280 versus 260 main jets (2013)

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L.E.N.

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My '73 850 Dunstall with KN airfilter and 2-1-2 exhaust came to me with 280 main jets. I live at 3000' elevation and have the needles in the 2nd notch of 5. I'm about to put anodized slides in and wonder if I should be thinking about switching to 260's ?. I don't really run wide open throttles very often at all, mostly winding 3rd gear roads around here. Any thoughts out there?
 
Buy a V8 set of plugs, put in new down in valley then climb on normal to high throttle for a minute or so then kill and clutch and check plugs, repeat with 280 lowered on last notch and check plugs. The needle tapper is active till like 3/4 open throttle so if plugs look ok and not WOT may be just fine as is, but I'd be restless not to try 260 anyway to learn something. Btw the sleeve bushes only get lube when in 4th so may want to get smaller drive sprocket to get into 4th more or start saving for new bushes when clutch wobble gets too much. I've a steep straight climb out place almost a mile long so cut at top then coast back to the
truck brake cool pull off parking to check and change where I've left tools out.
I think it takes more like 4000 ft to actually need a needle change.
 
Pretty sure all Amal carb needles have only three grooves. I think someone cut some extras on yours.
 
Biscuit said:
Pretty sure all Amal carb needles have only three grooves. I think someone cut some extras on yours.
Click to expand...

The needles in my 36 mm MK2 Amals have 5 grooves.

Ken
 
But on the other hand, just took another look at L.E N.'s bike and it does have Consentrics.
 
270s . :p

The Olde , at a fair lick , applying choke ( slightly ) .If it goes faster / better , Its To LEAN .

Air filter , mufflers , fuel etc , affect it all . If in doubt Veer RICH . Check plugs. any molten deposits . :shock:
 
They are concentric 32mm's (932) , I'm guessing that it has stock needles. I've just never seen recommendations for 280's on this forum or the other sites I've looked at. However I also don't know what I'm trying to improve here because it pulls hard without seeming to run rich, fluffy, or loading up. my problem has been with the idle. Also I don't have enrich enters and the plugs look tan-white at the center and brown-black nearer the threads.
 
Ha, I live in the gold country foothills of Northern California in Magalia, about 4-1/2 hrs northeast of San Francisco. A lot of great riding here, but not especially high speed at least by me.
 
LEN your DPO put a lot of his life in your ride so suspect hotter cam and bigger valves and efficient merged headers that may be happiest with 280's while other lesser developed engines might choke on 280's. Look up the mufflers or wipe with a white gloved finger :roll:, should mostly be dry dark greyish color not full sooty back. Look at piston tops for sense of over rich coke layer. New age lean burn gas is a good thing w/o booze in it so plugs tend a lot lighter even with rich enough. There should be soot on the rim and light tan deep in ceramic post and a thermal stain change just before the hook bend if timing spot on.
Did a quick search to see a rule of thumb on Miki carbs to reduce one jet size per 2000 ft rise. As I said you have stepped out of the ordinary with that Commando so we are waiting to learn what you can teach us about it specialness.
 
The 260 vs 280 should effect the WOT. The 260 should lean it up slightly at WOT, offer a few more MPG, and kill top end performance a bit.
 
Once more about the needles then I'll shut up. Just thinking about it and without any particular experience on this particular carb combo, but.....Mk2 needles are quite a bit longer than concentric needles no ? If they are indeed used in a concentric, seems to me at wot and at all throttle openings, more needle will be down in the jet than should be. I don't know what a MK2's needle taper is but could not that needle, too far into the jet, mask the fact that the main jet may be too large? Idle wise, I'd still have a good look at the tiny little idle passages, like someone first suggested here.
 
L.E.N. said:
My '73 850 Dunstall with KN airfilter and 2-1-2 exhaust came to me with 280 main jets. I live at 3000' elevation and have the needles in the 2nd notch of 5. I'm about to put anodized slides in and wonder if I should be thinking about switching to 260's ?. I don't really run wide open throttles very often at all, mostly winding 3rd gear roads around here. Any thoughts out there?
Click to expand...

The clue is in your question, a main jet at sea level will be too rich at 3000ft
The only way is to fit a smaller main jet and do a plug chop on near full throttle going up a slight hill, so that the motor is pulling and check colour of plugs to see if it is over fuelled.
HTH
but if you come down to sea level after fitting smal;ler jets it WILL be too weak :!:
 
LEN, I had it in my head that the factory fitted 260 mainjets to my Commando (mk3 850), however I was mistaken. 230s were fitted, according to both the Shop manual and the Riders manual. I see from an old Polk Cycle bill that the previous owner had them changed out to 260s, which work great on this engine.
280s are getting pretty large tho, a long way from standard issue, could be on the rich side at higher elevation.
If your carbs have drains on the bowls, then the main jets can be changed thru the drain holes. An 8 mm 1/4" drive deep socket will reach right up in there and do the trick.
It is possible your bike will pull better with smaller mainjets. When running too rich or too lean power drops off considerably.

Or you could just put a single Mikuni on there and make it into a Grampa bike! :mrgreen:

Glen
 
Thanks Glen, that's a great tip about chaging the mainjets through the drain holes. Mine do have this. I'll certainly swap some around and test it out. I just recieved my new anodized slides from Jerry ( and a good looking t-shirt) today. I think I'll leave the 280's in for now and test the new slides by themselves first. Too many changes at once and you don't know what did what.
 
Good plan, I'm doing similar with a Venhill clutch cable and an extra plate to change the clutch stack height. Was going to make both changes before riding again, then thought better of it. Might as well do them one at a time to get an accurate idea of the effect of each change. If nothing else it makes better info to pass along.

Glen
 
Somthing of interest. Metering needles with 4 rings ( not grooves) for later Norton spec Amals concentrics with the cutaway spray tube are 70mm long measured from top groove. Needles with 2 rings are 68mm long. Needles with three rings of which I have two examples are only 59mm long. Interestingly enough one of my 3 ringed examples has 5 grooves, I think it's from a two stroke application. I have no Mk 2 needles to compare. How long are your needles?
 
I will be taking my carbs apart this weekend to put new slides in and I will check out my needles thoroughly at that time. I'm hoping that I haven't led anyone astray with my claim of the number of grooves in the needles . That was my recollection from the last time I had them apart a few months ago when I pu stay up floats in. I'm less than an expert, but it did seem to me that there were 5 grooves. I'll fess up when I know for sure. I really appreciate all the input and generally love the site overall. All the info and especially the characters. :mrgreen:
 
My Seeley runs on methanol, but to get it right is just as difficult as with petrol. The motor is fitted with 34mm Mk2 Amals in which I use my own home made needle jets and Mikuni 6DP6 needles. The hardest part of getting the carburation right lies in the combination of needle and needle jet at 3/4 throttle. If the combination is too lean, the metering will be on that rather than the main jet,when the throttle is wide open. The first thing to do is to use the bigger main jets then lower the needles until the motor coughs when you are riding it around twisty roads. Once you have established that combination, raise the needles one notch and the bike should accelerate cleanly and swiftly right through the whole range of throttle openings. Mikuni needles are ground with three tapers, and using them you can get the tuning right, the amal needles tend to be a bit more crude. After you have the tuning right at 3/4 throttle, and through the rest of the range, it is then easy to get the main jets right by reading the plugs after a high speed blast on a long slightly uphill road. Your plugs on a road bike should have a 2mm black ring on the porcelain right down where it meets the metal. If it disappears you could burn pistons. It is possible that the metering never comes onto the main jet if your needles and needle jets are too lean, so if you fit the bigger main jets the motor should slow slightly in top speed, and the plugs should get darker. If this doesn't happen, you have a problem. In all of this, lean is good but always remember your bike will go its fastest just before it self-destructs. It is worth reading the Mikuni information on getting the tuning right . Most of it is about two strokes which are much more critical.
If you can get the carburation really lean right down the needle, and the main jets still work and are optimised, you will find that the bike really performs, and is lovely to ride until the weather turns really cold or hot. When you start it from cold, it should need the choke, and should spit and cough a bit, but be OK once it has warmed up. Most of your riding should not involve using the main jets, it will be on the needles and needle jets. On my own bike, the mains are rich and it does not matter - better to be safe than sorry.
 
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