The 2/3 secondary looks ok, but the 1/4 is a bit out of spec. I pulled the caps to see where the discrepancy lies, and it looks like both caps are well over 5000. However, I see this note in Len's info:

NOTE: some sources specify that XJ650 models, XJ750 air-cooled models, and XJ900, XJ1100 and XS1100 models used 5K caps on the outer plug (#1 and #4), and 10K caps on the inner plugs (cylinders #2 and #3). Other sources say that all 4 caps were 5K ohms. And other sources say that 10K caps were used on the outer cylinders (#1 and #4), while the two center cylinders use 10K caps…….whew! Our recommendation is to use the 5K caps on all 4 cylinders.

So, if it's true that they used 10K caps on outer cylinders, then both of them are within 20% of 10K.

But, no matter how you slice it, the PO put resistor plugs in, so add another 10K on to all those numbers. I think the best place to start is getting the correct plugs and see what happens from there.

 

I think I found another issue that might actually be my smoking gun...

I happened to be looking at http://xjbikes.wikidot.com/carb-specs and noticed it says the pilot FUEL jet is a #40. That jarred something in my brain and got me thinking...I went back and looked and realized I had written down it was a #32.5 in my notes, and that's what I ordered to replace the incorrect "drilled" ones that the PO had installed. (Which also happened to be #32.5, giving false confirmation of my mistake)

But that's the size for the STARTER jet built into the bowl! I went to Len's pages and looked it up there too...sure enough, he also says it's a #37.5 or #40, depending. Where the heck did I get the notion that 32.5 was correct?

So bottom line, I've been trying to get it to start with pilot jets that are waaay undersized. That could definitely account for what seems to be a super lean condition with all the backfiring. Ugh!

 

Good find. I still wonder whey your bike will not run on spray. That would take the carbs out of the equation, as you are just burning off the spray you add at the time you are trying to start. You mentioned before, your bike will not even run on spray. Maybe it is the spark, not to sure what your spark looks like. Lights off plugs out and on the side of the engine should be a good blue or what color when they snap. If they are orange or red, that is not very strong spark.

 

Like I said, not sure I was getting enough spray in there for it to do anything. There is no direct shot to the carb throats through the airbox. You can really only get to two of them, plus my can was almost empty so if I tried to tilt it to get around the corner, not much came out. Probably wasn't a good test.

Now I get the joy of pulling the carbs out again. Heading for another cold spell here though so it may have to wait. Even with the heater going full blast it's hard to get the garage warm enough to be able to wrestle those boots off and on.

 

Pulling carbs on these bikes is par for the course. My first refreshed XJ took me six times. Yes, six times of pulling and cleaning before they were right. Now, when I do them, I really take my time. The carbs are the heart of the system in my view. I have had stuck rings, and other weird things, but nothing like them carbs.

 

I've probably pulled the ones on my 750 at least a dozen times over the years. I think this is the 4th or 5th time for these. It never gets any funner.

Wish they had extended that slot to let the airbox slide back another 1/2" or so. You wouldn't have to wrestle the dang airbox boots back in place. That's the worst.

I'd love to see some vintage footage of how they put them in there on the assembly line back in the day. I'm sure they had some means of cheating. Some special tool or something.

 

Woo hoo!



Now all that's left is a carb sync, front brakes, front tire, clean the tank, fix the crack in the fairing, and put it all back together.

 

Hat falling off the fender means a running synch is in order.

Congrats! Sounds and looks good.

P.S. what was the magic trick that got it running.....just changing out the pilot jets?

 

Pilots and plugs were the only changes since the last attempt. I think mostly pilots. Explains the backfiring with being so lean.

Thanks for the help with the parts. I was beginning to have my doubts I'd ever get it going.

 

For the GS or XJ?

 

Id be interested in knowing the VIN of your 900 so I can see if it’s in the 900 registry or not. I’ll sent a private message

dave Fox

 

Dangit! (censored)

I got all the boots wrestled on, air box scooted back in place, and was getting ready to put the last clamp on, when I remembered seeing something that said the two larger vent hoses are supposed to route up and over the airbox...

Guess which way mine are pointed?

First thought was, no big deal, I'll just pull them off, flip around the T's, and stuff 'em back on. Nope. No matter which way you try to turn, the T hits something and prevents rotating. I'm going to have to pull them all the way off and split the rack AGAIN!

On a similar note, which way is the fuel supply supposed to route? On mine the wire clip to hold it in place is on the front (cylinder) side of the hat, but when I route it that way the hose seems to take kind of an unnatural bend. Anybody got a picture of how it's supposed to run?

 

I went back and looked at the pictures I took before I split the rack for the first time, and sure enough, the PO had them routed out the bottom too. So, no clues about where they should be going from that. I did see how he had the fuel line routed, but I don't trust that was correct either. He had it coming up behind the rail where I have the vent line now. Both of them won't fit in that space comfortably though.

Here is where I am at right now. I ran the fuel line (red F) outside of the rail and the vent lines (red Vs) inside the rail. But the wire clip for the fuel line (green arrow) seems out of place. The fuel line would have to bend way over to go through it. Maybe that's not what it's for?

The clutch cable clip (green C) seems good where it is.

All the skinny overflow lines are coming out the bottom and seem fine.

 

Looking at the manual, it's hard to make out but it looks like it might be calling that clip (green arrow) a Vacuum Pipe Clip, which could help explain things. It also shows a separate part called Fuel Hose Clip, which I am guessing might be one of those ones that are wider and vinyl coated that I've seen on other bikes. Maybe I am missing a part. It also kinda looks like the vent pipe routes down?! Or maybe that's supposed to be the fuel line. Doesn't look like it even shows the vent lines. Not the clearest diagram.

 

Yup. Apparently, I am missing one of these. I'll have to dig through my parts bucket and see if I can find one.