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PostPosted: Sun May 29, 2016 10:06 pm 
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On the street
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Joined: Sat Oct 06, 2012 10:54 pm
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Location: Over the hill in Fairfield NSW AU.
Country: Australia
Bikes owned: TR750, GSXR750, GT750, RGV500
To all those people who think you need to replace these BS40CV's to get performance out of their GT 750. My 738cc GT made 95.7 RWHP recently and last weekend ran 11.9 et at 110 mph which backed up a 12.1 at 112 mph at Sydney Dragway. Bike and rider weight was 689 lb race ready. The bike is in full street trim except for expansion chambers with silencers.
While this is a long way short of Steve Reynolds' time 40 years ago, Steve ran 11.5 @ 117 mph again with BS40CVs, people seem to be able to relate to more resent performances.
No doubt, if you are aspiring to over 95 hp you will need to fit higher flow carbs but let me assure you 95 hp on a street GT 750 really gets your attention:-)))
Bob Prior.


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PostPosted: Mon May 30, 2016 12:15 am 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 5979
Location: Illinois
Congrats Bob. Those are impressive numbers. 11's on a bike that looks so close to stock is quite a feat, I was expecting to see a higher MPH with such low ET times. You are making a lot more HP than mine, but I need to do more learnin'..........

Mine was only 13.1 at 107, so I may have to pull the barrels off again to slip a spacer in - and learn to ride too.....

I also need to fit the late model 77 swingarm which is similar to a GS arm but with GT brake anchor... :wink:

Clearly those carbs work at those speeds.


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PostPosted: Mon May 30, 2016 7:11 pm 
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On the street
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Joined: Sat Oct 06, 2012 10:54 pm
Posts: 24
Location: Over the hill in Fairfield NSW AU.
Country: Australia
Bikes owned: TR750, GSXR750, GT750, RGV500
Thanks for the kind words Richard but there is not much credit due to me here:-( I can't ride mine either, I put Kyle Major a young, fit, light and very accomplished bike rider on it and it dropped a full second under my pb and put on 9 mph. Kyle hadn't ridden anything older than a 1999 model Hayabusa before so it was quite an education for him. He feels it has more left in it with more seat time, he's only made 5 passes on it. On the day he was riding the GT from the front of the bike line up then back up the return road to jump on his turbo Hayabusa at the rear of the line:-) I can't even claim any credit for the porting:-( it's a Doug Flannary ported cylinder block and expansion chambers:-) Doug and Dave D. do really high quality work:-)
Bob


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PostPosted: Mon May 30, 2016 11:49 pm 
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On the street
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Joined: Sat Oct 06, 2012 10:54 pm
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Location: Over the hill in Fairfield NSW AU.
Country: Australia
Bikes owned: TR750, GSXR750, GT750, RGV500
Cyzygy wrote:
To all those people who think you need to replace these BS40CV's to get performance out of their GT 750. My 738cc GT made 95.7 RWHP recently and last weekend ran 11.9 et at 110 mph which backed up a 12.1 at 112 mph at Sydney Dragway. Bike and rider weight was 689 lb race ready. The bike is in full street trim except for expansion chambers with silencers and is fitted with BS40CV carbys.
While this is a long way short of Steve Reynolds' time 40 years ago, Steve ran 11.5 @ 117 mph again with BS40CVs, people seem to be able to relate to more resent performances.
No doubt, if you are aspiring to over 95 hp you will need to fit higher flow carbs but let me assure you 95 hp on a street GT 750 really gets your attention:-)))
Bob Prior.

Sorry I had forgot to mention that my GT 750 is fitted with BS40CV carbys.
Bob.


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PostPosted: Thu Jun 02, 2016 8:58 am 
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Yeah Man, the Interstate

Joined: Wed Apr 01, 2009 11:47 pm
Posts: 554
Location: NM USA
A lot has to do with tuning parts availability, and the fact CV's work so weird. They are just a lot more difficult to tune than standard type carbs.

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PostPosted: Fri Jun 03, 2016 9:35 pm 
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Joined: Sat Oct 06, 2012 10:54 pm
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Location: Over the hill in Fairfield NSW AU.
Country: Australia
Bikes owned: TR750, GSXR750, GT750, RGV500
ja-moo wrote:
A lot has to do with tuning parts availability, and the fact CV's work so weird. They are just a lot more difficult to tune than standard type carbs.


I've fitted the largest needle and seats I could find and drilled out the fuel passage ways as a precaution, fitted the air bleed restriction jets for the off idle banging / jerking and changed the main jet to 127.5, stock needle on middle clip, stock needle jet and stock pilot jet, no mods to the slide lift at all. Steve Reynolds's GT 750 that ran with CV40CVs ran 11.5 @ 117 mph 40 years ago, in a very similar bike, carbys were totally stock except for mainjet change. My GT 750 altered in 1976 ran 11.2 at 121 mph with CV40CVs totally stock with only a mainjet change. Nothing weird or difficult was experienced, but both my sets of carby, in 1970s and the current set were new carbs. Steve Reynolds's carbs were near new. Worn carbs may get weird or difficult. Neither of these 1970s engines would have been making near 100 hp though. I've had a BS40CV flow tested and was told by the flow bench operator they flow enough to make 116 crankshaft HP. All 3 of these bikes ran with no aircleaners, not even pods. This is all first hand info, not assumed, not hear say or read somewhere. I hope it can be of help to someone contemplating a 95 hp streeter, although my 11.9 second streeter is exciting to ride when you are "up it" it is a pig to ride sedately.
Bob Prior.


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PostPosted: Sat Jun 04, 2016 8:02 am 
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Expert racer

Joined: Tue May 18, 2010 5:38 pm
Posts: 1403
Location: New Hampshire
Country: USA
Bikes owned: Suz, Yam, Honda, Kaw.
If a CV is "weird" it's not a WOT. Here is just fine. For example, at WOT a CV is just like a VMaccept the flap in the front but, that is laying horizontal adding tad of restriction due shaft dia.

Where it becomes a little more challenging is "aggressive street cruising" tune For example, if you take a VM and hold for example 1/8 throttle, let engine climb in rpm range nothing changes at the carb, making tuning that circuit or position straight forward. Now, what happens if you do this with a CV? The Butterfly stays fixed at 1/8 open but, what is going on with the slide? Are the staying put our is it moving up some? Are they all moving the same? When moving you have dynamic and static behavior to sort out.

Take a large engine like the GT750, street riding you are barely use any throttle. Point being tune of small throttle positions is what is important not WOT.

I would take a VM over a CV any day...I'll control the slide for either power or mileage as I feel. For those that don't know how, worried about mileage , or feel to go faster or slow you must always twist throttle a CV is fine. Why I love 2 strokes is you can crack just a little throttle and the engine will climb RPMs (change road speed) oh so smoothly but, only if you are tuned correctly. Having to constantly work the throttle to adjust speed is too laborious to be enjoyable.

My standard 2 cents

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PostPosted: Sun Sep 10, 2017 9:05 pm 
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On the street
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Joined: Sat Oct 06, 2012 10:54 pm
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Location: Over the hill in Fairfield NSW AU.
Country: Australia
Bikes owned: TR750, GSXR750, GT750, RGV500
Took my GT750 to a Nostalgia meet yesterday at Sydney Dragway with Kyle Major riding it, new pb et 11.825 (701 lb bike and rider on track digital scales), Kyle was runner up in racing. First time out for 16 months, hope to do a few more meets this coming summer and lower the pb more:-))).
Bob.


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PostPosted: Mon Sep 11, 2017 10:20 am 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 5979
Location: Illinois
That's a spectacular time Bob. My PB last year was 12.8 @111 - a full second slower than yours. :oops:

Time for more dyno and seat time.

Hardest thing to do with CV carbs is to get the rack back on the bike. That is a serious PIA.

When I had the transmission out last week to replace some very worn gears, I noticed the barrels were full of "holes", so I tried to lighten it up a bit by making them bigger. We'll see if the extra lightness helps. :roll: :wink:


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PostPosted: Mon Sep 11, 2017 1:40 pm 
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Joined: Sat Oct 06, 2012 10:54 pm
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Location: Over the hill in Fairfield NSW AU.
Country: Australia
Bikes owned: TR750, GSXR750, GT750, RGV500
Richard,
Thanks for the kind words and no, your 12.8 is 1/10th quicker than mine but a second slower than Kyles:-)))
Suzuki Dave,
Can you send me an email, I've changed computers and lost your email address. I have a few pics of my new project to send you:-))))
Bob.


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PostPosted: Mon Sep 11, 2017 3:19 pm 
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Location: Illinois
Bob,
That's interesting. Could you bottle some of Kyle's genes and send them. That may be what is missing here.

What fuel are you running/can you run there? Up to that last PB run, by best time had been with U4.4 at a test n tune day but we can't use that for records here. It has to be non oxygenated, so for that run I used some old C11 premix that Kevin gave me and topped it off with regular street gas so I didn't run out on the run. The problem with that sort of mix is impossibility in reading plugs. Some black unleaded and some light gray leaded in differing proportions is not a scientific approach and repeatability is out the window.

Next week I'll try straight Sunoco Maximal which is 116 octane. It's more knock resistance than I need, but it's the fast burn rate that should help and it's not oxygenated.


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PostPosted: Mon Sep 11, 2017 7:14 pm 
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Joined: Sat Oct 06, 2012 10:54 pm
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Location: Over the hill in Fairfield NSW AU.
Country: Australia
Bikes owned: TR750, GSXR750, GT750, RGV500
Richard,
I just use premium unleaded, 98 octane allegedly, from the BP service station on the way to the track. PUL 98 is what all late model street cars and bikes use here now. I still use the oil pump. I haven't got your email address anymore, send me an email or pm.
Bob.


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