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CV vs Flat slide carbs (A learning exercise)

  • Thread starter Thread starter Oldman99
  • Start date Start date
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Oldman99

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I've started to not like the bs34 carbs that came stock on my 450. My complete lack of knowledge on Cv carbs and tuning is a major part in this and is only rivaled by alllll of the things that you can and cant do with/ around them and make them not work.
I just want simple carbs that will send air and fuel into the engine. I can buy 34mm flat slide carbs and the new throttle/ cable assembly to make them work for under $100.
What would be the personal views on rebuilding stock cs's vs buying brand new flat slides?

34mm carb.jpg
 
The carb you have pictured is a Chinese knock-off of a PWK scooter carburetor. I don't recognize the slow jets so you need to be careful as this carb may not be optimal for a 450cc machine; with two carbs, each is fueling 225cc. A scooter's jettting may be much less as they usually are fitted to a GY6 (I think either 125cc or 150cc) Honda scooter engine Chinese knock-off. I have used a similar carb with the automatic scooter choke and they are actually made decently but on a scooter, performance is a bit harder to judge. The GY6 engine isn't used by Honda anymore and the patent ran out so the Chinese are cloning this engine for almost every cheap scooter they make.

I would recommend you spend a few more bucks and get Mikuni round slide 34s. You can pick these up for under $90 each on eBay and they cost less than the TM series flat slides (about $140 each). Jets are easy to find and they are easy to tune; only have air screws to adjust the pilot mixture.
 
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My suggestion is to take the time to learn about your stock CV carbs. They work great when cleaned and tuned properly.
 
geol said:
I would recommend you spend a few more bucks and get Mikuni round slide 34s. You can pick these up for under $90 each on eBay and they cost less than the TM series flat slides (about $140 each). Jets are easy to find and they are easy to tune; only have air screws to adjust the pilot mixture.
Click to expand...

Thank you, I thought if the bore and jets where close to what the stock carbs are running; it would work. You where also correct about Real Mikuni's on ebay.
I found some Mikuni VM34's for $89 each. They would work better, right?

Mikuni-VM34.jpg
 
The stock CVs are hard to beat on daily drivers. I agree with JJ; take some time to learn how to tune them and you find that they work very well. I would start by cleaning them thoroughly and then replacing anything rubber. There's plenty of documents showing the factory settings. One other thing to consider is whether or not the valve clearances are correctly set. Valve clearances is fairly critical in making these GS motors run well.
 
I get that you don't like the carbs you have, but what is wrong with them? :-k

If it's simply a matter of not getting them to run right, the problem is probably rather simple, it's just that you don't know what to look for.

Have you posted your issues, asking for any help or do you just want to get new carbs and go through all the hassle of mounting them, jetting them, trying to synchronize them, etc., etc.

If it's just a matter of cleaning them and getting them proper on a somewhat stock bike, there are a few of us here that offer a carb cleaning service that will get your bike rolling again rather soon. If you really want the adventure of new carbs of a completely different style, enjoy yourself. :encouragement:

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I am not here to dispute the merits of the VM series carbs vs BS series. There are good reasons that Suzuki and Kawasaki went away from VM carb and switched to the BS carbs. Either can be made to work on your bike. I personally think the 34s are a tad on the large side. Once a venturi gets too large for the displacement of a bike air speed through the venturi becomes slow. This requires a use of smaller pilot air jets and much larger pilot jets, depending on the slide cutaway. Not sure what specs these 34s have but I would start a tad smaller venturi; say 30mm or so for the sake of ease of tuning. It will be a trial and error and has been suggested, there is more base of knowledge on tuning the stock carbs (BS series) than tuning VM carbs for your bike. I wouldn't hesitate to buy Mikuni VM carbs but I have some experience with tuning and would need complete specs before guessing at the jet sizes that would do the job.

As has been mentioned, why not tell us what issues you are having with the stock carbs and a little about the bike. Do you have the stock airbox and pipes? Etc.

Another thing I forgot to add is that you will likely have to change throttles to accommodate the throttle cable needed to pull both slides. The cable will have to be a Y type. I am not sure how easy it would be to sync the two carbs using this arrangement and a close bench sync might be all that could be accomplished. I think slide height can be manipulated using the cup/locknut type thing atop each of the VM carbs of this sort but have no experience with this issue. Not sure where a cable could be had. I see kits sold for other twins such as the XS650 using these type carbs and the seller basically is charging as much for the cable as the carbs. You will also have to figure out carb holders based on OD of venturi. I think SUDCO sells generic Mikuni carb holders for this purpose but some research needs to be done.
 
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If you plan to ride anywhere with large elevation changes, you can't beat the carburetors you have. The older VM series carburetors has a much better throttle response but I couldn't ride to Denver without stopping and tuning for altitude differences. As said before learn to adjust your carburetors properly. Your bike isn't worth the price of a good set of flat slides and if you are frustrated with tuning what you have now you would be buying an expensive nightmare for yourself.
 
If you're having problems tuning the stock carbs, I'd stay away from flat slides. Flat slides while offering allot more tuning options are much more complex to tune. CV carbs are simple compared to flat slides.

Manufacturers typically provide carbs with the same body (model #) but different, internals (air/fuel jets, slide cutaways, needles, emulsion tubes) as well as different features (choke, accelerator pumps, pressure equalization, air bleed). Thinking that a carb of the same bore size should work for multiple applications, ignores all the special internals and features that manufacturers put into carbs to make the work for different engine applications. . Bore sizes for CV carbs which rely on vacuum to control slide movement versus flat slides which use a mechanical linkage to control slide movement don't directly translate due to the differences in the way the carbs control slide movement

I have a Yamaha dirtbike which uses the exact same model and size Keihin flat slide that I have on a couple of Kawasakis. Almost every component aside from the body is different between the two carbs which are generically known as 39mm Keihin FCR flat slides. Thinking that a carb of the same bore size will work in an application which uses the same bore size is like thinking an ECU with the same plug connector will work on any engine with the same ECU plug connection
 
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