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Discussion Starter · #1 ·
It's hard to find time at the moment, but this was part of my original cafe racer plan, I thought I had better bite the bullet and start swapping out my carbs for a single mechanical throttle body.
So hopefully by the end of the week it will be done. And depending on how well, or not it works will dictate the next step e.g. the tank etc.
What do you think a flat air filter under the tank hidden of a hole in the tank flush to help air flow or whole hog a ram tube thru the tank.
I may even dyno it when done just for kicks
IMG_20201114_160449.jpg IMG_20201114_160341.jpg
 

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But why? From what I understand you're gonna get worse fueling, worse throttle response and probably a shit ton of other issues. Is there any advantage besides not needing to sync two carbs?
 

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Discussion Starter · #3 ·
But why? From what I understand you're gonna get worse fueling, worse throttle response and probably a shit ton of other issues. Is there any advantage besides not needing to sync two carbs?
We will see// On a Cafe Racer less is more

Maybe wishful thinking, but I am hoping for better mileage and better throttle response than the standard cv carbs and better mid range torque and if my manifold is good no loss of top end. These things rip. David Vizard famous engine tuner loved these things, look it up.
I have used these on cars for years and am always amazed. First time on a bike however. So it is a experiment.

This one is a 1 5/8" version from the 60's, I know California smog testing was still good with these in the late 80's
P.s. It started to day first go but much to do.
 

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Looks a bit like you've halved the flow cross section? not sure how that throttle body size compares with stock, but I'd just put in here that the problems you list as what you're trying to solve, aren't really problems...

Better Mileage: Yeah the SV isn't great, but is this really a motivating factor for anyone?
Better throttle response: sure the SV is a little snatchy but it's a V-twin...the stock butterflys move smoothly and you can always get different throttle spools or jetting.
Better mid range torque: this is like, what the SV is known for...

All that snarkyness aside, I fully love the experimentation, would be interested to see some dynos or videos. Good luck!
 

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Discussion Starter · #5 ·
A picture
IMG_20201130_091751.jpg
of progress
The TB has only one gasket and it was leaking so I ordered some appropriate gasket paper from ebay but I couldn't wait so I made a Kellog's gasket.
Got it going, and was able to find with the fuel metering in the minimum position it is supplying enough fuel for 5000 rpm with no load. so being a mechanical system if I can mod it to close of more it should work. not surprising since its 150-200hp setup, I do have smaller ones but I want to give this a go first.
55847
 

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Discussion Starter · #7 ·
Ok. Time for an update,
If idle speed is correct too much fuel is being delivered thru out the whole range. If I adjust for a 3500 rpm Idle I can get it all good.
So I am going to try a 2nd spray bar I have for the 100 hp version which has smaller holes I am not sure of compatibility with this larger TB but it's the best bet at this time.
 

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Ok. Time for an update,
If idle speed is correct too much fuel is being delivered thru out the whole range. If I adjust for a 3500 rpm Idle I can get it all good.
So I am going to try a 2nd spray bar I have for the 100 hp version which has smaller holes I am not sure of compatibility with this larger TB but it's the best bet at this time.
Just a thought, but maybe a 2nd carb would help.
 

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Less is more, but I kept both on mine. I did get rid of all the excess stuff on the outside. (and removed a bit inside as well :cool:)

55937
 

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Discussion Starter · #11 ·
Success is not guaranteed This is not paint by the numbers.
I fitted the spray bar with smaller fuel holes and found the air holes are the same size in both. so now the throttle plate is close a little more compared to the fuel metering so when its shut all the way down in idle position it now idles a bit over 2k rpm.
But since there seems to be no secret ratio between the fuel holes and the air holes I have now drilled 1 of the 3 air hole out 0.2 mm.
Next job reassemble and try again.
 

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Discussion Starter · #12 ·
I through myself a curve for a couple of days by not pressing to rear of the manifold all the way home. but I should be back on track now.
 

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Discussion Starter · #13 ·
Rear cylinder is still not playing the game. With idle set 2000 rpm It idles 5 seconds then the rear cuts out And runs on the front for 10 seconds at about 1000 rpm and keeps doing this. "Seems" to run ok 2500 and over.
The coil checked ok I put a wire across from the + front coil to the rear to ensure both has the same voltage supply.
new HT wires, plugs, and end caps, also swapped coil from a later model bike just to test(same)
The intermittent nature of the fault makes testing a bit inconclusive.
I put a inductive timing light on the rear and it continues to flash when not running on the rear but as the revs increase back again the light stops twice while revs go up (how weird is that)
I reduces the plug gap to it spark easier to see if that helped it didn't.
I pinched the vacuum pipe going to the fuel pump and the petcock with no change

Any thoughts
I am stuck at the moment
Thanks
 

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I had a similar issue lately (with stock carbs though), the rear cylinder wouldn't work at low RPM. Turns out it'd been flooded with gas and couldn't burn it. Is the rear plug all wet after idling for a few minutes? Is your exhaust spitting flames? If not, maybe it's running too lean?
 

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Discussion Starter · #15 ·
I had a similar issue lately (with stock carbs though), the rear cylinder wouldn't work at low RPM. Turns out it'd been flooded with gas and couldn't burn it. Is the rear plug all wet after idling for a few minutes? Is your exhaust spitting flames? If not, maybe it's running too lean?
Did yours start and stop while idling? mine is weird you could set a watch to in the rear runs and stops at regular intervals. I did wonder If it could be getting wet then drying and repeating.
I checked valve clearances they are all min spec but that should be ok.
I think it must be the manifold but I can't imagine why. next step will be sealant on all mating surfaces.
 

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Discussion Starter · #17 ·
It seems the rear cylinder is running lean at low rpm I suspect I its to do with the uneven firing order the airfuel mix has more time to stagnate and stop flowing as opposed to the front where the flow is still going stared buy the rear. I am thinking of running a 12mm tube from the manifold branch base of the rear cylinder to the front of the manifold.
It won't completely get rid of the air flow pause but it should keep a air/fuel mix in the rear branch while the front is pulling in its mix.
Interested to see what others think
 

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I'm not an expert on fluid dynamics, but my hypothesis goes like this: the rear cylinder is running extremely lean and thus not firing until enough fuel gathers inside. Since the front cylinder is almost horizontal, its intake is vertical and gravity helps the air-fuel mixture to take that path, starving the rear cylinder. Could be verified by somehow restricting the front intake.

Edit: my bike behaved differently, the rear cylinder wouldn't fire at all at idle, and I had to keep it in the upper RPM - otherwise it would get flooded and start misfiring even in the midrange.
 

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Discussion Starter · #19 ·
I put in a deflector favouring the rear but no noticeable change, I had thought about gravity etc. but I don't think there is much I can do about it. the inside of the plenum offers a blunt 90deg knife edge to spit the flow evenly
 
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