Running on 3 cylinders....

My 81 Xj750 seca have been nothing but a nightmare for the past 2 weeks.
Sporadic misfires started before that, but about 2 weeks ago it began getting worse and extremely difficult to start at times.

Some of you may have been involved in my last diagnosis which didn't solve all the problems....
http://xjbikes.com/Forums/viewtopic/t=40968.html

I cleaned the carbs already. wet set (floats were fine) and just did a vacuum sync. unfortunately I don't have my hands on a colour tune yet.

Few days ago I found that one of my ignition coils were dead.... Got a new one in there yesterday, but didn't fix everything.

It starts up fine now. But only idling on 3 cylinders.

Cylinder #2 is not firing from idle to about 1/4 throttle, then fires but feels week until full throttle when it'd stop firing as well.

I'm completely stumped. I cleaned the primary circuit. I blew compressed air into it and it's flowing through just fine.

I'm also getting fuel into the cylinder as the spark plug is getting black.


I'm out of ideas and so sick and tired of dealing with this. Just ready to push it off a cliff at any point now..... please help me

 

It could be as simple as a loose or bad resistor core in that spark plug cap.

Have you installed a BRAND NEW SET of correct, properly gapped spark plugs?

Barring that, we need to back up and start with the very basics. I believe you've said previously that you put the valves in spec?

DO A COMPRESSION TEST; post results. Let's go from there.

 

It may be something as simple as the spark plug being fuel fouled, especially if it's been "sporadic" for a while. Try pulling the plug wire on the missing cylinder about 1/4" to 1/2" with it idling. Do you hear a good spark? Sometimes holding it off that way for a few seconds will get it firing for a bit. A good cleaning of the plug will still be needed though.

 

Checked plugs. Got spark. Good gap. About 1500 km old, swapped with other cylinder and didn't do a thing.
Resistor was swapped with cylinder 3 and didn't do a thing.
Tried swapping the actual ignition wire from 3 to 2 and vise versa. Didn't do a thing

It's not ignition this time

 

Oh and yes, valves are in spec.

Compression test was done and in spec 3 months ago..... Will repeat today

 

145 cylinder 1
140 cylinder 2
143 cylinder 3
145 cylinder 4

Compression is good

 

Did you try swapping plugs?

 

That was a 5 stroke test

Maximum compression test shows 175, 162, 165, 175 (cylinders 1,2,3,4)

Still in specs

 

TCI? At this point you are running out of other options, but I would put in a brand new set of plugs. Once I start having any issues with fuel or ignition I try to replace the plugs once the other issue has been solved. They shouldn't be so temperamental but they can be. I realize you swapped and saw no difference, even so I would start fresh.

As you fight this battle, keep in mind that ultimately the process inside the cylinder is rather simple-compression, good fuel+air, timed spark -that's pretty much it-so one of those is not proper somehow and compression is def. not your problem.

 

If you are getting spark and have got compression then you are looking at lack of fuel, air or a bad mix.

Is the #2 plug wet with fuel after doing your tests? Pull it out do a sniff test.

I had an issue with synchronisation where it was so far out that one cylinder was not firing at all until pushing up the throttle. That could be said for the mixture being way out also, of course.

 

Any improvement without the air filter?

 

I would recommend that you swap the spark plug out with another cylinder as well as swap the plug wire with number 2 and 3. it is possible that the spark plug has a small crack in it and the spark is jumping through he porcelain to the block and not making to the end of the plug. If you inspect the plug you may see a small hairline crack that has brown around it. Also it is possible for the spark plug to leak compression between the metal part and the porcelain. you will see a brown burnt stain at the bottom of the porcelain.

If swapping the wires does not help then you at least know the wire that was going to number 2 has good and properly times spark.

Next move the spark plug to another hole and see if the problem moves, this will eliminate a problem with the plugs.

Also is the coil that you replaced the one that is for 1 and 4 or 2 and 3, if it is the one that is for 1 and 4 then try swapping the new coil to non running cylinder. as well as the connectors of coarse.

 

Not swapped around, or tried to clean. Not plugs that have been in it during all the fighting and diagnosing. BRAND NEW NGK's, gapped correctly?

If not, spend the $8.

 

The plugs were hardly 2 months old..... and i don't see how it could be a plug's problem if i can swap them around or throw in a different set of resisted plugs I had ran for a week (cudn't get non resisted here, took a week to ship in to the the closest shop in washington then i went to pick it up) And the resisted plugs didn't make a difference.


I did figure out the problem

I was soooo fed up and I KNEW it wasn't ignition for sure this time otherwise either both cylinders 2&3 would be failing or when i swap the ignition coil between 2/3 and 1/4 or when i just switch the wires between 2 - 3, the cylinder that misfire should have changed to a different cylinder.


Anyways
I was so fed up and I knew my compression was good and air filter is good. So I had no choice but to rebuild the car again..... which is when I noticed the main jet and the idle jet were mixed up on cylinder 2.......
!)@(*#$#*)$&%&*(^ .................

I thought they had different threading and were interchangable, and even then I still paid attention when I rebuilt it last time. But this bike had been re-jetted and the two are not nearly as easily identified as the original jets. I know what the original jets look like cuz I've got a rusty nasty spare carb in the spare parts box the PO passed down to me and I tore that one up to learn how to rebuild mine



Anyways

I'm glad this night mare is over.

Running so well. Exhaust pipes look a little more purple than before.... I'm going to pull the plugs today to check if it's running lean...

I'm 4 turns out on the idle adjustment screw already

are there other ways to tune the air fuel ratio other than using colortune? or some way of doing it roughly so it'd run healthier for now as it should take some time for color tune kit to be shipped over...

 

I'm glad to see that you finally sorted it out! I've been following this thread step by step and I was feeling bad for you.

 

rd,

^x1.. Glad you found it.

Gary

 

Thanks everyone.

I know i lost my cool at some points in time. Sorry about that. I've had too many nights where I only got 4 hours of crappy sleep. And too many days where my brain was gonna blow up.
Failing to diagnose an issue really does get to me sometimes.

It's nice to finally be back on a motorcycle again. And not just any, but the one that I fell in love with the moment I saw it. Oh and running smoothly on all 4 cylinders with plenty of power really makes the ride a lot more enjoyable!

Even through all this hell, I still love her and hopefully we will never part ways!

I love riding a motorcycle so much, and it has been only 6 months since I first started riding. I can't imagine myself ever giving up riding. The only thing I can imagine would be having 4 motorcycles. My Xj750. A harley with trailer for long road trips. A Yamaha Vmax. And a CBR 1000rr Repsol edition.

One day my son will want to start riding a motorcycle and as much as I want to tell him he's not allowed to till he's at least 20.... I don't think i'd ever be able to stop someone from having this much fun!

Thanks for everyone's help and support!

Now it's time to fine tune my air fuel ratio........... one step at a time.... haha

 

Until you get your ColorTune, best practice would be to set all 4 at 2 1/2 ~ 3 turns out and keep an eye on your plugs.

 

If you do a plug chop, meaning that you do a full throttle acceleration and hit the kill switch under full load, stop on the side of the road and pull a plug and inspect it, It it is white and and almost chalky it is too lean, if it is black and sooty it is too rich, if it looks a good running color then you are good on tune. This can also be done at idle to test the idle mixture. If you google reading spark plugs you should be able to find some pictures that will help you decide if you are rich or lean.