#2 Cylinder is Playing With My Mind

Hi,

I've just finished a rebuild of my XJ900F and all and everything electrical works as it should including a good spark to all 4 plugs.

On initial installation the bike started up but it seemed #2 cylinder was not firing or was not as strong as the others. #2 exhaust pipe directly out from engine was warm but others were hot (very). Soon....took of the carbs, the job I hate most and went through them again

I have cleaned the carburettors almost obsessively so that you could just about sip a gin and tonic out of them on a warm day! Other things that have been done are as follows:

* carbs dismantled, cleaned, new brass T fuel connector installed between #2 & #3 carbs
* new manifold boots between engine and carbs
* idle jet screws (and passages) cleaned and reinstalled. Turned out from seated position 2.5 turns
* all internal jets and passages cleaned, new carb diaphragms installed on existing carb slides. All 4 slides move up and down as they should
* new float needles installed. Float valves dismantled and cleaned out
* floats tested for correct height/measurement

Carbs reassembled taking care with floats. Bank of carbs reinstalled back on the engine (no...I think this is the job I hate most!).

Gravity feed of gas only at this stage. Started the engine and she burst to life. But still it seems #2 is either not firing at all or perhaps only weakly. Spark plug for #2 rested on engine and engine turned over and it has a clear spark. Plenty of compression too by the way.

Am I missing something??? Is it at all possible the weak gravity feed of gas (from a small receptacle) might in some way contribute to fuel being drawn away from carb #2?? Bearing in mind that the bike was going in its aged and dirty carb situation before dismantling some months ago. Any...ANY new perspective on this would be greatly appreciated.

 

Did you check valve clearances?

 

Try un treading plug cap from the plug wire cut off the end and re thread back in...I once had a mint 81 CB 750 that had a "cold cylinder" check it would jump spark , but found green corrosion in wire end , cut it off put cap back on and wow ran great after wards.

 

Hi,

Valve clearances checked and shims replaced as needed when working on top end of motor.

Will try plug wire as suggested.

Thanks!

 

Was float height measured, or were fuel levels measured?

Swap plug wires around (put #3 on the #2 plug). Does the problem follow the wire or stay with the plug? Even new plugs can be faulty and not fire (or misfire) under compression.

 

Good suggestion k-moe.

When the rain stops here and I can wade out to the bike I'll certainly give that a go.

Cheers,

K.

 

Oh and yes...float heights were checked and all four are to spec.

 

Wet?

 

No.

Dry.

I'll try everything possible before taking them off again. But it's on the agenda ):-((

 

Did you bench sync the carbs? Did you try opening the needle valve to see if it helps? My carbs need 3 to 3.5 turns out to run good. Dry plug should mean no gas or lean mix. Are you using the choke ? Did you get it warm or ride it?

 

You can check the fuel levels with the carbs still on the bike. All it takes is a length of clear fuel line and a screwdriver. Ideally the front tire will need to be on a board to get the carbs level.

 

You are referring to the idle mixture screws. It should not take more than 3 turns out to have a good running engine. Typical is to end up at 2.5 to 2.75 turns out, regardless of operating altitude.

 

To answer the questions:

Carbs were bench synced as close as I could get them using the sectioned business card method under the butterfly valves).

The carb drain screws were impossible to remove so at this stage I left them in place. Which, I know, makes it impossible to wet check the float levels with the method suggested.

Idle air mix screws are exactly 2.5 turns out. The other three cylinders seem to be running fine at that adjustment.

I'm starting to suspect coil or spark plug wire. I've got aftermarket coils which were on the bike and working before dismantling. The leads were swapped out for new ones during the rebuild. These coils allow for the replacement of leads without having to open up the casing as was the case with the original coils.
Swapping the leads on #2 & #3 is on the agenda tomorrow to see if the problem shifts to cyl #3. Will report back with results.

 

I am just saying that My bike likes it a bit rich at idle. I find it eliminates deceleration pop and just runs better for me on my bike.

 

Hi All,

Think I've found the problem...

Cylinder #2 was the problem, still is the problem. Decided to gently supply gas via a clear fuel line to the carbs. I kept slowly injecting gas into the fuel line via large syringe hooked up to the line. And kept injecting and kept injecting and couldn't figure out why it wasn't eventually showing an increased and steady level in the line. Looked under the bike and...gas dripping out of overflow lines for carb #2 (ok understandable) but also a little from #3 and #4.

This might explain the rich dark looking NEW plugs in #3 and #4 but also seems to explain #2 not firing as it's possibly flooding the cylinder.

Can I assume that we have a problem with either float levels or new float valve needles not seating or shutting off fuel supply properly?

 

sounds like float needle is hanging up. Are they new solid brass or rubber tip? You might swap them around to see if problem follows.

 

Rubber tipped. New from Chacal here.

Carbs to be taken off the bike tomorrow for investigation.

 

Seems like you need to get those screws out of there somehow... Without being able to open those up, you won't be able to monitor the consistency of fuel flow from the fuel supply to it's ultimate destination of a properly filled bowl. It took me several "openings and closings" of those screws (after my carb rebuild) to resolve my air pockets/bubbles that sometimes form in there after rebuild/install. I purchased several feet of clear tubing and connected all drain ports to fuel cans via the tubing at the same time. This way I could really open all screws and let the fuel do it's thing. Only when I saw a constant stream from a drain without bubbles, did I tighten it back down.

If you get the screws out, replace them with the stainless steel allen head screws that Chacal carries. You will not be sorry and you will never have a stuck bowl drain screw. It also couldn't hurt to but just touch of anti seize paste on the threads.

 

I think you're right.

I contradicted my own rule of thumb.

"There are no shortcuts!".

Live and learn. Off to do my most hated task...remove and eventually reinstall the carbs...

 

Just took the carbs off (again..!) and going through the inspection process. Will double check wet and dry float levels but was wondering if the cause of my leaky #2 carb is the float valve. Recently disassembled, cleaned out the crud, reassembled with new O rings for the float valve seat.

Used locally sourced O rings that are suitable for the purpose. But...Just wondering as per photo attached from the web...if the O ring(s) that I used might not be sealing properly and even when the needle is seated in the valve the gas is getting past the O ring as pictured? Is this a likely scenario?

 

How snug of a fit is the o-ring?
ID matters as much as OD here.

 

Snug.

You can feel the O ring give resistance as you push the valve into its housing but once in it does wobble a little and I reckon with dry hands I could pull it back out just with my fingers.

 

Hi Again,

Just now had time to troubleshoot this #2 cylinder conundrum.

Stuff I've done recently:

* Took carbs off (again!) and ensured they were clean and operating smoothly. Dry set and wet set float levels. They appear to be working OK as they fill up with fuel, hold the fuel steady in the clear fuel line, and do not overflow as one or two were doing before.

* Checked resistance of primary and secondary coils as per Haynes specs. A little high with and without plug caps (plug caps are new and fine and have a 5k resistance in each). These are aftermarket coils and came off the bike when it was working previously, before the breakdown and rebuild. The have easily removable HT leads and I swapped out the old leads for new appropriate HT leads.

* I then swapped the HT leads for cylinders #2 and #3 to see if the #2 HT lead was a problem. Still, #2 exhaust header is only luke warm at best compared the the hot hot headers of #1, #3, and #4.

* In that configuration I then swapped the spark plugs from #1 known to be good with #2 (tested to be sparking anyway when held outside and grounded on the engine). #2 header still only warm.

Conclusions...something in clean #2 carby still restricting fuel flow, maybe pilot circuit.

Or...even though valves were checked during rebuild, something went out of whack and maybe valves aren't closing properly.

Or...I'm stumped. Happy to accept any comments/criticisms to isolate the problem here. I feel I'm left with fuel or compression as the source of #2's problems.

Over to you knowledgable ones.

 

Have you tried a doing a compression test on #2 cyl ? maybe you have low compression on that cylinder.

 

Haven't done a compression test on #2 cylinder yet. I didn't take the head off during the restoration/rebuild and it was running fine beforehand.

I'm thinking about focussing on #2 carb pilot circuit and making sure it's clear. If that checks out then I'll pop the valve cover and recheck the valve clearances and compression.

Hopefully along the way I'll solve this conundrum.

 

Low compression can make it difficult to burn in the combustion chamber , I once worked on a small block chevy was running good until... the spark plug electrode post broke off ( actually damaged exhaust valve ) tried putting new plug in still ran rough , after trying everything did compression check 150 psi on 7 cylinders, 45psi in cylinder 8 Not saying you have a problem so severe, but valves shims not right or worse a burnt valve could cause no end to problems .

 

A compression test takes far less time than just removing the carbs does.
If it's low, revisit your valve clearances.

 

^^^+1 He should do a compression check ..will help rule out cylinder problem.

 

Thanks. Agree. Compression test far easier to do than those damn carb removals..!

Will report back.

 

SOLVED..!!! 'Twas a clogged pilot jet on #2 Carb.

For the benefit of future XJers who might experience a similar problem with one cylinder not firing...or appearing to not fire with any great power, the following is a summary and might help locate problems:

* Carbs cleaned inside and out some months ago, including removing jets and cleaning them thoroughly (or so I thought). Carbs then sat for several months while I worked on the rest of the bike.

* Mounted the carbs back on the engine, fired up and...sounded great until it became obvious that #2 cylinder was not firing or only doing so with extra choke/enrichener and the engine would not idle properly.

* Had some fuel level issues in the carb bowl(s) and replaced 4 x float needles (from Chacal). Problem solved.

* Remounted carbs and...#2 still not firing grrr

* Tested resistance in coils/HT leads etc. and tested fine.

* Swapped HT leads for #2 and #3 cylinders to see if it was coil/lead related and #2 cylinder/exhaust header still only very lukewarm. That is, swapping leads made no difference.

* Removed carbs. Again. Even though I was confident I cleaned them spotlessly months ago decided to check the jets for #2 carb.

And lo and behold, the pilot jet seems to have picked up some loose crud that must have been lurking in the carbs somewhere and blocked the hole in the jet...small as it is...to the point where this was only a teeny weeny really small sliver of light barely showing through. Carb cleaner, a fine wire and patience brought the inside of that pilot jet to the "Church of the Clean" specifications.

Haven't had a chance to remount the carbs yet and while they are off I'll check all the jets in the other 3 carbs before they go back on the bike.

Hopefully it will run normally now...will post results soon.

Thanks ever so much to contributors who provided their thoughts and advice along the way.

Cheers

 

Inline fuel filter is the key. You probably have a bit of rust in the tank.
If you have not installed an inline fuel filter yet, do so before remounting the carbs.
You will need a small one, preferably with a sintered bronze filter element.

 

Nice to see a detailed explanation of the resolution. It makes it a lot easier to use the thread to help trouble shoot similar problems in the future.