No "Big Bang" on Cylinder #3

I'm kind of lost. Bike was running rough, no idle, exhaust 3 was cold (no bang) and I decided to rebuild the carbs. Replaced all seals and o'rings, cleaned all the passages, clunk tested, fuel level adjusted, bench sync'ed and put it back on. She fired up right the way and everything was running good but cyl #3 was doing the same.
I got my colortune today and when I installed I found out that that cylinder for some reason is starbing. Sporadicly explosions but most of the time spark only.
I don't know why it's not getting fuel, I checked compression and it read 150psi in the dry test.
any suggestion? all the carbs were cleaned and tested in the same way.
thanks in advance

 

If your plug is getting spark then a Carb sync is in order, The throttle blade for that cylinder may not be open enough to get fuel for idle and low speed.

 

That or..."CLEAN" cannot be a 'relative' term in XJ carburetor land... It has to be a religion.

 

sure the bowl is filling up on the bike?
or a mass air leak after the carb

 

bowl gaskets could cause above.... did on mine.

check for air leaks with propane torch.

is the plug wet with gas when you remove it?

 

bowl is full (tomorrow I can take some pictures). I don't think an air leak will cause that without rev the engine up. I don't know, it's frustrating.

 

plug is completely dry

 

I'm still new and no expert, but after reviewing all the replies, I'd go with what Bigfitz and MN-Maxims say, plus try rapping the float bowl with a rubber mallet. Maybe the needle is stuck in the seat.

 

How are the valve clearances on the dead cylinder?

 

If the valves were far enough out of wack to cause non-combustion wouldnt that most likely throw off compression? Not trying to be rude, honestly asking.

 

Closed intake would prevent intake of fuel, not prevent compression. Compression only requires a closed valve. XJ valves tighten as they wear.

Compression does not equal combustion.

 

True. The valves don't tighten, the CLEARANCES do, as the valve pounds itself into the seat. Eventually, at ZERO clearance, the valve will begin to not seat fully and create a loss of compression.

All of which has nothing to do with the fact that #3 is not firing and the plug is DRY. Tight valve/compression issues will not cause a dry plug. The only immediate answer is that #3 isn't getting fuel.

 

I was discussing compression and not dry/wet.

I agree that if the plug is dry then the problem is in the carb.
Maybe clogged air passage or clogged fuel passage or stuck float or stuck butterfly.

 

. . . and an easy way to confirm that, since you have a Colortune, is to back off that carbs boot and hold a tiny gas soaked rag at the carb inlet.
See if that cyl. comes to life.

Sounds like a plugged idle, or pilot circuit on that carb.

 

I will try that Time. When I open the throtle the cylinder make one or two explosions but that's it.
you are talking to back off the airboot between the airbox and the carb right?

 

Yes, it'll slide into the airbox. That CYL should run for a few seconds until it dries out the rag. Be cautious about a back-flash, that's why the rag is tiny.

 

Fouled plug?

 

I pulled out the carbs again. Inspected everything in #3, all passages are clean and jets are clean. Enrichment circuit clean. Put them back in the bike and started right up and through the colortune I could see that #3 was firing and exhaust started to warm up. I said "OK this is my lucky day", turned off and moved it to the driveway. Started back up and guess what...cylinder #3 was dead again, I drove it for 5 miles and didn't improve a bit. Still running in 3... I'm totally lost.
any suggestions are very welcome, thanks

 

I vote for a fuel supply problem with that carb.....plugged fuel supply tube, clogged float needle valve beanie-screenie (if so equipped), a float installed upside down on that carb, or a float valve needle that is hanging up in it's brass seat somehow.

 

Maybe an intermittent spark. Check your coils and make sure they have power to them and make sure your wires arn't loose.

 

The bowl level is constantly good, the pick up tube is clean.
Weard ah?

 

Lets narrow it down. If that plug is truely dry and shows no signs of wet gas. Go take a deep breath and lets try it again. Fire it up and if it is not running on number three. OK pull the vacuum line to the petcock and spray some carb cleaner in the vacuum port. Does it fire now??? If yes its a fuel deal in that carb. So lets see whats is doing. I would also do the unlit propane torch trick. Flood the area around that carb and carb boot and see if you can get the cylinder to fire. If it does your looking at a serious vacuum leak. An out of place / Blown intake gasket can do what your describing.

 

I was going to say #3 has the vacume on it, that could be leaking.
But if the plug is dry that doesn't make sense.

 

I second checking the coils. Maybe an easy way to check coils out would be to swich the coils. If the dead cylinder moves, you've got a bad coil. Could also pull the #3 plug and check for spark. If you don't have a nice strong blue spark, bad plug or coil.

 

I see this post has been dead a while, but I was wondering if a solution was ever found. I have an 82' xj750 that is doing the exact same thing. My carbs have been rebuilt by a guy that I trust, have been bench tested and sync'd. I have replaced the 2nd 3rd cylider coils, which was omhs tested and spark tested. The bike will not start cold. It will start with starter fluid. Once it starts up and warms up, if it is shut off it will start right back up again. I'm sure I've left something out feel free to ask any question. Any help would be greatly appreciated. thaanks

 

Did you verify the starter jets are perfectly clean? This is very common problem and rebuild does not always include a surgical cleaning. Hard cold start is almost always this problem....

Clean enrichment circuit
http://www.xjbikes.com/Forums/viewtopic/t=8918.html

 

Zane,
that thing happened on my bike as well. Everything, absolutely everything was clean but some of the cylinders didn't fire up properly. Then it turned out that the original jets were too small for my bike. I've put larger jets into the carbs and she's fine now.

It might be the case when you're running with pods, open mufflers.

 

I'll have to check the starter jets.

ZsoltK...I've got factory exhaust on it that is in near perfect condition. but I suppose it could still be restricted fuel flow of some sort. Thanks guys I'll give these things a look, and see what happens.

 

Zane,
I couldn't find the problem yet, I haven't touched her in awhile. Mine start with no problem but cyl #3 will not fire up. I will take the carbs out again once I fix a few personal issues .

 

I got cyl #3 firing. May be a common problem. It seemed the It was flooding out. The float was not shuting off the fuel. All floats were set to factory. All floats did float ( I checked them). I switch the float, needle and seat with a working carb on a working cylinder and the problem did not follow. strange huh. With the carbs off the bike I pulled the bowl for the cylinder 3 carb, hooked up the fuel line, held the float with my finger and it worked, so I set that one float 1mm lower the the rest and it fired right up. Why that one cylinder wants to be lower that the rest I do not know. By lower I mean 17mm as the rest are at 16mm I know that seems high, but since you set the upside down, when its right side up its lower. But it still will not start cold. It wouldn't start after pool league last night, I had to leave it. Going back today to try it while its warm out. If not I have to truck it. Needless to say I am not happy. Its going to be 75* here in Baltimore tomorrow.

 

I know I just Posted but I have to add, the enrichment circuit is clear Its been check twice now, and in addition to what I posted above the float levels have been check and even set a touch higher to allow for more fuel flow, as advised by my motorcycle guy.

 

Mine is doing the exact same thing on #3??? the pug is black and dry???

 

It has probably been mentioned already but most of the time the vacume for the petcock goes to #3.
If the diaphram in the petcock is leaking it can pull extra fuel into the #3 carb causing flooding issues.

 

My petcock was leaking, i replaced it with a on-off-res type.

 

When I had this problem on my Nighthawk 650 I found the cause to be my #3 wire. It had hollowed out a hole in the wire where the pin from the coil fits creating a bad connection. My plug was dry because of the other cylinders vaccum created pulling the #3 fuel into the air cleaner. The fix was trimmimg 1/2 inch off the spark plug wire going into the coil creating a good connection. Hope this helps

 

OK, i exchanged #2 and #3 wires, #2 was hot and #3 cold. so the wires are OK, I also swaped the plugs and same thing, so the plugs are OK. I pulled the carbs and tore them apart. i am cleaning them up again. They were clean, but now they are going to be REALLY REALLY clean and shiny, i am polishing all the brass, inside and out. I am going for the surgical clean this time!!! One thing i did notice is after i bench sync'd them i sync'd them on the bike. the #3 butterfly looked "more closed" than the other 3??? Maybe i had the sync wrong on the bike???

 

pull your resistor caps and check the contacts while your there they have a tendancy to corrode at both ends dont forget to put down some white grease... one other thought, i ran into a problem where i had 3 boots leaking at the motor. remove re-seal... now all good.

 

i checked those, they were all loose, #3 actually came apart, there was a very small brass "washer" and the resistor (small white tube), i put the washer in first then the resistor. is that correct? i did not clean them up, i will clean and lube!!
EVERYONE, THANK YOU FOR ALL THE HELP!!!!
WHAT AN AWESOME PLACE!!!!!

 

now what you'll find is that you will get an inconsistant burn on them ... you'll see in my signature that i replaced my wires with 7mm accel copper by chopping into the coil packs and replaced my resistor caps .. thats the only way i could get a consistant even burn on the plugs.. total cost should be under $40