CV Carb Mid-Range Tuning - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 27 (permalink) Old 08-20-2019, 08:24 PM Thread Starter
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CV Carb Mid-Range Tuning

When my brother passed away, I kept two things...his dog and his motorcycle. The dog is another story. The motorcycle is a 2007 Triumph Bonneville Black. Of course, he had swapped the stock mufflers for so-called TORS (Triumph Off Road). Living in Denver, the “Mile High City", he had wisely not installed the larger 115 main jets that came with the mufflers. So...carburation was stone stock.
Once home in Oregon, I rode it to the local DMV to register the bike in my name and discovered a bit of a lean hiccup in the mid-range running. Then, I tried removing the snorkel from the air box, like conventional wisdom dictates. Trick “performance" exhaust pipes and chopped up air-boxes (or pod air filters) to compliment....should make the beast run better. Yet, the mid-range hiccup became a mid-range stumble. Almost no “pull" when the throttle’s whacked open...unacceptable! Trying to flatten my learning curve on this new (to me) machine and it’s issue, I was surprised to find practically everything I read online, advocated doing exactly the wrong things! Jet richer, shim the stock needle, throw away the air-box altogether, install a jet kit, etc.

I realize that hiring tuning work ain’t gonna be cheap and might not be done right the first time around. After all, re-jetting carbs, besides being a fiddley task at best, is not an exact science. Matching the mixture to the hardware can be fraught with potential error. Getting it precisely correct was something the factories with all their resources, couldn’t quite manage back in the days of carburetors. I don’t claim to be a mechanic, so how could I do better? Well first, identify the problem.

The EPA (And CARB) put the lid on motorcycle emissions (including noise) back in the late 70's. That’s 40-years of incremental engine strangulation. To meet these ever-tightening regulations, meant manufacturers had to make motorcycles that run “lean”...for the sake of cleaner air and at the expense of mid-range power. The simple fact is, a motorcycle cannot be ridden at full throttle for more than a few seconds at a time. Whereas, they idle a lot and are ridden at part throttle (cruise) most of the time. No prize then, for guessing that efforts at lowering emissions appear in the carb circuits that supply air/fuel mixture to the engine at part throttle. Namely, the jet needle. It dominates from just off idle to almost full throttle.
In a nutshell, you could ignore the low speed jet and the main jet because they are not “smog” lean...if you could only get the right needle. Good luck!

Hybrid CV (constant velocity) carbs should be referred to as “constant vacuum" because unlike the other carbs, the diaphragm in the top controls things, based on vacuum, not your right wrist. The so-called “nail-head” needles in these carbs are only adjustable by means of shims and even then still have a “lean” taper. Good for emissions, not so good for performance.

Air, as we know, moves faster than fuel. No trouble when you are just cruising, but when you yank the throttle open, the worst problem a lean midrange presents to the rider...is the dreaded, gutless, lean bog! In a carb with emissions-legal needles the mixture goes too lean and the engine won’t run on thin air alone. CV carbs work around the problem (somewhat sluggishly)...unless you remove or cut up the air box...which is where the vacuum is generated.

Of course, the aftermarket has jet-kits. Usually, including a different, adjustable clip-type needle, but prone to going too far the other way, resulting in marginal improvement at the expense of lousy mileage and potentially fussy ride-ability. Leaving (seemingly) that internet advice about installing huge main jets and pilot jets in vain attempts to work around this lean mid-range issue. Not an ideal solution for improving performance in daily riding.

Bottom line, manufacturers have nearly shut us out, when it comes to proper “performance" tuning of the midrange with emissions-legal, carbureted motorcycles. A virtual guarantee that in any attempt to “improve" performance, the circuit ridden on most, will be compromised. One way or another.

So, short of tossing stock carbs in the trash and coughing up a fortune for tunable “performance” (illegal) replacements... there’s really only one way to solve the mid-range muddle and it’s been hiding in plain sight for decades. So, how about we leave the jets and needles alone and add another circuit instead?

Pic 1 - This instructive graphic shows the basics of carburetor functionality. Stock emissions-controlled carbs are set-up to run at legal air/fuel ratios at partial (cruise) throttle openings. For the record, this means “on the needle” 90% of the time. Idle and WFO air/fuel is usually left at best performance ratios, because you simply cannot street ride at either extreme for more than very short periods of time. (In other words...not...an emissions problem.) The trouble comes when we amateurs try to outsmart the system by messing with circuits that were correct in the first place!

Pic 2 - Most of the time, “outsmart” results from “open” exhausts and air filters. Installing either... affects mid range air/fuel ratios. Of the two, the air filter/air box choices make the most difference. (Since they rely on the strong vacuum provided by the air box, open air filters and richer jetting can actually cripple performance on CV carbs!)
Conventional thinking involves changing the jets. This is inherently flawed! Especially if you cannot match jetting to the factory needle...often the only needle available...particularly in CV carbs. As you see here, increasing jet sizes, results in too much fuel to atomize well. Resulting in lots of reversion (fuel spit back), potential surging at “steady state” cruising, and crap throttle response! If you’ve messed up this badly...nothing will help except to revert to stock specs and try again... the right way!

Pic 3 - In other words, we’re not talking about simply adding yet more fuel. Rapid response to snap throttle, is virtually impossible to achieve with bigger jets and the factory needle...shimmed or not. To deal effectively with the situation, you need an atomized mixture, instantly applied at precisely the right time, in precisely the right amount. Since factory carbs don’t have a way to do that, you add it! Using Thunder Products Dial-A-Jet or Intelajet is the ticket, because it provides both air and fuel in a pre-atomized mixture, on demand and correctly balanced, with no need to disassemble or re-jet the carburetor. The outcome feels like this graph looks! Smooth, rapid response to the throttle, good fuel efficiency, no flat-spots in the mid-range...right...everywhere it needs to be right. Visit thunderproducts.com or call Lonn (320-587-2700) for applications,advice, details and prices.

Pic 4 - My brother would be proud! Installing Dial-A-Jets on his Bonnie took just about the same amount of time as he spent putting on the TORS. No disassembly required! Put the side panels back on and the only visible clue is the thin blue hoses. Two short rides to “dial-in” the Dial-A-Jets setting and problem solved! Mid-range is now an arm-yanker and adjusts automatically for virtually all riding conditions.
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post #2 of 27 (permalink) Old 08-21-2019, 11:53 AM
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That's about the best year Bonnie, in the best color. Nice one

Nice write up. My bike runs fine for my purposes, but when I nail the throttle I do have to wait a couple seconds, then the afterburner kicks in. Honestly not sure if it's fuel starved at that point, or too rich, but it takes a second for the carb to adjust. Maybe this would help.

Found this interesting write-up https://thunderpress.net/editorial/c...2018/03/21.htm
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post #3 of 27 (permalink) Old 08-23-2019, 05:51 PM
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I run 3 old carburetted Triumphs. Get them set up right and you have no flat spots. Mine don't. IMHO Someone will always be willimg to sell you one silly thing or another that the college educated, highly paid engineers at Triumph were just too dumb to figure out, right ? Don't think so myself. ...J.D.
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post #4 of 27 (permalink) Old 08-24-2019, 10:15 AM Thread Starter
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What "old" Triumphs would those be...specifically? And just how did you get yours "set up right"...specifically? Simply "sniping" with no specifics doesn't help anyone,
Also, if you read the article thoroughly, the point of the "silly thing" is to keep NOT set up right modifed motorcycles using stock carbs from having flat spots. Also, a "flat spot" doesn't always show up on a butt dyno. You practically have to ride yours back-to-back with another that is carbureted correctly, (preferably with an on-board A/F meter) to be certain. Lastly, those "college educated, highly paid engineers" deserve full credit for being bright enough to get it as right as they did (TPS and heaters notwithstanding) and still meet emissions standards Doesn't mean it's perfect and doesn't mean there's no room for improvement.
PS - For the record...the "silly thing" was invented by a PHD. Just sayin'!
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post #5 of 27 (permalink) Old 08-24-2019, 01:16 PM
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Pages apon pages have been written on the subject (probably wrote some myself) on this site alone. No sense in taking up more space on the subject. Use the search feature. Carburettors were invented by pre-historic man. How hard can it be ? He didn't even have a computer.

As for my bikes (ONLY because you asked):

2002 America (Keihin carbs, pipes, and jetting mods)
2000 Legend 900 3 cylinder (Keihin carbs, jetting and airbox mods)
1995 Sprint 900 3 cylinder (Mikuni carbs, jetting mods, air box mods, big exhaust, ignition mods).

But, on the other hand, I don't have a PHD, and I cannot play the guitar
...J.D.

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post #6 of 27 (permalink) Old 08-24-2019, 03:22 PM
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The Thunder Products web site would inspire more confidence and credibility if it explained how this wonder gadget works.

Marty
2005 Bonneville Blue 790cc, AI removed, Staintunes RC, no snorkel, inlet enlarged, 118/40/NBZT needles/1 shim/3 turns, Ikon 7610s, Ricor Intiminators, Barnett green clutch springs, Michelin Pilot Activs, D9 gauge panel, tachometer.
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post #7 of 27 (permalink) Old 08-24-2019, 04:54 PM
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Quote:
Originally Posted by Baltobonneville View Post
The Thunder Products web site would inspire more confidence and credibility if it explained how this wonder gadget works.
I DID look at thier website. I have been working on carburetors since the late 1950's. I was a professional mechanic for some years. I have 5 years of engineering at L.A. Trade Tech. In my now retired years I play with motorcycles and sports cars. All I can say is what a LOAD of cow excrement THAT site is ! Just my ignorant, uneducated opinion of course. ...J.D.
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post #8 of 27 (permalink) Old 08-24-2019, 06:50 PM
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Looks like it's gonna grab some fuel from the bowls and spray it into the air entering the carbs, so enriching the mixture. Works by vacuum, with the little dial controlling at which vacuum pressures the thing opens and closes? That it, am I close?

Seems to be about $200 for a pair of CVK 36's ... you can get a Bonnie pretty damn close with jets, needles and shims. That's a lot of jets, needles and shims to experiment with. I mean, it's still gonna ride like a stock Bonnie with TORS either way, not a bored out motor with open pipes and a hot cam, which would require something more than a CVK to run fast anyways.

I get why someone would drop the money to play with something like this. I get why folks spend time and cash to get their 50HP Bonnie up to 55 or 60. I mean, it's still a Bonnie and not a sport bike, but the tinkering is fun. Personally, me, I like my performance modifications like I like my women: Cheap.

Plus, when I just asked daughter if I should buy a Dial-a-Jet she asked if it would allow me to make phone calls to airplanes, and when I said no, she said why would you buy one then.
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post #9 of 27 (permalink) Old 08-24-2019, 07:15 PM
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Probably pretty close. Still kind of a stupid idea in my book. Jets are cheap. I used to have an adjustable main jet for the old VW Solex carbs laying around the shop. I ran it years ago on an engine, but I found that once you set the mixture, you were done. Never touched it again. Would have been just as easy AND cheaper to just put a bigger jet in it in the first place. Took it off an threw it in the junk when I rebuilt the carburetor. It layed around in the junk box in my shop for years. Don't know if it is still out there or not. Expensive scrap brass. ...J.D.

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post #10 of 27 (permalink) Old 08-25-2019, 11:03 AM
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Quote:
Originally Posted by Wire-Wheels View Post
I DID look at thier website. I have been working on carburetors since the late 1950's. I was a professional mechanic for some years. I have 5 years of engineering at L.A. Trade Tech. In my now retired years I play with motorcycles and sports cars. All I can say is what a LOAD of cow excrement THAT site is ! Just my ignorant, uneducated opinion of course. ...J.D.
Yep, it doesn't explain anything. The reader is just expected to take it on faith. If the product were as good as they claim, we'd all be using it already, I think.
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2005 Bonneville Blue 790cc, AI removed, Staintunes RC, no snorkel, inlet enlarged, 118/40/NBZT needles/1 shim/3 turns, Ikon 7610s, Ricor Intiminators, Barnett green clutch springs, Michelin Pilot Activs, D9 gauge panel, tachometer.
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