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Discussion Starter · #1 ·
I have this 72 CB 350 that I bought at the beginning of summer. Compression on both cylinders is around 170. Points are set properly. I’m having some type of fuel delivery/carburetor issue. I put on brand new petcock, new in-line fuel filters and cleaned the carbs. When I turn the fuel on I only get a trickle in one of the fuel filters coming from the dual lines in the petcock. I was under the assumption that the carbs create a vacuum and will pull fuel into the float bowls but I think I'm wrong. When I ride the bike I hear a very weird sputter that comes from the left side. It’s almost like a sucking sound coming from the left pod. After the bike has been running for a while the idle hangs when I blip the throttle and I’m assuming it’s not getting enough fuel. What are my options here? What do I need to do to fix this?
 

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Your bike runs on gravity feed, carburetors Never suck fuel in, fuel needs to be pushed. Yours sounds plugged likely from rust because your fuel tank was constructed from cheap steel.
 

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I had to clean a carburetor 7 times before a race once, the first 6 attempts were unsuccessful and after the 7th. dismantle (and some help from a thorn tree) it worked like a charm. Thanks to the wonders of modern fuel technology, it takes great patience to own a motorcycle with carburetors. Clean them again, all of the problems will be in the very bottom of the carburetors (where the floats are) the top half of the carbs contain mostly air.
 

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Discussion Starter · #6 ·
I unfortunately bought the bike with the pods. Ill try to remove the fuel filters and run straight lines to the carbs but I don't thinks that's the problem. When i run the straight lines i'll loosen the float bowls drains and see if we have any fuel in there.
Thanks for the replies!
Joe
 

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Lots of learning opportunities here. First off, CB350 carbs are known to be a PIA to get right and older bikes with stale fuel are a real problem to keep in shape.

OK. Let's start with the theory: Fuel feed from the tank to the carbs is gravity as Trials mentioned. So that fills the float bowls and teh jets sit in a pool of fuel. As air passes through the carbs, it speeds up and that causes a lower pressure in the venturi compared to the atmospheric pressure in the float bowl and that pressure difference causes fuel to come out of the jets into the airstream. Some call that a vacuum and others say it's the pressure in the float chamber and it's both. It's the difference in pressure that does the work.

Others will tell you that the carb needs a restriction on the inlet to allow the motor to suck fuel out of the jets but that's not really what's going on. For sure, a lack of filters or restrictive filters will potentially change flow a little and cheap pods usually restrict airflow enough to kill performance.

I have run CV crabs with stacks, and foam filters and super short drag race bellmouths and they all can be made to work with a little work. Nothing much fixes cheap pods though.

Your problem could be any number of things. So start at the beginning. New fuel tap may actually be the problem. Remove the fuel tubes from the carbs and put them into a container and turn on the fuel tap. It should flow freely but will usually only flow out of one tube at a time. If it does that, pinch the line that's flowing and if the other side starts to flow it's good.

That happens, I believe because as the flow starts in one side, it creates a low pressure on the other outlet and pulls air in instead of letting fuel out. That's my theory and I'm sticking to it - at least until I see a better explanation.

If the petcock is OK, the next place to check is the float bowl and the float valves to see if they are flowing. Remove one bowl and turn on the fuel and see if it comes out of the float valve. BTW, do this outside and with a container to catch the fuel. repeat on the other side. Fuel doesn't come out as fast as you might expect but as long as its flowing freely, you are OK.

Try that an report back please.
 

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Also if one hose has an uphill section as it is going toward the carb, you will in essence, have a trap, and since the gas has another path to take, that’s the way it will go (path of least resistance and such). When carb one fills up and the needle valve shuts off the flow, the line to carb 2 will have an air bubble with in the trap keeping the flow backed up.

This is why in-line filters are troublesome on old bikes. Usually they add length to the hose and create air traps
 

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Discussion Starter · #9 ·
ok, so I didn't have time to drop the bowls and check fuel flow but I did remove the fuel lines from the petcock and its flowing from both sides and shuts off securely. I'm kinda thinking the droopy fuel lines and inline filters may be the cause of my issue. Here's a pic of my fuel lines. For some reason the filters are pretty full of fuel. They were not like they yesterday when I was running the bike.
Automotive lighting Motor vehicle Hood Automotive tire Bumper
 

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First off there must be a reason the fuel filters would not be full of fuel, gravity should take care of that as air rises, so if it's up hill to the fuel tank supply the tank is where all the air should end up.
& "didn't have time to drop the bowls and check fuel flow" Once the fuel enters the carburetor body, all of your problems will be inside those float bowls, those are the first thing you should be servicing not left until you try other things.
Air trapped in your inline filters indicates your gravity feed fuel system has a restriction above those filters.
 

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Discussion Starter · #12 ·
Ok, so I pulled the bowls with the original fuel lines and I was getting fuel on both sides. When I raised the floats fuel stopped so I think that is all good. I put on straight clear fuel lines anyway just to make sure fuel was flowing properly. When I fired up the bike I'm having the same issue. The bike seems to run better with the choke on. Im assuming the carb/jets are not getting enough fuel. Right? I also checked the mixture screws, just out of curiosity, and they were both at almost 1.5 turns out. What should I try next? The bike was not warmed up, but as it was running I sprayed some carb cleaner on the boots and those seem ok as well.
 

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The screw sets your low speed fuel to air ratio, the position of the main jet needle makes the huge difference in anything above about 1/4 throttle, raise the needle by one cir-clip position if so equipped. It might need to get up to engine temperature to run half decent.
 

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Choke circuit on a CV carb is a fuel jet that is completely in addition to the normal running jets, so it's a fuel enrichment circuit that uses a completely different fuel path. It's like dumping more fuel straight into the intake manifold as close to the valve intake as possible, so yes more fuel compared to air seems to be in order.
 

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So far so good. It's a process of elimination.

CB350 has Keihin CV carbs and the "mixture screw" is on the motor side not the air filter side, so it's moderates fuel flow and not air flow. So more turns OUT makes things richer and turning IN makes it leaner.

As Trials mentioned, the motor has to be warm befor adjusting things, but it sounds like yours is lean. You could try turning the mixture screws out another 1/4 and then 1/2 turn to see if that cleans up the idle and off idle situation.

All of this is fine if the carbs are scrupulously clean but yours may have blockages in the jets or fuel supply. So first step is to clean the carbs and second step is to contact someone who knows those carbs. CrazyPJ and ask him if he can fix your carbs for you. The carbs on a 350 and 360 have their own quirks and PJ knows how to clean them up and make them work.
 

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Discussion Starter · #16 ·
OK, so I’m trying again to clean these carbs. And I thought I would add this little bit of information to this thread. The jets that are on there are size 38. Are these the right size jets for these stupid pods?
 

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OK, so I’m trying again to clean these carbs. And I thought I would add this little bit of information to this thread. The jets that are on there are size 38. Are these the right size jets for these stupid pods?
The jets need to be sized for the engine not the air filters. The ones that were in them to begin with are likely correct for the engine and stock exhaust and the carburetor manufacturers provide additional information on changing the jetting to sit in their tuning manuals. I was looking at some of those charts just earlier today in the sudco catalogue also.

... you really need to lose the notion that pods create significant less restriction on the intake so as to need re jetting and dumping more fuel into the carbs, it just ain't so, 'significant' changes in exhaust might need fuel and air adjustment but the pods, all they contribute is turbulence and reduced air flow into the carburetors.
 

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Discussion Starter · #18 ·
I haven't had much garage time this week but I was able to pull off the carb and clean everything again. I double checked the float heights too. I put everything back on and semi synced up the carbs and the bike seems to idle way better, although the idle does seem to hang just a little. I still get a little sucking from the Pod but I haven't tried messing with the mixture screws. Later this week my friend is giving me side covers and the stock air boxes from an old CB. Once I pop those on I'll try to do and mess with the mixture screw.
One other thing, I tried to sync the carbs turning out the throttle cable barrel adjusted until the idle set screw starts to move but I can only do that on one side. The other side doesn't want to work that way so I've been adjusting both sides until the both actuate at the same time.
 

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Discussion Starter · #19 ·
Today I had a chance to play around with the idle mixture screws and the right side seems to be running smoothly but I can't get the right side to run on one cylinder. It will run with the throttle wide open but it's kinda rough. I also tried to run it on choke to see if it ran better or worse. It ran the same with the choke on and off. Im thinking a can rule out carb issues and maybe look as spark/ignition coin. Whats my next step? Im thinking I should try to run the right side wire on the left to see if it runs better.
 

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Today I had a chance to play around with the idle mixture screws and the right side seems to be running smoothly but I can't get the right side to run on one cylinder. It will run with the throttle wide open but it's kinda rough. I also tried to run it on choke to see if it ran better or worse. It ran the same with the choke on and off. Im thinking a can rule out carb issues and maybe look as spark/ignition coin. Whats my next step? Im thinking I should try to run the right side wire on the left to see if it runs better.
Switching things from one side to the other where they are interchangeable is a good trouble-shooting technique, but spark wires are often easy and cheap enough to just replace or to meter test.
Normally you would rule out the ignition problems first and then work the carbs after, but whatever works.
Your bike has a battery-coil ignition system so don't over-look the battery and charging system.
If it's running rough at full throttle, you're likely still only running on one cylinder.

Is the motorcycle still 100% original or have things been changed on the electrics?
You said you set the points, did you do that just by the size of the point gap or did you use a test meter?
... use a meter, you will achieve far better results.

Condensers were last replaced when? They don't last forever.
 
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