running rich on 2nd cylinder -puzzling

[mr blackstock]

Hello all,

So a quick history of the bike. A year ago I found pot 3 had no compression, so I removed the head, found a valve not sealing, replaced valve, lapped them all back in, re-installed head.  I also installed a lumpy cam.

The issue is that pot 2 is not running from cold, the exhaust pipe is cold until the engine warms up, during warm up it back fires.  Once warm the pot is fine, the bike rides normally.  So the bike has a lot of kms, around 360,000.  So i decided the replace all the jets and needles with high quality ones, keyster from Japan.  No change.  I checked for air leaks, none. 

I then swapped out no.2 carby with a spare one that was sparkly clean! No change.  I checked the valve gaps, all within range.  I checked the choke plunger, not sticking out, I then checked the idle jet, all good, all set at 2.5 turns out.  When I unscrew the idle jet, no change in behaviour.

So I considered a bad coil wire, so i swapped pot 2 lead with pot 3, no change, the same cylinder is not running when cold, still backfiring.

So the cylinder is running too rich, the plug is wet and smells of petrol when checked, backfiring is a big indicator.  I re-checked the needle height, in the middle, just like all the others.  I checked float heights last year, all within recommended range.

No I know the most logical explaination would be blocked air passage in the carb, but I cleaned both bodies really well, ultra sonic clean as well.  If I missed something, the odds are small I missed the same passage on both carb bodies.

Has anyone come across similar issues?

Thanks, Gareth 

[Pat Conlon]

Many years ago...I had a cracked insulator on a bad plug that drove me nuts.
 At low rpm's the plug was cold only to later warm up and fire off.   

[mr blackstock]

Hello,

Yeah I replaced the plugs too... I am resigned to dismantling the carbs... again.... and i checked compression, all good and healthy..

[Pat Conlon]

Float level you checked, but did you check the actual fuel level in the bowl?
Are your emulsion tubes (aka needle jets) ok? Often the vibrating jet needle can wear an oblong hole in bottom opening of the tube causing all kinds of flooding/rich running gremlins.

Although....If this cold misfire happened with 2 different clean carbs, I would look for something ignition related. I've seen wet plugs from the lack of spark.

[mr blackstock]

So when i drained the fuel bowls, all were pretty much even.  I cannot remember the amount right now.  All the needle and emulsion tubes are new.... and I swapped over the spark plug leads, 2 and 3.... and no change...

The other day I cleaned no.2 carb body again, and tonight I will put it through another ultrasonic clean....

Agravating process!

G

[Pat Conlon]

So when i drained the fuel bowls, all were pretty much even.  I cannot remember the amount right....

Nope, not how you do it.

Here is a post from our Carb Files on how to check the actual fuel level in the carb bowl.

https://fjowners.com/index.php?topic=3592.0

Checking the fuel level *confirms* your float level settings. It doesn't take much to flood or run rich.

[Old Rider]

I had similar experience on a kz900 one exhaust bend did not get hot on cold starts.
After a lot of investigations with new coils rebuildig carbs and several float adjustments  i found that the the  aluminium intakeport on the cylinderhead on that cylinder that did not fire when cold had 2-3 pinholes in the aluminium near the valve.
I found that when did a valvejob on the head. when i had the head standing on it side and filled the intake with solvent to check if valves was sealing. i found that when filled it leaked solvent not from the valves but from 2 tiny pinholes in the aluminium intake duckt on that cylinder.
When filled the tiny holes with jb weld the cylinder fired and runs normal.
symtoms before i got it fixed was: cylinder did not fire from cold starts .very clean sparkplug
compared to the others. when giving it throttle the cylinder fired. and when hot the cylinder fired. The sparkplug often smelled fuel.
so that is a thing to check but it is not easy with the head on the bike. you can maybe try to
spray some starterfluid around the intake and se if rpm drops.

First thing i would try is to let the bike idle then set idle low as possible then try to screw the mixturescrew out slowly and se if the cyl fires .it can just be that easy.
the mixture screws can sometimes vary alot in turns. i once had to turn the screw 6 turns out
cyl 1 on my fj to get the plug not look too lean and backfire the other screws was about 3 turns.

[Sparky84]

Hello,

Yeah I replaced the plugs too... I am resigned to dismantling the carbs... again.... and i checked compression, all good and healthy..

Have you tried reterminating the spark plug cap, take cap off and cut wire back about 10mm and refit cap back on.
Or fit new caps .

Good luck , it's the little problems that are the hardest to find.

[Pat Conlon]

Hello,

Yeah I replaced the plugs too... I am resigned to dismantling the carbs... again.... and i checked compression, all good and healthy..

Have you tried reterminating the spark plug cap, take cap off and cut wire back about 10mm and refit cap back on.
Or fit new caps .

Good luck , it's the little problems that are the hardest to find.

Gareth (op) ruled out #2 plug wires/cap by switching the #2 with the #3 plug lead....the cold cylinder problem stayed on #2...if the cold cylinder moved to #3 then the problem would reside in the plug wires or cap.

[mr blackstock]

Thanks for heaps of good suggestions... and yeah I swapped leads too... so I cleaned the carbs and ultra soniced them absolutely silly... reassembled and measured the floats too, they were 25, 25, 24.5, 22.63 (all mm) So three of them I altered to between 22.5 and 23mm

Ran the bike again, and no.2 was still not kicking in until warm, but less backfire this time.  I screwed IN the idle screw until it is 1 turn out, from the colour of the plug, the cylinder is still running rich.  The others are all 2.5 turns out.

So.... now I am going to re-check the valve clearances and aim for the absolute minimal gap.  My reason being is that the motor now has a lumpy cam installed and with the shorter opening time, perhaps the valves are staying open too long, causing imperfect ignition.  Th valve clearances I used were for the stock camshaft.

So I am hoping by installing shims to take the gap to minimum, that ignition will improve.

Its all I have left i guess...

Cheers, Gareth

[Bones]

Gareth, this may sound like a simple question but do you sync the carbs properly after each pull-down and install. The reason I ask is at a Manshed day a few years ago there was an 1100 there that ran terrible, it was barely running on three cylinders coughing and farting and had us thinking at first it was electrical, we checked a few things out and all seemed ok so we turned our attention to the carbs. Hooked the gauges up and they were way out, it took quite a bit of turning the screws before it started to make a difference but bit by bit it ran smoother, in the end it ran great after a sync and adjustment of the mixture screws. Having clean carbs is one thing but having them work in harmony together makes a big difference.
Tony.

[Sparky84]

Thanks for heaps of good suggestions... and yeah I swapped leads too... so I cleaned the carbs and ultra soniced them absolutely silly... reassembled and measured the floats too, they were 25, 25, 24.5, 22.63 (all mm) So three of them I altered to between 22.5 and 23mm

Ran the bike again, and no.2 was still not kicking in until warm, but less backfire this time.  I screwed IN the idle screw until it is 1 turn out, from the colour of the plug, the cylinder is still running rich.  The others are all 2.5 turns out.

So.... now I am going to re-check the valve clearances and aim for the absolute minimal gap.  My reason being is that the motor now has a lumpy cam installed and with the shorter opening time, perhaps the valves are staying open too long, causing imperfect ignition.  Th valve clearances I used were for the stock camshaft.

So I am hoping by installing shims to take the gap to minimum, that ignition will improve.

Its all I have left i guess...

Cheers, Gareth

Any chance that there's someone nearby with a Gunsons colour tune plug to adjust your mixture screw.


sounds like that lumpy cam could be main problem...