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Discussion Starter · #1 ·
I have had this issue ever since I bought my bike last year and now after 10K miles and 2 dyno runs the cough/misfire is just just as apparent and still very annoying. I think however I have narrowed it down to a carb issue (but please add your input).

The run down:

when bought - dunstall exhausts, TTP breath, K&N replacement filter, 3mm drilled slides with thruxton needles +2 shims, mains was 115 with a 42 pilot

dyno run 1
“A bit rich at the bottom and lean at the top, got up 1 or two sizes on the mains and down a few on the pilot”
and with that information 38 pilots were on order and in the the end the bike settled with 130 mains.
(the bike didn’t like 38s. wouldn’t idle or run properly low down, So stuck with 42s for a little bit longer)


dyno 2
verified the mains and needles (apparently). and now has a set of 40 pilots in.

I’m properly rambling but this is my graph.


the question
can any off you brains tell me why my bike may be running rich at 2000 rpm? offer any info at all, let me here it all. I have played with the jetting a factorial number of times and this cough is still happening…. right where I would like to use the bike.

the needle jets have been replaced too as it was suggested that the bike wasn’t atomising the fuel properly, fouling a plug, coughing and clearing it before moving up the rev range.

I am very mechanically mined but this one has me stumped, nothing seem to work and I need more input. The gill coil seems to be in spec, the pick up could is within spec what am I missing?

Chris
 

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From what you described - This is just a shot, but if it were me this is what I would be doing.

I would take the shims out so the needle is dropped down into the leaner taper and try it. The cough is right when you are coming off idle at 2k rpm. That is on the pilot and very tip of the needle and the throttle plates opening. I would synchronize the carbs to be certain they are not fighting each other on the initial throttle opening and giving you that cough. Also, check for air leaks also around all the inlets.

Hope that helps you a little and gives you some ideas.
 
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Clearing it's throat

Mr. teletecher,

Usually a cough means to much fuel at once. I also would say to get rid of at least one shim. Stick with the 42's till you know the shim difference.

Has the air box baffle plate been removed? If not you might as well get it out.
For the price of a couple of dyno runs you can get a O2 bung welded into the exhaust and do your own checking with a meter.

PS Don't tell Moto I said that.

Namaste,
Charles
 

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Discussion Starter · #6 ·
carb sync is spot on (y)

just tried the 42s again with out changing anything else. worse.
tried the 40s, no shims, still rich. but better. - think my bike prefers the 40s rather than 38s or 42s. (sea level 10-15 degrees c on a usual day so may differ from other peoples bikes)

Im thinking maybe the enricher (choke) is some how leaking fuel in.
or the overrun enricher stuck open maybe? Just trying to think out the box as seems to rich to be a jet or a needle shim to me and isn’t a leaking manifold as it would show up lean and since the bike wont ‘blip’ smoothly without hesitating or just cutting out makes me think I have something wrong with the slide springs?

No idea anymore.

Chris
 

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Discussion Starter · #8 ·
my bike serially doesn’t make sense….. more than an MZ… and those have pretty unique problems in themselves.

theres not a huge amount to the carb: mains, needles, pilot jet, idle mixtures. there all reasonably in the right area and not to drastic.
maybe the needle shims need a little work as o shims it runs like crap and taking it to the extreme with 4 shims (.5mm ea.) it runs better but still like crap.

so something else must be letting/metering in fuel.
float level is 2mm lower than standard so it should have fine tuned my running at 2Krpm…. but it hasn’t, the needle jets have been replaced so that rules that out. maybe I have the wrong needle? I dont know the difference in fuel metering between the standard B and the thrux needles. anyone?

other symptoms are that when its bucking like a pig and I pin the throttle to rase the rpm… it kind of jumps back into life as if the slide was stuck open/closed because of the change of vacuum.

might rename my bike the mystery machine...
 

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Discussion Starter · #10 · (Edited)
will probably nick a few bits of my friends bonneville.


however in the meantime I have poorly made this:

This is an over lap of throttle positions and my a/f dyno results. (The dyno test by running at 1/4, 1/2, 3/4 and WOT. Its not all WOT runs..)

by overlapping and making this thing above it looks as if I need to play with the needle and find one with an increased diameter at the top and a decreased diameter in the middle so Its a bit leaner off take off and I dont have to use many shims in the middle.

any one know the differences between the needle codes?

NAGB
NFCD
NBTZ
NBAD
NBZY
N3RL

?\

edit - http://www.duncanracing.com/TechCenter/KeihinCarbJetting.pdf talks about needles
 

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Discussion Starter · #12 ·
stock carbs will work.. ish for now, until the breeze of insanity has totally gone though my hair and I take the engine out and set upon a large amount of modification and then put a o2 bung in the exhaust.
 

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Don't know about all the needles you listed but the NBZT is a Thruxton needle which you probably need especially if you remove the air box baffle. the NBAD needle is fatter, leaner, than the NBZT and came in a 04 790 America, among others. You have listed a NBTZ which I think was listed incorrectly in the Jinks guide.
 

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Discussion Starter · #14 · (Edited)
should have done this when writing the other post, apologies. according to Jenks guide Version 6a :

my stock Bonnie needle (01-05) - NAGB
790 2006 bonnie - NFCD
790 thruxton - NBZT

790 american&speedmaster - NBAD or NBZY

865 speedmaster - NBZY
865 thrux - NBZT
865 scrambler - N3RL

im in the process of deciphering the codes via http://www.duncanracing.com/TechCenter/KeihinCarbJetting.pdf and some other tables.

edit: to stop thread bumping

needle code lowdow:
the needle is broken down into 3 stages- taper, length & diameter (in that order)
for some reason the tables generally show 3 digits however our bikes show 4. not to worry as the N properly stand for a common dimension, like the diameter where the shims fit or something. so its discounted for the minute.

the needles are measured on a scale from A-Z where A is rich (so smaller diameters) and Z is lean.
this is true for length and diameter.

taper for some reason is measured with Z being lean and A is rich and corresponds to the degree of the taper angle.

however my bike needs a leaner taper. But since its already on Z what the hell do I do?
It has dawned on me that this is why people like fuel injection some how making me feel and idiot and yet clever at the same time.
 

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Skimmed through this, but my 2cents - I had similar though not to his extent on my 05 790. thruxton needles best with 0 shims, 1 is ok, never 2. I've alternated between thrux needles and stock, but it is happier with the thrux (NBZT). Pilots either 40 at 3 turns or (where I have ended up) 42s at 1/4 turn. I wouldn't start trying new needles - sounds like a rabbit hole you'll never get out of. Persevere with the tuning, you just have to know that every part of the carb setup affects every other bit to some degree. I'd put the floats back to normal height - you have too many variables / unknowns that make it much harder to diagnose.

130 mains is a tad rich with thrux needles and the airbox baffle still in. I was at 125's.


Sent from my iPhone using Tapatalk
 

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Discussion Starter · #16 · (Edited)
I came to the same conclusion as you n1ckel while staring at the A/F ratio graph. I am at leanest needle for that area and that there wasn’t any point.

while I was also steering at the A/F ratio graph I had one of these moments


here it is again as I’m going to talk though my thought process and you guys can help me to see if it actually makes sense.



to clean up the wave at the top of the line I would need to go down a smidge on the main in, 130 to 125.

I keep the needs the same as I already have a nice flat line

and since the main jet effects pretty much the whole rpm/throttle to a degree that should lean out my rich spot a small amount.

I tried this last night to a varying degree of scussess (I went down to 120 mains, but will go back up…. to 125)


however, I screwed in the pilot screws so that the turns out was <1, seemed to almost cure my rich spot but has left me with a super lean idle, so lean it wont give a nice even thumping sound.

Edit: this strange new configuration I have ended up has given me my throttle response back, nice and snappy like new undie elastic
 

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Teletecher, you're overthinking the jetting with all the different needles, you will more than likely get in to a mess. You need to get to a good basic jetting setup to see if it eliminates the cough/misfire before you move on.

The NAGB needle was designed for the 790 engine characteristics and the NBZT for the 865. The mods on your bike won't change the required needle significantly. To get your bike to run how it should with the mods you have, this is the jetting I have used in the past:

NAGB needle, 128 mains, 40 pilots, 1 shim and 2.5-2.75 turns out on the pilot screw. ( a good setup, but you may get excessive deceleration popping)

NAGB needles 125 mains, 42 pilots, possibly 1 shim and 0.5-0.75 turns out on the pilot screw. (a bit rich at the bottom, but good reduction of deceleration popping and a smooth ride)

Both of these will get you pretty close to where you need to be. If you still have a cough/misfire with these setups then you need to look at something other than the jetting to fix it.

Many have had success using the NBZT needles in a 790, but this does make a 790 engine run quite rich at the bottom end and if you add 42 pilots and a shim, it will be very rich at the bottom.
 

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Discussion Starter · #18 · (Edited)
taking this into account

"my 05 790. thruxton needles best with 0 shims, 1 is ok, never 2."
I've alternated between thrux needles and stock, but it is happier with the thrux (NBZT). Pilots either 40 at 3 turns or (where I have ended up) 42s at 1/4 turn.”



"NAGB needle was designed for the 790 engine characteristics and the NBZT for the 865…...Many have had success using the NBZT needles in a 790” apart from me it seems :S


"NAGB needle, 128 mains, 40 pilots, 1 shim and 2.5-2.75 turns out on the pilot screw. ( a good setup, but you may get excessive deceleration popping).
NAGB needles 125 mains, 42 pilots, possibly 1 shim and 0.5-0.75 turns out on the pilot screw. (a bit rich at the bottom, but good reduction of deceleration popping and a smooth ride)"


125 mains swapped in this morning.no shims. 40 pilots with 3 terns still with thruxton needles.
I will Persevere with the tuning above, but maybe I need to find me some NAGB needles.
 

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taking this into account

"my 05 790. thruxton needles best with 0 shims, 1 is ok, never 2."
I've alternated between thrux needles and stock, but it is happier with the thrux (NBZT). Pilots either 40 at 3 turns or (where I have ended up) 42s at 1/4 turn.”



"NAGB needle was designed for the 790 engine characteristics and the NBZT for the 865…...Many have had success using the NBZT needles in a 790” apart from me it seems :S


"NAGB needle, 128 mains, 40 pilots, 1 shim and 2.5-2.75 turns out on the pilot screw. ( a good setup, but you may get excessive deceleration popping).
NAGB needles 125 mains, 42 pilots, possibly 1 shim and 0.5-0.75 turns out on the pilot screw. (a bit rich at the bottom, but good reduction of deceleration popping and a smooth ride)"


125 mains swapped in this morning.no shims. 40 pilots with 3 terns still with thruxton needles.
I will Persevere with the tuning above, but maybe I need to find me some NAGB needles.


I know Pieman prefers the NAGB in the 790s, and I bow to his experience :) I've tried both, and good good results with both, but I found the NBZT a bit more forgiving. My bike is older and I'm sure has many imperfections and slight vac leaks despite me searching endlessly for them :), a slight bit of richness isn't all that bad. Once you make mods like more open pipes and airbox changes, you are changing the overall characteristics of the engine anyway. You do have to be careful though with the bottom end, especially if you have 42 pilots.

Every bike seems to be different - I know others run their 42s more open than mine. If everything else is pretty good, try a bit more on the AF screw and see if it helps.

N
 

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Discussion Starter · #20 · (Edited)
well today I spent the whole day playing with jets

set upon 125, thrux needle with no shim as per given advice.

so tried that main set up with the

42s all matter of turns - still a cough
40s all matter of turns - still there
38s all matter of terns - yup, still a cough, splutter, miss fire whatever you want to call it.

after sealing intakes for any leaks + checking no valve were left open to to check condition (surprisingly all good).

so I talking with my brother (who rides and ancient CX 500 with 75K miles on) about the problem and

- how an engine will still run whether slightly richer or leaner and shouldn’t cough
- being somewhat intermittent but more noticeable with a hot engine
- To make it a authentic triumph several electrical week points that probably haven’t been changed
we concluded that It could be an intermittent electric problem.

The rundown on the ignition:

there are noticeably three main faults to our bikes: Coil, igniter and pickup coil/crank position sensor.
I have swapped around with new and used coils and it still happens so I think I can rule out the coil.
from what I have read from faulty igniters is that they leave you at the side of the road and you can’t start the bike. Hasn’t happened to me yet so I think I can rule that one out… maybe.

Now the pick up coil. Gap was 0.8 so resized gap to 0.6 (did that a while ago so that might be a bit fuzzy with the numbers).

so with a hot engine I did a resistance measurement.

hot engine = 673 ohm
warmish engine = 640
a slightly cooler warmish engine = 638
35 degree c engine = 622omh….. almost in spec :D
edit: cold engine = 588

so, quoting Forchetto "560 ohms +/-10% at 20ºC” I’m out of spec. You guys agree with a dodgy pick up coil as jetting doesn’t hugely effect my 2K cough?
I did do a extensive check on my bike for HP, not stolen…. etc…. It also told me if it has never been in for any recalls. It hasn’t. Ever.



on a side, kind of vague note, will a iffy pick up coil cause a tacho counter needle bounce at a steady motorway/highway speed (60mph)?


thanks all,

Chris
 
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