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Old 02-28-2013, 02:31 PM   #16
Beemerboff
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Plaka -air flow aint the only thing that affects the slide position - ever wondered why SU sell eight different slide springs?

They fit Bings------.

BMW list two, and it is always worth trying the late, stiffer version.
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Old 02-28-2013, 03:07 PM   #17
Rob Farmer
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Originally Posted by Beemerboff View Post
When Rob Farmer was chasing a mixture problem he fitted a "O2" meter to his RS? which he could read out on the road, and found that the flat filter didnt flow enough at high revs , and stuffed up the mixture.

He fitted a old style round filter which flowed better, and didn't affect the mixture as much at high revs, but it still wasnt perfect.

I spent some time on the Dyno with my PD seeing what worked and didn't work

The flat airbox was spot on for bikes fitted with 32mm carbs but just didn't flow enough air for my 40mm carbed bike.

The problem I had was the lack of main jet sizes for the bings. The early airbox made the mixture too weak at the top end and I couldn't buy main jets in the right size to get it right.I really want to fit a set of Amal mark II carbs before I start messing around again.

What you do appear to get with the early airbox, as long as you have the higher comp pistons, is a much more linear power delivery.

My PD going though it's paces. You can see the early airbox.



I also had it rigged with a air/fuel ratio meter for road testing




This is an interesting read













Rob Farmer screwed with this post 02-28-2013 at 03:14 PM
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Old 02-28-2013, 04:24 PM   #18
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Originally Posted by Rob Farmer View Post
I spent some time on the Dyno with my PD seeing what worked and didn't work

The flat airbox was spot on for bikes fitted with 32mm carbs but just didn't flow enough air for my 40mm carbed bike.

The problem I had was the lack of main jet sizes for the bings. The early airbox made the mixture too weak at the top end and I couldn't buy main jets in the right size to get it right.I really want to fit a set of Amal mark II carbs before I start messing around again.

What you do appear to get with the early airbox, as long as you have the higher comp pistons, is a much more linear power delivery.

My PD going though it's paces. You can see the early airbox.



I also had it rigged with a air/fuel ratio meter for road testing




This is an interesting read












Since your post about your findings is often brought up in airbox discussions Rob, I think is important to point out that it is actually the airbox top and not the airbox itself or the filter that is actually restrictive. Well, that and those stock velocity stacks. For instance, in '95 San Jose's race bike Wrecking Ball broke ALL BMW Daytona track records and it was using a square airbox. Without the top, of course. After modifying my bike to a higher level, I run my square airbox with no top but the filter itself doesn't effect the mix any appreciateable amount.

I suspect it might work better than a claim shell in that you can have two big bell mouth stacks underneath a filter in a square airbox. My box is now basically a velocity stack support with a filter on top at this point.

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Old 02-28-2013, 04:42 PM   #19
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Originally Posted by Beemerboff View Post
Plaka - ever wondered why SU sell eight different slide springs?



BMW list two, and it is always worth trying the late, stiffer version.

They can't get it right? I thought you were supposed to poke a few pinholes in the diaphragms to tune slide motion?? You can file down the needles too to reduce the weight on the piston.

Seriously, if you went with a stiffer spring would you be going after an altered torque curve?
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Old 02-28-2013, 05:05 PM   #20
Kai Ju
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Lift holes

Don't forget the lift holes in the slide.
That's why modern jet kits come with not only different springs but quite often a drill bit to change the lift hole diameter.
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Old 02-28-2013, 05:31 PM   #21
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They can't get it right? I thought you were supposed to poke a few pinholes in the diaphragms to tune slide motion?? You can file down the needles too to reduce the weight on the piston.

Seriously, if you went with a stiffer spring would you be going after an altered torque curve?

SU carbs have very little in common with Bings. They have no idle circuit. no main jet, no low speed or idle jet. Everything is controlled by the needle and needle jet. The slide is oil dampened to provide acceleration enrichenment, the slide spring assists. Hundreds of different needles adjust the mix through the range.
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Old 03-01-2013, 12:26 AM   #22
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SS,
The square air box with the 32mm carbs was nigh on perfect on the dyno. The American spec gs I bought that had been imported from the states was spot on as well its just the 40mm carbs on the square air box that doesn't seem to work properly. moorespeed make a higher volume air box to get round the issue and I believe that Hpn raise the lip of the air box lower by 1cm to increase the volume of the box below the filter.
The clamshell reduced mid range torque but flowed more air at the top end, to the extent that on the standard jetting the mixture went so weak it felt like the ignition was being turned off, the lack of readily available jets in the larger sizes meant that it was difficult to get the jetting right. I fitted 170 main jets but these were too big, 160 were the next size down but these were weak.

Personally my ideal would be the square air box with 32mm carbs. After spending lots of money and dyno time I cannot see any advantage to the 40mm carbs. The bikes run so much better and have a much cleaner midrange with the 32mm carbs and the top end doesn't appear to be compromised for normal riding.
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Old 03-01-2013, 01:16 PM   #23
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SS,
The square air box with the 32mm carbs was nigh on perfect on the dyno. The American spec gs I bought that had been imported from the states was spot on as well its just the 40mm carbs on the square air box that doesn't seem to work properly. moorespeed make a higher volume air box to get round the issue and I believe that Hpn raise the lip of the air box lower by 1cm to increase the volume of the box below the filter.
The clamshell reduced mid range torque but flowed more air at the top end, to the extent that on the standard jetting the mixture went so weak it felt like the ignition was being turned off, the lack of readily available jets in the larger sizes meant that it was difficult to get the jetting right. I fitted 170 main jets but these were too big, 160 were the next size down but these were weak.

Personally my ideal would be the square air box with 32mm carbs. After spending lots of money and dyno time I cannot see any advantage to the 40mm carbs. The bikes run so much better and have a much cleaner midrange with the 32mm carbs and the top end doesn't appear to be compromised for normal riding.
The main jets go 170, 165, 162, 160, 158.....

With some wire gauge drills or reamers you have even more sizes available.

Speaking of Mikunis, the smaller carbs are known for higher intake velocities and better mid range torque. The larger ones for more high end horsepower. I would gather the same holds for the Bings, albeit with more lag.

I'm dealing with 5000'+ altutude for all my riding. I need raw breathing and everything I can get. I stick to the old butt dyno. if I can't feel it I don't care about it.

Plaka screwed with this post 03-01-2013 at 01:28 PM
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Old 03-01-2013, 01:29 PM   #24
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Originally Posted by robtg View Post
SU carbs have very little in common with Bings. They have no idle circuit. no main jet, no low speed or idle jet. Everything is controlled by the needle and needle jet. The slide is oil dampened to provide acceleration enrichenment, the slide spring assists. Hundreds of different needles adjust the mix through the range.
I was talking about stiffer springs in Bings.

I used to have a '66 MGB. I remember the SU's.
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Old 03-01-2013, 01:37 PM   #25
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Quote:
Originally Posted by Rob Farmer View Post
SS,
The square air box with the 32mm carbs was nigh on perfect on the dyno. The American spec gs I bought that had been imported from the states was spot on as well its just the 40mm carbs on the square air box that doesn't seem to work properly. moorespeed make a higher volume air box to get round the issue and I believe that Hpn raise the lip of the air box lower by 1cm to increase the volume of the box below the filter.
The clamshell reduced mid range torque but flowed more air at the top end, to the extent that on the standard jetting the mixture went so weak it felt like the ignition was being turned off, the lack of readily available jets in the larger sizes meant that it was difficult to get the jetting right. I fitted 170 main jets but these were too big, 160 were the next size down but these were weak.

Personally my ideal would be the square air box with 32mm carbs. After spending lots of money and dyno time I cannot see any advantage to the 40mm carbs. The bikes run so much better and have a much cleaner midrange with the 32mm carbs and the top end doesn't appear to be compromised for normal riding.
I think the 32mm Bings work better too. I am not much of a fan of the 40mm Bings. I am a big fan of bigger than 32mm carbs, just not Bings.

Whatever the issue is with a square box/40mm Bing combo I don't think it is volume. If 40mm Bings pulled that much more air than 32mm Bings they would run that much stronger with 40mm Bings and they don't. Besides, I know from experience that the stock airbox can pull a lot more air than 32mm Bings on a stock engine for my bike making way more power with a stock square box and 38mm Dellorto's, a sport cam, mild port work, and a sport exhaust.
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Old 03-01-2013, 02:03 PM   #26
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I think the 32mm Bings work better too. I am not much of a fan of the 40mm Bings. I am a big fan of bigger than 32mm carbs, just not Bings.

Whatever the issue is with a square box/40mm Bing combo I don't think it is volume. If 40mm Bings pulled that much more air than 32mm Bings they would run that much stronger with 40mm Bings and they don't. Besides, I know from experience that the stock airbox can pull a lot more air than 32mm Bings on a stock engine for my bike making way more power with a stock square box and 38mm Dellorto's, a sport cam, mild port work, and a sport exhaust.
Methinks it's the dellortos. just sayin'...
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Old 03-01-2013, 11:48 PM   #27
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So, if I have a GS with 40 mm Bings and I want to fit 32mm Mikunis what inlet stubs do I fit - is it just a case of getting the 32mm carb stubs from an earlier bike or is the thread in the head bigger too.?

And my previous post was wrong - SU have fifteen different slide springs not eight, although in practical terms probably no more than half a dozen would be relevant to any given carb set up..

The needles are also not plain tapers but are ground in 16 stages to enable precise selection in the midrange.

Does it all matter? - well, when Truimph did a 24 reliability trial on three stock production SU equipped, unfared, Thunderbird's they managed over 90 MPH and over 90 miles per (imperial) gallon - a bit better than any Bing equipped Airhead is ever going to do.
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Old 03-02-2013, 01:42 AM   #28
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So, if I have a GS with 40 mm Bings and I want to fit 32mm Mikunis what inlet stubs do I fit - is it just a case of getting the 32mm carb stubs from an earlier bike or is the thread in the head bigger too.?

And my previous post was wrong - SU have fifteen different slide springs not eight, although in practical terms probably no more than half a dozen would be relevant to any given carb set up..

The needles are also not plain tapers but are ground in 16 stages to enable precise selection in the midrange.

Does it all matter? - well, when Truimph did a 24 reliability trial on three stock production SU equipped, unfaired, Thunderbird's they managed over 90 MPH and over 90 miles per (imperial) gallon - a bit better than any Bing equipped Airhead is ever going to do.
Bing needles are also a complex taper. Lay one along a straitedge.

Look over the kits from Sudco. They have all the parts and you want not only the transition pieces but the jetting already set up and the correct cables. Ditto Rockypoint Cycles. There are a number of different Mikunis to choose from. I think Rocky point only has the VM. Sudco has the TM as well. San Jose BMW also has experience with the TMs. You might consult them if you go that way.

A friend of mine put some TMs on his 90/6. I rode that bike a few times---it was like there was a stiff steel wire connected to the power. Very crisp and nice. But in the end he was disappointed. The Mikunis didn't handle crummy gas the way Bings will and he got stranded a few times. He eventually gave up on them. I've gotten wet gas from a bad gas station and the bike was missing a lot but it kept going until I could replace the fuel (I gave it to a gal in a gas station who was filling some cans for free--she was very pleased). Bings are so easy to service that popping a float bowl and draining the water every 25 miles is trivial.

If you tell me that a 1600cc open bike running carbs got 90MPGi at 90 MPH...um...I'm gonna ask you what you smoked for lunch. That's about 76 miles/US gal. poking around on Triumphs site, the 2012 tbird gets 38/56 mpg but they don't mention the fuel delivery system. The 2013 Tbird is also listed at 38/56 and is fuel injected. I've never known any carb setup to beat fuel injection. My ancient type III VW squareback always got 40MPG with stock Bosch injection and this was long before "economy" cars. Small station wagon, 1600 cc air cooled motor, high altitude.

At 90mph at sea level my RS was getting high 30's low 40's. If I cut speed to 55mph mileage was in the high 40's low 50's. I used to run from LA to well north of San Francisco (Vallejo) up Highway 1 a lot. It's a 525 mile run and if you have to stop in someplace like Carmel for gas you get seriously bent over. I can carry about 8.6 gallons in the tanks and it's a slow road for the most part so I could make for cheap gas on the peninsula to finish the trip---roughly 400 mile range.

BTW, I'm not sure those intake spigots are threaded. I got a couple lying around and they ain't got threads on them. They do have slots for a flat bar so you can twist on them to help get them out. Also, you might consider going to 38mm rather than 32mm. I gather you have a 100GS. Consult the people that sell them, they will know more than anyone else and they have no particular interest in selling one over the other.

Plaka screwed with this post 03-02-2013 at 01:52 AM
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Old 03-02-2013, 01:50 AM   #29
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"If you tell me that a 1600cc open bike running carbs got 90MPGi at 90 MPH...um...I'm gonna ask you what you smoked for lunch. That's about 76 miles/US gal. poking around on Triumphs site, the 2012 tbird gets 38/56 mpg but they don't mention the fuel delivery system. The 2013 Tbird is also listed at 38/56 and is fuel injected. I've never known any carb setup to beat fuel injection. "


I think that he is referring to a mid 1950s 650cc single carb Triumph twin!

Some people fitted 750 Norton Commandos with single SU carbs with good results in terms of fuel efficiency and tractability.
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Old 03-02-2013, 01:40 PM   #30
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The later 32 and 40mm carb spigots have the same threads into the head.

Bing CV's jet needles have a straight taper.
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