Topic: upgrade from standard carburetors

[jason41987]

i was curious.. if i wanted to upgrade from the standard 550F carburetors for something more efficient in either power production or fuel economy... what options do i have?

[Steve_K]

If your carbs are working OK and are setup properly you should be as good as it gets.  I brought 36mm Mikunis for my GIXXER and they were $700.  That's a lot cash and you may find carbs off another bike, but for better performace maybe one else may chime in on what is possible.
Steve

[LesterPiglet]

I wouldn't have thought that would be the most efficient/cheap upgrade. 650 cam but make sure it isn't the 50hp version you get in some markets.

[jason41987]

well, one of the reasons i wonder is because one of the carburetors does appear to be in pretty rough shape, the post on the inside of the float bowl that holes the main jet is erroded on top and im not sure at this point how well itll be able to hold that main jet.. ive considered re-shaping the top of that post with JB-weld, but im not sure how well itll hold up to gasoline, probably well enough since they repair fuel tanks with it... also ive concidered replacing this carb alone, but ive also been wondering what good i can achieve if i replaced them all with newer, modern ones...

since im not able to do anymore work on the bike till a couple carburetor parts arrive ive been searching the aftermarket for possible upgrades and replacement parts

[LesterPiglet]

If you search the forum there are guys who repair everything for SOHC's. I know carb posts are repairable. Use the google custom search above left

[Xnavylfr]

I know some of the 750 guys try to find and run the 29mm Mikunis but the boots to get them to match the head are harder to find than the carbs. A good set of 29s ready to run is going to cost about 5 benjamins (100s). The 28mm are just as good without the adjustablity of the main jet without removing the carbs..

These Mikunis are run mostly on early KZs

Xnavylfr(CHUCK)

[LesterPiglet]

The standard carbs are more than needed. It's only when you improve breathing i.e. bigger valves, bigger cam that you need a bigger carb. Changing carbs first is a waste of time and money.

[Motoguy23]

VM28's or 29's may be just right for 750's, but Ive been told are way too big for the 500/550.  You'd lose all your vacuum in the carb and not pull enough gas out of the bowl for acceleration but at top speeds you'll be fine.  I have a set of VM26 roundslides off of an 81 KZ650 with an accelerator pump that I'm currently making intake runners for. 

As a bonus, the KZ carbs are spaced almost perfectly to match straight to the head compared to having the "Kinked" stock intake runners.

[jason41987]

well, what features are present in the VM26 carbs vs the standard 550 carbs? im curious as to what the benefits would be

[bollingball]

Your best bet is to buy a spare rack of the same as you have. Unless you are going to do a lot of motor work.
Ken

[jason41987]

im wondering.. would it be possible to use JB weld to repair the eroded top section of the one carburetor, and reuse the carb without any negative effect?

[jason41987]

ive discovered that i am incredibly under budget with an SOHC/4 than what i had set aside for rebuilding an old bike... so ive found myself with a considerably larger amount of money to play with than i thought i would have at this point..

so im not closed to the idea of engine mods, i would absolutely love to make this bike able to tear up a road even more than it can now, but i also wouldnt mind better fuel economy as its a bike im looking to have for decades, not years as my primary source of transportation, so the amount of money i would save in fuel over that period of time would add up for me

i should admit though... besides the CX500 i worked on before which was in such good shape i never had to take the carbs apart, ive worked entirely on fuel injected vehicles... so while you guys are familiar with needles, jets, and slides, im equally familiar with crank sensors, injectors and ECMs... so ill admit im a bit out of my element with carburetion, and have even thought about converting to fuel injection for more power, better fuel economy, and a system im more familiar with, possibly even replacing the points with a crank positioning sensor setup so the ECM can operate and control the electronic ignition as well

but, the reason im trying to learn more about carburetors mainly is because id like the kickstarter to still be practical

so with my extra money, im looking to play around with this bike a little more than rebuilding it, id like to take it a step further and am already serious considering a new cam, valves, electronic ignition, etc and was looking at what i could do with the carbs as well

[harisuluv]

You might be underbudget now but you aren't truly until you are totally done with your rebuild.  Keep in mind that a few bucks here and there add up.  Your carbs are probably ok but it just sounds to me like you want to tinker with toys.  If that is case, that's fine, but remember it's just buying toys. 

I'm gonna guess that your float post is ok, it's common for them to have some slight corrosion.  Keep in mind there is not that much stress to that little pivot point.  If it's not ok you can always just get a new carb body.  People like me have a bunch of them lying around. 

[jason41987]

the posts for the floats are fine.. i did mangle the pins a fair bit getting them out.. so much that the pins wouldnt go back in.... i placed them in my dremels chuck and let them spin over some 240 grit sand paper till they fit right again and i was able to re-use the pins for the float... ill entertain the idea of getting a new carb body to replace the one that eroded a bit on top

at some point im going to begin tinkering with EFI by fabricating new flanges thatll contain a throttle body and nipple for the injector, but thats later on down the road....

im curious though... it seems a lot of bikes did use the mikuni VM26 carbs so finding an old set and rebuilding those wouldnt be too bad, so im curious, compared to the standard carbs, what improvements or advantages does the mikuni have over the stock carb?... also, what advantages/disadvantages would there be with a CV carburetor like the CB450 uses?

[dave500]

just use the standard carbs and save yourself a lot of bother,its only a two valve head.

[matt mattison]

I'm not sure what your after here Jason. Previous posts you made we're about being as frugal as possible on these carbs and trying to rebuild these with hardware store parts. Now your talking about dropping (possibly) several hundy on new/different carbs. As stated before, your not under budget until you are done. I'm assuming you don't have this machine running yet. Get it running first. If your after fuel economy and the bike running perfectly, don't change anything in the fuel delivery or exhaust department. In stock form I hear of people getting over 40mpg on a 550. If you want to start adding exhausts, pods/stacks,cam, etc etc expect that mpg to drop off significantly. And all that hi-Po magic doesn't necessarily mean that your engine will run any better than in the factory trim ,at cruising speeds. Like Dave said, use the stock carbs. They are quite capable of even handling some minor engine modifications. If your not satisfied with the way it runs, then worry about changing your carbs out. Don't put the cart before the horse, make your life easy. Getting better fuel economy and picking up engine performance don't generally go hand in hand with these SOHC 4's. There is going to be a performance trade off. That trade off is fine, if you realize what your gonna end up with. If your after better fuel economy and performance, start a thread about ignition improvements. There you will find options and opinions to have both.

[jason41987]

actually, i was looking into a number of engine modifications as well for this bike.. looking at aftermarket cams, pistons, valves.. i mean, i did say i wanted a cafe racer, and by that definition something much more sportier than stock... so if i was able to find a good used set of carbs that would give me better performance and throttle response, i see no reason not to consider it to go with the engine upgrades i eventually want to do... thats pretty much what i purchased this bike for

[jason41987]

after attempting to re seat the main jet into the carb with the erroded post... its obvious to me that this carb just isnt going to work properly.. i need to atleast find a new carburetor body... but with the engine modifications i would like to do... such as a new cam, new pistons at some point, new valves, springs... i want to make it into a higher performing cafe, so i do intend to do a bit of engine work on it over the next few months

so my question was what carburetors should i look at as a replacement to the stock carburetors i have now that can handle these future upgrades?.. preferably something that can give me better throttle response with an engine that will then be capable of demanding more fuel and power, so an accelerator pump

also.. ive been following some other aftermarket modifications.. one includes a fabricated 4-into-1 carb installation, i believe someone on advrider was doing this to a 550.. and another thing i seen was making 4-into-2 manifolds to use vacuum carbs off something like a sportster or a CB450 (but those might be too small for a performance CB550 engine.... which will probably be bored to 600cc at some point)

[harisuluv]

I'm confused, is this the bike you are under budget on?  Cause it looks like a money pit

[jason41987]

put a seat, tank on that, put the carbs back on a new harness and itll look normal... just looks bad because it doesnt have a seat or tank. a lot of the other small parts i have but in a box.. and i was budgeting for a complete rebuild into a cafe bike.. in total, i have about $5k free that i could spend on it should it go that high.. really depends on how far i want to go with it... and i do intend on keeping it for many, many years when its finished

i could go into more detail, but thats best left to another thread.. point remains, almost all of my knowledge is focused on fuel injection systems, not carburetion, i know how carburetion works but havent been familiar with these older bikes long enough to know the different brands out there, different carburetors available, or the size needed.... and its hard to speculate just how much more power ill be able to get out of the engine when its all finished

but i can say one thing about my stock carbs.. i do NOT feel comfortable running this bike for very long given the erosion on the post of what i believe is the carb furthest to the right of the bike, after putting the new gasket on the jet and putting it back in theres BARELY enough metal left to cover the o-ring.. putting it side by side with another carb it looks like between 1/8 and 1/4 inch might be missing off the top.. so at a minimum even if i use these carbs for now that one would have to be replaced

[trueblue]

CB 550/500/XXX carb measurements anyone?

[matt mattison]

after attempting to re seat the main jet into the carb with the erroded post... its obvious to me that this carb just isnt going to work properly.. i need to atleast find a new carburetor body... but with the engine modifications i would like to do... such as a new cam, new pistons at some point, new valves, springs... i want to make it into a higher performing cafe, so i do intend to do a bit of engine work on it over the next few months

so my question was what carburetors should i look at as a replacement to the stock carburetors i have now that can handle these future upgrades?.. preferably something that can give me better throttle response with an engine that will then be capable of demanding more fuel and power, so an accelerator pump

also.. ive been following some other aftermarket modifications.. one includes a fabricated 4-into-1 carb installation, i believe someone on advrider was doing this to a 550.. and another thing i seen was making 4-into-2 manifolds to use vacuum carbs off something like a sportster or a CB450 (but those might be too small for a performance CB550 engine.... which will probably be bored to 600cc at some point)

Need more specific info in order to answer your question. Which cam? Which pistons? Which exhaust? Are you porting the head? Stock air box or " other".  The stock carbs might be ok depending on your specific application. They might not. Your throttle response won't neccesarily get better  if you really open up the breathing of any engine, including a SOHC four. It usually will get worse, and you will lose low end torque and response in normal running conditions. You would need to operate the engine at higher RPM's in order to realize the benefit of any changes. This is engine building 101. Know what you want the engine to do, in what operating range. Then identify the parts to accomplish this. All we know is you want a more "peppy" cafe bike. This is a very general term, which leaves much to interpretation. You could be road racing it, which lends you to one type of modification. If that's the case, you are not gonna care if your engine don't produce any power under 5000 rpm, and granny in her walker pulls away from you at a stop light. Maybe your drag racing it? Different modifications again. Maybe your just looking for someone to justify the purchase of a set of CR29's for you? The question you should ask is " I'm gonna ride the bike in (insert rpm range here). What engine engine components should I use to accomplish this task and make the most power under these conditions? I'm sure you know everything must be matched up in order to work properly. Carbs are only one piece to the puzzle.

[Xnavylfr]

+1 on what MATT said,If you plan to use this bike as a daily commuter and have some extra pep for freeway speeds, it will need to be set up different than a bike you'll just play with in the twisties on the weekend. I know of 1 550 here in NC that was built as a bobber/cafe style and he ran with us 750s (836cc) all day long and NEVER lagged behind.
I (personally) feel the vm26s are better responding carbs then the Keihins and you can buy a set for the same price as the stock carbs. The only way YOU'LL be happy with it, is build it the way you want it..

Xnavylfr(CHUCK)

[jason41987]

ill be using the stock pipes, id like to keep them since theyre the only pipes that set it apart from a standard 550... just K&N air filters, and if you go to dynoman.net it would basically be their stage 2 kit for the CB550 which includes 61mm 10.5:1 pistons, webcam 358b camshaft, KPMI valve springs, DP SHP clutch kit, chrome moly cylinder studs, vesrah top gasket set, and CFM or MLS head gasket (just listing it off)

[harisuluv]

Jason with all due respect you are kind of all over the place with your build.  People have taken the time to give you some really good, sound advice and you don't seem to be interested at all.  If you are going to just do what you want to do then that's cool, but don't ask for advice on this or that.  You seem like you have made up your mind on most things.  You say you want fuel economy then want a performance engine kit and carbs.  Frugality in one post then say you have up to 5k to spend.  You say you are under budget on your project but haven't really even started yet.  We are trying to help you out here, brother.