intermitant miss -- nearly solved!

[Lucky]

I sent the following to Tdub, but i'd like your imput as well.

I need a fresh view of things, i think i'm too close to the forest to see the trees.  i'll give you the symptoms & what i've checked, tell me where you'd suspect.

audible intermitant miss, sounds mostly like the rear cyl. (i have macs, seperate exhaust ft & rear)  it will miss several strokes & fire several strokes, (bap-bap-bap-bup-bup-bup-bap-bap-bap-bup-bup-bup) makes no diff hot or cold.

it wants a lot of choke till really warmed up, but the choke is really touchy. too much & it'll peg 3k easy, too little & it'll stall, & it's one or the other, i.e. there is barely any "middle ground". i have to be very sensitive about it.

spark checked with gap jumper, looks constant, but unable to say 100% for sure. wires/caps/plugs new within the last year.

pulled carbs, all jets, etc clean. synqued not long ago

no change wiggling pick-up coil wires (was thinking skinned wire)

i'm thinking either somehow the synq has gotten way out of whack or i have a coil going south...

don't think about it too much, i'd like your first instinct.

thanks, --Lucky

oh, also, electronics gurus:

if i put a multimeter on a very low voltage reading attached to the pick up coils, shouldn't that give me a reading (or needle if i go analog) jump when the pick up fires? once per rev...  i would think this would be a good visual test for the pick ups if an analog meter is used, a miss might be easier to spot....
sound reasonable?
--L

[Lucky]

or instead of an analog needle, a low voltage piezo buzzer might be easier to hear an intermitant miss... or am i now getting out in left field?

[Extent]

I don't know if an analog meter would have the response time, the needle moves really slow and even at idle you're talking about 25 pulses a second, and very short pulses at that I would assume.  using a piezo is pretty clever, no idea if it would work tho.  I had always thought about recording a signal like that straight into a computers mic or line in, gives you roughly 44khz resolution, poor mans oscilloscope.  Huge potential for damaging delecate electonics though.

You sure it has to be electrical?  I wouldn't think that the choke would have any effect on things.

Do you have a stethescope?  Or you could try using a screwdriver to listen to each cyl and see which is missing.

[MotorPlow]

I don't think the choke has anything to do with it. My choke is exactly the same way, always has been. Do you have spare coils that you can try. If you don't have spares, you now have Dave living near by... Try swapping the coils from his bike temporarily.

[Night Vision]

for what it's worth.....my first instint is the carb settings.... have you adjusted them lately?

the reason I say this is- I just went through a bap bap bap, bup bup, bap bap bap bap bup ordeal.
the tricky choke is another symptom I had too.

I tried the center between rpm changes method several times.... tried 3 and 3 turns out... etc.. etc.
I could have the screws set where the choke worked like a dream and the bike ran like crap... lots of different settings.... not happy

Then I put it on the center stand.... adjusted the idle knob to around 3500 - 4000 rpm and adjusted the bup out with the screw settings...... that was pretty darn close... but of course... I still wasn't happy.

I had that darn split second lag/hesitation with a quick roll on around 5 or 6k rpm......

soooo.... I went back to the 130 top jet in the front carb / 135 rear setup that was the original jetting with the '82 flapper mod.... AND added 1 nickle to the flapper.

readjusted carb screws at 4k on center stand and fine tuned doing test runs from there (5 1/4 out front, 2 rear) and the choke now has some middle ground too.

Is it perfect? no... never will be.... is it good enough? no..... it's better than good enough  ;D

[don_vanecek]

I only say this due to a recent problem I had-unless your really sure you've eliminated this, spend the five bucks and put two new plugs in. I didn't think my plugs were too old either.

[Lucky]

Don, I just put NGK irradium plugs in this spring, i know it's not that, but i'll dbl check them anyway (maybe one is loose..)  i'll start re-checking the mentioned items below. i'm leaning towards synq... i've played with the pilots & they don't really make a difference, but if the synq is off for some reason then everything else will be off too.

I don't have spare coils, but i can swap them side to side & see if that changes anything.

btw Nightvision, these are 83 carbs.  i think the choke thing may be incidental too, but later on i want to chase that down & find a way to improve it...

good idea on the stethiscope, i have one..packed somewhere.. i'll dig it out.

[haunter]

double check your the voltage across the battery, I have been getting an intermittant miss, and my RR is bad, I guess it didnt always fire one of teh coils b/c it isnt always getting the volts she wants  ???

[Lucky]

Ok, so here are todays results:
--started by checking the synq, close enough, not perfect, but within specs.
--played with the pilot settings with no difference.
--checked the spark with an inline spark tester. you can see the light flash as the spark fires while the engine runs.  it appeared as if the flash would skip along with the miss. a little hard to tell.
--I usually clamp off the YICS with hemostats when synqing & noticed that the miss seemed better when i clamped off the rear cyl YICS hose.  sure enough, my repaired YICS is leaking.  pulled it & sealed the ports on the head for now, HOWEVER. the miss soon returned.
--98 degrees today & 1/4 tank of gas (which is fine, 'extreme' conditions should be tolerated) i noticed after idling in the sun for 30 min the gas cap was 'sucking air'. i had gone thru the gas cap earlier in the season, & it too was in good shape, never the less, i removed the cap gasket(the thin maze like one, not the rubber ring), check valve, ball & filter.  no change in the miss....
--before discovering the cap problem i noticed that the miss seemed to go away if the petcock was turned to prime & would return after a few moments when returned to "on". i was going to retry this 'test' after freeing up the cap, but ran out of time in the day. i'll let you know what i find out next time.

Also while listening to it run in the hot driveway (listening is diagnosis) i also seemed to hear a plug wire arcing. i'll have to rig up a pony tank & run it at night to see if i can spot an arc.  once again, plugs caps & wires are all recent. (i must be a sucky mechanic! lol)

more later, --Lucky

[Tdub]

Sorry I haven't been on lately...busy packing and all that other crap associated with moving.
Any way it definate sounds ignition related. The choke can effect a weak spark. Coils rarely go bad, but then again we are talking about 25 yr old coils. can you disassemble the vacuum petcock? Will it continue to run OK on Prime? HMMMM?

[supervision]

 Lucky, never trust an NGK spark plug!   Stands for No Good Kind    No, that just a joke    But I would try a different plug cause even a new plug could have come up with a deposit... 

[hfarley]

Lucky, I am probably completely off base here but figured I would throw this out there. It may just get you thinking in a different direction or eliminate one more thing for you. I noticed that when I took the mac exhaust off my bike to do some repairs and put it back on with new washers it took a lot of fiddeling with tigheting the bolts/nuts to get the bike to sound right again. My pipes were how should I say this, odd shaped at the ends and the washers had to be positioned just so. I ended up having to undo everything and redo everything a few times before It sounded right. As I stated probably not your problem but thought it might help ya in some way.
-Heather

[Lucky]

I'll keep it mind Heather, i think it's ignition, but if it's not, to bastardise Sherlock Holmes: "after all other options have been eliminated, what remains, however unlikely, must be the solution"

--Lucky

[kiawrench]

ahhh grasshopper,,,, welcome to the world of different altitude!!
1. your bike is set up to run it's best at or near sea level.

2. you are no longer at or near sea level

3. your bike may be in need of a change i had to make, just in reverse!

  i moved from high altitude to lower, and ended up running way too rich- you went from low to high, and may now be too lean,   i rejetted, based on dyno readings during a bike week, and run great-- hate to say it, may be time to rethink your jetting, maybe try stock jets and see where you go.

  i know is a lot of work,,  but then , the dragon is so close, and the bike does love to roll in front,,,,,,

[Brian Moffet]

ahhh grasshopper,,,, welcome to the world of different altitude!!
  i moved from high altitude to lower, and ended up running way too rich- you went from low to high, and may now be too lean,   i rejetted, based on dyno readings during a bike week, and run great-- hate to say it, may be time to rethink your jetting, maybe try stock jets and see where you go.

Actually, at higher altitude Lucky will be running rich, not lean.  Atmospheric pressure is lower at higher altitudes, which means that the carbs are going to allowing in too much fuel.

Unless motorcycle carbs are unlike airplane carbs.

[Walt_M.]

Does the miss go away if you top up the fuel tank? I'm thinking fuel pump here.

[admin]

guess I'll weigh in here also, Brian's right about altitude changes, although I
don't know how much higher elevation you are at, it's probably not the cause of the miss.
although I wonder what jets you are running, did you end up rejetting for the macs at all ?

The tci is such a damn weak system that you know if your battery volts are down you're
gonna have a symptom like this.  Don't think you have a plug issue unless you're running
champion plugs  ;D  personally I like ND's in the vision, but it's fairly rare to have bad ngk's also.
more likely to be a plug cap.

so, did you end up pulling out a plug ? a fuel fowled plug could give you an occasional miss as described....

-Ron

[Lucky]

--The jetting is stock (82 engine, 83 full fuel system) has been for years, never had a problem.
--plugs, caps, wires are all basicly new (within a year, & it ran better after the replacement) .
--swapped TCI's (forgot to mention it) made no difference.
--problem presented itself before i moved & has not worsened since, so i don't think altitude is a factor.

Fuel pump has not been checked, but i think the petcock thing in the last post was due to the cap. i can run it without the cap & it will miss with the petcock in any position. i believe that the reason it ran 'better' when turned to prime is that with the cap restrictive & the petcock on "on" it was starving for fuel, making the SYMPTOMS appear worse.  turning it to prime allowed fuel to free flow back into the tank (cooler fuel mind you) so then there was fuel to burn & the cyls would 'pick up' again. 

checked it tonight & with the cap fully vented (or off even) it now misses in either petcock position.

so next on the list for elimination is ignition. i am hearing a definate spark like click.  it's hard to pinpoint with the tank on & lowers in place,even with a stethiscope or hose, i'm not even sure it's not a normal engine sound at this point, so my next step is to remove the tank & lowers, inspect the 'new' wires for arcing, and inspect the coils.  who knows, i may a find arc trail from the coil...

i'll rig up a pony tank & run it in the dark & see if i can see any arcing.  i now think that i hear the arc click in time to the miss.  whether i'm just imagining it, or it has actually gotten worse to the point of being able to notice this, i'm not sure... (enjoying watching me spiral into Vision diagnostic delerium?)

if it turns out being the 'new' wire, i'm defenatly going to rethink the stock routing of the wires...running tucked in along the frame with metal clips holding it tight has never seemed like a bright idea....

--Lucky

[Lucky]

oh, & no Ron, i haven't pulled a plug yet, it seems less likely the irradiums would foul, but i should check.  i don't think it can be described as an occasional miss, it's pretty regular for a miss, lol

if i don't find an ignition problem, i'm going to start thinking burned valve & do a compression check.  if i find that's so, i'm blowing the engine & annoying Tdub untill he sends me a 750, lol

[Phatjap]

Hello
Just thought I'd add something here.....

I have been having a bit of trouble with the running of my bike....what seemed like missing every so often. Checked for spark etc....all seemed well, in fact two new spark plugs very recently, 2 months ago-ish.
Carbs re-set. I did the spark plugs test out of the cylinder and they sparked great. However, when I put the rear spark plug back in, I got to what I thought was the last of the thread, did another turn to tighten it and it just kept turning......! Oh merde, I thought!! The centre of the plug came out ok of course, and luckily as I had copper greased the plug on re-fitting it, that wasnt too much trouble either.Seat off, large screw driver into pot did the trick.
But the important bit of the story is, that when I inspected the pug there was a black mark going through the threaded bit just where it broke off. It was a bout 6 mm long and went from inside to outside. And looking closer, the plug was tarnished around the chrome with a dull grey matter.

So the comments previously on this thread regarding crappy NGK's hold up. Don't always trust a spark plug just becasue its new. Of course the chances of it being the same in this case a remote, but worth concidering. Incidently, putting the choke on seemed to smoothe out some of the miss-firing, not sure how that would work.

Cheers

Phatjap