Tons of ?s about my 1980 750-four

My first post off the "newbie board" so bear with me.

I now have a mild understanding of cleaning and adjusting carbs, but the vacuum system is throwing me off.
Everyone is talking about pods or other mods for air intake. Although the performance upgrade would be nice, I just want it easier to take those carbs off when necessary.
Can someone explain to me in simple terms how the vacuum system works? I have the service manual (awesome book) but it doesn't get into it other than show where the vacuum switch is. (but not it's purpose.)
Another example; if getting rid of the old air intake box, I have no idea what everyone is doing with that large (3/4"?) vacuum hose that is attached between the #2 and #3 boot holes. OR what to do with the gas overflow tubes.
What the heck are the #1 and #4 vacuum lines off the carbs doing anyway? I know #2 is needed for petcock flow. And I read the third MUST be capped. But after that I'm lost.
HELP!

I'll take a shot at explaining but hope I dont confuse things in the process.
First a word of warning. If you are in the US and have US spec model you may have whats called an emmissions system with tubes coming from the head and tubes going to the airbox. Otherwise here goes.

During the intake stroke piston goes down creating a vacuum and drawing in the gas/air mix from the carbs. Each carb has a vacuum nipple. The main function of these is that you can hook up a manometer to each one when synchronising the carbs.
Under normal running the vacuum nipples on #1, #3 and #4 will be capped. On #2 it is common to run a line from the nipple to the petcock - assuming it's a vacuum activated petcock. With this type of setup the slight vacuum in the line sucks a diaphragm in the petcock opening a channel and allowing the gas to flow to the carbs.
Some people use a straightforward on-off petcock like a Pingle which does not require a vacuum and so will just cap the nipple on #3. In other word they will have all 4 carb vacuum nipples capped.

You should have overflow lines runnning from the nipple on the bottom of each carb and you should route these down between the swingarm and the engine. Idea is to prevent gas overflowing on to a hot engine.
I cant picture a 3/4" line between the 2 & 3 boot holes. Maybe its part of an emissions system. Is this coming up from the crankcase ? Regarding emissions system we never had that up here but I have heard people say take it off. Hopefull others can chime in here.
Good luck and welcome to the forum.


Post edited by: JR, at: 2007/10/02 20:25

del

No doubt about it, kraygh. If you have a US spec KZ750E1
then you DO have the EPA-mandated "air switch" valve and associated hoses for emissions reduction.
If you look at your valve cover on the front L/H side just above the exhaust headers for cylinders 1 & 2 you should/should see a spigot/hose barb protruding from a little "sub-cover" bolted to the top of the valve cover itself. The large hose you mention should push onto this spigot and then run over to a round black canister-type valve bolted up to the frame backbone. Then another large hose runs from the back of this valve to a spigot on the air box. There is an additional smaller hose that runs from the air valve body to a tee fitting in the petcock vacuum line. This smaller line is the air switch valve actuation line.
I won't bore you with how it actually works but basically it functions as an A.I.R. (Air Injection Reaction) valve, taking fresh air from the air box and injecting the air into the exhaust just downstream of the exhaust valve. This fresh air mixes with the exhaust gases and helps to complete the combustion of any unburned fuel in the exhaust stream.
You can safely disconnect the whole system if you so desire but you MUST block off the spigot/hose barb on top of the valve cover so that dirt/water can't get into your exhaust headers.
If you want to leave the system hooked up but also want to run pod filters then you'll have to install a small crankcase breather-type filter on the end of the large hose that was originally connected to the air box spigot. UNI makes a line of small cleanable filters for just this kind of job. They come in 4 different sizes of spigots to fit most sizes of rubber lines.
I hope this clears up some of the mysteries that are baffling you.

Post edited by: H2RICK, at: 2007/10/02 21:52

Double post....sorry....:S :whistle:

Post edited by: H2RICK, at: 2007/10/02 21:50

Terrific info. thanks.
I thought I might just remove the system entirely, but when I tried idling with the large vacuum intake hose plugged, I was getting weird "popping" out of the number 2 carb (intake boot side). It was like a back-fire or something with a puff of white smoke and gas spray. very weird. Could be unrelated and sign of something else wrong, but since it doesn't happen with the vacuum intake unplugged, I guess I'll just find a breather for it.
It's a shame though, because the engine is so much quieter with the vacuum hose plugged.