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Discussion Starter #1 (Edited)
This is probably a redundant set of questions/concerns as I think I know what the issue is, but nonetheless it's better to be safe than sorry.

Context: Just finished rebuilding and bench syncing my carbs two or so weeks ago, but it's been raining and I've been busy so they haven't been able to get on. My impatience got the better of me today and I decided to put them back on and fire the bike up under the carport despite pouring rain since I'm eager to see how she runs after a whole tune-up and sitting for a season. Much to my pleasure she fires right up first kick at half-choke, but I quickly have to go and lower the throttle-stop screw to keep the revs from going crazy at idle. From here all is dandy as she's warming up for five or so minutes but as time goes on I notice the revs start to slow down until it kills completely. I play with the throttle-stop screw and get her back up and going but I cannot seem to find a mid point: either the throttle screw is in too much leading to excessive idle rpm or it is too low and gradually sinks down from 2000rpm to zero.

I know the carbs still have to be vacuum synced but shouldn't the bike at least idle long enough to get sufficiently warm enough to actually sync them? I was thinking that perhaps the bike was being fuel starved since I had the gas tank off the bike and connected to the carbs with extra long hoses that may have been preventing adequate fuel delivery. I also noticed that it would sometimes begin to kill after grabbing hold of the throttle without twisting (like it wasn't getting enough fuel or something).

Should any of this concern me; do I just set the throttle screw high enough so it idles without killing to get warm enough to then proceed with a vacuum sync, or is this possibly a larger issue that will preventing the sync from even being achievable. Thanks for any insight guys.

Bike is a CL360 '75 by the way
 

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I would check for an air leak first. Spray some WD40 or carb cleaner around the mounting to see if it affects the way it runs.
 

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Discussion Starter #3
I would check for an air leak first. Spray some WD40 or carb cleaner around the mounting to see if it affects the way it runs.
I'll go ahead and give that a try on Tuesday as I'll be too busy at work all day tomorrow to try. I have my doubts that an air leak is the problem considering I have brand new intake manifolds with new gaskets as well as NOS Honda air filters. I maybe could have had a leak at the exhaust because they were mounted in a hurried fashion but once again I have my doubts.
 

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Discussion Starter #4 (Edited)
Given the information I've found in this post https://www.hondatwins.net/forums/49-fuel-supply-carburation/122722-68-cl350-dies-after-1-2-minutes-broken-cross-over-tubes.html I've decided to check the plugs as soon as I can for any possible fouling. His bike's symptoms sound eerily similar to what I am experiencing, but before jumping the gun I need to properly set the mixture screws and vacuum sync the bike. If I'm lucky that will fix the issue all by itself.

For reference I set my floats to 19.00mm as an in between from Common Motor's suggestion and Motor Mayhem's suggestion, » Honda CB Carburetor Rebuild &raquo Motor Mayhem .

In the meantime any further advice is appreciated, but I will be updating this post as soon as I find the time to work on the bike.
 

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I would make a guess that you have set the float wrong.

What model Carbs do you have?
19mm seems small.

There are couple of things to consider here.

1st is the inverse relationship to float hiehgt and fuel level in the bowl.
The smaller the gap in the float hiehgt the higher the level of fuel in the bowl.
This is because the float hangs upside down. As you set the gap smaller the float rises and so does the fuel level.

2nd is th TIP of the Needle that rests on the Float Tanfg is spring loaded.
If you are not holding the carb at the 45 deg angle as specified in the manula you will compress that spring tip and have a bad setting.
Again this typically will result in a fuel level that is too high and will cause flooding condition and one that the idle mixture screw seems to have no effect on.
 

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If you suspect the float's too high, turn the petcock off after you start it, see if that makes a change.
 

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Discussion Starter #7
I would make a guess that you have set the float wrong.

What model Carbs do you have?
19mm seems small.
I have the stock Khelin carbs for a '75 CL360, which the manual states to have at 18.5mm. So that we're not shooting around in the dark I'll be checking for any fouling/gas smell on the plugs tonight after work. If any is present I'll go ahead and raise the floats a bit to around 20-22mm.

If you suspect the float's too high, turn the petcock off after you start it, see if that makes a change.
I would be doing that after it warms up and begins to lower in rpm, correct?
 

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Discussion Starter #8
The plugs are black as can be and smell slightly of gas, so it seems my suspicions have been confirmed. Will be adjusting float level tomorrow several mm higher and let you guys know what's up.
 

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My thought on the Fuel Level being too high was the Spring Loaded Tip on The Needle Seat.
The Needle that contacts the Float TANG is Spring Loaded.
If you don't hold the Carb Body at an angle as prescribed in the FSM, the weight of the the float can compress the spring giving you a false reading that is too large.
The end result is to high a fuel level.
 

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Discussion Starter #10 (Edited)
My thought on the Fuel Level being too high was the Spring Loaded Tip on The Needle Seat.
The Needle that contacts the Float TANG is Spring Loaded.
If you don't hold the Carb Body at an angle as prescribed in the FSM, the weight of the the float can compress the spring giving you a false reading that is too large.
The end result is to high a fuel level.
Whenever I adjust them this time around I'll be extra careful to hold them at the proper angle and not accidentally compress the spring whenever measuring them. I'll also measure them before bending to ensure that I am actually setting them to a different, higher value, than what they are now. Higher value equals less fuel in the bowl correct?
 

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Yes - Correct.
The float hangs upside down.
So a greater distance means it is suspended at a lower point when the needle shuts off the fuel flow.
Hence a lower level in the fuel bowl.
 

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Discussion Starter #13 (Edited)
Hey guys well I'm back after finally getting to it, some stuff popped up as it always does.
Well anyways I raised the floats to a little under 21mm each and the bike now runs for about 30mim before dying completely and being unable to start at all until it's sat a good while.

Couple of issues I noticed this time around:
The butterfly on my left carb is worn to the point where it cannot get tight enough during bench sync to match the right. To compensate I made them both similar, but not perfect, at a looser setting.
I was leaking a good bit of air by the right exit exhaust- the copper rings that butt in are present and it wasn't doing this last time so kinda at a loss there-maybe I didn't screw it on at the correct angle.
The right spark plug is still fouling, but new issue is the left plug doesn't seem to be used at all. It looks brand new (put a new one in after changing the floats). It doesn't look like anything on a plug chart-just looks brand new.
Misfiring badly this time around as well you could audibly hear it struggle on one side on the engine.

Well I think that's about it, getting a little frustrated as I've doing everything in the full tune up by the book, but I'm stating to wonder if I bought a bike that needs serious bottom end engine work which I quite frankly don't feel comfortable doing and don't have cash to pay the non existent vintage mechanics around here. Lemme know what you guys think

For reference the killing after 20-30 minutes of idling has been there from the start. The jack ass of a previous owner didn't tell me about it at all and seeing as I only took a 10min, or maybe even less, ride on it the issue never surfaced until I tried to ride in the following days. I figured it just needed a general tune up but doesn't seem that way anymore.
 

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25-30min. then dying could easliy be as simple as a clogged Fuel Vent in the Gas Cap.

Does it have the same problem if you don't secure the gas cap?
 

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Discussion Starter #15
25-30min. then dying could easliy be as simple as a clogged Fuel Vent in the Gas Cap.

Does it have the same problem if you don't secure the gas cap?

Gas cap is brand new so I don't think it's clogged, but I can give it a try. I'm still using those extra long fuel hoses as well. Think that could be a factor?
 

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I think I saw the Sintered Brass type fuel filters in your hoses.
These are typically used on Pressfeed systems that have a Fuel Pump.

FYI - they will restrict the fuel flow.

Gravity feed fuel to the carbs needs a very free flowing supply.
The Filter Screens in the Petcock valve and pickup screens in the tank should be all you need.

If you believe you MUST have filters use the pleated paper types.
They are MUCH-MUCH less restrictive.
 

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Sounds like it’s a fuel issue but I’ll throw it out there as it sounds like a similar problem I was having . . .
As per your spark plug not being fouled - are your coils good and getting a good spark in that cylinder? I had fuel problems but once I solved them I found out that my coil was fried - probably my own fault for leaving the key on while working on it.
 

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Discussion Starter #18
Back again, I took off the brass filters, strapped the tank to the back of the bike, and cut some proper length fuel line. Not backfiring like it was before, but bike still kills after 5-10mim of idling.

I'm thinking I'll have to raise the floats more but they're already at a little under 21mm and stock is 18.5mm for a cl360, so I'm a little worried there's a bigger issue here.

Left plug still looks clean as well going to check the coil soon.
 

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On the 360 carbs, the balance adjust mechanism between the carbs is pretty finnicky. If the spring is collapsed, the mechanism is binding, or the aliminum pieces that connect to it are bent, you will not get it to run right. The locknut and special bolt can strip etc. Kinda hard to explain but make sure that all the parts are correctly installed and working freely.

I'm battling this problem on my 360 at the moment. Once adjusted the spring is not providing enough pressure to keep the two butterflies synced all the time and one of the throttle shaft ends is bent. trying to get it sorted.
 

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"The butterfly on my left carb is worn to the point where it cannot get tight enough during bench sync to match the right."
Carb butterflies don't wear out, if the butterfly is not a good fit in the carb body someone has had it apart and put it back together crooked, loosen the screws and re-align it so it can meter fuel/air properly.
"The right spark plug is still fouling, but new issue is the left plug doesn't seem to be used at all. It looks brand new (put a new one in after changing the floats)."
Pull off the
air cleaners and squirt a small amount of gas or carb cleaner into the carb throat while it's running. If it picks up a little bit doing that it's too lean, if it stumbles it's too rich.
What does the battery voltage read, after it quits? Long periods of idling is a guaranteed way to kill the battery, it doesn't put out enough watts to keep up with the current draw under 3,000 rpm.
 
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