Help understanding our carb basics

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juntjoo
Posts: 247
Joined: Fri Nov 10, 2017 6:19 pm

Help understanding our carb basics

Postby juntjoo » Sat Jan 06, 2018 8:02 pm

1st time rebuilding carbs. I think I've figured out about 55% of how they work, so help me out a little on a couple things. so what it's come down to, the most difficult parts of this carb to figure out is the different jets. These two pics

https://mega.nz/#!jRAgWQLR!SP_Cwxde01je ... BZxecbaeF4

for the cb750 shows the main jet the one on the left, close to the pilot screw, whereas this one:

https://mega.nz/#!eUQBzJIK!NcAj17ORV_ui ... rdG09iNIi4

for the 81 cb650 has the main jet on the other side closest to the float needle jet, the one that catches the piston needle.

Now the carbs look the same, but the 750 uses the needle jet hole as the "secondary" jet, whereas the 81" 650 uses it as the main one which makes sense, as so should mine right? That would explain why I've not been able to pass any carb fluid through any 4 of these holes. So what... they just make them all the same for manufacturing costs and confuse noobs?

other than the jets, how does the pilot screw work? I see it is opposite a little hole in the throttle body. Does it connect to a channel going to that (useless?)first jet down the way?

WHAT IS THAT JET FOR!?!? excuse me. It's been driving me mad.

And please direct me to a source for a service manual specific to my bike if available. I've got Clymers and it just glosses over the carbs
Thanks
82" cb650 Nighthawk

Riche
Posts: 28
Joined: Fri Sep 22, 2017 5:27 am

Re: Help understanding our carb basics

Postby Riche » Sun Jan 07, 2018 3:40 am

Can’t see any of your images?

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Folsoml
Posts: 1630
Joined: Sat Apr 07, 2012 9:02 pm
Location: North Florida

Re: Help understanding our carb basics

Postby Folsoml » Sun Jan 07, 2018 9:47 am

I made an edit to the original post so we can see the pictures.

The images you've provided do show the correct jet positions. The CB750 has primary and secondary main jets, while the CB650 only has one main jet. They bodies are not all the same. Your bike, for example, should have a VB44C carb bank. This is the only bike that used this carb bank.

As far as getting your carbs right, I think I recall you mentioning the MacGregor Manual in another post. Even though those carbs are slightly different, if you follow his cleaning procedure, you will clear the internal passages in the carb body.

Lastly, the pilot screw controls the mixture of fuel to air.
Have a problem with your CB650? Have a technical question? Click here!


My Current Bikes: 2005 HD FLHTCUI Electra Glide Ultra Classic, 2007 Yamaha Vino,

juntjoo
Posts: 247
Joined: Fri Nov 10, 2017 6:19 pm

Re: Help understanding our carb basics

Postby juntjoo » Thu Jan 11, 2018 6:12 pm

Folsoml wrote:I made an edit to the original post so we can see the pictures.

The images you've provided do show the correct jet positions. The CB750 has primary and secondary main jets, while the CB650 only has one main jet. They bodies are not all the same. Your bike, for example, should have a VB44C carb bank. This is the only bike that used this carb bank.

As far as getting your carbs right, I think I recall you mentioning the MacGregor Manual in another post. Even though those carbs are slightly different, if you follow his cleaning procedure, you will clear the internal passages in the carb body.

Lastly, the pilot screw controls the mixture of fuel to air.


thank you. then what is the jet for that is the primary on the 750 on the 650 that i cant seem to pass anything through?
82" cb650 Nighthawk

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Folsoml
Posts: 1630
Joined: Sat Apr 07, 2012 9:02 pm
Location: North Florida

Re: Help understanding our carb basics

Postby Folsoml » Thu Jan 11, 2018 8:20 pm

Strange as it may seem, I am not as familiar with the carbs on the CB650 as I am with the DOHC Honda carbs. I'm not really sure what that hole is for on those carbs.

Have you tried pushing a length of a high E guitar string through that hole?
Have a problem with your CB650? Have a technical question? Click here!


My Current Bikes: 2005 HD FLHTCUI Electra Glide Ultra Classic, 2007 Yamaha Vino,

juntjoo
Posts: 247
Joined: Fri Nov 10, 2017 6:19 pm

Re: Help understanding our carb basics

Postby juntjoo » Thu Jan 11, 2018 8:52 pm

Folsoml wrote:Strange as it may seem, I am not as familiar with the carbs on the CB650 as I am with the DOHC Honda carbs. I'm not really sure what that hole is for on those carbs.

Have you tried pushing a length of a high E guitar string through that hole?


yes, it's like a fishing string or related, something in the garage. I lucked out and it fits all the little holes perfect, a little snug in the slow speed jet, and maybe I read it in that pdf or some thread but used my razor handle with a portion of this string, hooked the end and unplugged one of those accelerator pump jets. good thing I made note of the warnings not to miss those and go through that PITA to wiggle that in from behind with a tight angle view through the top only one bad eye could get a shot. And no, nothing goes through that main jet. That's why I'm thinking it's a single cast used for these two carbs and they just plug the holes of that circuit in the float side and the vacuum side too. Couldnt be all four of mine totally blocked.
82" cb650 Nighthawk

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Folsoml
Posts: 1630
Joined: Sat Apr 07, 2012 9:02 pm
Location: North Florida

Re: Help understanding our carb basics

Postby Folsoml » Fri Jan 12, 2018 6:24 am

I'm not sure fishing line is stiff enough. When I say guitar "string," it is actually very thin wire.

I'm not sure how long your bike sat up before you got it, but I can easily picture all four carbs being equally blocked. Did you try soaking them in anything? Or ultrasonically cleaning them? This is a good product for soaking them:

Image


What I do when I'm rebuilding carbs is first remove all the jets and soak them in this product for a half hour or so. Then I soak each body--one at a time--for half an hour. I choose the half hour time only because I have a small ultrasonic cleaner with a 30 minute timer. After I soak the jets, I put them in the cleaner for a cycle. While they are cleaning, I'm soaking something else. In the ultrasonic cleaner, I have a mixture of Pinesol and water (Note: If you choose to use Pinesol and water, make sure that when they are done soaking, you rinse them well in clean water, and then spray WD40 on them. If not, the aluminum on them will oxidize.) Even after all this, I still use the guitar string to clear out the jet passageways.

That's how I handle the inside. The outside I will clean using a soda blaster. I'll then buff the vacuum caps, choke plates, fuel rails, air cutoff covers with a buffing wheel on a bench grinder. I'll shine up the outsides of carbs 1 and 4 (the sides visible when it's on the bike) with a dremel tool and a carbon steel brush tip.

This is a carb bank I bought off ebay for $1.

Image

This is it when I was done. I think I sold this bank for $250. I buy O-rings and stainless steel hex cap bolts in bulk, so I always have plenty on hand.

(ignore the red arrow)
Image

Image

Image
Have a problem with your CB650? Have a technical question? Click here!


My Current Bikes: 2005 HD FLHTCUI Electra Glide Ultra Classic, 2007 Yamaha Vino,

juntjoo
Posts: 247
Joined: Fri Nov 10, 2017 6:19 pm

Re: Help understanding our carb basics

Postby juntjoo » Fri Jan 12, 2018 9:46 am

Folsoml wrote:I'm not sure fishing line is stiff enough. When I say guitar "string," it is actually very thin wire.

I'm not sure how long your bike sat up before you got it, but I can easily picture all four carbs being equally blocked. Did you try soaking them in anything? Or ultrasonically cleaning them? This is a good product for soaking them:

Image


What I do when I'm rebuilding carbs is first remove all the jets and soak them in this product for a half hour or so. Then I soak each body--one at a time--for half an hour. I choose the half hour time only because I have a small ultrasonic cleaner with a 30 minute timer. After I soak the jets, I put them in the cleaner for a cycle. While they are cleaning, I'm soaking something else. In the ultrasonic cleaner, I have a mixture of Pinesol and water (Note: If you choose to use Pinesol and water, make sure that when they are done soaking, you rinse them well in clean water, and then spray WD40 on them. If not, the aluminum on them will oxidize.) Even after all this, I still use the guitar string to clear out the jet passageways.

That's how I handle the inside. The outside I will clean using a soda blaster. I'll then buff the vacuum caps, choke plates, fuel rails, air cutoff covers with a buffing wheel on a bench grinder. I'll shine up the outsides of carbs 1 and 4 (the sides visible when it's on the bike) with a dremel tool and a carbon steel brush tip.

This is a carb bank I bought off ebay for $1.

Image

This is it when I was done. I think I sold this bank for $250. I buy O-rings and stainless steel hex cap bolts in bulk, so I always have plenty on hand.

(ignore the red arrow)
Image

Image

Image


wow, thanks for sharing, that's some nice work. now I'm starting to regret not using my dremel. I just wanted to get this bike working first before I started cleaning it but that is easy pretty work with the right tools. But I did soak them in that Berryman stuff for a good day or two and used regular carb spray where needed(internal parts) but didn't immediately wash the stuff off so I'll keep that in mind for next time. And I see your point about the guitar string. mine wasn't as strong but the perfect size and I guess it's not a regular fishing line as it's a wire itself, just not as strong as a guitar string. I just got it back when from the fishing isle. As far as those blocked jets, I'll keep looking, but until someone can confirm/deny the are supposed to be blocked, we won't know for sure and if I get impatient well we'll find out soon enough. Currently soaking the boots...
82" cb650 Nighthawk


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