Exhaust change from MK 111 stock , jetting.

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Ditching the stock crossover pipe bean can system for straight through down pipes and straight through peashooters. Any jetting tips ? I'm getting backfire pops on fully closed throttle decelerations.. I'll be watching this thread , stock MK 111 settings 1976 model at present.
 
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3 1/2 slide on Amal 32's. Needle on highest leanest position. 928 /104 needle jet , 260 main , Stay up floats new. No choke system , closed off at carb tops. Everything stock for 76 model including air filter. The new non crossover downtubes come in shortly from U.K. for the soon MK fitting. Again with crossover pipes and straight through reverse con mufflers she is popping on closed throttle off the gas. Adjusting idle mixture screw does not change that. Ideas ?
 

MichaelB

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My 850 runs better with needle in middle position.
I'm at sea level...
 

Fast Eddie

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3 1/2 slide on Amal 32's. Needle on highest leanest position. 928 /104 needle jet , 260 main , Stay up floats new. No choke system , closed off at carb tops. Everything stock for 76 model including air filter. The new non crossover downtubes come in shortly from U.K. for the soon MK fitting. Again with crossover pipes and straight through reverse con mufflers she is popping on closed throttle off the gas. Adjusting idle mixture screw does not change that. Ideas ?
104 needle jet ?
 
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I'm just reading what is in my old Haynes manual as to needle jet for the MK111 model. Chapter 4 Carburation. I agree it's likely 2 106 jets. A printing error likely. DHL just called so must pay tax and duty now.
 
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Its not the size of the needle jet but the part number for the needle which matches to the cutaway spray tube

NORTON COMMANDO 850 THROTTLE NEEDLE 4-GROOVE (1) 928/104
 
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MK 111 straight pipe exhaust headers landed in Toronto this morning from U.K. and message from DHL says it will be delivered today. $30 in Duty and taxes paid through them this morning. That's 3 days ! WOW !
 
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I had to go to 270 mains when I converted to single pipes with up-swept pea shooters on my 74.
But I am running a Megacycle cam which is the closest to stock. 560-NR
Here is what I have: RH-10 head, 270 mains, 106 needle jet, 4 groove needle @ mid position. cut away spray tube and 3.5 slides, which Chessel sleeved. Stay up floats and aluminum float needle.
Best to do some full throttle pulls up a hill and then back off the throttle slightly. What does the engine do? Plug chop and inspect spark plugs. Report back.
 
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I had to go to 270 mains when I converted to single pipes with up-swept pea shooters on my 74.
But I am running a Megacycle cam which is the closest to stock. 560-NR
Here is what I have: RH-10 head, 270 mains, 106 needle jet, 4 groove needle @ mid position. cut away spray tube and 3.5 slides, which Chessel sleeved. Stay up floats and aluminum float needle.
Best to do some full throttle pulls up a hill and then back off the throttle slightly. What does the engine do? Plug chop and inspect spark plugs. Report back.
Just unwrapped them ,better than x-mas. Looks like I'll be using the collet system not the crush ring in the exhaust port. I'll fit tomorrow and report back for sure. Less than 3 days to my door ,impressive in Covid times.
 
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Do they fit? and if so who supplied them? I will be looking for a Mk3 set shortly.
Thanks Al
 
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Popping usually indicates a leak in the exhaust pipe. You should know when your jetting is too lean because you get the cough when you ride the bike changing up through the gears. It will happen as you wind the throttle on. When you get it, raise the needles one notch and you should get good clean acceleration. If the cough persists keep raising the needles one notch at a time, until they reach their highest, then you might need the next size needle jets. You won't usually burn a piston unless you are too lean on the mains, so you need to have a look at the plugs after you have got the midrange right and start to use full throttle.
 
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maylar

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Popping usually indicates a leak in the exhaust pipe.
I totally agree with this. With a street bike you notice this on dropping throttle between shifts and on decel. I make it a habit to wiggle the end of each silencer after a ride while it's still hot. There should be no movement at the head or at the silencer/headpipe joints.
 
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Do they fit? and if so who supplied them? I will be looking for a Mk3 set shortly.
Thanks Al
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Stonepeg Limited , The Old Racing Stables , Nine Views , Cranborne BH21 5PW , U.K.
Trying the fit now.
 
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Just fitted the left primary side. The only side that matters. Used copper silicone to the collet nut -headpipe retention system. I wish they would just sell the primary side pipe which is the only one that really matters. The right is a no issue.
It fit perfectly. The pipe goes inboard of the primary and the oil drain protruding screw. Someone really thought about this. I was worried the sidestand would hit or snag up but it retracted clean and free.
Now onto the right side once the heat goes down a bit.
 
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Fitted the left side. Piece of cake. New everything. The nice thing about the later collet system in the exh. ports is a flexibility for system fitting.
Fired her up and all went well , retightened after hot. No Poppings at all but will go for a spin tomorrow , it's 7 P.M. and a couple of G and T 's with the wife means no test drive. But all good.
 
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Fitted the left side. Piece of cake. New everything. The nice thing about the later collet system in the exh. ports is a flexibility for system fitting.
Fired her up and all went well , retightened after hot. No Poppings at all but will go for a spin tomorrow , it's 7 P.M. and a couple of G and T 's with the wife means no test drive. But all good.
Right side was easy. As a side note: cracked the shitty plastic cap on my tube of Copper hi heat silicone. Used a 7/16 1 inch screw to insert into tube end , fit perfect , then duct tape over it as a final.
 
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Great fit. People showed up with booze to party and hear it run , which I did. No poppings. Will drive tomorrow. Over .08.
 

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