Going Single Carb? Read this first.

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dan filipi

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My Bike Models
1983 Interstate
2018 KLR 650
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https://classicgoldwings.com/forums/dan-filipi.122/
This thread is a collaboration of many years of information built from member experiences and studying the history of carb tech.

There has been plenty of completely wrong info on what a carb is and does, and what it takes to be one. This makes no difference whether you're talking stock carbs or others as it applies to oldwing motors ...

Carbs have been around for over a hundred years ...a basic carb is a very precise tool for metering the correct amount of air and gas to burn efficiently in an internal combustion engine.

Many have experienced how the stock 4 carb system has degraded to such a point that the bike flounders so bad at low rpm that slow RPM riding becomes an ordeal. With a stock CV carb system, they rely on vacuum to pull the slides up to increase air flow and then gas charge. This is a relatively slow acting process.
Honda provides for sync tuning of carbs so at a certain rpm you can adjust vacuum. This sounds great but does not fix uneven cylinders. As soon as rpm goes away from the synchronized spot it's right back to uneven. This is just a very poor arrangement and is a degradation to decent gas charge in the engines' capable rpm zone. A good carb system will provide a good fuel/air mixture at idle and stay even to top rpm.

Vacuum like a lot of things on engines are not consistent, it is different cold than it is hot like oil is different from cold to hot, so its not a very good thing to build around at all...especially x4 in a stock set up.

Vacuum added to carbs got big in the sixties and was added to two stage carbs as in 4 barrel carbs. They added vacuum to engage the throttle plates to open the back barrels of the carb.

This is when carbs started going into less than a carb territory. It wasn't long before they started making non adjustable carbs making them so owners could not tune or modify them. Oldwing carbs made stock were deep into this user unfriendly induction system, as the years went along it also spread to ignition timing...

These two factors of stock oldwings made many want something better, as well new parts are getting more scarce, and no service by Honda for these bikes..

This has ushered in many who want something simpler and adjustable, and more performance driven..........
 
With the lack of mother Honda support for these oldwings, SCC (Single Carb Conversion) has gained a lot of interest.

One problem is most of the so called single carb conversions available use no better carbs than the stock Honda 4 carb induction system that use CV slides and vacuum enricheners.
Many carbs from the 70's on are user unfriendly and made to not adjust, run lean ...they put devices on them to shoot gas in under load so the lean condition wouldn't trash the motor, mostly meaning engineered not to dial in right for an internal combustion engine, especially not an oldwing.

Many put these carbs on thinking they were carbs that simply metered air and fuel with very little if any understanding what is going on in that carb. Fact is carbs made in the early 1900's were much better than these user unfriendly carbs.

Thus started the whole myth on the internet that it was the manifolds fault not the carb...pushed by some who actually know very little about carburetors.
 
As things went, many were convinced by others that single carb oldwings were not going to be as good as stock carbs. This myth was pushed hard by a seller of aftermarket parts for stock carbs and many others on the forums. Some would say "you can't mess with Mother Honda".

Interestingly this was mostly true as most simply bolted on a car carb that has very similar faults as the stock carb system, and the car carbs were no where near set up for an oldwing.

Joedrum on his first try to use a single carb, he cobbled together a manifold that leaked. With some tweaking of the 32-34 carb he had he "tricked" the carb into running decent and saw right then that all the discounts of the single carb set up were false, the 32-34 carb was revving the oldwing motor like no stock system ever could. From that point on he never considered stock carbs as even in the same game.

Dan Filipi and others got interested to see what could be done, with similar results especially in the low RPM torque zone. This started a collaboration at CGW that was totally different from all other forums as we worked together, compliant not defiant, to kill the myths about SCC.

It all boils down to learning the straight carb and what it's secrets are. Like almost all manufactures used through the fifties before dumbed down tech started reinventing the wheel, so to speak and ruining the basic carbs simple functions.

A straight carb without the gadgetry is the only carb that can go from one motor to another if motors are reasonably close in size. HUGE FACT THIS IS.
 
A "straight carb" as used here can be defined as a device that only needs air flow to work and almost no moving parts, and from that will produce a near perfect charge for an internal combustion engine. Nothing else needed but air flow.

The Weber DFT carb is in no way a straight carb and is not "bolt and go" as led to believe on the forums. Yes it will run but it's not capable of producing the results of a well calibrated DFT.
One thing tried in jetting to make it run right had bad effects elsewhere. but after yrs of trying things we (mostly Joedrum) finally chartered a path that essentially turned the DFT into a highly adjustable straight carb, and have learned what a well calibrated carb is capable of.
Dan purchased a 40's era jeep carb reproduction. When we made the right jet move, the carb was dialed in. It took very little experimentation with all that we learned.
From here all things are about straight carbs and what it takes to dial them in and how simplistic means best especially up against FI and computers, which are anything but simple.
 
Most carbs out there being used on oldwings do not adjust enough for oldwing use to run acceptable especially at idle around 1000 rpm. In particular focus here is the Weber DFT models.

The DFT idle circuit and off idle circuit is combined together.
This can be okay on cars it was originally built for because the engine was required to power accessories like A/C and power steering at idle which can take a heavy load at idle and not run over rich.
The oldwing motor simply idles and spins a stator for it's idle load.

Idle is the hardest running point to dial in on a oldwing engine as it runs the slowest here. The idle circuit has a very narrow window the engine draws from. The most precise adjustment is here, which is also the most difficult.

if idle mixture is not right ..going further is tough as it has to overcome the bad idle first. On a DFT, a rich mixture is tough to get past. Running, the carb will load up and die if left idling very long. When revving engine it will stumble as it clears this over-rich mixture. Spark plugs black.
Some have had great success with DFT models, others not so much and scrapped the idea.

The following is what Joedrum has to say about his experiences tuning the DFT.
I have helped him with spell check and readability......


To be short, the idle and off idle circuit on a DFT is way too big to idle a oldwing ...it takes a certain pull to makes this circuit work and once that happens it's already way to rich ...they also have fixed air bleeds or non changeable in a place where you need to be able to adjust everything.
If you try and cut the circuit down to lean it ..the motor just quits as you have cut so bad it wont operate the circuit anymore.
I'd say there are many DFT carbs running around operating too rich at idle ...heat added to some to help band aid this problem ..runs okay elsewhere and gets poor mpg ..as we spend most of the time running in this to rich idle circuit....

On my carb i've drilled the idle gas holder to have a air bleed right there where the gas is drawn from the carb primary well tube ....so i'm getting more air and it makes the idle circuit just like the primary and the secondary wells ...with air bleed.
I also have separated the idle circuit from the off idle circuit by putting in a jet just below the off idle feed ...this cut flow to the idle circuit ...and this changed the need of a mix screw totally ....in fact it is totally gone and i have an air adjuster put in its place for even more control ....result bike runs perfect down low and all thing are totally adjustable ....

From there the dft carb is stripped of power valve and bowl venting mods are done ..this carb can be adjusted per motor needs..my bike runs great and suffers no errors anywhere in its rpm range ...none ...

There are many reasons i like weber carb set up over slide carb set up ...mostly slides have many moving parts and Weber emulsion tube set up have almost none ...im not here to push anything other than it takes a strait carb to really do a good SCC conversion ...whether it is bought like dan did and all it took was dialing in or it is made like i did on the hooch bike .....

It just must be known when its SCC your after ..its the carb that has to get right ...
All one has to do is look at manifolds and it's obvious just about anything works if the carb is right.
 
I forget what the so called perfect air mix is . To me its plug color is the best indicator of carb tuning. Its very precise and cant be 1/2 assed. So I'll speak of this as i had to learn it all with Hooch ...Hmmmm 14 to 1 i think is proper mix numbers ....

As stated earlier, idle is hardest part to get right. If the carb you're dealing with has lack of adjustments here as in air bleed and gas jetting or it's system is so big it doesn't work till its already too rich for motor, Then you are dealing with an epa carb or a carb with just too big a circuit. On the DFT carb both these factors were true. No doubt it took me a few yrs trying things to get there with mods that address the problem.

A carb has fast moving air through it. When like on a dft the throttle plate opens. At idle the plate is basically closed. On my carb the idle circuit situated below throttle plate I can get idle with plate completely closed. Once you start zeroing in on a carb you find there is a window of adjustment to lean a bit , richen a bit, or raise rpm .What you shoot for once here is how fast you can get the rpm up by going from idle to off idle ...

So one step leads into next step ...

Along the way on my modded DFT, off idle and accelerator pump blend to create fast revving into primary main circuit. Which consist of tube with holes with air bleed jet on top and gas jet on bottom. These two adjustments need to keep motor rpm climbing. This circuit empties right into the center of the primary barrel as it power to meter gas charge all comes from here. When you're jetting here its about the last place of reasonable riding and checking plug color as it is at this spot the hooch bike is flying past all speed limits ...i did manage to get good reading ...from plug color and feel of power ...the jetting turned out to be huge over stock car carb set up. Double or very close to that .....

Well from here I sort of looked at numbers and tried stuff as I was in the land of opening the secondary to the game .Secondary has an idle feed ,mostly because this carb is made for an economy car there was going to be times the secondary was going to be open just barely. Stock carb set up is 60 gas jet. I'm thinking mine is right at 100 or 90 in that range my memory has faded. I have completely shut this circuit off and it still works but bounces hard and it bounces hard now. So I've seen the advantage. It works good on Hooch to the point secondary hits so hard and smooth hanging on is all you can think of. From here in the main secondary circuit all my test were mostly done in second gear. It the only place on Hooch where this testing is not life threatening. Again not sure but I think secondary jetting on the hooch bike main is 190 gas and 160 air bleed. Stock set up for car was I think 95 gas jet ...and 220 air bleed ...So this is way more than double the car set up .Even though the motor was bigger than a oldwing its less rpm capable .....

I've made other mods like secondary kicking in sooner than stock and other things like the stock gas bowl venting was way down from what the oldwing demanded. Lol this one thing took awhile to figure out. So dialing in a carb isn't stone age crapola at all.....It is the height of perfection and simplicity.

I'd put hooch up against any of todays tech when it comes to induction and faster than all in execution time to revv to top rpm. Also driveability is second to none....

It is totally independent system that only needs air to work ...and is super reliable as Hooch has a real carb not a hack dft as manufactured. Plain and simple rules ...and it makes the biggest power the quickest ...

Comments welcome.
 
until this type of thinking prevails SCC will always be mixed results ...and thats a shame ...

the definition of dial in is ....good MPG....plug color of good burn .....and no bad spots fron 0 to top rpm ...... :rtfm:
 
Wow really nice read I had no idea the involvement in the carburetor you are a very smart intelligent talented person I wish I had your skills and knowledge as well as others maybe someday I'll have a little bit more I can only hope now do you guys set these carbs up and sell them cuz I sure would like to buy one but once again thanks it was a really good read lot of good information y'all are geniuses thank you for information to keep these machines running it's nice to know that I can trust this website with honesty no gimmicks just truth and facts just awesome I have so much to learn thank you all I can't wait to try some of these out on my own bikes thank you
 
Wow I had forgot about doing this thread with the help of dan ....I once thought to modify dft carbs to sale ...but I just don’t have the resources or stable place to operate from to do this ...life has it way of being a stick in the spokes ....I can tell you I’m nobody special ....anyone can do this ...my biggest asset is smart hands ....
 
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