Possibly not firing on one cylinder

Hi guys,

I have an 81 XJ650 that I’ve been tinkering with for a few months now with varying degrees of success.

Firstly, Its been modded with pod filters (previous owner, let’s not start on this one) it also has a dirt bike exhaust welded on and some new electrical components.

It was idling very rough and hanging on higher revs at stop, then on a ride it only barely made it home limping (didn’t want to start and barely ran)
When I pulled the plugs they were all very black
so I have:

- Rebuilt carbs (everything apart from idle enrichment circuit) including slide diaphragms and intake boots and took jets back to stock (they were 45, 128)
- new spark plugs
- lubed the throttle cable

Took it for a quick run and it got significantly worse once it was hot.
Now when I start it backfires a lot and shoots bluish flames out the exhaust along with running shit unless on about half choke (looks great but doesn’t run great)
When I rechecked the plugs, they all look a nice grayish colour now except one which doesn’t look like it’s firing at all (checked and it still has spark and looks wet from fuel)
Also, the exhaust coming from that cylinder is much cooler than the rest

Can someone please guide me to the next logical step?
Should I be checking the coils? Valve clearances? ‍

Ps If there’s any members around the Gold Coast area with some spare time and know how, I’d be very keen to get some help with this one

 

The cylinder that appears to not firing , unthread the plug cap fron the plug wire trim a short bit off the end, rethread the plug cap back on and put back on plug start it up and see if pipe heats up . Try this first.

 

The PO rejetted to suit the pods, and hopefully did it correctly.
Stock jets are contributing to the issue, but not the reason that one cylinder isn't firing.
I agree ith Jetfixer. Start by confirming spark on that plug.

The plug color and need for enrichment to be on mean that she's jetted lean (because of the combination of pods and stock jets).

 

Checking the valve clearance isnt a bad idea at all, not likely the reason for a cylinder not firing but is often a neglected maintenance item.

 

Hi Jetfixer,
Thanks for your reply. I cut off around 1-2cms and threaded the plug cap back on. No change unfortunately. It would’ve been great if that worked though!

 

The PO put in 45/128 jets. According to https://raggedmoto.com/carburetor-jet-size-calculator/
I need 41/124 for current setup. It was running rich before going back to factory.
Also, I thought grey was ok, not lean. Pretty new at this so happy to be corrected though.

Anyone had any experience with rejetting by drilling out?
Alternatively, does anyone have a source for hitachi jets, or a jet pool?

 

Thanks Chitwood, it’s on the list. I’ve got the shim removal tool on its way

 

The plug color is good, but not quite ideal. It may be contributing to the issue since the condition worsens when the engine is up to temp. You should never have any popping or a hanging idle, both of which are indicitave of a lean condition regardless of cause.

 

hatachi jets are listed by the metric drill size used
I think the drills would cost more than the jets

source for jets www.xj4ever.com our supporting vendor

 

Ok, thats good to know.
The hanging idle was also present before I took jets back to stock, but the plugs were definitely carbon fouled (with 45/128’s)
I’ve also checked throttle cable returnand it looks like it’s snapping back nicely

 

Thanks, I’ll see if I can get a price on both options. I’ll post the outcome

 

Just

Just having a look at jets vs drill bits:
New Jets will be usd29.95 each set of 4, so around $60 total.
Drill bits are around $3 for 21 piece set including a 0.41mm (0.0160”) off eBay.
The 1.14mm (0.45”) I need is usd$4 from grainger.com
Just wanted to double check that is what you meant by metric sizing on hitachi jets eg #41 pilot is 0.41mm?
Has anyone tried drilling their own jets?

 

The problem with drilling jets is that you cant go back when you realize you went a size or two too big. I would recommend spending the money on the jets themselves that way you can swap them around until you find what works for your set up. After which if you really wanted I'm sure you could sell any remaining jets

 

you need metric drills not converted sizes from inches

 

True, the PO had it running 45/118’s so I might put those back in and attempt to create laminar flow with the pods I have to try to match the jets

 

I still need to work out why it’s not firing on one cylinder. Any suggestions?
Replace coils?


I’ll resistence test the spark plug wires/caps to eliminate those in the next couple of days

 

swap the mating spark plug wire see if the problem follows the wire seta 1 and4 , 2 and 3.
if not swap the spark plug.

 

Thanks xj550h,
I just swapped 2 & 3 leads (4 wouldn’t reach 1) and still the second exhaust isn’t heating as much as 1,3&4.
This is the second new plug since owning and I’ve swapped 1&2 spark plugs. The plug looks like it’s firing in 1st cylinder but I might redo that test.
Any other suggestions?

 

Possible PO already drilled out...ALWAYS be weary of what the PO did or did not do.

 

From the reading above you are saying it is not getting warm? You have pulled the plugs and visually seen the spark of all four plugs? If you have verified that you have spark on all four, then you will have to check the carb for that cylinder. Just guessing if it has spark but the exhaust pipe on that cylinder is not getting hot then there is no gas/air to burn in the cylinder?

As for drilling out the jets, is it really worth maybe not getting them perfectly bore sized and or smooth? When I first stared my first XJ project, I pulled my carbs six times, yes SIX times, before I got them correct. I learned and now really take the time to do them right the first time, no trying to find short cuts or maybe that is good enough.

As Jetfixer said the PO might have done some good and some bad, you will just have to check over the bike in all safety aspects to have that warm fuzzy feeling about taking it down the road.

 

Thanks

Thanks for the reassurance.
I’ve jut found another thread here of a guy who had a similar problem and found he’d accidentally mixed up pilot/main fuel jets and the cylinder was flooding. Worth looking at...
I’ve ordered a compression tester, colourtune plug and carb balancer.
I’ll update when I’ve done some more testing

 

Could a torn slide diaphram cause a cylinder not to fire?

 

Would it be possible for a torn slide diaphram to cause a cylinder not to get fuel?

 

I would not help that is for sure. I would guess someone else will chime in here and get more technical. You will have to seal the torn diaphragm before that carb will work correctly. If the slide inside the carb will not move up, the pin that goes down into the main will not move. As you roll on the throttle the pin will not move up so the fuel mixture will not increase. There are some good links here that show you how to check (hold them up to light and look for holes, tears) and seal them with form a gasket or other RTV type sealer. Not the best but it will work to get you on the road.

 

That would definitely be unhelpful but I already replaced all four slide diaphragms with new ones when I rebuilt the carbs.
I’ll pull the carbs again and quadruple check them

 

I don't know if you can just pull the rear carb boots off when you start it and see if all the slides are working when you roll the throttle? That would be better then taking them off again and having a look. As long as they all function the same way when you roll on the throttle that would tell you the diaphragms are just fine.

Using your hand on the back of the carb/cylinder that you think is not firing will also force gas up from the bowl and then into the cylinder (and on your hand) to see if that is in case the issue. I don't want you to pull the carbs off unless you think you have jet issues and have to replace them.

 

If the carb and cylinder do fire with a hand blocking the intake what does that tell you?

 

Thanks for the tip timbox,
With pods off I can see that the slides are working fine and when I cover the rear of the carb it does indeed have vacuum and gas comes out onto my hand.
Exhaust on second cylinder still not as hot as other 3 which sizzle when touched with wet hand.
Spark plug wet with fuel when pulled (from cutoff at high revs)

 

Have another thought swap the plug caps from a hot cylinder to the cylinder that is cold see if the cold cylinder heats up ...could be a bad plug cap .

 

Thanks jet, I already swapped the spark plug leads 2&3 without success

 

No. The main jets don't come into play at idle.

 

you could just have a bad plug

 

The plug has been changed twice and checked on a different cylinder

 

Well f*** me sideways with a chainsaw!
In frustration I thought I’d better take one last look at the carbs.
Pulled them last night and somehow (I’m guessing I just didn’t tighten it properly) the pilot jet was sloshing around by itself in the fuel bowl
Just took her for a run. Still not quite 100% there but at least no backfiring blue flames.
Colourtune plugs and compression tester in the mail, hopefully I can get her running properly before too long.
Thanks to everyone that posted

 

Way to stick with it and get results! Glad she is running again and at least putting a smile back on your face.