Which slides - 3 or 3 1/2 ? Also thoughts on a change to single Mikuni

A bit of info to share- I have been doing a few plug chops with a recently rebuilt Atlas fitted with 928 Concentric's [new std #3 slides needles etc] I too had different colours between the spark plugs [NGK B7ES], on inspection I found that even though the needle clips were each on the same grove the measurement from slide to the tip of the needle was different. It turned out that the size of the recess in the slides where the needle clip sits were slightly different sizes and one clip was sitting slightly proud. I have never looked for or come across this before!

Great spot !

I am reluctant to give "advise" about carburetor settings ( too many variables..) but I feel that the importance of the cutaway is underestimated.
It has an effect on the mixture maybe 90% of the time on the road, like cruising at 70 - 90 kph.
850 Mk 2A came std with 3 1/2 slides, but with less restrictive air filter and exhaust, IMO a # 3 slide is a better starting point.
If needed, you can always increase the cut-off by careful filing.

One reason most don't experiment too much with slides is because they aren't cheap.

Fast Eddie said:

I find it hard to believe, that if all else was equal, a bike that runs perfect on 3 1/2s would be rich enough to foul plugs using 3s.

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There was a member here a little while ago who complained that he was constantly fouling spark plugs. He did a lot of low throttle riding and had #3 slides. He was advised to change to 3-1/2 and that fixed his issues.

maylar said:

There was a member here a little while ago who complained that he was constantly fouling spark plugs. He did a lot of low throttle riding and had #3 slides. He was advised to change to 3-1/2 and that fixed his issues.

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Maybe something else was changed at the same time.

Getting the cutaway right is important, no doubt about that. But the difference between a 3 1/2 and a 3 just isn’t enough to be the difference between perfection and plug fouling.

Just to ensure we’re talking about the same thing here... ‘Plug Fouling’ doesn’t just mean having slightly darker plugs, it means plugs fouling to the point they don’t work, causing bad running problems. This is caused by either rather excessive richness or oil burning or a combination of both.

Fast Eddie said:

Maybe something else was changed at the same time.

Getting the cutaway right is important, no doubt about that. But the difference between a 3 1/2 and a 3 just isn’t enough to be the difference between perfection and plug fouling.

Just to ensure we’re talking about the same thing here... ‘Plug Fouling’ doesn’t just mean having slightly darker plugs, it means plugs fouling to the point they don’t work, causing bad running problems. This is caused by either rather excessive richness or oil burning or a combination of both.

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Yes, this fella was complaining that brand new plugs would start misfiring after a few miles of 35 mph riding.

maylar said:

Yes, this fella was complaining that brand new plugs would start misfiring after a few miles of 35 mph riding.

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I stand by the hypothesis that something else was amiss and corrected during the fault finding process.

My 750 , OK in a f/bed, ended up with 3 and a quater slides 2x 3 sides filed to between 3 and 3 1/2.

Fast Eddie said:

Just to ensure we’re talking about the same thing here... ‘Plug Fouling’ doesn’t just mean having slightly darker plugs, it means plugs fouling to the point they don’t work, causing bad running problems. This is caused by either rather excessive richness or oil burning or a combination of both.

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+1

Carburetor settings and issues have been discussed countless times, engine health, electrical system and ignition maladies can all bear on fouled plugs.

When carb question(s) are asked responses often well meaning posts about: primary chain too tight/loose, exhaust system too open/restricted, yada, yada. When pondering the seemingly endless compound possibilities it's a good time to rebuild a Reynold's chain or to replace the filaments in the various light bulbs. When frustration with wasting time rebuilding items that are better off replaced sets in the carb/ignition/electrical issues will seem a pleasure. Wax on, wax off.

A tip to the wise (would be wise?): do a search, read the responses, look for information that is in common, distill your results and you will be amazed at how simple the solution is.

Best.

Lineslinger said:

Good thread . Lots of of solid/reliable guidance. Thank you.

I am curious what spark plugs are you running?

Just thinking out loud here but consider an infrared thermometer before purchasing a colortune.

I own and have used both.

Using an infrared reading and adjusting my carbs to the header pipe temps I was able to synchronize both 932's on my 74' 850 to within a final tweak of matching in mixture/plug color.

I got them close enough so that I could not detect a difference in performance...but of course I kept on screwing around with them until I was satisfied with the color match on the plugs vs. overall performance.
An infrared will run you around $18.

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Do you remember the temperature of your headers and what the largest difference between sides was?
I just measured mine on a Combat engine with MK2 Amal concentrics and got readings of around 170* F on right side and 130*F on left side. Temperature reading was after stable idle at 1000rpm for a few minutes with a big fan on it. This was right after performing idle mixture adjustment procedure from AMR.

I would say that the head temps. of my engine would not necessarily be the best reference for yours...different engines, fuel, plugs, ignition etc.

What I did was establish a roughed in starting point with my engine from idle to wot based on the recommendations found in the various offerings here in AN.
I would then take my temp readings, make an adjustment to one, the other or both, and ride it again. This method got the mixtures ratios in the two carbs very close.
After a few times you get a sense of the settings the engine likes while giving the spark plugs a look for color indications. I wouild then start tweaking the carbs in tiny increments seeking a matching header temp./optimal plug color with my final reference still being riding performance.

The mixture ratios on my reworked/overhauled Amals are not an exact match, but very close. The final decisions I make on my carb settings are based on overall engine performance and rideability. The header temps are now within maybe 3 to 5 degrees of the other with one cylinder burning just a tiny bit richer than the other according to plug color and the bike likes it this way.

The Norton starts on kick one or two, idles rock steady at 900 rpm and pulls strong all the way up thru 5K without hesitation or stumble.

Hope this helps.