I have a MK3 850 with a single Mikuni and Bean Can mufflers. Does any one know what size main jet should be fitted for this set up. Also I am looking to fit pea shooter mufflers with my balance pipe headers, I assume I am going to need a new main jet and if so what size is recommended. Any ideas????
What size main jet.
- Thread starter nnnrh
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nnnrh said:
You might want to fit the mufflers then "tune".... If your not racing and just doing normal stuff, the plugs will tell you what you need... Thats all assuming the the Mikuni is up and running now and you may find it not necessary for a jet change.
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olChris said:
The plugs can't talk, you'll be at it forever using this technique, if you don't burn out the pistons in the meantime. I'd contact either or both below for a sensible starting point.
Tech@allensperformance.co.uk
sales@motocarb.co.uk
Phil
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If you can tell us what the size of the mikuni is and what your current pilot/slide/needle and jet sizes in it are are it would help immensely
Needle Jet 159-P2 although I have run the Q2. Start with the P2.
Jet Needle 6DH3, this seems to be popular across the board,
Main Jets are really relative to your preference, 240 to 280 and beyond. I have run up to 320 for better top end but 280 was my norm. 240 for economy.
Pilot jet, again, relative to your machine's needs 30, 35, or 40.
These carb are so adjustable but when right are solid. The way Nortons are, a fresh motor may have to be jetted differently than a motor with 10 thou on it. Find out what works best for you and save your spares.
Jet Needle 6DH3, this seems to be popular across the board,
Main Jets are really relative to your preference, 240 to 280 and beyond. I have run up to 320 for better top end but 280 was my norm. 240 for economy.
Pilot jet, again, relative to your machine's needs 30, 35, or 40.
These carb are so adjustable but when right are solid. The way Nortons are, a fresh motor may have to be jetted differently than a motor with 10 thou on it. Find out what works best for you and save your spares.
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pete.v said:
pete
When nnnrh swaps to pea shooters, being far less restrictive than the bean cans, should he go up in main jet size or down? I have the same consideration but with twin Amals fitted to the second MkIII. These will be replaced with twin FCR's from Matt at CNW when his next batch arrive in. But I'll run the Amals in the mean time (not much choice but to). My understanding is, derestricting mufflers should be matched with fitting larger main jets. I had a mate many years ago who went the other way after fitting short (and very loud) megaphones to his Bonneville. It wasn't very loud for very long. It was running beautifully then suddenly stopped dead and had no compression. We discovered it had no pistons either. Peering into the spark plug holes with a torch, there was a neat hole in each piston like someone had burned through it with a cigarette. But geez it was running beautifully until it's sudden tragic end. The bike shop had told him to go down in main jet size when it should have been up. I will never forget seeing those holes in those pistons, we were in disbelief and in the middle of no where.
Phil
Forget the GD main jets. They don't mean shit till you are @ WOT. Getting smooth running through the range is the trick. You can almost run any or no main jet at all.phil yates said:
As mentioned, I ran any main jet I felt comfortable with, a bigger one for hitting the upper range and a smaller one for economy. But if you are hitting the throttle hard then you will want to throttle chop it to see what your bike wants. Otherwise go for something in the middle and don't be so concerned with the GD mains.
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pete.v said:
Well my mate and I were hitting the throttle (not bottle) hard.
His Bonneville "throttle chopped" itself.
It wanted new pistons.
The only thing he'd changed were his mufflers and smaller main jets. I imagine the serious pinging would have been drowned out by the very loud megaphones, then suddenly there was nothing but the birds chirping in the trees.
I shall explore the matter further and let you know what I discover.
Phil
ps what is WOT?
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