01 ZX6R- Misfire under load until 3500RPM, okay after 5000

Z

zx6rj2

Member
Hello all

Hoping to get some feedback with an issue Ive been trying to tackle over the last couple of days.

Issue:
Bike will misfire and sound like a v twin all the way up to ~3500RPM. After this, theres a surge and the sound and power delivery seem normal. This happens in all forward gears. I also noticed that cyl#4 seems a little cooler (water evap test on headers). However, bike will start and idle fine. No overheating. Issue will only occur under load.

Troubleshooting done so far:
2nd set of new plugs
Checked resistance of each coil (2 stages)- OK
Checked for spark- OK
Swapped coils/plugs across cylinders
Checked battery-OK
Fresh fuel
No fuel/oil in airbox
No kinked or pinched hoses
I will do a compression test tomorrow

Bike has ~35K km and in the last 6 months:
-valves adjusted
-new fuel filter
-new plugs
-new air filter
-oil/filter
-carbs cleaned and synced

Assuming the compression checks out tomorrow, what else could this be? Vaccuum leak? Fuel issue?

Banging my head on this one :confused:
 
-Crank trigger for ignition? (Is the pickup working correctly, has something skewed the timing)
-Ignition module? (Usually nearly impossible to test outside of swapping with a known good part)

zx6rj2 said:
Bike has ~35K km and in the last 6 months:
-carbs cleaned and synced
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How confident are you that this was done thoroughly and correctly? You could have a clogged or misadjusted pilot jet or two.
 
TwistedKestrel said:
-Crank trigger for ignition? (Is the pickup working correctly, has something skewed the timing)
-Ignition module? (Usually nearly impossible to test outside of swapping with a known good part)



How confident are you that this was done thoroughly and correctly? You could have a clogged or misadjusted pilot jet or two.
Click to expand...

I think you may be on to something with regards to the carb clean.

I pulled the carbs and the boots for cyl 1 2 , 3 had some residual fuel. Cyl 4 was bone dry.

I know cyl 4 is getting fuel since the bowl was full when draining. It is not delivering it to the motor. At th point what should I be looking out for?

Thank you for the reply
 
zx6rj2 said:
I think you may be on to something with regards to the carb clean.

I pulled the carbs and the boots for cyl 1 2 , 3 had some residual fuel. Cyl 4 was bone dry.

I know cyl 4 is getting fuel since the bowl was full when draining. It is not delivering it to the motor. At th point what should I be looking out for?

Thank you for the reply
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It's weird that the bike seems to idle okay, but you did say you balanced the carbs, so it's possible that #4 is opened a lot more at idle to compensate. At minimum I would yank the pilot jet and inspect it. I can't really give you a useful explanation as to how, a manual will serve you a lot better for that. It's the first thing I thought of since you say the bike seems to run fine once you're above 3500 RPM (presumably now on the needle jet).

But hey, at least you have the carbs off. You've pretty much done all of the work already
 
You should rebalance the carbs after #4 is fixed, of course
 
TwistedKestrel said:
It's weird that the bike seems to idle okay, but you did say you balanced the carbs, so it's possible that #4 is opened a lot more at idle to compensate. At minimum I would yank the pilot jet and inspect it. I can't really give you a useful explanation as to how, a manual will serve you a lot better for that. It's the first thing I thought of since you say the bike seems to run fine once you're above 3500 RPM (presumably now on the needle jet).

But hey, at least you have the carbs off. You've pretty much done all of the work already
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I pulled the pilot jets from all 4. There is a pinprick of light going through each of them in roughly the same amount.

However, the jet looks like its tapered in the middle...I dont know if this is the reason why there is only a pinprick. I am wary of poking a needle or something through it. I soaked them down in carb cleaner.

One thing I noticed was the jets were easy to unthread...almost as if they were put on by hand. Would this cause an issue?

Sidenote/Question: did not have the chance to to run a compression test as I wasnt able to get a tester today. Would a dead cylinder not create the vaccuum to pull the fuel in?
 
zx6rj2 said:
I pulled the pilot jets from all 4. There is a pinprick of light going through each of them in roughly the same amount.

However, the jet looks like its tapered in the middle...I dont know if this is the reason why there is only a pinprick. I am wary of poking a needle or something through it. I soaked them down in carb cleaner.

One thing I noticed was the jets were easy to unthread...almost as if they were put on by hand. Would this cause an issue?

Sidenote/Question: did not have the chance to to run a compression test as I wasnt able to get a tester today. Would a dead cylinder not create the vaccuum to pull the fuel in?
Click to expand...

Yeah, poking a wire through a jet is not something I am a fan of. I dunno if the jets being easy to unthread is really a problem - I have had carbs where they were practically welded to the body and getting them out without completely stripping them was tricky - but I imagine if you ever adjusted them they wouldn't be super tight anyway.

Low compression I don't think would create that problem. *No* compression might, but that would mean like a holed piston or something, which you definitely don't have.

Hoping someone more carb smart than me chimes in here
 
its lean on the low/pilot circuit. this could be due to a restricted jet. pipe, aftermarket air filter. put new pilot jets in, (if piped) go 1 size larger. adjust mixture screw in 1/2 turn
 
racepro171 said:
its lean on the low/pilot circuit. this could be due to a restricted jet. pipe, aftermarket air filter. put new pilot jets in, (if piped) go 1 size larger. adjust mixture screw in 1/2 turn
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Thank you for the reply. Out of curiosity, would this lean condition on the pilot circuit have any impact at idle?

I ask because the header remains cool at idle
 
yes, the pilot effects idle aswell. if you cant get new pilots. soak them in pinsol for 4-5 hrs. they will be like new. and a carb sync and proper jetting are probably needed if it has a pipe and or filter. you can also do a test. tape up 95% of the filter. if it is better. its lean.
 
racepro171 said:
yes, the pilot effects idle aswell. if you cant get new pilots. soak them in pinsol for 4-5 hrs. they will be like new. and a carb sync and proper jetting are probably needed if it has a pipe and or filter. you can also do a test. tape up 95% of the filter. if it is better. its lean.
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Thank you for the reply.

I managed to get my hands on larger (by one size) pilot jet. I do have a question however.

If I turn in (clockwise) the mixture plug (have to drill through caps to access them), would that not make the system leaner?

Sorry if this is a silly question.
 
no, those screws control air, in cuts the air. makes for a richer mix
 
racepro171 said:
no, those screws control air, in cuts the air. makes for a richer mix
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Good news!

The misfires are gone!

What I did:
one size bigger on the pilot
reset float heights
1/2 turn on the mix screw
carb sync

From idle to mid, it pulls like a beast again.

What I did notice now however is:
-a little bit of a stumble/flat spot from 5-6k when giving it. However cruising at that range is fine
-after a 30-40 minute ride, the bike will idle slightly higher (~1500 vs the factory 1350 +/-50)

Im not really concerned about the slightly higher idle too much but the stumble is a bit disconcerting; especially when needing that acceleration

Thank you all once again for the help!!!
 
meh, carbs? its a dieing art of black wizardry. your welcome:cool: you should be thankful you are not equipped with fcr carbs, the amount of adjustments are endless. as for stumble, raise the needle a bit, and or you could drill the slide hole a bit bigger
 
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Have you checked that you have a good seal on your carb boots? I would have initially guessed you number 4 was sucking air. Typically tested by spraying boots while running to see if idle changes occur.


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racepro171 said:
meh, carbs? its a dieing art of black wizardry. your welcome:cool: you should be thankful you are not equipped with fcr carbs, the amount of adjustments are endless. as for stumble, raise the needle a bit, and or you could drill the slide hole a bit bigger
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I am not going to say I like carbs but this was definitely a challenge and learning experience to say the least.

Its great to know we still have wizards amongst us normies lol :p

The stumble seems to have corrected itself..oddly enough? I don’t know how to explain it but after a couple spins, the issue disappeared.

Now I do have one more question…the bike smells lean but doesn’t seem to running lean. I don’t know if that makes sense but theres no misfire, rpm hangs or other lean symptoms but if you keep your hand over the exhaust at idle and take a sniff…it smells similar to ‘burning metal’ but wont water your eyes. :confused:

Should I leave it be? lol
 
superseven said:
Have you checked that you have a good seal on your carb boots? I would have initially guessed you number 4 was sucking air. Typically tested by spraying boots while running to see if idle changes occur.


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Click to expand...

I checked for leaks around the boots...seemed to be fine. No rpm change while spraying it down with brake clean
 
I am assuming the bike is still lean, on your second ride was the outside air cooler? I would tape over a percentage of your air box opening and see what you experience, if you are close to correct a/f this should help you determine what is happening. If lean more coverage should rich richen up the mixture.


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