Joined
·
1,687 Posts
Just when the airbox thought it was safe from any further prodding, poking,cutting and grinding!!
Part 2 of the airbox opening-up to give a double-entry filter was covered excellently by JohnyC here:
http://www.triumphrat.net/twins-technical-talk/97291-scrambler-airbox-mod-part2.html
The following thread and advice by Twinfan, plus the additional input from Gob-ny-geay, set the ball rolling
as to where to proceed next in improving the stock airbox, but without actually removing it.
http://www.triumphrat.net/twins-technical-talk/103270-velocity-stacks-in-the-airbox.html
So, a few clicks on the PC brought two sets of these to the door........
Mikuni RS velocity stacks which can connect directly to the 55mm O/D of the stock CVK carb inlets.
I bought the 70mm and 50mm length stacks so a little “playing around” can be done.
Now we all know what the original airbox/carb rubbers look like, with their internal step and alterations
in shape............
After removing the side of the airbox, (which can be done fairly easy by just removing the battery and the 4
small securing bolts in the battery box, plus the two mudguard torx screws (adjacent to the igniter) to loosen
the box enough to get at the difficult screws) and pulling out the original rubbers, the 50mm stacks slotted in
like this....
You can see I had to grind off a portion of the circumference of the bellmouth to allow it to sit next to the
central bulkhead of the airbox.
A little Dremmeling of the oval opening was required and a section of the supplied sealing rubber was cut
and siliconed into place...............
The airbox was then all reassembled and the double-entry Unifilter replaced.
Outwardly the airbox looks no different from original, other than the connection to the carbs are now shiny aluminium!!
I decided for now to just fit these as a direct replacement to the rubbers, without playing with the plenum
length by adding to or removing the rubber spacers (Insulators, as Triumph call them) at the cylinder head
entry. I can save that for another day!!
And to the results.....
Before this mod I was running 135 main jets with the airbox modded to allow double-entry.
Filter cover opened and restrictor plate long gone. Unifilter opened at both ends.
These 135's had the A/F reading spot on and good results on the dyno.
http://www.triumphrat.net/twins-technical-talk/103145-modded-scram-dyno-results.html
I decided to test run the bike first, without altering jetting, to see what difference the stacks made to A/F ratio.
Testing showed I was now getting an amazingly lean reading of over 16:1 at everything over 3000 revs!!
In other words I’d dramatically increased the airflow to the engine, by the biggest marginal increment of all the different freeing-up mods so far!!
I have now had to step up to 145 jets to regain A/F parity of 13:1, which equates to the same size needed when I ran the bike, while experimenting, with no airbox and NO filters!
The assumption is that an ARK’d Scrambler would require 140 jets. And I’ve now surpassed that!!
(maybe some ARK'd Scrambler owners could confirm?)
I’ll leave you all to draw your conclusions on that, and invite the comments which will follow.
Next step is obviously the dyno to compare the results and get some comparisons.
Testing the bike by SOTP would certainly indicate performance gains across the board.
Also I'll try to play around with differing plenum and stack lengths.
On that I’ll keep you posted!
A big up to Twinfan for his guidance and suggestions. Those guys in Germany are so knowledgable in the stealth-mode modifying of the engine (due to official restrictions imposed on them). It would be great if they could impart some of their vast knowledge on us here.
Cheers also to Gob-ny-geay for the very detailed and informative description of his version of the above.
Dyno here we come!!...................
V.
Part 2 of the airbox opening-up to give a double-entry filter was covered excellently by JohnyC here:
http://www.triumphrat.net/twins-technical-talk/97291-scrambler-airbox-mod-part2.html
The following thread and advice by Twinfan, plus the additional input from Gob-ny-geay, set the ball rolling
as to where to proceed next in improving the stock airbox, but without actually removing it.
http://www.triumphrat.net/twins-technical-talk/103270-velocity-stacks-in-the-airbox.html
So, a few clicks on the PC brought two sets of these to the door........

Mikuni RS velocity stacks which can connect directly to the 55mm O/D of the stock CVK carb inlets.
I bought the 70mm and 50mm length stacks so a little “playing around” can be done.
Now we all know what the original airbox/carb rubbers look like, with their internal step and alterations
in shape............

After removing the side of the airbox, (which can be done fairly easy by just removing the battery and the 4
small securing bolts in the battery box, plus the two mudguard torx screws (adjacent to the igniter) to loosen
the box enough to get at the difficult screws) and pulling out the original rubbers, the 50mm stacks slotted in
like this....

You can see I had to grind off a portion of the circumference of the bellmouth to allow it to sit next to the
central bulkhead of the airbox.
A little Dremmeling of the oval opening was required and a section of the supplied sealing rubber was cut
and siliconed into place...............

The airbox was then all reassembled and the double-entry Unifilter replaced.
Outwardly the airbox looks no different from original, other than the connection to the carbs are now shiny aluminium!!
I decided for now to just fit these as a direct replacement to the rubbers, without playing with the plenum
length by adding to or removing the rubber spacers (Insulators, as Triumph call them) at the cylinder head
entry. I can save that for another day!!
And to the results.....
Before this mod I was running 135 main jets with the airbox modded to allow double-entry.
Filter cover opened and restrictor plate long gone. Unifilter opened at both ends.
These 135's had the A/F reading spot on and good results on the dyno.
http://www.triumphrat.net/twins-technical-talk/103145-modded-scram-dyno-results.html
I decided to test run the bike first, without altering jetting, to see what difference the stacks made to A/F ratio.
Testing showed I was now getting an amazingly lean reading of over 16:1 at everything over 3000 revs!!
In other words I’d dramatically increased the airflow to the engine, by the biggest marginal increment of all the different freeing-up mods so far!!
I have now had to step up to 145 jets to regain A/F parity of 13:1, which equates to the same size needed when I ran the bike, while experimenting, with no airbox and NO filters!
The assumption is that an ARK’d Scrambler would require 140 jets. And I’ve now surpassed that!!
(maybe some ARK'd Scrambler owners could confirm?)
I’ll leave you all to draw your conclusions on that, and invite the comments which will follow.
Next step is obviously the dyno to compare the results and get some comparisons.
Testing the bike by SOTP would certainly indicate performance gains across the board.
Also I'll try to play around with differing plenum and stack lengths.
On that I’ll keep you posted!
A big up to Twinfan for his guidance and suggestions. Those guys in Germany are so knowledgable in the stealth-mode modifying of the engine (due to official restrictions imposed on them). It would be great if they could impart some of their vast knowledge on us here.
Cheers also to Gob-ny-geay for the very detailed and informative description of his version of the above.
Dyno here we come!!...................
V.