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Discussion Starter · #1 ·
I noticed some things with the carbs I rebuilt for my 81 cx500. It had the stock idle jets but the other two don't seem to be stock now that I looked up the sizes. The set I am replacing these with don't have sizes marked at all. I was hoping they were stock jets but that doesn't seem to be the case. It's real likely whoever used these carbs before me was running pods.
I have a 120 and a 90 in there. Is this alright with a stock setup? I'm not real knowledgeable on jetting with these bikes. I know with ethanol it can help to go larger and get less lean but that's about it.

Right now my choke doesn't seem to be working. I think its little spring is too lose. The cable engages, the linkage moves, both plates come up, but it's like there's no choke at all. It doesn't really push the throttle up. It just touches. So I need to figure out what's wrong with that linkage and sync. I have my old set to compare with. I'll figure it out. I just want to know if I should be using something different for jets. I get the feeling my previous carb set had the wrong size jets in it since none were marked. It runs better currently by far with the current rack and 90/120. I'm using VB25As without the vacuum tube. I just bypassed the seal on the petcock. No big deal.
 

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Discussion Starter · #2 ·
Ps I noticed people were saying 3 turns out? I always started at 2.5 and went a little further. Maybe another 1/4 turn? 3 is better? I'm going to try and idle drop when I get these carbs synced to see if I can find the sweet spot but 2.75-3 seems like a good place to be. My plugs never look all that lean.
 

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Discussion Starter · #4 ·
Do you have the Vac blocked the carb nipple? With stock AB and H-box/exhaust 90/120 is probably to much. Believe stock is 78/115?
My carb doesn't have a nipple for the vac. Hence converting the tank to free flowing.

Yeah 78/115 is stock I believe. That's why I'm asking.
 

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Discussion Starter · #5 ·

Looking at that it's a lot of extra fuel over stock. I'll see what's actually in the carb kits I have. They might be the right jets. Or I'll just order them. I'll double check to see what I have in the carbs tomorrow. Indo remember noticing that the one was definitely 120. Too big.
 

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My carb doesn't have a nipple for the vac. Hence converting the tank to free flowing.

Yeah 78/115 is stock I believe. That's why I'm asking.
yet you have a Vac petcock? is there two or three nipple there? If two one is a vent/drain not a Vac.
 

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Discussion Starter · #8 ·
yet you have a Vac petcock? is there two or three nipple there? If two one is a vent/drain not a Vac.
I had a vacuum petcock haha. I disabled the vacuum lock since my new set of carbs lacked the vacuum port. I see there are some variations on the 25s. Yeah I have a FSM. If I have to, I'll just run it for a couple of days until I can get the proper jets in. Worst case I'll probably slowly foul the plugs. I doubt I'll want to ride like that for long.
 

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1982 CX500C (US)
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The “choke” part of the choke refers to restricting of air if I’m not mistaken but a second function of pulling it is usually a fast idle mechanism that nudges the throttle a little.

Mentioned in many posts here like Fast Idle Adjustment and others, and definitely outlined in the FSM, but now that you know what it’s called you can look into it!

It might be as simple as the black idle adjustment is pushed so far in the nudge can’t happen, or maybe it’s out of spec.
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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My carb doesn't have a nipple for the vac. Hence converting the tank to free flowing.

Yeah 78/115 is stock I believe. That's why I'm asking.
VB25A is the stock carburetor on an '81 Custom. You have the vacuum nipple on the right carb.
 

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Discussion Starter · #11 ·
The “choke” part of the choke refers to restricting of air if I’m not mistaken but a second function of pulling it is usually a fast idle mechanism that nudges the throttle a little.

Mentioned in many posts here like Fast Idle Adjustment and others, and definitely outlined in the FSM, but now that you know what it’s called you can look into it!

It might be as simple as the black idle adjustment is pushed so far in the nudge can’t happen, or maybe it’s out of spec.
I'm going to look at my old rack which is still intact and see what's different. I think I compared the rocker arm you describe and they were identical. But yeah it just touches and doesn't push. So that needs to happen. I think the clearance spec is rather small and no I didn't check when they were on the bench haha. I just made sure it pushed the rocker arm and that the butterflies closed. The spring on that set is all sloppy and unwound looking. I think that's part of my problem. I read those posts earlier. Perhaps all of them. And they consistently mention the spring and to check it. I'm moving in the right direction. That bike wasn't firing on 2 at idle very well. #2 was running much hotter than #1. Very noticeable when the condensation steams out of the pipes in the winter. That's what prompted me to tear down the carbs. Which resulted in a ruined carb because of trying to do something dumb with a pressed jet. I got another rack and pulled out both those jets cleanly. I had a hard time starting but once I got it running it was much, much better with evenly hearing pipes.

Then I started to sync by ear a bit and it backfired and blew out an intake boot. I ran it around a little like that and it was still pulling better like that. I was probably pretty lean on #1 though. I limped it back to the garage and I'm waiting on some boots that may or may not fit and some gauges so I can really see what's going on with my vacuum and so I can also hopefully see any leaks I might have missed. And then I started thinking about the jets and looked up the jet sizes out of curiosity. Yep. Like 112/78 I think. So that's what I need to start with. I'm moving in the right direction.
 

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You might be correct. The FSM shows the vacuum petcock being introduced on the 1981 models, but the '80 Custom has the VB25As.
 

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Ps I noticed people were saying 3 turns out? I always started at 2.5 and went a little further. Maybe another 1/4 turn? 3 is better? I'm going to try and idle drop when I get these carbs synced to see if I can find the sweet spot but 2.75-3 seems like a good place to be. My plugs never look all that lean.
Per "Pilot Screw Adjustment" on page 23-9 of the FSM, initial setting should be 1 3/4 turns from seated. If you follow the procedure and don't notice a perceptible change in engine speed, you end up at 1 turn out. Three turns would be way too much.
Of course, if you've abandoned Honda's carefully engineered stock system, this might not apply.
 

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If you have disconnected the vacuum feed to the petcock - have you swapped the internal spring inside the petcock to the other side of the diaphagm? If not fuel wont flow at all.
 

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Discussion Starter · #18 ·
If you have disconnected the vacuum feed to the petcock - have you swapped the internal spring inside the petcock to the other side of the diaphagm? If not fuel wont flow at all.
Yes. Reversing the spring is the only way to make the petcock flow.
 

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Discussion Starter · #19 ·
VB25A is the stock carburetor on an '81 Custom. You have the vacuum nipple on the right carb.
It has no vacuum port. Earlier carbs. My current set is off an 80 or earlier I would guess. Otherwise that is the only real difference. It's raining today so I'm not going to deal with it until tomorrow. I will make sure the linkage pushes the throttle some and measure the gap.
 
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