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Intake mods 2017: bits and the bite

Old 04-28-2017, 03:07 PM
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Intake mods 2017: bits and the bite

As many of you know, my VTR has been, and remains, somewhat of a mobile laboratory for my modding (ie. OCMD) ideas. Part of this process has led me down the road of some rather extensive mods on the intake side of the engine (including, among others, filter in lid mod, gutted and enlarged airbox, HPower-replica velocity stacks, custom X-wing intake diffusers, streamlined butterflies, HDJ needle jets, HRC needles, revised lift holes, Flo-Commanders on main and pilot circuits,....). The results were great, but alas, my curiosity pushed me yet again. So, for 2017, a few more mods aimed at improving airflow further:

- replaced screws holding carb butterflies in place with lower profile units (which were ground down as much as possible)
- built a new airbox lid (ie. "Filter-in-lid design) with a larger filter element (area is up 40% over previous version) which also increased volume slightly and increased clearance over the stacks
- raised the tank a bit to improve airflow into filter/airbox/engine
- matched carb throats/intake collars/intake ports (minor dremmeling to latter two to provide a smooth and uniform channel for the air)
- put thermal barrier on underside of airbox and carburetors (cooler airbox and carbs mean cooler air, better air density for improved power)

To improve response, I drilled a small additional lift hole in each slide (was running single hole before, just added 18% more area)

Having taken a couple rides for the purpose of tuning, I had to richen the pilot circuit (easily achieved by turning F-C screw) and go up 3 sizes on the mains (from 155/158 to 162/165) to give the bike enough fuel.

The end result? A noticeable increase in intake noise (not a bad thing) and some new-found response and power. The bike pulls better everywhere (response is sharper, the pull more authoritative) and dropping down a couple gears at cruising speed and rolling on the gas now has the bike surging ahead with more fervour (had the front come up on me!)

A true win-win (win for a successful experiment, and win for more power on the bike). Another success day in Mikstr's OCMD-land
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Old 05-04-2017, 03:14 PM
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i was just reading through that and seen your main jet sizes? do you guys get a different carb setup from us brits?. im running 185/190 in mine.
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Old 05-04-2017, 04:00 PM
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nope, standard Keihin jets... (ran Dynojet for the longest time but am back to Keihin as of last year). My jet size requirements (main and pilot) went way down when I switched to the filter-in-lid mod (had to lean the needle positions significantly too). Only explanation I can come up with is that it flows so much more air, that it greatly enhances the vacuum pulling fuel through the jets. Bike runs great and is definitely not lacking in power... I should mention that I am running with the Flo-Commander screw (which controls air to the main air jet) almost closed, which has the effect of greatly enriching the mixture (would expect to go up another 2-3 sizes with the F-C to get it running "normal"). I may go up in jet sizes to allow the F-C screw to be eased off at some point, but am ok with it for now...
Also worthy of mention is the use of Factory Pro HDJ needle jets (along with custom X-wing diffusers, HPower-replica velocity stacks, streamlined butterflies, gutted and expanded airbox, Mori Stage 1 exh. cams in intake position, ported heads, HRC needles, and much more...) Pretty hard to compare apples to apples when mine isn't exactly an apple anymore...

FWIW, I am running 38 pilots too and the needles are a half a clip position (obtained via washers) from being as lean as possible

EDIT: re-thinking this entire matter, I am wondering why I am having to go to richer jets if the airflow is better with the new filter. Is it possible that the larger area is slowing down intake velocity? Is there such a thing as too much intake area? Hmmmmmmmm I will re-try the previous FIL (with Triumph Explorer 1200 filter element) and see what happens to the jetting... This will also allow me to isolate the effects of the non-filter changes I made (ie. matching intake collar and port as well as streamlined butterfly screws)

The fun never ends in OCMD land... lol

Last edited by mikstr; 05-05-2017 at 08:53 AM.
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Old 01-11-2018, 02:33 PM
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Originally Posted by mikstr View Post
EDIT: re-thinking this entire matter, I am wondering why I am having to go to richer jets if the airflow is better with the new filter. Is it possible that the larger area is slowing down intake velocity? Is there such a thing as too much intake area? Hmmmmmmmm I will re-try the previous FIL (with Triumph Explorer 1200 filter element) and see what happens to the jetting... This will also allow me to isolate the effects of the non-filter changes I made (ie. matching intake collar and port as well as streamlined butterfly screws)

The fun never ends in OCMD land... lol
Late to the dance but the answer to this question is that you have finally achieved 100% flow through the jets you have, so now you have to finally start going larger with the jets.
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Old 01-11-2018, 05:28 PM
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Originally Posted by 8541Hawk View Post
Late to the dance but the answer to this question is that you have finally achieved 100% flow through the jets you have, so now you have to finally start going larger with the jets.
so you're saying I turned my bike into a super sucker? lol

BTW, I modded the (latest) airbox lid slightly a few weeks back, creating slightly more headspace over the velocity stacks (the goal being to create 6-7 cm of clearance, this based on the findings of a chap who did some development work on Porsche race engines and found this area to be critical to airflow). This also bumped volume ever so slightly, which shouldn't hurt either.

I won't learn how it works out until the spring as the bike is in hibernation at the moment... (snowmobile season)

Last edited by mikstr; 01-11-2018 at 05:37 PM.
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