Revisiting those Vacuum hoses again

I'm tryin to track down a vacuum leak. My question - on the KZ750 -4 what is the purpose of the vacuum hoses on the far left and right right carbs? Can they be capped off??

The KZ750, like almost ALL bikes of this vintage use reed valves located in the valve cover to collect unburned crud and burn it by depositing it into the runner that has egress in the exhaust runner just outside the combustion chamber on the exhaust side. This area is HOT. Anyway, the reed valves are operated by a vacuum switch which connects via a big hose which tees into the ports above the reed valves. The switch itself is controlled by vacuum from those two fittings on the #1 and #4 carbs (carbs and cylinders numbered 1 to 4 from left to right as you sit on the bike). The EGR or ASS system (called either tends to make the bike backfire on decelleration and doesn't do much for performance one way or the other unless the system is compromised. There are many hoses and many chances for the system to leak air. Since raw air into the vacuum lines kind of ruins the function and causes the carbs to race (rev high), the system is best pitched. To get rid of it, take off the vacuum hoses to the vacuum switch from the #1 and #4 carbs and use vinyl vacuum caps; rubber deteriorates within weeks. Remove the switch and hoses to the valve cover and the hoses to the air box. You can plug the airbox or patch the hole as you like; it isn't critical. You MUST not let the holes over the reed valves stay open because if the bike backfires, you will suddenly have flames coming out these hole... under the tank isn't all that amusing. I use BLACK PERMATEX RTV to plug these holes. Just fill them up and let it dry. You could use JB Weld or run a hose between them (interconnect the two snouts). Removing this junk will clean up the area atop the engine and be less prone to vacuum leaks.

Great answer WG!

The air injection systems on these bikes uses pressure pulses in the exhaust system to pump unburned air into the exhaust port right behind the exhaust valve. The idea is that the presence of additional oxygen in the exhaust tract will help combust unburned fuel. The reed valves serve to block the positive exhaust pulses and open to let the fresh air in every time the pulses go negative.

Pretty Cool Right? Not quite, whenever the rider chops the throttle closed at speed the intake vacuum pressure goes high and with the air cut off, the mixture goes rich. If the air injection system continues to pump fresh air into the exhaust tract during times of high vacuum the result will be backfiring in the exhaust. To avoid this the air injection system monitors the intake vacuum and closes off the fresh air path till the intake vacuum level returns to normal.

To answer your question yes you can cap those hoses off if you also remove the air injection system.

KZCSI

Ok, cool.

If the opening on the exhaust cam cover are left open (or vented another way), will that affect performance? (as opposed to using RTV).


Also, can you use plate covers like on the european bikes to cover the area?


Thanks for the wisdom!

I plugged those two carb ports and the bike runs alot better. Not great, though.

I have breathers on the exhaust valve/cam covers. Could this be affecting me adversely?

Hi BS,

If you've fitted breathers to the exhaust cam reed valve covers your air injection system will still function except that you will have lost the ability to shut down that system during times of high vacuum. If it's a problem you would experience backfiring in the exhaust on deceleration.

By breathers do you mean air filters? Not sure it's really that important but the air injection system gets only filtered air from the airbox.

KZCSI

Well there really isn't anything left of that system on my bike. It's actually sitting underneath a broken Honda on the floor of my garage.

I have small breathers in place where the hoses connected to the valve cover. I think I'm going to get (or make) solid plates to go in their place.

In the meantime RTV silicone is an instant solution.

KZCSI