Topic: 1980 cb650 carbs

[pbrom]

So this is the first time on the site. I've read a similar post about a carb swap from different bikes but still have a few questions.

My goal is bobbing out my cb650 400$ project. First thing stock carbs need rebuild. Parts are spendy and jets are press in and parts seems harder to find. So I want to go to a set of carbs from a 71-76 cb750. Seem better in long run over the stock 650 ones. Or should I go with stock 650 ones, but I want to run pod filters and 4 individual 1 inchish exhaust.

When I buy the 750 carbs what are the best years and also does it matter on if I buy a set off a dohc or sohc 750? Plan to buy used stock set off eBay then re build them. Thank into anyone who can help and excited to find this forum!

[MiGhost]

To start off. The DOHC carbs will be the CV vacuum slide carbs. They will not match the port spacing on the 80 650 head.

The PD carbs do not need to be expensive to rebuild. All the PD carbs use the same gaskets, and o-rings with the exception of the accelerator pump rail. The PD50/51 series are the only carbs that use the metal tube with o-rings. You can use the cheaper gasket kits for the 77-78 CB550K. These will have all the necessary gaskets for rebuilding the carbs. You will only need to get (6) -7 (dash7) o-rings at your local hardware store for the accel rail.

The pressed in pilot jet is removable. You need to pull it out with pliers (use a scrap of leather or similar to keep from marring the jet). A careful twisting motion as you pull outward will remove them. There are tiny holes in the jet that must be clean, or the engine will not idle, and run correctly.

The combination of 750 carbs along with the pods, and exhaust is going to be a tuning nightmare. Unless you have a buddy (he won't be for long) with a dyno that is willing to spend the time necessary to dial in the jetting. You can plan on weeks, to months of trial, and error testing. Make sure you understand the changes you are planning. It is not a simple bolt it together and run it deal.

Unless you are spending the $$$$ to build a high performance engine. You will not gain enough from the bigger carbs to make it worth the effort. Cam selection to support the bigger carbs for the 650 is nonexistent beyond the stock cam. There are options, but the $$$$ involved to get it right must be considered.

Do not be fooled into thinking that the stock carbs on the 650 will not give you good performance. When you get done cleaning the stripe out of your shorts because the bike walked out from under you at 70+ mph you'll understand.

[pbrom]

Thank you so much, cleared a lot of confusion up as well as saved me money. I think I will just run the stock carbs then with the rebuild kit from the 550 and the -7 o rings. So just to make sure your saying ill be fine to re jet it right for the straight pipe exhaust and pods with these carbs? That's the only mods I plan to do for clean look purposes and some style. Cause I won't mind cleaning my shorts haha. One last thing to is the needle valve seat is press in as well but that should be fine I think right?

[MiGhost]

Starting with the float valve. It would likely be destroyed by trying to remove it. There are no replacement pieces available. As long as you are getting flow through it there is no reason to go any further than cleaning the inner bore, and seat.

Jetting is as much an art, as it is a science. The only thing that I can say with confidence is that the open exhaust with pods is going to create a severely lean condition. You will stand a high risk of burning and valve, or piston. There are to many unknowns to be able to say whether you will be fine with the rejetting.

[SOHC]

You have a few better, but a bit more expensive options for the Air Cleaner, You can get a brand new "breadbox" setup from these guys http://www.steeldragonperformance.com/air_boxes_7.html
or, there are some vintage "breadbox" air cleaners, for less that pop up on ebay from time to time.
I did confirm with steeldragon about the spacing being the same for the 750 pd carbs, since the guy wasnt sure, so the 750 pd carb breadbox will work.

IMO the reason the breadbox is better, than pods is because the breadbox "seems" to cause less turbulance to the air going into the carb, which in theory should lessen the "tuning nightmare" a bit vs. the pods.
There are a few guys over on the 650 forums that have used a breadbox, with little to no issues, one member even made one himself.

[pbrom]

okay float valve ill just make sure its smooth and the fuel is flowing. just wish it was changable in case if it does fail since one guy said something about the unleaded fuel eating at carbs overtime and pitting them. dont think itll happen soon but still sketches me out a little.

I was concerned about lean condition forsure since im cutting the exhaust silencer out haha. But to compensate i was gonna run small pipes. worse case i was thinking i could do mini silencers per say, more just a restrictive cap/plug with smaller hole in each pipe to set it back to stock exhaust flow per cylinder. and to find that out i was just going to divide the diameter of the stock silencer hole in half since its a 2 to 1 set up and that will give me the correct plug hole size silencer thing i put into each indivual exhaust pipe to set it back to stockish spec for exhaust flow per cylinder right???

As far as the breadbox idea im digging that and think ill go with that because it still looks sleek but does look more restrictive and less filter surface area compared to 4 pods you know? then i should be able to tune out whatever leaness is left over id say. not looking for more power just want to be able to fit exhaust style to overall bike

[sinister902]

76-78 cb750A carb kits will be entirely the same besides needle and main jet. The main jet is a 105 instead of a 95.....which might be great if you're doing anything with pods or exhaust as it will be closer to what you'll need jetting-wise. as for the pilot jet, on hondacb650.com it's very common practice to upgrade from a #35 to #38, even on a stock bike to cope with difficult cold starts from the factory. I forget which seller on ebay had them, but you can get the 750A kits as a set of 4 for around $50 on ebay, and the #38 jets are cheapest from SCI, all 4 for $22.

[pbrom]

Nice! I'll check out those carb Kitts as well. Idk as bout eBay ones I feel they'll be #$%*ty but I'm going to look still. Jet wise I read a similar build threads with pods and exhaust and dude was using 120 mains and 40 pilots. Gotta get the pressed in pilots out as well as mains and tap to fit new screw jets or drill out press ins. Havent decided what way I'm going to do it. I'll giver sci a look for jets as well.

[MiGhost]

76-78 cb750A carb kits will be entirely the same besides needle and main jet. The main jet is a 105 instead of a 95.....which might be great if you're doing anything with pods or exhaust as it will be closer to what you'll need jetting-wise. as for the pilot jet, on hondacb650.com it's very common practice to upgrade from a #35 to #38, even on a stock bike to cope with difficult cold starts from the factory. I forget which seller on ebay had them, but you can get the 750A kits as a set of 4 for around $50 on ebay, and the #38 jets are cheapest from SCI, all 4 for $22.

I had forgotten about the cruzinimage kits on ebay. He does carry them for the 650 PD carbs also. Out of stock at the moment, but I am sure he will get more.
http://stores.ebay.com/cruzinimage-co1484/_i.html?_nkw=cb650&submit=Search&_sid=526414511

[pbrom]

Awesome thx for the link and help!

[sinister902]

76-78 cb750A carb kits will be entirely the same besides needle and main jet. The main jet is a 105 instead of a 95.....which might be great if you're doing anything with pods or exhaust as it will be closer to what you'll need jetting-wise. as for the pilot jet, on hondacb650.com it's very common practice to upgrade from a #35 to #38, even on a stock bike to cope with difficult cold starts from the factory. I forget which seller on ebay had them, but you can get the 750A kits as a set of 4 for around $50 on ebay, and the #38 jets are cheapest from SCI, all 4 for $22.

I had forgotten about the cruzinimage kits on ebay. He does carry them for the 650 PD carbs also. Out of stock at the moment, but I am sure he will get more.
http://stores.ebay.com/cruzinimage-co1484/_i.html?_nkw=cb650&submit=Search&_sid=526414511

I asked him for a buddy, he said he'll get more end of january