CV vs Mechanical Carbs

[ARons512]

Hi boys, sure this discussion has been made but currently turning my Sr400 into a cafe racer. I have a Mikuni CV carb on there now and after slapping a pod filter on it's running quite lean. I just want the bike to run healthy as I use it daily for work. Will also have my gf on the back every now and then. Running at sea level pretty much. All else appears stock and not looking to make any other performance enhancements (MAYBE a new exhaust next year).

I think the routes I want to take are as follows and need some guidance:

1. Have Deus see if they can tune the current carb on there. They estimated 1 hr but could be more (i.e. $120+). Current set up is 170 main, 25 pilot, jet needle 5d132, needle jet (unknown) and slide 2.

2. Buy a VM34. Based on my readings I think I'd do 230/240 main jet, 27.5/30 pilot, P4/P6 needle jet, 6DH3 needle and 2.5 slide. Would need new throttle cable as well. My research suggests I could get this all for around $280.

Don't mind buying a new carb if it's going to be easy for me to set up and more enjoyable on the road.

Much love,

 

[GLJ]

If you are willing to "set up" a new carb why don't you want to work on the you have? No reason it won't work.
By the way nice looking bike. You do realize this is a XS650 forum. Not that we mind you asking questions or showing pics of your bike.

 

[5twins]

I think $120+ would buy you lots of jets, lol. Re-jetting for pods and pipes isn't that difficult or that radical of a change. Most bikes only require a size or 2 change up on the pilots, maybe 1 to 3 on the mains. Sometimes you need to fool with the needle setting too but being a newer bike, your needle may not be adjustable.

 

[grizld1]

There's going to be noise from the gallery on this, but I'll offer it anyway: pods aren't going to deliver more performance, a vacuum carburetor works better with the still air box in place. An aftermarket carburetor, in the absence of other motor work, will not make a mild-mannered stocker split its paint and turn into the Incredible Hulk. What you'd lose, if it's a concern, is the ability of the vacuum carb to respond to atmospheric changes. Peace.

 

[ARons512]

Yes I do realise haha apologies but it did say other models could be discussed! I've heard mixed reviews about whether the CV will work. Some people are saying a stock CV is really only made for a stock set up and once you remove the airbox it turns into a POS.

I already went up on the mains from 162.5 to 170. I tried WOT testing around the Sydney side streets but really not ideal. And could get pinched by cops which would be $200 fine no doubt haha.

I do think I'd need to go up on the pilot as well on the CV. Needle is adjustable with 5 positions. I tried 3rd from bottom and 2nd from bottom and both seemed quite lean.

I simply just don't know if I'm in a good area with the CV. I have to order jets online and get them shipped which isn't ideal. My plug after 15 mins of some full range work was very white. Tried to stay in 3/4 - 4/4 but very difficult in Sydney unless your on the highway (even then the speed limit is 60kms).

I don't care much about elevation. I'll be keeping it at sea level 99% of the time.

 

[FLEA]

if thinking about a new carb , why not take it to someone with a dyno and get them to set it up as you wont get good plug readings on the streets of syd .. also as said cv carbs arnt the best on pods but you can get running ok (just not great) but if you do go for the foam ones not the cheap pleats

 

[ARons512]

I don't think Dyno is in for me. Chatted with a few places and they can't guarantee prices. They said if they nail it first attempt could be $250. But could require multiple trips back which could ring up $1K+ bill. I have a single carb bike so confident a little hardwork and focus can get the right set up (along with some stress of course haha).

 

[FLEA]

dont know where you go but shop in cairns gives 3 goes on the dyno for $180 and you just swap out the jets inbetween goes .. just takes a bit of time waiting your turn ea time or multi trips back .. if its deus exsplains a lot .. even their shits gold plated .. but your right single carbs just a bit of mucking around , id take it to a mates with some more open rds then you with a few jets and have a play

 

[grizld1]

Pics show the OE side covers, so obviously you haven't mounted a pod for cosmetic reasons. Why do you want to ditch the still air box, which you know will deliver better performance and less headache? If you must use a pod filter, use the largest cylindrical item that will fit. Small cones and tapered filters will cause you grief. If you've left the exhaust alone and only changed the intake and are using a large volume filter, drastic jetting changes should not be needed--no more than one or two steps up from stock on the main. If air screw adjustment yields a good idle with no more than around 250 rpm increase from first steady idle at startup to full warmup and no backtalk from the exhaust under engine braking, leave the pilot jet alone. Click the Tech button at the top of the screen and find the link to the XS650 Garage USA Carb Guide, and go to the section on Tuning for Modifications. 5twins wrote it, and he did a fine job. Since you're concerned about plug reading, go here and click on the spark plug icon: www.strappe.com . Forget everything you think you know about plug reading first; any traces of urban myth in your head will explode, and severe neural damage could result.

 

[ARons512]

Hi Griz. Much appreciated feedback. The pic is a bit older just added for your viewing pleasure haha.

The plan is to remove the side covers, install a battery tray and filter a pod filter. I'll be then moving the ignition barrel under the seat and installing a smooth top triple clamp from cognito moto. Going to redo the switches, grips and install an LED headlight (new control box will be needed in the new tray). I also just want to learn as much as I cann on my first bike. When I get my second I'll determine if carb tuning and moving things around is worth it!

I got a fairly large K&N pod filter recommended by Deus with what appears to be a small rubber stack. It's probably the size of a blender bottle.

Just asking if a CV carb will get me to a healthy running bike for daily commuting. And if so, what jetting I should go with but I think I can get away with a bigger pilot and potentially one or two more sizes in the main.

Or would a VM be less of a headache and better / safer performer.

Not fussed either way just want to learn and not spend more than $200/$250 on this carb project.

Cheers

 

[grizld1]

Yes, you can tune the CV carb with a pod and have a reliable ride. Some guys prefer a large foam filter to K&N pleated fabric filters, but my take on the matter is that they're comparing outcomes with small cone or tapered oval K&Ns to outcomes with large volume UNI foam filters. If you're using a large volume K&N it should be OK. I don't know anything about the weather in Sydney, but if you get much wet weather at all you might consider a rain cover for your filter. A company called Outerwears supplies covers to K&N, but K&N offers only a very few options. Outerwears will make anything you want on special order for a very reasonable price: www.outerwears.com.

I've written it before, here it is again. The only reasons for using aftermarket carbs are performance, and replacement of an OE carb that is past hope due to corrosion, abuse, or wear on throttle shaft pivots. Most buyers of aftermarket carbs have some fiddling to do--sometimes a little, sometimes a lot. With only a change of air filter to deal with, your tuning job on the OE carb shouldn't be much of a challenge. Re. the pilot jet, I gave you the tuning procedure. You only increase PJ size if you can't achieve those results within the range of adjustment of the air screw--on your carb, no more than 3-1/2 turns out.

 

[ARons512]

Thanks Griz. You are the bomb! I'll have a think about route I want to take. But if I'm close with the CV I may just order a couple new pilots and mains and have a crack this week and see how I go.

I will NOT be riding around in the rain. Just not safe or fun. Might get a boot and carry around with me tho just in case I get dumped on.

Big thanks!

 

[stinky78]

what year is your SR? I’ve never seen an SR with a CV carb, it must be a later model, I’m really only familiar with the 500s and early 400s, I’m no expert but I think CV carbs are fine on a stock bike, the factory engineers were not surprisingly, quite good at getting them to run well, but as soon as you start changing things that’s when things go downhill, at least that’s been my experience, I find tuning CVs really frustrating especially when they’re getting old, VM 34s & 36s are such an easy swap on an SR and so many people have done it that there’s heaps of info out in the weboshpere, VM34s especially, are so simple and easy to dial in on an SR, unless you’re doing a heap of other performance mods I’d go with a VM34, just make sure you get it from mikunioz or someone like that, you don’t want to end up with one meant for a snowmobile or a 2 stroke dirt bike (not all VMs are the same) otherwise keep the CV and return the intake/exhaust to stock.