1981 Honda CB750c Carb Hose Connections

ezridermn

ezridermn

New Member
I recently took the carbs off my 1981 Honda CB750c to rebuild them. I reinstalled the carbs and am going to use pod filters instead of the factory airbox. Yes I know people say it is going to run poorly with pod filters. But I am going to find out for myself.

Now that the carbs are reinstalled and the airbox is removed I have two small hoses that I am unsure what to do with. One comes out from the middle of the carb bank and the other comes out of the right top of the carb bank.

I would appreciate any input you can give me on what to do with these hoses. Thanks in advance for your help!
 

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After you realize that everyone was correct and just trying to save you a lot of pain by telling you that your DOHC with CV carbs runs like crap with pods and is not tunable, you will be plugging those back into your air box after re-installing it. So don't cut them or discard. You will need them.,

You could save yourself a lot of trouble and put the stock air box back on now. But just in case you don't believe the pod story and really want to find out if the fire is hot by testing it with your face, here is what your mystery hoses are :

In your top photo, that is a vent line for the vacuum petcock ( the module it plugs into- there is a vacuum line from the carb #2 that operates the valve opening your fuel flow to the rail from the tank ) Make sure this hose does not get clogged or kinked. and same with the line in the bottom photo, that is the master line for the carb vent rail. Keep them open and clean and away from debris.

You also unhooked the breather pipe that was plugged into the top-front of the air box. You don't want that end left open. You will need to get a longer breather hose and run it down behind the engine and install a little air filter on it. Or install the filter anywhere you want-up to and including right at engine. Again, save the complete original breather hose for when you put the stock air box back on.
 
Let me get this straight... you ditched the airbox and went with pods... but kept the Spawn of Satan (vacuum opperated fuel valve)? :o
 
With re-jetting and setting the air screw at 3 1/2 turns, I am running pods on my 81 CB750c... but maybe I just got lucky
replacing the 68's with 75's and the 102's with 110's and no adjustment to the needle valve height... I've ran it over 2000 miles, mostly at highway speeds... sometimes a little cold-natured to start... but runs like a top!
 
Thanks for all the input. Always get good feedback on this forum. Can anyone fill me in about the vacuum operated fuel valve? What issues does it present? Is there something I should do to deal with whatever problems it creates?
 
The main issue is that over time they tend to leak and that causes all sorts of problems. They can usually be fixed with a repair kit (depends on the bike though). Or find a new one from a more modern bike and use that for the next ten years until that is ready to be replaced.
 
Or discared it. Two phillips screws and send it to the dustbin.
Extended crank times, fuel shortages, and vacuum leaks that is why it's called the SOS or Spawn of Satan.
To by-pass it, run the line from the tank to the carb fuel inlet (with a inline filter of course) and cap off the vacuum port at the carbs that supplied it manifold vacuum...

Where the hell is Notlob in all this. :D
 
So I fired the bike up just now. Used a temporary auxiliary gas tank to supply the fuel. Started right up but dumped a bunch of gas out of the overflow tube(s). Couldn't tell if it came out of more than one. Do you think this is a problem related to the auxiliary gas tank or something else?
 
ezridermn said:
So I fired the bike up just now. Used a temporary auxiliary gas tank to supply the fuel. Started right up but dumped a bunch of gas out of the overflow tube(s). Couldn't tell if it came out of more than one. Do you think this is a problem related to the auxiliary gas tank or something else?
Click to expand...
Either the fuel valves are stuck open / not seating, float height is set too high (my 81's were not adjustable) or the overflow tube in the float bowls are cracked (not as uncommon as you would think).

crack-in-pipe.jpg


Pull the float bowl, remove the float and fuel valve and clean the seat with a Q-tip with a little buffing compound followed by spray carb cleaner. Check the neopreme (sp?) tip of the needle valve itself while you have it out. And inspect the overflow tube pictured...
 
Update. When I had the bike running with the auxiliary gas tank hooked up fuel was coming out the overflow lines and the bike acted like it was flooded. I unhooked the gas tank and let it sit for about 20 minutes. I started it again without the gas tank attached and it seemed to run perfect on the gas remaining in the system and there were no leaks. Does that shed any more light on the situation?
 
ezridermn said:
So I fired the bike up just now. Used a temporary auxiliary gas tank to supply the fuel. Started right up but dumped a bunch of gas out of the overflow tube(s). Couldn't tell if it came out of more than one. Do you think this is a problem related to the auxiliary gas tank or something else?
Click to expand...

Using an auxiliary tank and then fuel dumping out of the overflow tubes, suggests either faulty float valves or a incorrectly adjusted float heights.

ezridermn said:
Update. When I had the bike running with the auxiliary gas tank hooked up fuel was coming out the overflow lines and the bike acted like it was flooded. I unhooked the gas tank and let it sit for about 20 minutes. I started it again without the gas tank attached and it seemed to run perfect on the gas remaining in the system and there were no leaks. Does that shed any more light on the situation?
Click to expand...
I'd say this suggests that you probably dont have a cracked vent tube as it would of leaked as it did with the tank attached. But your problem still exists when you attach a fuel supply? so check out the float valves and float heights. They should be 17.5mm from gasket face to highest point of float when resting closed on the spring.

You could try this official Honda modification for the carbs to run with stacks, should work with pods but you will still have to fine tune the jetting. Probably best to download the images so you can enlarge it.

8964691060_be4c68db60.jpg


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glenhunn said:
With re-jetting and setting the air screw at 3 1/2 turns, I am running pods on my 81 CB750c... but maybe I just got lucky
replacing the 68's with 75's and the 102's with 110's and no adjustment to the needle valve height... I've ran it over 2000 miles, mostly at highway speeds... sometimes a little cold-natured to start... but runs like a top!
Click to expand...

When Honda RSC (forerunner to HRC) instructed further modifications other that jetting to make the carbs work well enough for production racing, suggest to me they wont work properly just with a jetting change. Many have claimed to have the magic jetting formula but kudos to you, thats the 1st time I've seen someone state what their jetting is.
They are cold natured even with the airbox attached. I dont have a 750 anymore but your info could be useful to other owners who want to try it and maybe post up their results
 
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