CB360 Carburetor Swap - Idle Way Too High!

krukster86

krukster86

Been Around the Block
Hey all you CB360'ers and carb enthusiasts! Help me out here.

Today is a lazy sunday...and I hate lazy sundays. I stopped in the garage to satisfy my curiosity with the carburetors on my CB360. Right now I am running CB450 carbs more or less tuned for a CB360. I had spare set of CB360 carbs from my parts bike. I took out the cb450 carbs and put in the cb360 carbs.

Backstory: The CB360 carbs have been cleaned inside and out and the diaphragms are in EXCELLENT condition. I set them at stock jetting settings and thanks to another forum member, I set the floats to spec.

I throw the carbs on the bike and decide, what the heck, and start the bike up without adjustments from stock. WHAMO she starts up on the first kick in 40 degree F weather, but the idle speed shoots up to 3k rpms and stays there. Mind you, I have the idle screw set all the way out (aka it is not touching the throttle cable wheel at all). I tried moving the air screws in a few turns and out a few turns and it looks like there is no difference.

What is the culprit here? I think that I may try to stick with these carbs, they are looking pretty damn good.
 
I dont have this bike or know the carbs, but do you have an airmix (pilot) screw? On my bike, its at the bottom and its tiny, and the idle screws are on the side of the carbs. I think on your bike the idle screw is shared, which means you turn one and it adjusts both carbs, but i might be wrong.

What id do anyway is set all the adjustments to factory, then start with the pilot screw, turn that in using 1/4 turns until the idle starts to faulter. Then slowly turn it out in 1/16th turn increments until the idle stabilizes. Wait a few seconds after each 1/16th turn for it to take effect. Once you have the idle stabilized using the pilot screw, move onto the idle screws and turn them out until you get the idle you want. If the bike starts to die out before it gets to a low enough idle, turn the idle screws back in using 1/8th to 1/16th turns until it stabilizes, and then go back to the pilot screws. Repeat until the bike is running as it should. Once this is done you should balance the carbs with a manometer (You can make one if you dont own one way cheaper than the pro-tool).

Because you said you set everything back to stock (Jets, needle, float), the above should work for you. The important thing to remember is when making changes, make the EXACT same changes to both carbs, at least when youre adjusting without using the manometer at the start. Once you bust out the manometer for syncing, its ok to make uneven changes to each carb.

*edit* i just re-read and you do have air mix screws. The thing here is that you need to use SUPER fine adjustments and you really need to listen to the bike, each cylinder seperately. Turning them may have no effect on the idle showing on the tach, especially at 3k rpm, but if you listen, you will hear the change in the idle. Remember that if theyre truely air mix screws, out is leaner and in is richer. It also really helps if you have them set to the factory spec, even though most likely that will not work for you and will need adjustment. At least at factory spec you have a reference point for how the screws need to be adjusted, either richer or leaner. You will be able to hear when you have them set correctly, as the engine will purr along with absolutely no hiccups and will sound, for lack of a better word, "perfect". It takes some time to get the feel for it, but just take your time, dont rush it, and use really small turns (1/16th i found is the best for my bike). You can also use new plugs when you start, and pull them after letting it idle for a minute and see what they look like, it will give you an idea if its running lean or rich and what adjustments you need to make.
 
If bobjohnson's idea doesn't work, check out the throttle cable as well. If it moves freely, then it's not kinked. But double check where it meets up with the throttle sleeve too. Those couple of adjusting nuts around there can keep your idle up as well.
 
Just FYI:

I have tried the carbs out with JUST the fuel lines attached. There is no binding occurring at all. I think what the problem might be is the floats or the fuel valve. I noticed that upon inspecting the carb, it always seems to be puking fuel. I will let you guys know whats up after I get this carb rebuild kit.
 
I just have to ask, do the throttle sleeves have small tapers on the bottoms? It is possible to put them in backwards, and that can cause a high idle too.
 
Throttle sleeves? Inside the carbs, they move up and down with the cable.

Maybe they are called throttle slides? Been a long day - sorry
 
Oh ok gotcha, nah they are in correctly. The diaphragms have a certain "tab" on the lip that needs to be inserted the right way to close the "top" of the carb
 
krukster86 said:
Just FYI:

I have tried the carbs out with JUST the fuel lines attached. There is no binding occurring at all. I think what the problem might be is the floats or the fuel valve. I noticed that upon inspecting the carb, it always seems to be puking fuel. I will let you guys know whats up after I get this carb rebuild kit.
Click to expand...

I am having this exact same issue. High idle when running, and constant overflow of fuel from the carbs.

Please let me know if you figure out the issue.
 
This week I am going to see if the float (fuel) valve is to blame:
Take off the float bowls, pour water down the fuel intake tubes. Flicking the float bowls up should stop fluid flow. If not, I will try this again with the new set in the carb rebuild kit from eBay.

I wonder if the two problems are related. Would excess fuel in the carbs cause a high rpm idle? I would think that if anything it would enrich the mixture so much the engine would cut out or the spark plugs would foul.
 
Big R said:
I just have to ask, do the throttle sleeves have small tapers on the bottoms? It is possible to put them in backwards, and that can cause a high idle too.
Click to expand...

Not with CV carbs, airflow is determined by throttle plate.
Cables are on right way round?
The opening cable locknuts are probably in the wrong place and cable is adjusted too tight, it can still feel OK but isn't
Same thing can happen if someone has messed with sync screw and one throttle plate is open

PJ
 
crazypj said:
Not with CV carbs, airflow is determined by throttle plate.
Cables are on right way round?
The opening cable locknuts are probably in the wrong place and cable is adjusted too tight, it can still feel OK but isn't
Same thing can happen if someone has messed with sync screw and one throttle plate is open

PJ
Click to expand...
PJ,
There were no cables attached when I tried this out, just the fuel lines. I just wanted to see how the bike idled. I am tearing down the carbs a bit right now and inspecting all the internals. I am inspecting the carbs right now (literally) and the throttle butterfly valves are are set up correctly. They might be out of sync a bit, but not enough to tell by eye.
 
JustinLonghorn said:
When the bike is off, and the carbs are spitting fuel everywhere, are the carbs making any noise?
Click to expand...
No I do not hear any noise. When the bike is shut off, the carbs are silent. I think I exaggerated about the "spitting fuel part". They carb bowls on one of the carbs leaks (crappy gasket). I did a test of the float valves with water and it looks like they are not holding up too well. They will be replaced. I hope this is the solution.
 
Just trying to cover all the bases here so lemme know if anything is strangely off:

AirJets.jpg

Air Jets - I am pretty sure I have them set up the right, but not 100%

IMG_1653.jpg

Diaphragm springs. One is slightly shorter than the other. The cylinders spring back really well, but I am wondering if this is an issue.

IMG_1655.jpg

Butterfly valves closed

IMG_1657.jpg

Butterfly valves open

Other than a possible issue with this stuff above, I think the float valve is to blame.
 
Left butterfly looks open in your 'closed' pic?
You don't have air leak at manifold do you?
BTW, those carbs have fuel screws, out is richer, in is leaner
 
crazypj said:
Left butterfly looks open in your 'closed' pic?
You don't have air leak at manifold do you?
BTW, those carbs have fuel screws, out is richer, in is leaner
Click to expand...
I think its just the lighting/shadows that make it look open, there is no leakage through (tried it out with water)
The carb boots and gaskets are brand new (Had engine rebuilt a month ago)
 
you'd be surprised how just a tiny crack can screw with the idle, I was having almost the same problem with my 360 last year and it ended up being partly the throttle plate to blame. When I visually inspected the carbs the first time, they looked exactly like yours, but the right one had just the tiniest crack at the top, wound up being the throttle plate was just slightly out of whack and needed to loosen and retighten the plate screws
 
Interesting, I will try that out.
EDIT:
So I inspected the throttle plates with a micrometer I had and sure enough they were off in the "WOT" position! I expected a couple turns of the adjusting screw would even them out, but it was a lot more than that. I would say even 3-3 1/2 turns to get it correct. This might be ok for now (micrometer indicates both plates are "even"), but I will try using a carb tune to get them tip top.
 
Ok update time!

CB360 replacement gaskets arrived today, so I replaced the float valves and gaskets. The floats needed to be reset to spec, but I set them to more or less the 18mm-20mm range. I saw what a pain in the ass the gaskets were. Somehow I lucked out and pressure fitted them into the grooves. I plopped the carbs onto the bike and decided to give it a shot.

Observations:
1. The float bowls no longer leak
2. The bike idles at 1,300 rpms decently well, but I will have to use a carbtune to sync 100%
3. When some throttle is given, the RPMs slowly drop back down to idle (probably because of #2)
4. I get sputters of backfire out of one of the right muffler this time. I feel that the exhaust is not "warm" like the left side, and the headers do not get as hot.

Facepalm moment:
After inspecting the rebuilt carbs once and retightening, I cause a pinch point in the bowl-gasket assembly and cut my almost brand new gasket in 1/2. I am going to see if I can run with it (and try to use a method to really hold it down in place), but if not, is there a chance I can just use something from a hardware store, if they have o-rings this big?
 
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