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Discussion Starter #1
Bike is an 82 CB450SC Nighthawk. I was hoping to get some help from the list on some oddities that I'm finding. Bike is new to me, with 9000 miles on the odometer. The PO said that the last time it was "worked on" that the mechanic said that the carbs were "finicky". When I turned on the gas, the carbs both leaked out of the drains and the accelerator pump housing, so I made sure it started and ran (poorly, but it ran on both cylinders), so now it's time to clean the carbs and get them right.

I got the carbs off the bike this afternoon and took a preliminary look. It's no wonder that they didn't run well. The o-rings on the air valves were missing, the accelerator pump was clogged (and leaking while on the bike). Throttle plates massively out-of-sync. Choke plates out-of-sync (bent actuator). One pilot screw all the way in, one out 5 turns. Yikes.

What has me puzzled is the installed jet sizes. The carbs are VB22G

Main primary: 72
Main secondary: 102
Pilot jet: 35
Needle: D329

These sizes all seem quite a bit smaller that what should be there stock. I'm going to keep the stock airbox/filter. What sizes should I use to replace the above jets?

Does the Honda gasket kit for these carbs come with the air-cut valve o-ring, or do I need to buy complete air-cut rebuild kits. The diaphragm on the ones in the carbs now seem fine.

Also, looking at this pic, should there be a rubber plug on here somewhere? The FSM shows a picture that points to a "slow jet plug" but the pic is hard to see. Am I missing this plug?

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Also, any idea why someone would do this to an airbox? It looks like they locked a hungry rat in there and made it chew its way out:

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One other thing too. I noticed that when I removed the jet marked 72 (primary main?) that there looked like something underneath it that could be unscrewed and removed. I didn't have a screwdriver that fit, so I haven't touched it yet. What is that under there?

Thanks in advance for any help guys. I appreciate it. I know it's riding season for most.
Regards,
Mike M.
 

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Discussion Starter #4
Thanks JT and LDR.
I've been trying to line-up all the parts I need and was looking on Jets-R-us for the correct replacements. I noticed something strange. I think that the pilot jets that are installed in these carbs are not only the wrong jet size, but the wrong type of jet all together. As you can see this is an authentic Keihin jet, but I think it is too long. This jet measure 28mm long, and it looks like the correct pilot jet should be 23.5mm. Can someone please confirm this for me?

Considering the jets are not only the wrong value, but the wrong size, perhaps someone tried to use a carb rebuild kit for a different carburetor? With the pilot jet 4.5mm too long, perhaps the rubber plugs didn't fit, so they tossed them?

Looks like this carb rebuild is going to cost about 1/2 what I paid for the bike :)

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Here's a closeup of the marking showing the "K star" that Keihin uses.

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The Honda fiche identifies #48 in this fiche as a 99103-420-0380 or 0400. 38 is the standard size on all these but I'd switch to the 40.
https://shop.westernhonda.com/fiche_section_detail.asp?section=387190&category=Motorcycles&make=Honda&year=1982&fveh=12281
Jet-R-Us shows the 99103-420 with the measurements you list and since they are a very trusted source I'd believe them
https://www.jetsrus.com/a_jets_by_carburetor_type/jets_keihin_99103-420.html
Sounds like the PO or someone working for him just threw anything that looked sort of close at these carbs.
 

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Discussion Starter #6
The folks who I bought the bike from were the original owners. I'd like to think it was the PO and not a professional mechanic who made this mess.

My orders are placed from Honda for carb gaskets, mixture screws and float needles. Jets-R-Us for all new jets (one size up). 4-into-1 for accelerator pump and air cut valves. I had to resort to Ebay for the 61C needles.

Here's hoping for a well-running bike when I'm done. Thanks again for the advice.
 

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Discussion Starter #8
Even with the stress and anxiety of an impending hurricane, I'm still excited. All my carb parts are here and I'm ready to start rebuilding. Wish me luck!

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Even with the stress and anxiety of an impending hurricane, I'm still excited. All my carb parts are here and I'm ready to start rebuilding. Wish me luck!
I'll wish you luck on both counts... we're about 50 miles north of you and not feeling too comfy about the hurricane either - not looking forward to spending most of a day putting my shutters up and taking down all the stuff in my yard, but I'm more concerned about my big pine trees staying upright. No flooding worries where we are, but high winds could be a big problem
 

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Discussion Starter #10
I got the carbs back together and back on the bike. The weather is stormy so I only just started it up in the garage for a few seconds. I'll know more tomorrow when I try to balance and adjust the mixture.

One thing that bothered me is the springs in the vacuum piston. Can someone tell me if these look like they've been modified (cut)? Thanks!

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Discussion Starter #12
That's really nice of you Jim. Thank you!!!

Dealing with this old bike makes me feel a little bit like a detective. Seeing the evidence left over by the "perpetrators", and trying to figure out what happened.

Was someone looking to improve performance? I assume that's why the top of the airbox was cut open. Then there's the jets that were installed that were all too small. Why did that happen? Maybe the bike wouldn't start after sitting, and the shop used a carb kit from a 750 because that's what they had on the shelf? Maybe after the carbs were "rebuilt" using the wrong parts, someone was trying to fix a lean-condition? There is also sealant on the seal on the end of the throttle shaft. So the bike was super-lean, then they tried to fix air leaks, then chopped the piston spring trying to get more fuel?

It looks like there are some "jet kits" that include instructions to modify the piston spring. Maybe someone got a hold of a kit for the wrong type of bike and just installed it?

Who knows. The bike has only done 240 miles a year since it was built. Obviously, it sat for long periods of time. I'm really looking forward to getting her on the road soon and breaking the cycle of neglect.

Thanks for the help in untwisting the mystery :)
 

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Discussion Starter #13
It's running great! I could only take it for a short spin because the tires are ancient, the tag is expired and I'm a little worried about the front brake.

I did noticed just a tiny bit of fuel at the drain on the left carb. Not dripping, but could see it when I touched my finger there. Not sure if that's an issue or not. But, it started easy, idled great and moved down the road with gusto! Thanks again to all who helped, LDR, J-T, Ancientdad and all the others who chimed in.

By the way, how do you turn the idle mixture screws when the carbs are on the bike? The right carb screw is directly above the neutral switch. Even a stubby-screwdriver wouldn't fit. Any tricks to this?

Here she is, running like a Japanese sewing machine! BTW, I do have the side covers. I just had them off while fooling with the carbs.

 

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Dealing with this old bike makes me feel a little bit like a detective. Seeing the evidence left over by the "perpetrators", and trying to figure out what happened.
What we do on our vintage bikes absolutely is like being a detective... and some of the stuff done by POs before us borders on criminal behavior to these bikes so yes, in some cases they could be called perps :D

Thanks again to all who helped, LDR, J-T, Ancientdad and all the others who chimed in.
Thanks, but all I really did was comment on the current hurricane situation... I doubt that contributed to your success :lol:

Glad it's running good
 

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The original mixture screws had caps on them with a limiter tab so they could only adjust almost 1 turn. I've been leaving the caps on, they're glued to the screw, and cutting the limit tab off the float bowl. Makes it easy to adjust with fingers plus the tab gives a reference point of positioning.
If you don't have the caps maybe try some small vacuum hose forced over the end of the screw
 

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Discussion Starter #16
I went down to the DMV today to get my title and registration for the bike, so I had a chance to run it a little more at higher speeds. Out-of-state titles require a VIN check so I had to bring the bike "in person".

I noticed that when going from part throttle to WOT that there seemed to be a little bog or delay, then it would pick up. It's not horrible, but noticeable. Once it was opened up it seemed to run as it should all the way to redline. Could that be because of the cut springs? I would think that the piston would raise up faster than stock, so maybe it is a little rich until the motor catches up?

I've been looking for replacement springs, and apart from buying a complete parts bike I haven't found any yet. I'm keeping my eyes open.

Thanks guys
Mike M.
 

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Springs could easily be the cause.
Also the accel pump. With the air box boots off you can watch the nozzle by the choke plate squirt with fast throttle twist. Real booger to clean those since they face backwards
 

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Discussion Starter #18
Thanks Jim.

I think my accelerator pump is good. I bench-tested that when the carbs were reassembled. Twisting the throttle plates gives a skinny stream of fuel straight back to the throttle plates.

Once I get my new tires on I'll be able to test more thoroughly. I was driving on rock hard tires with an expired tag yesterday, so that limited my shenanigans :)

I did notice that the on-off and off-on throttle transitions were buttery smooth. No flat spots or jerkyness, and better than many fuel injected bikes I've owned. The only issue I noticed was when going from 1/2 throttle to WOT. That's when I noticed the flat spot. Maybe my accelerator pump adjustment is off? I had trouble following the adjustment procedure from the manual. I couldn't get a feeler gauge in the spot between the pump rod and the actuator with all the other hardware in the way. Even then, the clearance is TINY. 0.01mm? That's 10 microns! Is that a typo in the manual?

Thanks again
Mike M.
 

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Thanks Jim.
The smallest one I have is 2 thousandths, and it is super flimsy. A 1 thousandth feeler gauge must be hard to handle.

I'm going to keep looking for replacement slide springs. I was also thinking that maybe that giant hole that someone cut in the top of the airbox lid might be having an impact too. That part is easy to find. The springs haven't been. I really don't want to buy a set of carbs just to get the springs.

I'll update when I know more. My tires arrive today, so I've got work to do in the meantime.

Thanks
Mike M.
 
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