1977 850mk3 Pattern Blackcap Silencer Main Jet Size

Lovel

Lovel

Joined
Dec 28, 2021
Messages
70
Country flag
I currently have peashooters fitted to my 1977 850mk3 with standard twin Amal’s. The silencers are starting to get a bit picky and was considering a change to Blackcaps from A/N. The bike runs sweetly on its current set up and perhaps I’m a bit reluctant to change something that is so well set up just for cosmetic reasons. However I’m still inclined to look at a change. The A/N website states that the blackcaps are not manufactured as per original design and maybe freer flowing, hence wondering if any members are using the current design of A/N blackcaps and what size of main jet or needle position are they using? Performance doesn’t really bother me although something a little less noisy is what I want.
 
Lovel said:
I currently have peashooters fitted to my 1977 850mk3 with standard twin Amal’s. The silencers are starting to get a bit picky and was considering a change to Blackcaps from A/N. The bike runs sweetly on its current set up and perhaps I’m a bit reluctant to change something that is so well set up just for cosmetic reasons. However I’m still inclined to look at a change. The A/N website states that the blackcaps are not manufactured as per original design and maybe freer flowing, hence wondering if any members are using the current design of A/N blackcaps and what size of main jet or needle position are they using? Performance doesn’t really bother me although something a little less noisy is what I want.
Click to expand...
The MKIII carbs are the most confusing, but I try to sort it out in the notes at the bottom of this page: https://www.gregmarsh.com/MC/Norton/CommandoID.aspx Then click on the PACK/ACK number 113 where you will find more notes. I would start with those settings. Your current main jets could be as big as 260. If you don't usually go over 3/4 throttle, the main jet size means almost nothing.
 
Lovel said:
The bike runs sweetly on its current set up and perhaps I’m a bit reluctant to change something that is so well set up just for cosmetic reasons. However I’m still inclined to look at a change.
Click to expand...

The AN (seamed or seamless) peashooters "with sound deadening" are supposed to be quieter.
andover-norton.co.uk

Motorcycle parts and spares for Norton motorbikes - Genuine Andover Norton Spare Parts - Andover Norton Motorcycles Parts - Original Andover Norton Parts

Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Genuine Factory Spare Parts for Norton motorcycles built before 2002.
andover-norton.co.uk
andover-norton.co.uk

Motorcycle parts and spares for Norton motorbikes - Genuine Andover Norton Spare Parts - Andover Norton Motorcycles Parts - Original Andover Norton Parts

Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Genuine Factory Spare Parts for Norton motorcycles built before 2002.
andover-norton.co.uk
 
marshg246 said:
The MKIII carbs are the most confusing, but I try to sort it out in the notes at the bottom of this page: https://www.gregmarsh.com/MC/Norton/CommandoID.aspx Then click on the PACK/ACK number 113 where you will find more notes. I would start with those settings. Your current main jets could be as big as 260. If you don't usually go over 3/4 throttle, the main jet size means almost nothing.
Click to expand...
Oh I do go up to the redline plenty times, but not all the time. 260 main jet was quoted for the original blackcap with its restriction.
 
L.A.B. said:
The AN (seamed or seamless) peashooters "with sound deadening" are supposed to be quieter.
andover-norton.co.uk

Motorcycle parts and spares for Norton motorbikes - Genuine Andover Norton Spare Parts - Andover Norton Motorcycles Parts - Original Andover Norton Parts

Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Genuine Factory Spare Parts for Norton motorcycles built before 2002.
andover-norton.co.uk
andover-norton.co.uk

Motorcycle parts and spares for Norton motorbikes - Genuine Andover Norton Spare Parts - Andover Norton Motorcycles Parts - Original Andover Norton Parts

Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Genuine Factory Spare Parts for Norton motorcycles built before 2002.
andover-norton.co.uk
Click to expand...
I do like the look of the blackcaps for some strange reason rather than the peas. Plus the do look more original. Perhaps if A/N modified blackcaps are straight through I needn’t worry about changing the jet size from the 210 or 220 that I have currently fitted?
 
Lovel said:
Perhaps if A/N modified blackcaps are straight through I needn’t worry about changing the jet size from the 210 or 220 that I have currently fitted?
Click to expand...

They don't seem to be the fully straight-through type like the RGM "straight-through" black caps...
www.rgmnorton.co.uk

BLACK CAP SILENCER STRAIGHT THROUGH BAFFLE SYSTEM

W603A STRAIGHT THROUGH BAFFLE SYSTEM BLACK CAP SILENCER 064170M
www.rgmnorton.co.uk
www.accessnorton.com

Black cap bean can modification

Considering putting stock exhaust back on my Mk III. Is there any modifications I can do internally to get better performance but have original looks. Same with plastic air box
www.accessnorton.com www.accessnorton.com

... you would also need the black cap mounting plates (2 of each).

www.rgmnorton.co.uk

BLACK CAP MOUNTING PLATE POLISHED ST/ST MAIN BACK PLATE

064148S RGM NORTON
www.rgmnorton.co.uk
www.rgmnorton.co.uk

BLACK CAP MOUNTING PLATE POLISHED ST/ST SILENCER MOUNT

064150S RGM NORTON
www.rgmnorton.co.uk
www.rgmnorton.co.uk

BLACK CAP MOUNTING PLATE POLISHED ST/ST FOOTREST GUSSET

064149S RGM NORTON
www.rgmnorton.co.uk
 
Lovel said:
I do like the look of the blackcaps for some strange reason rather than the peas. Plus the do look more original. Perhaps if A/N modified blackcaps are straight through I needn’t worry about changing the jet size from the 210 or 220 that I have currently fitted?
Click to expand...
Main jet size is has to do with the head, exhaust pipes (crossover or not), silencers, altitude, air cleaner, and fuel. Your MKIII left the factory with 230 as best i can tell. 220 or 230 are probably small for peashooters, especially with E10 gas and non-crossover pipes (if you have them). You shouldn't spend time at more than 3/4 throttle and lean!
 
Ok my apologies but I got mixed up with my main jet sizes. To confirm currently I have 260 main jets with the peashooter type silencers. If the A/N modified blackcap silencers were fitted would it be ok to go to 230 main jet or would it require something in between 230 and 260?
 
Lovel said:
If the A/N modified blackcap silencers were fitted would it be ok to go to 230 main jet or would it require something in between 230 and 260?
Click to expand...

As the internals aren't the same as the originals then it could need richer main jets than the standard 230s.
Unless AN can offer any jetting recommendations then start with the 260s and reduce the size as necessary.
 
I am running straight through RGM black caps on one of mine, it is slightly less noisy than my other one with pea shooters (both are Mk 3s). The restricted black caps will hurt the bikes performance significantly , as I found out a few years ago when I wasted my money on some.
 
Lovel said:
Ok my apologies but I got mixed up with my main jet sizes. To confirm currently I have 260 main jets with the peashooter type silencers. If the A/N modified blackcap silencers were fitted would it be ok to go to 230 main jet or would it require something in between 230 and 260?
Click to expand...
I think you will end up tuning your jetting to suit the silencers and the fuel available these days
 
I have only tested original Black Cap silencers, not these modified aftermarket versions.
The originals worked best for me when 230 mainjets were fitted.
At that the power was reduced considerably from open Pea shooters and 260s.
The engine really struggled after 4000 rpm.
The original Black Caps are also very heavy.
It's good that both AN and RGM have come up with free flowing versions of the Black Caps. Perhaps they are lighter as well.

Glen
 
Earlier this year I fitted a pair of Armours 'Black caps' (although the caps are chromed) in the hopes they would reduce the noise level
and for a more original 'Mk3' look...:
1977 850mk3 Pattern Blackcap Silencer Main Jet Size

www.armoursltd.co.uk

Silencer Norton Blackcap Silencer - 650 cc, Exhausts, Exhausts | Norton - Armour Motor Products

Silencer Blackcap, inlet 1-3/8" Price is for one, order 2 for a pair Our HD triple plated silencers are made by Armours, using our own design corrosion resistant construction, typical internal design is all metal non-restrictive straight through construction with an exit main gas stream...
www.armoursltd.co.uk www.armoursltd.co.uk
(850 not "650" obviously)
1977 850mk3 Pattern Blackcap Silencer Main Jet Size


"Our HD triple plated silencers are made by Armours, using our own design corrosion resistant construction, typical internal design is all metal non-restrictive straight through construction with an exit main gas stream diffuser. This design gives a nice tone without the use of moisture retaining absorption. Baffle design will vary according to type."

... that roughly translates to the front two-thirds Correction third of the silencer being devoid of any baffling, however, they seem quieter than peashooters at higher speeds with no apparent loss of power and didn't require any jetting changes although my Mk3 only needed 230 main jets with peashooters (possibly due to the S&B air filters or flat-top spray tube carbs?).
 
You only use the main jests when you are on full blast with wide open throttle going down an open road. Under normal circumstances most of your riding is probably done using the needles and needle jets. With a Commando you are given two choices of main jets. I would use the largest one, then adjust the needle jet size and the taper on the needle. A two stroke is not much different from a four-stroke. If you have your mixture adjusted fine enough to give you best power, you usually need to feed the throttle on to get macimum acceleration. Bigger inlet ports create the need for more rapid needle taper to compensate for loss of vacuum as you open the throttle.
My bike is different but similar to yours. I am forced to feed the throttle on a bit slower, but when I do that the bike accelerates much faster than if I just whacked the throttle open. If I did the latter all that would probably happen , would be I would get a gasp.
 
Road bikes that require slow application of the throttle to avoid bogging are a real downer to ride, although some guys will put up with that.
That's the fun of a properly tuned Commando, open the throttle and wham, it hits.

Glen
 
A lot of people do not seem to understand that normal carburettors operate in three stages. Unless it is miles too small, main jet size has no effect until the needles are almost fully withdrawn from the needle jets. It is however possible to meter the fuel flow by the tip of the needle in the needle jet, when the main jet should be operating. The main jet is monitored by doing plug readings after balsts at full thottle. If you fit a larger main jet, and the plug does not change to show richer, you are metering off the tip of the needle.
 
acotrel said:
With a Commando you are given two choices of main jets.
Click to expand...

Two?
acotrel said:
I would use the largest one, then adjust the needle jet size and the taper on the needle.
Click to expand...

Or, use the standard needles and needle jets which should be perfectly adequate.
Could you please try not to think of everything in terms of track racing/riding, for the sake of (nearly) everyone's sanity here?
 

Similar threads

M
  • Locked
Early Fastback main jet size qu
Replies
12
Views
701
macca49
M
A
  • Locked
relationship between main jet sizes and their diameters
Replies
10
Views
1,086
jrh2020
jrh2020
ILLF8ED
  • Locked
What main jet size are you using in your Combat?
Replies
14
Views
1,228
ILLF8ED
ILLF8ED
A
  • Locked
Main jet size
Replies
4
Views
2,914
ILLF8ED
ILLF8ED
Back
Top