Runs well, plugs foul....

Danno said:

More like two stone axes whirling in attempted synch. Once you get both sets of points breaking at 28 at full advance, then all you have to be concerned about is the equaliity of the springs and the friction on the bobweight pivots both being equal at all times. Now I'm hearing nightmare tales about cam runout a the seal surface influencing the points. It's all too much when you can plug in a box.

I can see where a hi-perf engine being run up to redline in each gear repeatedly would like a little less advance.

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Springs and bob weights only a factor until 1,500-2,500 where the AAU is full advance. Cam runout is a non issue as it is a constant. The scatter at full advance is cam chain snatch and pinion lash. Points may have some inherent attribute for scatter but that is a stretch. I have seen plenty of cases where I have swapped points for electronic off of the cam and still scatter. Switch to crank trigger and as solid as a rock. The Nourish was the worst with a cam triggered Hall effect trigger; scatter was horrendous and crank trigger gave solid performance.

Point being that AAU is really not a factor once at full advance. Turbulence affects timing requirements right up through the rpm/load range of an engine, it is not limited to high rpm WOT.

Has Lineslinger gained any additional insight to what is going on with his bike?

He said getting the Stay-ups properly set, along with the N4G plugs I gave him has it running as it should, maybe even a little lean, so raising the needles will be next.

Nice. Thanks for the feedback.

Danno said:

He said getting the Stay-ups properly set, along with the N4G plugs I gave him has it running as it should, maybe even a little lean, so raising the needles will be next.

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If it's a bit lean across the throttle range I would try raising the floats to closer to the upper limit of the fuel level range (4mm from bowl top) rather than move straight to needle change

Danno said:

He said getting the Stay-ups properly set, along with the N4G plugs I gave him has it running as it should, maybe even a little lean, so raising the needles will be next.

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I installed a set of hotter plugs. Champion N4G's I swapped out for from Danno, who offered some really helpful knowledge. Air screws 1 turn out, maybe just a little less. Needle clips at the top, furthest away from the taper. Ran great with better idle and response.

I got some really encouraging results with with the N4G plugs without changing any other settings.
But, at 4.5K + RPM's on a downshift and opening the throttle it hesitated, slogged, would not get going like expected.

Plugs showed a lean coloration.

I have four groove position needles.

I changed the needle setting from the top groove, the one being being furthest away from the taper, which I will call #1...down to # 3. Offering a richer mix.
Now it stutters or bogs down much sooner, 3.5K + like a rev limiter, when I crank it, making me think it is starving for fuel...not sure.
Probably raised the needle position too much?
It may just be asking for a leaner mixture...OR...more fuel for that setting?

I really studied the float bowl tutorials and along with Dans insight think those are now correct or very close. I have figured out close on float bowls is not close enough.
I will raise the clip up, one notch, away from the taper and see how it responds. If that shows positive results I will raise the float bowls, me thinks maybe I was too conservative on setting the new float levels?

Seems to me that if I created a condition in which the engine is more responsive while showing lean conditions, then it wants more fuel than the bowls/floats are supplying it.


I'm getting really good at pulling the seat and tank.
I am also excelling at walking around looking for one of the three 5/32 allen's I own that has been in my left hand the whole time.
I still need to find the other two.

I really hate those four screws you know with my big stiff fingers.

Every engine's a little different. I got sidetracked the other day starting to tell the story about re-adjusting the floats on the SS clone in the parking lot of a beach hotel in Daytona in 2018. Got down there and it was blubbering at sea level.

Did you get a chance to strobe the timing and see the advance curve for yourself?

Once I get a smoother rev response out of it I will put the strobe on it.

You have obviously never ridden the bike when it is running well. You should be able to tell when the mid throttle jetting is too lean or too rich. If it is too lean, when you ride the bike changing up and down through the gears - too lean gives you the cough or flat spot - too rich makes the acceleration performance sluggish. I would not raise the float levels to correct lean-ness. There is a specified level for the floats - set them both at that and jet against it. Also do not change the static ignition timing while you are re-jetting the carbs, or after you have finished. Strobe it as soon as possible. The slightest increase in the amount of advance will give the same effect as leaning-off the jetting.
If you have the mid-throttle jetting too rich, you still get tolerable performance - it is just less than it can be. But if it is too lean, you should know immediately because the bike becomes a pig when you ride it. You need to find that condition, then raise the needles one. When you do that, you will find the bike's performance becomes really snappy - easier to ride.

If you have put the clips in the top notches of the needles and still don't get the cough when you ride the bike, you need smaller needle jets.

I have a bunch of comments on different things on this thread so far:

*First, your initial post said you had "3 position needles", and now you seem to be saying you have "4 groove position needles". That strikes me as something to check on as the possible problem (maybe, swap the old needles and needle jets for the new ones to see if that cures your problem)

*regarding the float height comments, Slightly lower fuel levels in the bowl due to float height wont make your bike's plugs look too rich, which was your original problem. Lowering the bowl's fuel level height might make you run out of fuel at WOT, but won't make your bike's plugs richer looking... so I wouldn't chase float height adjustment, unless you know the fuel level is too high.

*Your discription of adjusting the air screw on your first post seems to indicate that you might think that the air screw adjustment is significant throughout the full range of throttle. It's greatest influence is at idle and the lower register of throttle. Set the air screw with the technique posted below and if you get a good idle and good transition to your lower rpms, then you're air screw is 'set'. Here's my technique to set it up.

-Start the bike, set the idle stop screw somewhere near 1000 rpms. Go back and forth from side to side to get an even sounding cadence for the cylinders.

-Turn the air screw in until the bike starts to run rough, noting the screw position. Then turn it outward until it runs rough again and note that position. Return the screw position to the midpoint of those 2 position. Now do the same to the other side.

-Return to the original side air screw. While the bike is running, and you have the screw driver on the air screw, gently lift and lower the throttle as you turn the screw in and out very small amounts to see if you can smooth out the transition from "idle jet only" running to the beginning of the needle jet/needles influence being added in. It's very subtle to find the smoothest transition point position. Do the same to the other side... (If you want, you can repeat this last process again to micro adjust each side)

*At this point, you've set the air idle screw, So any other changes you need to make to the bike's carburation in higher RPM ranges are not related to the air idle screw position, and the variables you're left with are cutaway size, needle size, needle jet size, and main jet size.

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*Hotter plugs seemed to help your idle, but now you're claiming that moving the needle up (clip down) has made the plugs look lean and bogged down sooner... If anything, moving the needle up should make the mixture RICHER, so that wouldn't account for leaner looking plugs.

........ If I couldn't get past 4500 rpms, with new hotter plugs and readjusted float levels then I'd suspect that maybe I have the wrong cut away size, wrong needles, wrong needle jets, or wrong mains. The stock recommended settings are the best place to start. Then make adjustments to one thing at a time only, then retest to see where your at.

In the past, I had 240 mains on my commando and could never get past 85 mph. It was like there was a wall at 85. I changed to 220's and zoomed right up to 100mph. Sure enough the 220's were the recommended size for my set up. If I was you, I'd start with all the recommended settings and make adjustments to components based on what RPM range seems off.

This is just my $.02 so feel free to ignore it...

After the initial idle adjustments I always adjust both throttle cables to have an equal amount of play before doing anything else.
My € 0,02 ;-)

If the Trispark was fitted at the time the fault appeared then I would be going back to the last ignition solution to rule out the possibility of a ignition and or a coil breakdown.

I have a 250 Ducati that would bugger plugs in about 30 miles and that was down to a weak spark.

Lineslinger said:

New slides, 3 position needles, main jets (standard 220's),

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Lineslinger said:

View attachment 9578 View attachment 9577

Dave....Crossover pipe...snug connections...no leaks.
Again a hotter plug is mentioned.
New floats showed up yesterday. I am setting them .04 from top edge of float bowl.

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I will admit, I very strongly disagree with LAB over the reason needed and justification for the cut away spray tube relating to balance pipes VS a mute. Incredibly few 850 have mutes but ALL 850 came with balance pipes AND all came with cutaway spray tubes.
Post #14 shows 750 spray tubes
I have not yet found what your cutaway is?

I see this as 6 pages of failing to acknowledge the effect that balance pipes have on breathing.

dynodave said:

I will admit, I very strongly disagree with LAB over the reason needed and justification for the cut away spray tube relating to balance pipes VS a mute. Incredibly few 850 have mutes but ALL 850 came with balance pipes AND all came with cutaway spray tubes.

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Then you have obviously misunderstood something I've said or you haven't read Bob Rowley's account of the actual reason why the stepped spray tubes were fitted.

It has been a while since I read Rowley's write up, but I excruciatingly dissected what he offered and his final conclusions. I have thought a lot about what: 1. balance pipes do to the low RPM breathing and the "open header'' effect and the resultant mixture/fuel requirement, then 2. How will the stepped spray tube affect the whole range of flow, to give the richer mixture at the lower RPM and throttle setting. 3. why then the 3-1/2 cutaway is necessary to start to lean it out after the open header effect fades. 4. the leaner taper of the 4 groove needle to hold back the final 260 main jet.
The whole process of components cascades to the end to get the extra fuel mixture to satisfy the open header effect only in a particular band of throttle/RPM.
If you run an 850 with single pipes then you can easily revert to 95% 750 carb settings. an 828 is only 10% bigger than a 750 and with the same exhaust configuration it should need virtually no mixture change over 32mm 750 carbs.
I feel the mute was the final straw that forced the needed fix but the mute was not really the main reason...IMO it was the balance pipes.

dynodave said:

Post #14 shows 750 spray tubes

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This photo? I can't see any spray tubes?
https://www.accessnorton.com/NortonCommando/runs-well-plugs-foul.27474/#post-414591

post #14 pix
not a stepped spray tube

I see what appears to be main jet, jet holder and needle jet assembled, and a second needle jet?

You're right, silly me??
Still did not hear it said there is a stepped spray tube anywhere in the text.
The 2 groove and 4 groove needle are absolutely the same up to 3/8 throttle. So the spray tube and slide cut- away controlling the fuel pick up to that point.


Interesting side story, not that it proves much about this thread.
Had a friend ride to a club meeting (maiden voyage) about 20 miles and while on the highway around 55-65 mph it started to break up. Ran ok if run slower. After we got home, we were going to check the size of the main jet that the professional bike restorer had put in....

there was NO main jet...

This was the first time I started to help him work on his bike. It went down hill from there..LOL