Bonneville America Forums Forums Technical Forums Performance Mods CR II carb problems

Hey guys. I have been running the CR II carbs now. Well I am having a problem with the acceliration. When I twist the handle to give it more gass it cuts out on me. Like it bogs down if the RMP's are low. If the RPM's are high it will work but kinda jutters a bit. Now it works fine if I "role" in to the gass it will work ok.
I just got these carbs last night and I do not know what size of jets are in it now but the idle mixture screw is out about 1 turn. Can anyone help

One more thing dont know if it will help but when it is running it sounds like it is "sucking air". sounds like when you suck through a straw. Now I have the K&N filters on it and there is Nothing around the airfilters.

Please help. Not sure where to go next. sorry for the spelling errors I dont have my spell checker today!!!

I don't have the CR IIs, but sounds like my carbs did when I changed to the TBS needles. I put a shim on the needles first and it bogged bad at rollon and sputtered when at a constant speed. I could slow rollon, but it really wasn't much better. I took the shims out and it ran perfect, which told me it was getting way too much fuel from less exposed needle. If your screws are only one turn out you probably need to drop a jet size or two to give you some adjustability. Jets are cheap, I'd buy a couple different sizes lower and try them all, then stick with what works best.

Get used to that, you CANNOT slam the CRS carbs open at low RPMs without you bogging down, remember these carbs don't have an accelerator circuit. My carbs are tuned pretty darned close, and I still can't slam 'em, you have to roll into the throttle. About the only thing you can do to help a bit, is richen up your needle setting, but even that won't entirely fix it. My best guess without constantly studying the tach, is anytime under 1/4 throttle below 4000 RPM, and they'll bog. Don't know anything about the whistling sound, but then again, my pipes are darned loud. For whatever it's worth, here are the jet settings that I'm using right now, this is with Freak, Cams, and Pipes:

Slow: 55
Air Screws: roughly 1-1/4 turns out
Needles: 5183, 3rd Notch Down
Main Fuel: 115
Main Air: 230 (I think)

Another thing to consider also, if your carbs aren't synchronized closely enough, it won't have helped the bogging any, and you'll have very rough throttle response at low throttle openings. Hope this helps.

thanks a bunch guys. I took it for a ride today. I turned the screws out to about 4 turns. I think it helped alot. I have more power before it bogs out. But even at 85 miles an hour it will start to bog out, (to much throttle i think.) I do not know what size the jets are because I just got them and I didnt look before I installed them. I will check on them Tomarrow.

Something I forgot to mention; Those air screws only affect the Carbs from Idle up to AT MOST 1/4 throttle, anything higher than that and you start going to the needle and then to the mains. Also, there are easy ways to check to see if your air screws are flowing symmetrically, counting out and matching each side equally is only a start and probably won't be your final setting. There's about a half turn difference between my right and left carb air screws, so don't get hung up thinking that equal turns are the best setting. have fun, when they're set up right, THEY ROCK!

If your air screws are turned out more than 2 turns you are not jetted correctly. The America (270 degree crank) will not run well with the CRs as the std Bonnie (360 degree crank). SUDCO has set these carbs for the std. Bonnie, 60 pilot, 120 mains, 5181 needle. Change to the 5183 needle, 2nd clip position - 58 pilots, 110 to 115 mains, air screw approx 2 turns out. You are over fueling the motor as the America is "Lazier" than the std. Bonnie, it wont take the fueling, furthermore these are cable direct slides, you hit the throttle the slides rise, no delay like the vaccum slides on the CVKs. Further info email me: frank@precisionlithoservice.com

NOTE: if you are running completely straight pipes, no baffles, you are loosing midrange power and torque, straight pipes are for WOT and the drag strip only!!!!You need a degree of baffling to create back pressure and allow for appropriate exhaust scavanging.

Yeah, I'd love to try out a Thruxton with CRS carbs, boy would that be a blast!!!

Ok Guys I checked it today. It has 58 and 110. I am not sure what the needle size is or what clip it is on. I ordered them from http://www.bellacorse.com/ He sounded like he knew what he was doing. Very nice guy. The only problem I found was that when I took the bowl off the 110 jet was laying in the bottom. Maybe the problem huh! Well I put it back in and I am going to go try it out today.

One other question. Would these carbs run even better if I put the Big Bore Kit on???

And with my straight pipes how do i get the power back and keep the sound. I love it this loud!!!

Thanks for all your help.

That's a huge problem to have a jet lying in the bowl, that jet is there to meter fuel through a very precisely drilled orifice, if it's not there, you're going to have some very lousy running. I'll bet that your carbs have 5181 needles, those are what mine came with. It's actually really easy to check the needles, far easier than messing with the jets, take the top off, slide the retaining clip off to the side, hold the throttle open with one hand, catch the circlip with either a knife blade, or a pick, or something of that nature, and then let the throttle close, and you'll have the needle by the clip. The size will be punched in somewhere. Yes, a big bore kit, with those carbs, will really wake that engine up, don't think of it as "the carbs running better", instead think of it like the engine running better with those carbs AND a big bore conversion.

I think that is what I will do this winter. I want to ride it now. I have been messing with it for 4 weeks now. It is time to ride. Thanks guys for all your help.

Yeah, the needles aren't too big of a deal when you get around to them, 15 minute job, not quite like a big bore conversion, they're fun as hell carbs though. Where I normally have alot of fun are the merges onto a highway, you can really let loose then :-)

Two areas to check on a carb, which I almost never see mentioned, are float level and transition mixture. The former is adjustable, but the latter is (usually) not.

A low float level will give you a hesitation on acceration because there is a delay in getting the gas to flow in the main system, as the carb goes from providing fuel from the transition portion into that main portion.

On the other hand, too high a float level can cause fuel flooding at worst, or bad/poor gas mileage at least. Try to set the level at the manufacturer's recommended point, though you could go slightly higher if needed.

I'm not familiar enough with this carb to say much about the transition curcuit, except that almost always this part has a fixed jet which meters fuel. If a guy really knows what he's doing, he could determine the size of this jet with a drill index (metric, I'm sure) and then go up one size to drill out the jet. Finding those tiny metric drill bits should be a real challenge! (I own the inch size bits in size #60 down to #80 and they are really handy for carb or hobby work.)

NOTE: Drilling jets is extremely tricky business! You can actually make the jet leaner by over-drilling it, as burrs and ridges left from drilling can cause turbulence, which the leans the fuel flow!

I suggest to anyone who is playing with carbs, whatever brand, that they find one of those old Holley car carb books and check out the first chapter or so, which covers basic carb theory and uses a down draft single barrel carb drawing to help in understanding what is really happening.

Once you get a grasp of the overall theory, the whole process of dialing in a new or different carb set up is much, much easier!

This other suggestion is so different from what I wrote above, that I decided to put it in a separate post.

Something I had on my old bracket dragster engine years ago, which really helped with tuning my large alcohol carb on a small displacement motor, was the anti-reversion function I built into my headers.

This neat mod tended to kill the reverse pulse coming back up the pipe and into the combustion chamber during overlap of the valve phase. This nasty pluse slowed down the intake charge velocity, especially at lower rpms, which made the signal (usually called "vacuum") lower on the fuel circuit. This causes a lean condition.

The way to build an anti-reversion device into your exhaust is to make a short (approx. 1 to 1 1/2 in. long) stub pipe which fits inside your main exhaust pipe, right up at the head.

You could do this by welding or even brazing the tubing onto a thin flange which would be sandwiched between the exhaust port head flange area and the flange of the stock or custom exhaust header pipe.

The added-on stub tubing should be the same size as the port (on the inside, of course), but smaller than the pipe it slips into. If the header pipe happens to be the same size as the exhaust port, then a larger end needs to be fitted onto it to allow the anti-reversion piece to fit down inside the main pipe.

Here's how it works: that stub pipe will catch the reverse pulse coming back up the pipe, in the area between it and the outer wall of the main pipe, thus killing it. It's like a oneway valve, in other words.

Yup, it's a fair amount of effort, but it works.

The other solution was mentioned above: get a carb with an accerator pump circuit. I've heard tell the Harley guys remove such a carb when going to a bigger one, and sell them cheap on e-Bay.

Ok -- I took it for a ride today and LOVED it.. There is no backfire and it runs GREAT!!! I still need to role into the gass but hey I dont mind it still hits 80 before I can mergerge onto the highway...!!!! the other thing is that it "Tops out" at about 85-90 (on the spedo). now I do not have a Tac so I can not tell but it will not go any faster. It like bogs out or cuts out if I give it more gass to go faster. but all the lower end problems are fixed.

Haven't figured out how to adjust Float level on the CRS carbs, there isn't any kind of transition metering device beyond the needle, the progression is slow-needle-main. Now the 39mm FCR carbs that are being advertised on one of the Triumph Dealer sites, they have two separate acceleration circuits, that would give alot more transition flexibility than the CRS carbs. Evanman, if you're still running those 110 mains that may be cutting off your top end a good bit. Those carbs can flow enought to support at least 110 MPH indicated on my Speedmaster, so they should be capable of seeing you over the Ton on yours.

I am still useing the 58 and 110 Do you think I should go with 58 114? Would this help?

Oh I forgot to tell ya I am at Evanston 6748ft.

When I was in Knoxville, TN last year (Don't know the actual height above sea level) I was still running the 118 mains w/5181 needles, bike was still rich, but not badly so, and not necessarily due to the elevation. Bike had good power and pull, and incredible gas mileage (50+) and that was going through the Dragon is 2nd gear screaming along. I would recommend the 115's, but also take a look at your main air jets too, those are actually the easiest to access. And don't forget about maybe doing a high throttle plug check, just be careful doing it. That's still the best way of checking your burn.

The needle is a metering device in the main circuit and can only work when the main circuit is working.

The idle circuit flows fuel because of the pressure drop ("vacuum") on the back side of the butterfly/slide. Once that slide starts to open, the signal on the idle circuit drops. But, there is not enough air flow speed yet, to start the main circuit, so fuel has to come from someplace else. The idle circuit is still supplying fuel, but it is not enough for the increased air amount, so the transistion fuel is added to it.

The transition circuit is usually a few holes or a slot, right behind the butterfly/slide. In fact, some carbs use the butterfly to partially meter the amount of transition fuel (Holley, among others). With your air cleaner off, open the carb and look for some holes right behind where the butterfly/slide seats when closed. That is, the holes will be closer in toward the cyl. head and will be on the floor of the carb throat.

The jet that controls the fuel to this circuit might be buried in the carb somewhere, behind a pressed-in plug, much as the idle mixture screw is covered up. It's possible that the passageway can only be seen with the float chamber removed. It's also possible that the holes themselves do the metering, but most likely there is a brass insert jet. Never the less, the transition circuit is in there somewhere.

ok I am lost..... that is as clear as mud... I am a network engineer not a carb guy. But I will try to print the instructions and muddle through it and see what I can find out.

Evanman, most of your worries are going to be confined between the slow, main jets, and needle. About the only other things that can be changed on these carbs are the starter jets (choke), and altering the cut-away to enrichen the throttle valve. Sudco and Carb parts warehouse (carbparts.com) both have pretty good documentation on CRS carbs that you can print out, that will help you tune on these suckers. It really isn't that bad. If you find the exploded diagrams, you can get an idea of where everything fits.

I got about 1/2 a notion that I should grab some cash and go for a drive.... Find one of you guys that know more than I do and have you look at it... but then I think what about the elevation change..... Oh well. I will be going to Idaho in 2 months maybe if I dont have it figured out by then I will have someone look at it there. Thanks guys for all your help I have lots to try and look at now..

Take a deep breath, and remember that it wasn't a rocket scientist that designed the flipping things. If your bike is reliably running from stop light to stop light without either dying on you, or spitting out plumes of black smoke, then you're not that far off. At this point, you're only refining and fine-tuning, so you can afford to take your time. It took me almost 4 months of off-and-on, back-and-forth tuning, before I got the things were I thought they were decently tuned, and even then the stupid things still could pull a "black magic" on me. Give a yell if you need any advice balancing them, that's fun too.

ya I need to balance them but I not right now. I will ask when I get to that. I am going to put the 114 jets in it today and see if that helps this weekend. thanks for all your help!!!

No worries, collective knowledge is the key!

A side draft Weber car carb with approx. 40mm inserts ought to be the cat's meow for these bikes. They have accel. pumps, too! Every circuit in these units is adjustable.

If the carb seems a little too big, pop out the 40 mm inserts and install something a little smaller.

But, as Red Green would say, "This should only be done by a professional!"

Oh I agree, a Weber DCOE carb would be the shiznits, and wouldn't that be a topic of conversation whenever I'd park the bike!!! But it would take someone with more skills and patience than I have to make the thing work right; these CRS carbs were so different from my little Weber Progressive 32/36 DFEV carb on my VW, I swore I was dealing with black magic, and my initital fumbling sure could have used some more rubber chicken voodoo. It was a very frustrating process when the things weren't running right, but I later realized that almost all of my problems were being heavy-handed with those ****** air screws.

Ok Boys!!! Well, ok, ok, Men if you so say... I adjusted the jets yesterday. I took the 110 out and installed the 114. Then off we went for a ride. I only work about .5 miles from my house. I decided to run to work and print out the Idaho Invasion flyer. Well guess what!! On my way there it was raining the whole way. So when I got the the High School here (that is where I work) I decided not to leave my bike in the rain. So guess what I parked it in the hallway. Now if that wasnt LOUD. My head is still spining. Then on my way home it was snowing HUGE snowflakes. So by the time I got home I was cold and wet. and you know I could not just take the snort way home. Stupid kids always gota do stupid things. This seemed to help quite alot.. I think a little more playing and a little more adjusting I will have it. The more I tweek the better it gets.

Tis good, tis good, one day (may as well say one year) I'd like to get those FCR carbs that Dinqua found on one of the sites and mess with those. They are unGodly expensive, and I'm not exactly hopping with anticipation about having to figure out new carbs from the ground up, so I can afford to be patient. Just looking at the pics though, they don't seem that different from the CRS carbs, they look to have the same method of synchronization, and the bowl looks pretty similar, with most of the jets probably being right there in your face. Those accelerator circuits would probably be a real trick to tune though.