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Twins Technical Talk Technical Talk for Hinckley Triumph Twins: Bonneville, T100, Speedmaster, America, Thruxton, and Scrambler.

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Old 07-25-2008, 07:11 AM   #1 (permalink)
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Air intake mods...... Definitive results

As some of you here may know, a while back I fitted an Innovate A/F gauge on my Scrambler.
This now gives me real-time readout of the air:fuel mixture, as I make adjustments and as I ride.
After much tinkering I had the fueling just about spot-on for my then set-up, giving as close as possible to 13:1 to 13.7:1 A/F across the board.
This definately being the ratio where optimum performance is obtained. Anything below 12:1 and above 14.5:1 can certainly be "felt" by the rear-end dyno!
This set-up consisted of the usual initial mods we all carry out i.e. free'd up exhaust (in my case Zard 2:1), snorkle removed and Unifilter fitted.
Jetting for this set-up was 115's and 40's, no shims, and 1.5 turns.

There have been so many questions in various threads over the years as to exactly what effect the various mods we carry out actually have. With the A/F gauge I can now quantify these effects.

Firstly I refitted the snorkle. The imediate result was to run WAY RICH (11.5:1) which shows how much the snorkle actually does restrict air flow. A rejet would certainly be required in this case if I was to leave it that way.

Next I refitted the stock air filter. This made NO difference to the readings, either with or without the snorkle. This shows that the Unifilter may be a more efficient type of filter, but it does not improve air flow to the engine.

I then returned to my normal set-up (Unifilter and no snorkle) and then pulled out the air-box restrictor plate.
This certainly caused the biggest effect, this time running WAY LEAN (15.5:1), and I lost a great deal of performance. This plainly shows how much restricting the restrictor is actually doing!
I've now left the restrictor out and had to step-up one main jet size and fit a shim to compensate.

This set-up, with no restrictor, no snorkle, Unifilter, 120's, 40's, 1 shim and 1.0 turn has now resulted in much improved performance and sweet throttle response across the entire rev/throttle range.

It's also worthy of mention that I found very little difference in readings between my original NH Off Road mufflers and the Zard 2:1's, which would indicate that there isn't a great deal between types of exhausts. If they have no baffle then they are very similar.

The above info may help in deciding which air intake mods to carry out, and what effect they will have on fueling, and the adjusments required.

Others opinions and experiences may differ for all this but, as I see it anyway, being able to actually see as well as feel the differences certainly makes for a more accurate means of tuning.


V.
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Old 07-25-2008, 07:28 AM   #2 (permalink)
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These are good measurements Ventura, and nicely presented. clearly shows how useful the A/F guage is. Reconfiguring finances now.............

Can you quantify / estimate what the effect of removal of the snorkel does? Let me be more specific. Right now I am running at a ratio of 12 pretty much across the whole rev range. If I pull the snorkel, this will lean the mixture. Any idea how far? Might this just take me the magic 13 and a bit?

EDIT: Yes you can quantify it, it's right there in your post.

Duh. Sounds like I should give it a go.
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Old 07-25-2008, 12:42 PM   #3 (permalink)
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Hi again propforward, you should certainly give it a go!

Without a doubt removing the snorkle will lean up your mixture.
And will take it slightly above the 13 which we strive for.
The only restrictive factor will then be the airbox restrictor plate which you should also consider.

This really is a simple job, especially if you remove the battery and the four air-box securing bolts. This allows you to move the air-box around by just enough to give access to the elusive three screws which would give a problem otherwise.

If both are removed then a step-up in jets will be required, and you'll certainly notice the difference!

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Old 07-25-2008, 12:55 PM   #4 (permalink)
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Regarding the restrictor plate removal:
There are some comments in some threads about adverse effects when removing the restrictor plate, i.e, poor mid-range, uneven idle, etc. Did you notice anything at all?
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Old 07-25-2008, 01:40 PM   #5 (permalink)
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I haven't experienced any adverse effects in removing the restrictor, nor have any been apparent on the A/F readings.
The mid-range A/F was slightly effected, which the shimming sorted, the lean running at top-end was sorted with the increased main jetting. Performance was certainly improved across the board.

The idling is spot-on and totally unaffected by the restrictor plate removal. I've found in fact that the idle mixture adjustments, when they are correct, results in very stable tick-over and also the fact that I no longer need the choke (enricher) when starting from cold.

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Quote:
Originally Posted by Pokeyjoe View Post
Regarding the restrictor plate removal:
There are some comments in some threads about adverse effects when removing the restrictor plate, i.e, poor mid-range, uneven idle, etc. Did you notice anything at all?
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Old 07-25-2008, 01:53 PM   #6 (permalink)
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Quote:
Originally Posted by Ventura View Post
I haven't experienced any adverse effects in removing the restrictor, nor have any been apparent on the A/F readings.
V.
Thanks. (My next mod).
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Old 07-25-2008, 02:13 PM   #7 (permalink)
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Fantastic post. As a scientist, I love to see quantification and I think it highlights a lot of what butt dynos tell us. Airbox is the biggest change, any free flow exhaust is good (I too have the Zard 2:1 and it's lighter too, so double good).

I assume you put the exhaust bung after the 2 pipes join together ? To what extent does this make tuning each carb possible ? Or did you just make equal turns on each carb ?
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Old 07-25-2008, 02:28 PM   #8 (permalink)
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Quote:
Originally Posted by NorthernThrux View Post
.............I assume you put the exhaust bung after the 2 pipes join together ? To what extent does this make tuning each carb possible ? Or did you just make equal turns on each carb ?
Thanx NorthernThrux for your comments.
I installed the exhaust sensor here:-



This obviously doesn't make it possible to easily obtain separate carb idle adjustment readings as it is the sum of the result which is displayed, but IMHO this would be negligible in the grand scheme of things, especially with the airbox restrictor removed, which must balance out the air intake to the individual carbs very well.

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Last edited by Ventura; 07-25-2008 at 02:32 PM.
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Old 07-25-2008, 02:32 PM   #9 (permalink)
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On an inline twin like ours I think it's not too bad, but on a V-twin the settings are definitely unique for each cylinder. I do find that there are subtle differences even on our bikes, but likely no more than 1/8 of a turn difference, so not worth worrying about I suppose.

I think I gotta get me that setup. Looks sweet.
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Old 07-26-2008, 01:52 AM   #10 (permalink)
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Great results... looks like a sound investment for tuning. Any plans on an air box removal kit?

on a side note, have you noticed a discernible change in MPG?
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