Motorcycle mechanic here: these carbs really really like to have a air inlet tube / velocity stack to make the airflow more directional , Just 2 inches can be enough with a pod filter, and this aids flow through the internal passages. also the low RPM air fuel ratio is adjusted by the mixture screw on the side, this can affect fueling upto 1/3 - 1/2 throttle (the CV slide not the butterfly)

You're missing a big tuning CV carbs: the needle. On variable venturi carbs, the needle taper has a huge influence on fueling. The main jet size will have a big effect on high RPM WOT fueling, the needle everywhere else. If you are lean at high RPM WOT, you need a bigger main jet. If that then causes you to be rich at low RPM, you need a needle that has a larger diameter at its base. There are tons of different needle tapers available. Many needles are also adjustable. If you take the diaphragm off, you will see the needle is retained with a clip. Adjustable needles will have multiple grooves. Setting the needle deeper into the carb body (a higher groove) will lean out low and mid RPM. Pulling the needle up (lower groove) will enrichen low and mid RPM.

Needs a fatter needle on the end , generally they are twin tapered. Drop the pilot one size .

Needle tuning is the most important on that style carburetor. You're overlooking a huge step. The main jet only does WOT

Tuning a CV carb is not just about the jet sizes, The needle shape is an important part of fine tuning. Also, washers can be used under the needle to raise it up if the needle does not have an E clip and adjustment grooves.

You need a bigger main jet! Main jet setting is for wot mixture. You need too drop the jet needle down or replace it with one with a fatter profile. The stock one should have a c clip and 5 positions to raise or lower. The emulsion tube is called a needle jet on that style of carb and usually isnt changed when tuning. You can also shorten or streach the spring in the slide to effect the venture sizing. Typically we shorten it when tuning for more fuel on modified bikes.

Definitely listen to people talking about needles, but also make sure you have an air box. On a bike, that air box keeps a large volume of relatively stable air for the engine to draw from. That gives consistency to the air part of the equation which is *far* less consistent with even a pod filter.

Most all older carbureted 4 wheelers/atvs use that enrichening valve setup versus an actual choke as well. My father argued this fact with me vehemently one day when I told him his wheeler didn’t have a choke but rather a small valve that lets more fuel in. He just refused to believe that the little lever that said choke wasn’t actually a choke until I had to literally pull the carb off and show him that there was only 1 butterfly in the carb and that was the throttle, and the “choke lever” he would move was just a cable that opened a fuel valve. 🤣

I don’t know about your carb but the SU type has a selection of different shaped needles for running. Different needle widths at different heights adjust the mixture for changing loads.. Great shows keep them coming.

you're an excellent narrator and a good script writer. concise, funny, and surprising

Man, the needle controls the mixture between 1/4 and 3/4 throttle. You have different needle tapers and, sometimes, you can adjust needle height by changing the clip that fixes the needle to the sliding cilinder (we call that the "campana"). What I can tell is that you should lower the needle a coupla points, or change to another with less taper. Opening the idle mixture valve (on the side) can also change mixture richness. You can screw it til it tops, without exerting any kind of torque, then unscrew, say, one turn. Usually it is a turn and a half. That will make mixture leaner when the throttle starts opening, but will also affect idle.

My favorite before church show to watch! I love how well thought out your projects are and how well they work.

Hey jimbo, lower the needle hight to remove the rich site of the curve and put a airfilter to make i run a bit richer and make the venturi work better. Keep up te good work!

a tear rolled down my cheek watching the Saturn get towed away.

Before we get to the end of this, can we at least contemplate a super charger pls

Hi Jimbo, I think the best way to correct the "lean/rich" curve you have is to try different postion of the needle, or buy one with a different curvature. But, as you say, if you can't get a rich mixture anymore with the main jet you need a new polverizer. Also, it's more easy to adjust carburation with an airbox...

I used to have a 65 Mustang with a Holley 600 on it heavily modified😮 I had it set so well that I could drive around on the primary two barrels all day without ever opening up the secondaries🎉 right around 30 miles to the gallon easy🎉 when I stepped on and it was like nitrous🎉 God I miss that car

The air/fuel screw is where you adjust off idle and deceleration AFR. If it’s on the motor side, it adjusts fuel. If it’s on the air filter side, it adjusts air.

Good bye dear Saturn. Your contributions to Jimbo's mad science church will not go unnoticed and unrecognized. You'll be missed and remembered. ✊😢

To the guys saying "bigger main jet" - the main alone doesn't control wide open fueling on these - its only allowed to if the slide pulls the needle far enough up. If this carb is set up for a bike its probably set up for a larger cylinder displacement, more compression and better high rpm head flow characteristics. That means its going to have a long, thin taper to the needle and a fairly stiff return spring over the slide. No watch what the engine is doing. Idle is fine so its in the ballpark for carb sizing as its set correctly without messing about with it. Then it goes rich at low load - suggesting as the throttle is open at lower rpm the fueling is still correct - though now not so thanks to the larger jet. This points to the carb behaving correctly and putting a normal sized main back in would correct this. Its then leaning out more and more as rpm rises. We know there is enough or too much fuel because it goes rich lower down so its a fuel command problem rather than one of availability. That means the slide is coming back down as rpm rises and the engine's flow dropes off. It only needs to come down a few mm and it will start closing that main which will then drop power and the cycle continues. The only way a bigger main fixes this is if its so large there is bypass of the needle. Thats not what you want on a cv because atomisation is then in the toilet and you're going to be on a good mixture but wondering why its got no response.

Use small washers to space tge needle from its sest on the diaphragm slide. 1-1.5 mm sould make a noticable difference for mostly free. Or get a set of adjustable needles. The 1st gen SV650s did the washer mod after exhaust to fix a up top lean bog.

The way you narrate your videos instead of talking in them is an odd choice for "cartube" but it really works. I'm not sure many other channels could pull it off, but it's kind of a staple at this point.

“…acceleration in a hurry” What a quote! 😂😂😂

I just love your sense of humor and how you teach us about what's going on so can fully understand your struggles

As a guy who's been working on bikes for 30 years I agree with the general consensus: Drop the needle, bigger main, play with the pilot and idle mixture screw to tweak the transition. Look at needle profiles as well. As someone who's put 2 48mm CV carbs from a VTR1000 on a Volvo B18 engine (It's LeMons, don't ask) the emulsion tube is one of the last things to change... Love the channel

I never fail to learn something while watching your videos, thanks Jimbo.

Short is ok. This is worth a second watch the view the very simple and easy to understand explanation of how a carburetor works. And the motor sounds really good.

Excellent work as always, Jimbo. Always look forward to your Sunday videos. Sure sucks to see 128k views and only 1k likes though. You deserve so much more for all the work that you put into your videos. Thanks for always keeping me entertained!

Loving your stuff, very interesting solution to the two bolt occupying the same place in the space time continuum. Learn something new every time I watch, looking forward to next week!!

Thanks for all your experiments Jimbo.. lots of subtle and not so subtle lessons to take away.

I don't know why I love your channel so much. Normaly I am a big bore 2 stroke guy but there is no Sunday without Robot Cantina. Thank you for the Videos and greedings from Germany.

I LOVE this channel, the work you're doing. I have learned so much (and I already knew a bit)! Please keep up the most excellent work!!

Great job explaining how the carb and venturi work BTW 👏

The variable carb thing looks interesting, i was wondering if that dip you see in air fuel has to do with how that venturi restriction opens up. Then you said emulsion tube and I sat back to learn more.. Looking forward to next week!

Jimbo. In addition to what others have said about different needles. The Keihin CVK is also very sensitive to air filters/boxes, so do you jetting with one fitted otherwise you'll end up having to do it all again

What a very good explanation about the variable veturi and why it matters to the small engines, thanks😊

Great work professor!

Jimbo you rock ! Thank you for entertaining the masses!!

Thanks for keeping your upload schedule. Nice to wake up and watch this with my Sunday morning coffee.

Once again I was left wanting more. Great work, the isssues will probably be solved by tomorrow and can't wait to see those numbers. Also RIP saturn. Died like a hero, teaching us very interesting stuff.

Thanks for another great video Jimbo. It may feel like you ended on a low note, but that's just part of experimentation and progress. Now you have more things to play with and make videos about. :) RIP Saturn. You were a great project car. You lived a good life.

I've essentially got that same pump/regulator/return line setup on my 350 S10. Works fine! I've experimented with a number of EFI systems and carbs on it over the last 15 or so years, and didn't want to drop the high pressure pump when I went from EFI (~55psi) to a carb (~7psi).

Some of the best edutainment on KZfaq! 🤘🏻💀🤘🏻

You need to lower the needle a couple of notches in order to lean off the part throttle mixture (raise the clip), you can then go bigger on the main jet in order to get the WOT right. The other thing to note is that you really need an air box on the back of the carb, the reason is that it is the slight vacuum in the airbox is 'transmitted' to the top of the carb via that hole at the back of the carb in order to lift the slide, at the moment I doubt your slide is lifting fully. This is the downside of a CV carb, there needs to be at least a small restriction in the intake side in order to create a vacuum at the back of the carb so that the diaphragm can lift the slide, a pure flat slide carb does not have this issue, of course on the plus side a CV carb will tolerate ham-fisted throttle abuse much better than a flat slide carb. Personally I would make sure that the slide is lifting fully at WOT (maybe via a camera), I'd then get the WOT mixture setup to around 12.8:1, I'd then use the needle to lean off the part throttle to around 14.5-15.5:1. If you can not get the part throttle mixture right then you are into using different needles which would get expensive!

Loving the Renault series

Love this channel! I too find nitrile gloves work best with holes in them!

Needle will help but you also need some sort of velocity stack on there. Even if it's a stock air boot. The turbulence at the mouth of the carb will scree with the jetting. Going from a pod type filter on the back of the carb to a velocity stack with the pod filter on the end let me drop about 10 jet sizes and it ran much more consistently.

Tuning small engine carburetors can be quite the art. I used to tune motorcycle carbs way back in the '70s, and it was always a delicate interplay of the main, pilot, needle and emulsion tubes. Of course we didn't have wide band ox sensors then, we had to make precision throttle runs and read the plug color, plus a lot of seat-of-the-pants measurements. I see people just throwing on a random carb without tuning and wondering why it doesn't run right without understanding why.

I had an SU carb on my turbo car in the early 80's - no EFI back then, really not so many turbos either! The tuner bloke mapped the power/AF Ratio across the band and honed the needle down - in a drill with sandpaper! Worked like a charm.

Been a long time since I fooled with a motorcycle carb, and actually, I never had jets to put in either car or motorcycle carbs. Always worked with what I had. I particularly liked the graphs you came up with. Now I know I need a lot of stuff if I'm going to tune a carb.

Jesus Christ Jimbo, that intake manifold fix was just genius. Hope you can solve the issues with the tune soon. Greetings from Spain!

Oh man!!! Now that’s what you call a cliffhanger. Can’t wait for the next Sunday.

5:37 I know it may seem like an obvious fix, but I love the solution you came up with. I don't think I would have come up with an idea like that if I was presented the same conundrum.

There's plenty of videos on youtube of lawnmower carbs on V8's gettting really good fuel economy. The higher displacement and small venturi make the air velocity very high and allows for good mixing.

Another great video, thanks for sharing 👍.

Thank you for another great video.

Bye bye Saturn. Good job on a ton of work, wonder what the results will be next week. Keep up the interesting videos.

We use to put a fine screen between the carb and manifold to annimise the fuel ! I think your leaning out because of your fuel routing system , all you need is around 3 lbs of fuel pressure , so put the restrictor between the card and pump and dial it down without the return line ! I'd ditch the governor also !

Another great video thanks for sharing

Geat video! I am using the exact same fuel system in my old John Deere 25 hp.

Plenty of comments have noted the importance of the needle and airbox / filter on a CV. I'm just here to +1 :)

Vroom vroom! Back at it again!

Nicely illustrated work on the carb explanations. Recycle in peace little Saturn.

I love ur channel, keeping doing this stuff its so entertaining

Love your videos. You are a great teacher and shovel worthless knowledge into my skull in every video. I still think you should have put the worthless Satty on E-Bay or BAT and made a few bucks off it. But you do you, Dude.

Great content again, thank you! 🫵🏻❤👍🏻

Really awesome stuff Jimbo, those motorcycle carbs are really interesting, anyways keep up the great work man

Your crickets are making my crickets cricket.

Love this series.😎Good carb choice it's really hard to oversize a CV carb. You should make sure you have a 5 or 6 inches of tube on the intake side of the carb to contain the fuel reversion . If this is not done you might have some issues with the transition from idle and may need to almost double the size of the idle jet. Your main is too small. 0 to 1/4 throttle is idle jet, 1/4 to 3/4 throttle is the jet needle and needle jet, 3/4 to full is the main. Set the main first and work backwards. You shouldn't need to change the needle jet. More likely you just have to change the height of the jet needle. But set the main first.

Love the new carb... You'll be able to get er dialed in! Much easier to access than a 4 cylinder motorcycle!

Great solution on that manifold Jimbo!

You really know how to keep us hanging. lol I can't wait to see the solution to your carb tuning problem. I'm guessing that once you get the 670 running like you want that performance figures might not be that much different than the stock engine. A few performance enhancements might even yield faster times than the Renault's factory performance. Thanks for sharing. Phil

That fabricobbled carburetor support is going to force the float off it's seat and hammer the slide. Youre better off leaving just the boot. Also, if the needle doesnt have multiple grooves shim it up off the slide with a tiny washer or two.

love these videos. only thing like it on youtube

This channel reminds me of Dave’s farm from long ago, except the only predator in these videos is the engine 😂

There is so much aftermarket support for these engines. We would all love to see you do valve springs and the biggest CAM you can fit. Or they also have race ready heads for these as well. Heads are best but cams are great as well or if wanna go cheaspeat and easiest then ratio rockers are good as well

Jimbo here's an experement to find out whats causing it. Jam the slide open and see what happens. If it fixes the lean issue you need a softer spring in the slide. Of course it will run like total garbage low down but its an easy way to figure out what to do next.

The needle taper on the plunger is also part of the tuning on the carbs. since it changes the opening as it goes through the motions of the oceans. Not sure if you interested, but I have a carb that I think will work well for your application. Its from my Buick/Opel by isuzu, comes from 1.8L engine. has tiny primary and a little bigger vacuum secondaries. Though would need to make the intake adapter that would be at least 500cc in volume to keep it happy.

I definitely think down a notch on needle for part throttle and possibly up a size or two on main jets.

Hi Jimbo. Love your work 👍

I like how you make brackets that mount to exsiting hardware. That's how I like to do it, but my friends just like to hobble stuff together that causes problems down the road.

Love your videos

Try stacking some washers under the needle in the cv portion of the carb. It will clear the main jet sooner. That with the smaller jet for off idle might help. Part of what we had to do to tune bikes after making changes to the intake and exhaust. Another person mentioned the air cleaner with a velocity stack. Cooler air might help.

I felt like saluting, as the Saturn was loaded up. Will go down in KZfaq lore.

you must have some serious ab muscles being able to hold that sensor forward like that during hard acceleration. lol really enjoy your videos. Keep up the great work!

First of all: I am by no means an expert in this style of carbs but if I understand correctly the negative pressure opens the venturi. If this is true, you could also try to get a valve in between the low pressure side and the system which would make the venturi adjustable. Just an idea..... Another idea: In theory it should be possible to rig up an injection system to counteract the lean mixture like Kugelfischer did way back when.

Great effort to gain the best performance on this cute little car. Jetting carburator sucks!!! I allready removed mine about 40 to 50 times to change jets or adjust floater...brrrrr. Cheers mate you do a great job. This little car is perfect for Farmer Markt shopping day in your village. Greetings from Barcelona...we Spanish love Renault.

Jet back down and raise the needle or get a lighter weight spring in there. Its going rich because the main is too large then its leaning out because as the rpm rises the engine isn't making enough vaccuum to keep the slide fully open. The emulsion tube is just helping atomisation - it has no real say on overall fueling until you're really far off what the carb is set up for.

Well done Mr 😎

The main jet needle insertion depth is another area the controls the A/F Ratio.

Thanks again Jimbo! 👍💪✌

Another great video sir. All I can do is "parrot" what other commenters have already mentioned, in that, you need a different tapered needle and an intake of sorts. Have fun projecting! I look forward to the next one.

Always like motorbike carb experiments... sounds good with the Jimbo-povic muffler

Absolutely fun journey

Years and years ago when I was racing an MGB with dual SU carbs, we would be messing with needle profiles at this point. It was pretty much guess work. I wish we had a wideband gauge back then.

lower the needle inside the slide, that would require you to raise the snap ring to a higher position on the needle. I'd go two for as rich as you were. Awesome content, and good luck with your project.

The needle is probably your issue, not the emulsion tube. the needles vary in taper and clip positions. :)

I got that same electric fuel pump for my mower due to the same issues on one of my garden tractors. .. I was also planning to add a return line. Wasn't planning on the 'dead head' regulator, was maybe going to put a valve on... But guess I'll add that to the list.

I gotta go to work so I'll finish the vid later but you need two of these carbs. I can explain why later but if staying CV you should find yourself loving that size x2 or one 45mm flat slide like an hsr would be great too. Also 2 32mm CV carbs is still choking this thing. 670cc ÷2 335cc and on motorcycles we normally see gains going bigger than stock and stock for honda would be closer to 36mm although I'll admit the 670 has trash ports and heads and the runner length matters dont make it too short if you do go with two

I was expecting the gnomes to return to wave goodbye to the Saturn, and maybe even go inside and leave with it.