1981 cb750k Carb fuel line routing

[PRIMEWORKS]

So long story short, I'm currently attempting to start my bike up after having it sit all winter and I'm having some problems. Over the winter I rebuilt the carbs, and I just recently put in new iridium plugs. I Re installed the carbs following this picture (forgot to take pictures of fuel line routing prior to removing the carbs) and I think I may have installed the lines incorrectly because when I put fuel into the line that would be connected to the tank the gas doesn't go anywhere. The bike wants to start but obviously without the gas being properly delivered to the carbs it won't. Any help would really be appreciated!

 

[amc49]

The vacuum valve hasn't opened yet, motor has to be running to do it. It assumes already enough fuel in bowls to at least start engine up for the second or so it takes to open the valve. You have to get some fuel in the bowls manually to let engine start. Then it'll work right assuming it does. They give lots of trouble and easily removed if you just train yourself like oldschool bikers to ALWAYS turn the petcock off when engine stopped. Personally I dump that valve instantly. Useless bullshit made necessary by modern 'eddicated' people who aren't smart enough to turn gas off but smart enough to sue. Oh!-how hard it is!!!!

 

[PRIMEWORKS]

The vacuum valve hasn't opened yet, motor has to be running to do it. It assumes already enough fuel in bowls to at least start engine up for the second or so it takes to open the valve. You have to get some fuel in the bowls manually to let engine start. Then it'll work right assuming it does. They give lots of trouble and easily removed if you just train yourself like oldschool bikers to ALWAYS turn the petcock off when engine stopped. Personally I dump that valve instantly. Useless bullshit made necessary by modern 'eddicated' people who aren't smart enough to turn gas off but smart enough to sue. Oh!-how hard it is!!!!

Is there an ideal amount of gas I should be putting in each bowl? also a couple of friends mentioned to put a little bit of gas in the cylinders to help in the first start up. Should I do that as well?

Thanks for the reply!

 

[Dave]

The ideal amount would be full.

You could temporarily bypass the valve.

You could remove the valve and file it in the rubbish container as AMC suggests and run a new line straight to the T fitting that goes to the carbs and plug the other fitting off.

Easiest, get some starting fluid, open the airbox, and spray it all over the air filter, immediately fire it up. Repeat this until it doesn't stall from lack of fuel.

DO NOT put gas directly into the cylinders and slap the friend who suggested it upside the head.

 

[PRIMEWORKS]

The ideal amount would be full.

You could temporarily bypass the valve.

You could remove the valve and file it in the rubbish container as AMC suggests and run a new line straight to the T fitting that goes to the carbs and plug the other fitting off.

Easiest, get some starting fluid, open the airbox, and spray it all over the air filter, immediately fire it up. Repeat this until it doesn't stall from lack of fuel.

DO NOT put gas directly into the cylinders and slap the friend who suggested it upside the head.

lmao sounds good! thanks so much for the advice! I'll check back in and update once I try filling the bowls with gas.

 

[PRIMEWORKS]

well she started right up! but I think I may have run into a new problem. So after I got the bike started I turned it on and off a couple times after letting it run for a bit. Turned it off to hook up my carb sync, turned the bike back on, fired right up, idled fine, then the idle dropped and the bike died. Tried to start it back up but no luck. Thinking I flooded it, I let the bike sit for about 30 mins and still no luck so I'm a bit stumped cause the bowls still have gas in them.

 

[PRIMEWORKS]

So after letting the bike sit over night I tried to start her this morning and nothing. Looks like were back to square one and I don't really know what to check at this point. I just find it interesting that when I put the gas into the bowl she fired right up with no problems and now even though there is gas in the bowls she won't... Still leads me to believe that she't not getting fuel some how but maybe I'm wrong. I do have the air box off but I doubt that would lead to the bike not starting at all.

 

[Dave]

Rule of thumb, always check spark 1st because that's the easiest thing to check.

Have you tried bypassing that vacuum valve yet? or even testing it?

Unhook the fuel line to the carb and put some vacuum to the valve, and see if it holds open and allows fuel to flow freely or slowly closes indicating a leak in the diaphragm. You could even do this by sucking on the end of the line and plugging it with your tongue if you don't have a vacuum tester. (I'd recommend using a brand new piece of line if you try this, the used line will taste like gas)

PS On my own bike, I sometimes have to wiggle the engine kill switch to get it to make contact. It must have a little dirt in it but I don't have any problem with it unless the kids or I turn that switch and I turn off the bike with the key so I never bothered to try cleaning it out.

 

[PRIMEWORKS]

Rule of thumb, always check spark 1st because that's the easiest thing to check.

Have you tried bypassing that vacuum valve yet? or even testing it?

Unhook the fuel line to the carb and put some vacuum to the valve, and see if it holds open and allows fuel to flow freely or slowly closes indicating a leak in the diaphragm. You could even do this by sucking on the end of the line and plugging it with your tongue if you don't have a vacuum tester. (I'd recommend using a brand new piece of line if you try this, the used line will taste like gas)

PS On my own bike, I sometimes have to wiggle the engine kill switch to get it to make contact. It must have a little dirt in it but I don't have any problem with it unless the kids or I turn that switch and I turn off the bike with the key so I never bothered to try cleaning it out.

Is the vacuum valve the piece that sits on the carbs that has the multiple hoses fitted into it?

 

[Dave]

Yeah, fuel in, fuel out, vacuum from carb, and the vent. That is the vacuum / fuel valve.

 

[PRIMEWORKS]

Yeah, fuel in, fuel out, vacuum from carb, and the vent. That is the vacuum / fuel valve.

So I asked a friend about the valve (he owns a cx500 so is a little familiar with bikes). He told me to just remove the valve (he referred to it as an accelerator pump) and run the fuel line from the tank directly to the fuel inlet between carbs 1 and 2. So I did so and the bike started right up! however the thing was smoking quite a bit (not out of the exhaust but from the engine) and would not idle by itself. So at this point I'm wondering if I have a vacuum leak and if weather or not I installed the exhaust properly or if the engine has a bad gasket of some sort. The bike also runs like shit and like it might possibly be miss firing but I could be wrong seeing as the bike just runs like shit atm. Going to the check the spark on each plug when I have a chance but on the bright side she started.

 

[amc49]

Accelerator pump is actually on the fuel bowl of the #2 carb as you sit on bike................that valve is a 'vacuum operated fuel shutoff valve'.

If bike has been sitting awhile the carbs will be clogged, they pretty much all do it to run like crap. Early DOHCs are the worst.

 

[PRIMEWORKS]

Accelerator pump is actually on the fuel bowl of the #2 carb as you sit on bike................that valve is a 'vacuum operated fuel shutoff valve'.

If bike has been sitting awhile the carbs will be clogged, they pretty much all do it to run like crap. Early DOHCs are the worst.

Alright well then i removed the vacuum operated fuel shutoff valve lmao. Is it alright to run the fuel line from the gas tank directly to the carbs (specifically to the large fitting between carbs 1 and 2)?

 

[dirtdigger]

There is a valve on the fuel tank correct? If there is then just splice the line together in place of the vacuum valve and just remember to turn the valve off.

 

[PRIMEWORKS]

There is a valve on the fuel tank correct? If there is then just splice the line together in place of the vacuum valve and just remember to turn the valve off.

no the petcock is just a standard petcock no valve attached to it. just on, off, and res.

 

[dirtdigger]

Thats what I mean. The petcock is a valve, thats all you need just remember to turn it on and off. Just eliminate the vacuum valve.

 

[amc49]

Hey anything that turns off or on something is a valve...................LOL.

 

[PRIMEWORKS]

Alright so finally managed to find some time to work on the bike and try and get her running properly but I'm definitely going to need some help from some more experienced people!

The bike has been sitting for a couple of days now without being turned on. So today I hooked up the tank to ensure there was a constant amount of fuel going to the carbs (fuel is entering the carbs via the fuel line in this picture), engaged the choke, and started her. She started with no hiccups. Leaving the choke on the bikes RPM went up to around 3000ish (possibly just slightly above 3000) and sat there running for a bit. There seemed to be some white smoke coming from the front of the engine where the exhaust connected to the engine. I'm thinking I may have installed the exhaust improperly but I'll explain the reasoning for that in a second. After the bike continued to run for about a minute or so I decided to slowly release the choke, as I did so the RPM went up quite a bit to around 4-5000 (not sure if this is normal or not to be honest I don't see why that would happen though. If anything I would think the RPM would drop). So i reengaged the choke to get it running as it was before right around 3000 RPM. After another minute or so the bikes RPM dropped slowly (took less then 30 seconds) and the bike died on me. I then let the bike cool down and I decided to pull the plugs to see what they looked like.


CYLINDER 1


CYLINDER 2


CYLINDER 3


CYLINDER 4

I definitely feel like the plugs shouldn't be that black..

So to basically number the current concerns.

1 If the plugs are that black, what might that mean and does it possibly relate the the fact that the bike will run and then die on me?

2 Regarding the white smoke coming from where the exhaust connects to the engine. The current exhaust that is on the bike is a new MAC 4-1 exhaust. I installed new copper exhaust gaskets, but I think I installed the exhaust incorrectly and thats whats possibly causing the smoke/leak The exhaust came with 2 types of flanges that look like this . I read on a forum somewhere to only use the small flanges with the flat rimmed edge facing the exhaust port of the engine. But I'm thinking this is wrong. Should both exhaust flanges be used together some how or just one type of flange? If only one flange, should I use the smaller flange or the larger flange? I'm also thinking that the flat rimmed part of the flange should be flipped around but I'm just thinking that based off some exhaust diagrams I've seen. So to clarify the current way the exhaust is installed is: Header with finned clamp, then the SMALLER flange with the rimmed edge facing the exhaust port on the engine, then the copper exhaust gasket, then the engine itself.

3 Shouldn't the RPM drop when the choke is disengaged rather then rise?

NOTE: The airbox is not installed and the carbs were cleaned and rebuilt (new gaskets and and new jets (jets would be the original spec jets)).

Thanks for reading and hope to hear from someone more experienced then myself soon! (I should probably make a build thread with this amount of questions hahaha)

 

[Dave]

#1 Black plugs too much gas. To be expected if it ran with the choke on the whole time.

#2 I dunno, never took my pipe off. Didn't it come with instructions?

#3 Sounds like a major vacuum leak, did you plug off the carbs where lines #4 and #5 in that photo hooked up?

NOTE: If carbs were just rebuilt, they need to be resynced and adjusted, on the bike while the engine is running.

 

[PRIMEWORKS]

#1 Black plugs too much gas. To be expected if it ran with the choke on the whole time.

#2 I dunno, never took my pipe off. Didn't it come with instructions?

#3 Sounds like a major vacuum leak, did you plug off the carbs where lines #4 and #5 in that photo hooked up?

#1 Good to know, Lets see what happens when I get the choke thing figured out

#2 the instructions were pretty poor to be honest and included no pictures. It didn't mention anything about the flanges.

#3 No i didn't... going to try and do that now. Whats the best thing to use to plug them? or should i just cut the hose to be shorter then put a screw in the hose?

Just to clarify, are the floats in the DOHC carbs adjustable ?