boring the venturi

[Anonymous]

i have a 1bbl carter YFA...actually aa couple so it doesnt matter if i screw one up...i was wondering if i could grind away as much meat as i can? it definitely wouldnt be round anymore but i could smooth it out still real good. does it matter if the venturi isnt round? should i go ahead and try it? will this help me out at all?

 

[rickwrench]

I have bored out many corvair carb throats to stinger specs. Round is the least restrictive, oval is close. Smooth is the key word, without obstructions.
Rick(wrench)

 

[Anonymous]

thanks alot rick...I'll be gettin to work on it right away then.

 

[Anonymous]

well i got it done and on the car...i managed to grind/cut away about 1mm-1.5mm of the venturi bump, and ground the bump on a steeper angle. i polished it up where i could and just used a knife and gouge to smoothen up the rest where i could barely fit it with my dremel, since i dont have small polishing bits for it. its installed and it runs a heck of a lot better than the other carb, although the other carb was definitely on its way out...i havent taken it for a test drive yet, but i certainly love the way it runs in idle too now...tomorrow i will take it for a test run, what kind of gains in cfm do u think it may have made? doesnt seem like it would be that much since the hole is only a little bit bigger, maybe like 15 cfm more?

 

[Anonymous]

:x you made me think........



















dang you now my head hurts and I still don`t know what I was thinking about , I need some sleep .

 

[Anonymous]

well i took it for a good drive...it needs some work...maybe u guys can help...i dont think the problem is cause of a rough venturi since its not really that rough....but it feels like it lacks power...not much but it feels more like it than the last carb...my last carburator was also a carter YFA, but was sorta messed up, the car was alot louder with the last carb...this carberator that i just did is better running; its a smoother and better running engine...i hear the manifold air being sucked even louder, so i dont think thats a bad thing...i've played with timing and have it about 3-4 degrees before pinging, so it runs smooth...actually it sorta feels like it bogs from about 10-25 mph...and every now and then it surges like its had a good breath of air...but other than that its smooth and acellerated without a dead spot or anything...

when i rebuilt my last carburator i thought that there was a little ball bearing in the hole where the fuel comes out before it actually goes through the main jet...is this right...i didnt have it on my last carb, but i dont se the difference it would make if it was missing or not, except possibly the fuel amount that comes out...i had grabbed a small bearing from my dads 2BBl carter (leftovers from a rebuilt) and stuck it in...it still quirts out fuel so its not blocking anything, but would that decrease the amount of fuel being squirted out of the jet? it still squirts a good solid squirt, i dont think the bearing is blocking the jet or not, but could this be my lack of power that i feel?

my new hood is built similar to a ram air for a dodge ram...i have a hole cut in the hood for putting in a custom ram air system, but the ram air setup isnt there yet...at the moment would the air blow right through the filter and sorta draw the air out of the carb rather than in...seems like thats how logic would have it, i'm figguring out a ram air setup for it at the moment..it might work out...but that, i'm sure, could be a problem as it is.

the problem may be both, but i'd like to hear others thoughts as well before i go disassembling stuff again.

 

[rickwrench]

You'll likely have to play with jet sizes a bit. The corvair main jets always get bumped up a few sizes (49 --> 52 or 53, for those that have a corvair) when they are bored. Even a small increase in venturi size makes a big difference in airflow. 1" to 1 1/8" inch is more than a 20%+ increase. Velocity will drop a bit a idle with larger venturis, but Falcons are way under carbed anyway. Yet it idles well... what vacuum are you hooked up to with your distributor?
Rick(wrench)

 

[Anonymous]

I didn't see your post until today. I would think it is a bad idea to bore out the venturi. Maybe there are some successful rodders that know how to do it properly. But the venturi or restriction is critical to the performance of the carburetor. It helps provides a differential pressure across the top of the carb and the manifold. This pressure difference is what some of the fuel circuits rely on to provide the motivation for fuel to enter the air stream. Venturi's are precision instruments designed to operate in a specific range of flow. Enlarging the venturi will decrease the air velocity and decrease both the pressure differential and the vacuum signal on some circuits. I have a feeling you'll be chasing your tail trying to sort out a carburetor modified this way.
Doug

 

[Anonymous]

Rick...i am not hooked up to any vacuum source at the moment, its blocked off...and the carb doent even give off 1 inch of vacuum at any RPM, so the only place i can hook it up to is the manifold...would this make a difference, i will get rid of the ball bearing just to see, and i will check out some larger jets...i should be able to get them at a Lordco i guess....what sizes do u think i should try...i dont know anything about jet sizes so i wont mind getting a few, i have taken out a jet on another carb so its not too bad....but i can idle at 400 RPM without a hickup...so you're saying that it will bog in the low end just a bit due to the increase in velocity?

I would think it is a bad idea to bore out the venturi. Maybe there are some successful rodders that know how to do it properly

the reason this forum is so successful is because people play around and do things themselves, and in turn they can give first hand knowledge on troubles and victories they have encountered...besides...i have enough extra carb parts for the YFA, if i screw one up it doesnt matter, i'll just throw on a stock one again. 8)

Enlarging the venturi will decrease the air velocity and decrease both the pressure differential and the vacuum signal on some circuits

what are circuits lol, the reason i love my car and engine is because there are no circuits...its just tooooo easy to work on, and modify without worrying about bloody circuitry....but thanks for your concern nonetheless

 

[Anonymous]

The circuits I was referring to are the idle circuit and cruise circuit etc. Various flow paths or circuits through the which the air (vacuum) and fuel meet. For the same amount of air ingested, the velocity by an enlarged venturi will be less and the signal will be less for the same amount of airflow. If the signal is decreased, the carb will think that there is less air travelling through the carb and the mixture will be lean.
(OK maybe carbs don't think)
Enlarging the venturi will benefit the upper rpm ranges more so than the lower rpm ranges.
Doug

 

[CZLN6]

Howdy Dave and all:

Enlarging and polishing the venturi is an old and new Hot Rodders trick. There is no smoke and mirrors here, just common sense tuning. Remember it's the area that makes the difference, not the diameter. A little diameter change makes a big change in area.

There are two, possibly three carb circuits you will likely have to fine tune because of enlarging the venturi. Some circuits are internal and difficult to access. Luckily all circuits you need are easily accessible. If you are familiar with how this carb works you should be able to get it very close to right by the seat of your pants.

1st will likely be the main jet, but because of your near sea level location maybe not. Read you plugs at WOT to assess the Main Jet A/F ratio.

2nd will likely be the accelerator pump system. I believe YFs have a rod system to figure out. Most likely, you'll need a bigger shot earlier on acceleration. You'll notice this most in traffic type driving.

3rd might be the needle seat rod setting, which will richen transitions at speeds above and off idle condition.

You may also reassess your knock thresh-hold. You may find that the increased flow has raised or lowered that setting over a stock YFA.

I am NOT as familiar with the YFA version of the Carter one barrel. Does this one allow you to richen your low speed idle screw? If so you may need to richen that too? Short of a good quality ($$$$) Air/fuel ratio meter, your plugs are your best indication of what's happening. Your driving satisfaction is also a good indicator.

Good luck and keep trying the cheap stuff.

Adios, David

 

[Anonymous]

i understand now 66fastback, sorry i didnt understand earlier...makes sense...at least there is a way to fix that...

thank u dave for the help...i connected my dist to manifold vacuum, that didnt fix anything but it idles a bit higher, i also just tightened the hex screw above the accelerator pump, not quite sure if i need to loosen or tighten, i know i will need to open up the carb a few times...i can adjust my low speed idle screw and i have just richened the mixture...i took out the ball bearing and that gave me a dead spot, so i put the bearing back in....i will phone lordco and ask them about jet sizes and whats available...i havent checked the plugs yet, but will do so soon. all in all its a bit better, but it still feels like it has power waiting to be unleashed (whether the power is there or not is another story)....i still feel i need a different filter setup for my car, will a raised filter with a back on it work? i have been getting frustrated trying to find things to make it work...i may try a mini 4" filter version of the ram air system that is typically done, but i dont know if thats suffice. Thank u all for your help, and i hope this works.

 

[Anonymous]

checked my spark plugs...all good, just a light brown color on all of em...
I couldnt get any larger jets for my carter since they dont make those anymore...i was thinking of drilling and tapping for the new style, but there isnt quite enough meat at the top of the carb and it needs to be flat all the way around the top of the hole or else gas will leak out...so i ended up taking a jet out of my other carb and grinding the tip closer to the crown of the pipe...the hole is almost as large as the main hole of the pipe and probably .003-.005 bigger than the origional hole...i think i can go a bit larger and just make a straight through...but the difference from just going from the origional size to what i have now is amazing...it ALMOST feels like ive unleashed all the power...maybe once the ram air is on it will feel better...i dont know if i want to enlarge the jet anymore because i dont know how it would effect my mileage...probably not too much more since i can only enlarge the hole about another .002. but thank u for all your help...i apreciate it...the setup is almost perfected, but its way better than it was...still cant burn rubber off my tires, but it'll take more than a carb to do that anyways.

 

[Anonymous]

is there a place in the states that sells the tube type jets still...what do u guys do with your 1bbls? do u chuckem when it gets to that point and just get a bigger carb? I'm sure some of u gus have changed jets in your single barrel carb...where do u get your jets and for how much?

 

[CZLN6]

Howdy back dave:

My brother and I have been collecting jets for the Autolite carbs, which are no longer being made either. Every time we come by a used/junked 1100 or 2100 we will, at least pull the jets. The Carters are a diffeent breed of cat and jets do not interchange.

With the Carters you might look at a YF off of a late 70s/early 80s 240/300 six. My guess is that they will have a larger jet then your YFA.

Adios, David

 

[rickwrench]

No jets for the Carter? Wow, how old am I again...?
I have a set of small hand twist drill bits that I sometimes use to enlarge the Corvair jet sizes with, among other things, when I don't have the right size handy (jets still available, cheap). It's a one way trip, though. If your plugs look good now, and your vacuum advance is hooked up, I'd play around with the pump shot for a while.
CZLN6 is on the right track with checking the jets from the big six YF.
Sounds like you are getting it sorted out fine. Carbs are fun to play with. It's fast becoming a lost art. If you get good and word gets out that you have the touch, you will suddenly have a lot of friends who need help.
Rick(wrench)

 

[Anonymous]

thanks guys...last time i was at the car parts yard i saw a carter YF, I will go down to see if its still there, carbs dissappear very quick over there though...my smallest drill bit just broke, I'll see if i can find a smaller one, but the small drill bits dont hold very well in the chuck, anyone have any tricks to getting small bits to fit in a chuck..tape? i think tape would throw of the center and the bit would spin wobbly...thanks again for your help.

 

[rickwrench]

The tiny jet bits I use are finger bits. They fit into a little pin vise and the whole set up looks like a screwdriver. The bits are about $1.50-3.00 each, the sets are usually 20 or 40 bits, size 42(.0935") to size 61 (.0390") to size 80 (.0135"). Tiny bits in a drill press always break. Using your fingers to supply the torque makes it so much easier.
Rick(wrench)

 

[Anonymous]

thanks Rick..i'll look around and see if i can find some. i appreciate all the help I've gotten.

 

[Lazy JW]

The main jet in my YF is a #146 which is just under .125" in stock form. I drilled it out to a full .125".
Joe