Carbs, does 1 + 1 = 2?

[Overlord Neil]

Been umming and ahhing over buying carbs for trixxie, scouring eBay for that illusive pair of 'cheap' FCR's... loads of dirt bike solo carbs, plenty of in-line 4's. I read that a bank of four has a slight problem of only one accelerator pump so the 2 surplus are basically expensive spare parts, but then i got to thinking, why not 2 solo carbs off a dirt bike? KTM's used FCR39's as OEM kit, why not get two and run them in parallel? There might be a bit of fudgery to get the throttle linkage working on one set of cables, but nothing that can't be fixed with duct tape and cable ties.

Any thoughts, someone must have thought of it before and theres probably a really good reason not too.... anyone?

[bagmaker]

G'Day Neil,
I pulled down a set of 39s from a bank of 4, wasn't a big deal, linkage mods, spacing correctness.
Cannot see any reason that 2 singles wont work, are they cheaper though??

[cobbadiggabuddyblooo]

Just be mindful of the needles and jets used..
you need OCEMP Jet needles and pilot jets are 48.

They are a little rich with the 48 pilot jets but this helps with cold starts and unlike the CV carbies, there is no air cut off valve so it helps when off throttle on higher revs to richen the mix just like what the air cut out valve does.

2.0 float needle and matching seat and float height is 9mm,
150 main fuel and 200 main air jet come std.
33mm inlet spigots

You'll have to set up a 2-1 cable set up on both push and pull throttle cables and look at your TPS set up and branch your fuel feed.
Acc pump will need attention too but some single fcr's have a leak jet fitted so you can tune it that way.
If no adjustable leak jet set up (like on the std TRX FCR ) you can use valve shims in the acc pump body to deal with duration and volume.
Look at an o2 sensor set up too.
For $250 you can set up your TRX with a wide band O2 sensor and you can easily tune things that way.
You'll pay $250+ in dyno time if you give it to someone to do the job plus it will help when it comes to tuning your ignition or any other modifications you may choose to add at a later date.

[Overlord Neil]

Good responses (as always), after a lot more reading (quiet day at work) I decided to stick with the standard CV carbs but show them a little love and splashed out on a couple of Factory Pro goodies as well a Litetek seal kit. I know I will probably eventually splash the cash and get some flatsiders but I need to sort the handling out first and get the R6 front end fitted, which will require some fettling.

I did toy with the idea of getting a couple of these:

https://www.aliexpress.com/store/produc ... 53171.html

(Chinese 'genuine' Keihin PWK 40)

I know they are knock offs but at £80 each delivered they are cheaper than the jet kit from the US. If I didn't rely on my bike for daily transport I'd be very tempted to have a go.

[cobbadiggabuddyblooo]

You really do want a PUSH/Pull style throttle cable set up Neil. The amount of intake vacuum will cause issues. I regularly had problems of the slides sticking open on my old T140 Bonneville with the vacuum locking the slides on the Mk11 Amals. AAAHH those where the days....

Good idea sorting suspension first .
Again 1 reason why I enjoy riding Rods bog stock standard TRX. (apart from the 0.9 springs and emulators in the forks/ rebuilt shock)
Suspension and fueling sorted it's a gem to ride..
It was only once things got over 6000rpm the FCR's would come into their own on a my own TRX when it had a std motor and you start to make good gains.
Your drive train and tyres will love you for it too.

[WooHoo850]

Hi guys I'd love FCR's too but for price ect.. I've looked at alternatives like another fella here a while back was curious about 44ida webber copies (I hope it worked for him). Genuine 48's from a rotary would take some fettling to work. Years ago I had a 38/38 Webber from an XF Falcon and a turbo from a Suzuki cappuccino then into the mix went an EA Falcon centerpoint EFI throttle body also 38/38. The project was shelved and bike sold but what has my attention now is the old 350 Holley still a bit small at 38/38 but the 500 Holley measures just shy of 41/41 like an FCR but the thing that is really cool is that speedway freaks love these carbs and race prepped ones can be had for under $500 bucks and plenty are set up for force feed (those suzi turbo's are a good fit for the TRIXY and hot up bits are all over Japland).
Any other left field carbs /EFI worth a look ?

[Overlord Neil]

All up for left field ideas. Just had a quick look at the 500 Holley, everyone knows Holley but they are a 'Yank' thing we don't see that much of in UK, they are butterfly carbs, isn't the whole point of flat sides that the bore is clear at full throttle; less restriction = more intake?

Loving where you are going though

[dicky]

All up for left field ideas. Just had a quick look at the 500 Holley, everyone knows Holley but they are a 'Yank' thing we don't see that much of in UK, they are butterfly carbs, isn't the whole point of flat sides that the bore is clear at full throttle; less restriction = more intake?

Loving where you are going though

Almost.
The point is that the restrictor in the throttle body, whether it be a flat slide, round slide or butterfly valve, is directly actuated by the throttle cable rather than by a vacuum diaphragm as in a CV carby. There may be less restriction in a slide vs butterfly but this is only a minor point.

[Overlord Neil]

Mikni HSR42's?

They seem to be sold almost exclusively as a Harley performance upgrade on big carb between 2 cylinders, but I also found this:

http://triumphbonneville.org/diy/mikuni ... 5-upgrade/

Its a massively modified (new generation) Triumph Bonneville 865cc paralel twin, started off with OEM Keihin CVK36's, then put on the Mikuni HSR42's then it all went a bit silly and engine capacity suddenly jumped to 1100cc. Some guys even turbo charge them...

You can pick up the carbs for as little as £2-250 new (each) so its not a massive saving but its a cheaper option. This is turning more into an carb alternatives thread, I'm just exploring options.

[dandywarhol]

Mikni HSR42's?

They seem to be sold almost exclusively as a Harley performance upgrade on big carb between 2 cylinders, but I also found this:

http://triumphbonneville.org/diy/mikuni ... 5-upgrade/

Its a massively modified (new generation) Triumph Bonneville 865cc paralel twin, started off with OEM Keihin CVK36's, then put on the Mikuni HSR42's then it all went a bit silly and engine capacity suddenly jumped to 1100cc. Some guys even turbo charge them...

You can pick up the carbs for as little as £2-250 new (each) so its not a massive saving but its a cheaper option. This is turning more into an carb alternatives thread, I'm just exploring options.

These are side draught whereas the TRX are downdraught.......................

[WooHoo850]

Ok Overlord Neil
"We are through the looking glass gentlemen" if you want truly Left field heretis': rotec TBI 40 (throttle body injection) it's made in Austalia for aircraft its kinda like an FCR and a CV's lovechild. They have a remote fuel regulator that is the brains of the unit it works similar to the breathing regulator in a SCUBA set up with a flat slide, the mixture can be adjusted on the fly(bad pun, sorry) and can support 100's of horsepower. It is designed for static rev's but they have been installed on Harleys and cars(not motorbikes just Harleys). The main drawback is they can't support ram air or force induction (It messes with fuel metering, I think there is a way around this but they take falling out of the sky very seriously)(the only downside I see is the impact)
http://www.avidfoxflyers.com/uploads/mo ... _thumb.jpg How good would this be on a TRX not sure of price, but its aviation the price may be scary.
As for downdraught on a TRX the intake is near enough to 45deg so split the diffrence. A cool experiment install the side draught of a pit bike downdraught and pissoff the manifold. They run great so long as the front tyre doesn't touch the ground the added bonus is extended front tyre life.

Are we having fun yet?

[WooHoo850]

Oh yeah PPS....
The butterfly vs flatslide is a moot point on the street. FCR light, awesome, $$$, short ram at WOT If your chasing 10th"s this is what you want, but for me I'll mod the butterfly axle and spend the difference on rider training(single most important performance mod), fresh rubber, regular maintenance.
Back on the Holley, the speedway blokes grind the butterfly axle including the retaining screws AAAGGGHHH! we tried this on a dyno and does yield 1 point 'ish but you can suck mettle into your intake but not for me thanks so we played around with this trick that yielded 1-2 points: remove 1 screw and tap the butterfly drill out the axle thread and install a larger screw. die-grind/ dremel some material from the axle behind the original screw and blend into the new screw head which is now opposite the original. The theory was that the yin and yang shape creates a low-high/high-low inducing a swirl to counteract the turbulence the butterfly creates. A lot of work for small yield but we worked for a bloke who said "I'll pay 10grand for 1 10th"( It is easier to replace the screws with grub screws)
You did well to follow all that and indulge my trip back down memory lane.
https://rspimages.holley.com/4412bk_xp.jpg?width=583 NASCAR carb on TRIXY for $500 yankybux WooHoo....