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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 07-18-2008, 05:17 AM   #1 (permalink)
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HSR42 installed on stock thruxton (photo)

Hi everybody,

I have just installed a pair of HRS42 (from Bonneville Performance of course) on a stock thruxton (no big bore, just pipes and K&N pod filters).
Many of you have installed those big boys on 904 engines or even more. I never dynoed my bike but I am quite familiar with carburetors tuning. I will show you some photos and I am open to discuss about tuning and jetting with those who have Mikuni on a stock motor.

Bonneville Performance configuration:

pilots 22.5
air screws: 3/4 turns from closed
needles 97
needle e-clip: middle position (out of 5)
main jets 145

Modifications:

The strongest idle (giving roughly the maximum rpm at idle) is reached with the air screws set at 1 turn out from closed (leaner than 3/4 of turns). However the bike pops a lot on decel with 1 turn out. So I came back to 3/4 of turns to reduce popping. Probably I can try to switch from 97 to 96 needle (richer) to lean out the air screws without getting to much popping on decel. I have still to try 96 needle.

I moved the e-clip from middle position to 2nd from top (leaner). I now want to try the 4th from top (richer) to get some sensitivity on needle height and motor response.

Tried the "roll-off" method and the 145 main jets seem right. I have got 147.5 and asked Bill Gately to ship 142.5 and 140 mains to have some different jets to try.

What I firstly noticed is more torque down low thanks to which I reduced shifting in the traffic (expecially from 2nd to 1st gear). With stock carbs, no airbox and open pipes (mototwin exhausts and swept-back headers) I noticed a slight reduction of torque below 3000 rpm. This loss is totally vanished (butt-dyno) with HSR.
I think that acceleration from down low to 4000-4500 rpm is perfect and strong. I feel that final acceleration to top-end (from 4500 to 7000) can be improved. That's why I want to play with needle height.

I can do nothing but confirm that those Miks are really "torque-makers"!

As a final comment these carbs seems quite easy to tune and have few systems to deal with:

- you've got a pilot jet and an air screw for idle to 1/8 throttle operation.

- four needle diameters (95 to 98) for 1/8 to 1/4 throttle operation (apart from the initial diameter the four needles are completely identical with each other).

- 5 needle positions for 1/4 to 3/4 throttle operation.

- main jets for 3/4 to WOT throttle operation (main jet easily replacable without removing the float bowl).

What??

Oh yeah, I forgot the photos...
Attached Thumbnails
HSR42 installed on stock thruxton (photo)-mik1.jpg   HSR42 installed on stock thruxton (photo)-mik2.jpg  
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Old 07-18-2008, 06:19 AM   #2 (permalink)
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Lets start a fight

My bike 904 with Mototwin pipes and K+N filters.

I'm fortunate in having both the 42mm Mikuni's and the 39mm FCR keihin carbs I have recently tried them back to back. I completely agree with your comments the Mikuni's are torque monsters and dead easy to adjust. (Fuel injectors disabled)
The FCR's are deceiving, with the fuel injection they let you believe they pull hard (which they do) but the Mikunis are head and shoulders stronger in the torque producing stakes.

I am using the FCR's at the moment. As I am trying to build up a new large bore motor and will be fitting the Mikunis to that

Both carbs are Brilliant and a fantastic step up from standard.

If I had to buy new carbs I would definatly go for the Mikuni's.

PS- I do not have any affiliation to any bike shop and source my parts both standard and performance according to what I am doing


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Old 07-18-2008, 08:22 AM   #3 (permalink)
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Quote:
Originally Posted by Phil-1100 View Post
My bike 904 with Mototwin pipes and K+N filters.

(Fuel injectors disabled)


Do you mean accelerator pumps or do you also have FI ??
Yes the FCRs are harder to dial in. I'm still playing. I think the best way to set the idle is somewhere to the lean side of where the idle rpms start to drop from being overly rich and to the rich side of maximum rpm. That is 4 or 5 turns of the fuel screw so lots to explore. I would assume setting to the lean side is optimal as the accelerator pump richens it up when you twist the throttle.

As you can see in the pic I made some Fuel Screws to make this easy.

The best way to set the air screws is a bit different. You actually want to pick 1/8 throttle for this (I just set my idle to 1850 rpm). The air screws determine how rich or lean the pilot/idle circuit gets as you get up to 1/8 throttle, while the fuel screws sets it for idle. Turn the air screw all the way in and the bike should stall (rich...no air mixing in). Turn them out 3 or 4 turns and the bike should have a lean hesitation at 0-1/8. I've been setting the air screw for max rpm right now.

Other suggestions ?
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Last edited by NorthernThrux; 01-27-2011 at 08:38 PM.
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Old 07-18-2008, 09:05 AM   #4 (permalink)
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Yes "Accelerator pumps"

FCR's are harder to tune, read this maybe it will help you sort them out better.
http://www.factorypro.com/tech/carbkei.html
http://www.factorypro.com/tech/carbkei.html
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Old 07-18-2008, 09:12 AM   #5 (permalink)
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Talking

Quote:
Originally Posted by Phil-1100 View Post
My bike 904 with Mototwin pipes and K+N filters.

I'm fortunate in having both the 42mm Mikuni's and the 39mm FCR keihin carbs I have recently tried them back to back.
Hi Phil,

hey, you are one of the fews with both sets of carbs!!!

Even if you have a 904 engine can I ask you your Mikuni jetting (air screw, needle clip, mains)?

When I installed the Miks I noticed that the slides were opening more than needed (actually they were raising above the throat height) so the last part of throttle rotation was useless. So I had to adjust the throttle stop screws on both carbs to reduce slide opening.

I also noticed than it's very important during installation trying not to put any side loads (up-down or front-rear) on the throttle shaft otherwise the throttle action will become stronger and it may also happen that the throttle is not closing all the way in (because the return spring force is reduced when the throttle shaft is almost closed). I am using a single-pull throttle cable.

I also fitted an extra fuel filter because the stock CVK carbs actually had an extra fuel filter in the black plastic orizontal tube connecting the two carbs.

Needle removal is a little bit annoying...also because Mikuni are much closer to the tank if compared to stockers.

On the Mikuni tuning manual it is said that accelerator pump must be tuned to give strong acceleration when suddenly opening the throttle from 1/2 to WOT. Actually I am not sure if I can open suddenly the throttle from 1/2 without hesitation ("bogging") so I ordered the smallest size A/P nozzles (50) to try to play a little bit with the accelerator pump. However I am expecting an overly rich condition since nobody is using the A/P not even with a 904...but hey! You've got to try to be sure! The ideal situation is to set the A/P in order to have it non-working when smoothly opening the throttle and to squirt some fuel when throttle is suddenly opened. Once the smallest nozzle is fitted i can only play with start and finish position of the A/P.
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Old 07-18-2008, 09:30 AM   #6 (permalink)
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I have a 904 bonnie with some headwork (bigger valves) and the Mikunis from Bill. The carbs made more of a difference in hp than did the engine work! This may be that they allowed the bigger engine to finally breathe whereas the stock carbs were a definite restriction.

I've been playing with the jetting a lot since I got them, and you're right getting to the needles is kind of a pain. Really not the carbs fault, more like the gas tank's fault!

I recently went to 142.5 mains (from 145's and larger) and I'm running the 96 needles in the middle position, and about 3/4 turns out on the idle mix. This combo really made a difference in the power from the previous jetting. It even sounds better.

Bill sent them to me with the 97 needles in the mid position and 147.5 mains, which felt really good, except at full throttle it seemed weak. I thought I needed a richer needle and leaner main, which is why I went with the above jetting combo, and it really runs good now.

I also get a lot of popping on decel, so I've been trying to gradually richen the idle mix, but I think part of the deal with these carbs is that they do pop quite a bit. If I tune the pop out completely then the bike is running way too rich.

I can almost assure you that you won't need the accelerator pumps. Once you get your needles tuned correctly it really eliminates the bog. You still have to be progressive in your throttle action, but above 3,000 rpms it's not an issue.

These carbs work really well! They are danm expensive, but for me they were totally worth the $$$.
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Old 07-18-2008, 10:07 AM   #7 (permalink)
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Quote:
Originally Posted by sweatmachine View Post
I have a 904 bonnie with some headwork (bigger valves) and the Mikunis from Bill. The carbs made more of a difference in hp than did the engine work! This may be that they allowed the bigger engine to finally breathe whereas the stock carbs were a definite restriction.

I've been playing with the jetting a lot since I got them, and you're right getting to the needles is kind of a pain. Really not the carbs fault, more like the gas tank's fault!

I recently went to 142.5 mains (from 145's and larger) and I'm running the 96 needles in the middle position, and about 3/4 turns out on the idle mix. This combo really made a difference in the power from the previous jetting. It even sounds better.

Bill sent them to me with the 97 needles in the mid position and 147.5 mains, which felt really good, except at full throttle it seemed weak. I thought I needed a richer needle and leaner main, which is why I went with the above jetting combo, and it really runs good now.

I also get a lot of popping on decel, so I've been trying to gradually richen the idle mix, but I think part of the deal with these carbs is that they do pop quite a bit. If I tune the pop out completely then the bike is running way too rich.

I can almost assure you that you won't need the accelerator pumps. Once you get your needles tuned correctly it really eliminates the bog. You still have to be progressive in your throttle action, but above 3,000 rpms it's not an issue.

These carbs work really well! They are danm expensive, but for me they were totally worth the $$$.
Thanks Sweat,

according to tuning manual the needle difference (95, 96, 97, 98) will affect only 1/8 to 1/4 throttle openings because the only change is the diameter of the cylindrical (not tapered) section. One should not feel any change with different needles above 1/4 throttle opening. So you strengthened full throttle response only with the reduction of the main jet (because you changed needle but didn't move e-clip from center position), or at least that's what theory says.

About the main jet tuning:

The "roll-off" method is mainly used to find out if the mains are too small (lean) because the mixture is richened for an instant when throttle is suddenly closed 1/8 of a turns from WOT. If the bike tends to accelerate then the mains are too small. But what if the mains are too big?

The manual says that a too big main can produce a bogging (overly rich condition) when throttle is suddenly closed from WOT to 1/2. So that's can be a way to check if the mains are too big.

Apart from this, there's always the good ol' butt-dyno
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Old 07-18-2008, 10:47 AM   #8 (permalink)
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Quote:
Originally Posted by wolverine View Post

The "roll-off" method is mainly used to find out if the mains are too small (lean) because the mixture is richened for an instant when throttle is suddenly closed 1/8 of a turns from WOT. If the bike tends to accelerate then the mains are too small. But what if the mains are too big?
I went all the way up to 155 and it got worse and worse, so I knew I was rich, then I thought I'd go leaner until I felt it accelerate on roll off, and even with the 142.5s it still doesn't do that, so I might even try 140s to see how it feels.

You're right about the needle position, and I still want to play with that some more. I need a weekend day where I can ride, rejet, ride, rejet, etc to get some good butt dyno results, and play with the needle position a bit. It's been tough to get a whole day (or few hours anyway) free lately.
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Old 07-18-2008, 12:39 PM   #9 (permalink)
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Quote:
Originally Posted by sweatmachine View Post
I need a weekend day where I can ride, rejet, ride, rejet, etc to get some good butt dyno results, and play with the needle position a bit. It's been tough to get a whole day (or few hours anyway) free lately.
Yeah, gotcha!

however...just thought of how hard it was going to be with the FCR with slow jets, slow air, tons of needles...) Just kidding!
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Old 07-18-2008, 12:51 PM   #10 (permalink)
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I have had my 42s for about a year now.I first used them on a stock 790 I played around with a lot of things back then.I ran brit cutoms intakes on mine with the stock motor cause i liked the way they held the carbs back then i ran 160 mains 96 niddles in middle slot and the big pilots and no pumps .Now with the 989 and the head work i am running bills intakes 145 mains 96 niddles one slot down from the middle slot big pilots and have pumps coming in at 1/4 throttle and going out at 3/4 throttle my pump nozzles arent marked for all i know they are the 70s that come with the carbs.I can tell you if the pumps will work with your motor they wake the thing up big time .I can nail mine off idle with no bog just bang its gone.One thing i can tell you about finding the right niddles get it on the road run at 1/8 to 1/4 throttle (where the sweat spot is for cruseing at steady throttle)mine is like 55 to 60 mpg.If your niddles are to small(lean) it will kinda surge it should be smooth as glass.try both niddles in middle slot like that to find what you need.then play with the clip to see where it needs to be .I dont know if the pumps will work on a stock motor or not,they seam to work better on the bigger motors.I am not real sure how you tell where wot stops should be set on these carbs with out a flow bench.One thing to keep in mind is when you make them go less open you allso lower the niddle at wot.That could change main jet size.
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