280 versus 260 main jets (2013)

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worntorn said:
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

Remember that intake restriction will have a PROFOUND effect on jetting, so depending on which air filter(s) you choose to run, the jetting will change.
 
There are two four stroke needles available for the Mark II carbs. The 2a1 has five grooves and is used with the .106 needle jet. There is also the 2c3 needle which is used with the .105 needle jet and has three grooves. The latter combination was used on T140e Triumphs after mid 78 to meet emission requirements. The 2c3 needle is fatter and longer than the 2a1. By the way the Mark II is also a concentric, ie the float is concentric around the main jet.
 
htown16, for the sake of argument I'm using concentric to mean the earlier style MK1 carb. I know the next generation Amal MK2 is also called concentric but believe most think of the earlier carb as the concentric and the later type wholly as MK2's. Do you have a length measurement on those MK2 needles by the way?
 
Sorry, they are already in the carbs. Here's a pic
280 versus 260 main jets (2013)

The euro is the 2a1 needle and .106 jet and the US is the 2c3 needle and .105 jet.
I'm currently converting my 78 Bonneville to the euro jetting to run Norton peashooter style mufflers on it. Like all 75 and later bikes the stockers are very restrictive.
I love the sound of the ones on the Commando so much, decided I had to have them on the Triumph too.
 
Well I just changed my slides out to the new anodized set I got from Jerry. I do have 280 jets but I was mistaken about the grooves in the needles as most everyone was correct in pointing out. The needles do only have the 3 grooves there are four narrow rings at the top of the needles above the grooves. My clips are in the top most grooves. I haven't started it up yet because it's 11:45 PM. Interesting also was the two slides that I removed were different from each other. Both were the same size with the same 3 1/2 cutaways, but the left slide was gray aluminum and the right slide was chrome. The aluminum slide was quite scuffed up and discolored and the chrome one was "not so much". I'm guessing the PO replaced the right one due to wear in the past. I've taken pictures but I can't remember my password for photobucket. I did turn 60 today and I guess the forgetfulness is a reality.
Lance
 
OK, those are the correct needles. Lots of other guys had good posts about determining the jetting for your application so it's all good. And happy birthday, I'm a bit older than you even so I know what your talking about..... ah you're the one with the tire question right?..... :wink: :?
 
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.
The leaner an engine runs, the more power it will make all the way to the point where detonation is a real threat from too lean.
 
The leaner an engine runs, the more power it will make all the way to the point where detonation is a real threat from too lean.
Near 8 year old post, with my cam and head work I been running 270 main jets in my old Amals and my bike runs great.
 
The leaner an engine runs, the more power it will make all the way to the point where detonation is a real threat from too lean.
2-strokes, yes....
These long stroke dump truck engines are more productive on the chubby side.
 
2-strokes, yes....
These long stroke dump truck engines are more productive on the chubby side.
It applies to 4 strokes too. Spent many hours on dynos having my '06 Gen III Hemi tuned. Stoichiometric A/F ratio is 14.7:1, theoretically the ideal A/F ratio. However it is the ragged edge and A/F ratios of 13.5:1 or at most 13.8:1 are the usual targets. There is not any significant increase in power above the 13.8:1 value and it gives a bit of safety cushion to prevent detonation.

Granted this is with EFI so a carbureted engine might want to have a lower A/F ratio. The point is, leaner makes more power but it is a mater of diminishing return.
 
With no knock sensors, no liquid cooling, etc. anything above 13:1 is too lean for these old lumps IMHO.

I‘m not disputing it might go faster at that, just that it might not do it for very long…

But I’m confused, you’re talking about wanting to be on the lean side… and upping the jets to 280?

Anyhoo, you seem to have it all covered so I’ll leave you to it.
 
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It applies to 4 strokes too. Spent many hours on dynos having my '06 Gen III Hemi tuned. Stoichiometric A/F ratio is 14.7:1, theoretically the ideal A/F ratio. However it is the ragged edge and A/F ratios of 13.5:1 or at most 13.8:1 are the usual targets. There is not any significant increase in power above the 13.8:1 value and it gives a bit of safety cushion to prevent detonation.

Granted this is with EFI so a carbureted engine might want to have a lower A/F ratio. The point is, leaner makes more power but it is a mater of diminishing return.
I fold my tent. Sounds great. ;)
 
With no knock sensors, no liquid cooling, etc. anything above 13:1 is too lean for these old lumps IMHO.

I‘m not disputing it might go faster at that, just that it might not do it for very long…
I agree 100% but the other poster cited a power increase with a fatter A/F ratio.
 
The needles in my 36 mm MK2 Amals have 5 grooves.

Ken
How many different tapered needles are available for Amal carbs, if you don't use Mikuni needles ? Half a thou of an inch in needle jet size makes a big difference when using methanol fuel, and the jets are much larger for methanol than they are for petrol. As a percentage of flow through the jet, that half a tho when using methanol becomes similar to having a needle jet which is a full thou too big when using petrol.
I suggest it is much more difficult getting the jetting right when using petrol. The needles and needle jets sizes are critical if you want performance. THe main jets are pretty much irrelevant, as long as they are large enough to stop valve or piston burning, when you are on full throttle. If your jetting is right at mid-throttle, you should need to feed the throttle on, as you would do with a two stroke. If you whak the throttle open, the motor should hesitate - gasp. An increase in taper rate allows you to whack the throttle open, but the motor will be slower spinning-up as you come up through the gears. Especially if your ports are too big.
 
Grabbing a big handful of throttle is often not the best way to get your bike to accelerate. If your mid-throttle jetting is lean enough, the bike will accelerate faster. When you use full throtle, the jetting should be rich enough so that the motor gets a reprieve. How often do you ride very long distances at full throttle ? What is more important is the motor should really get going as you feed the throttle on. as you come up through the gearbox.
Because I use methanol fuel, I do not need to play with different taper needles, but if I used petrol I would. If your motor is that bit sluggish, you often do not detect it until you lean-off your mid-range jetting and you get more go.
 
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