Over the previous several weeks, I removed the old carb rack from my cb750 (not an easy task) and also removed the airbox which I discovered to be a fascinating piece of engineering. 2 crankcase breather hoses attach to the airbox and shoot any solid particles (water/oil) to the back of the airbox where any condensate drains down to the catch tank. Remaining oily air/fuel mixture is then sucked back into the engine for burning. (I wonder how this recycled mixture affects octane rating or if the claims that this mixtures adds to carbon build up in the intake tract has any merit).
Picked up a heater hose from the local auto store and cut it into 2.5" straight sections to connect the new carbs to the engine (3" would be better to allow space for cuts). Since the carbs do not line up perfectly with the engine you must make some angled cuts on the connectors to get them to mate up with the carbs. My #2 connector was a little short and I will need to go back and make another one for it someday. I don't think it will leak, I just don't like how the connector is not fully seated on the carb spigot and engine.
Hooked up the throttle push cable (pull cable is not used) and finding that I've got too much slack to do WOT (wide open throttle). So I need to think about the throttle setup some more. I have a right hand euro switch coming that will have top exit cables and the lights off switch in case I have another charging issue in the future so I will worry about it later.
I'll need to make sure the throttle moves smoothly and doesn't hang up when I let go of the throttle. It should snap closed when I let go.
I managed to fire it up for the first time before finalizing things. The induction noise without airfilters are amazingly loud when the revs get above 3k rpm. The engine sounds different with these carbs. Sportier, faster reving and meaner.
I picked up some flexible clear yellow fuel line from mcmaster.com, it was a bummer that I had to buy the tubing I wanted in large quantity and worse that I needed 2 sizes. In the end, I used way more tube than I thought I would need after re-doing the tubing a few times.
The Mikuni carbs use a metric fuel line close to 5/16" but the cb750 uses 1/4" so I needed to find a reducing tee. Mcmaster had the 3/8 X 1/4 X 3/8 tee that was close enough to do the trick, so I bought 3, 1 for the fuel, 1 for the vent, and 1 for the breather system.
Fuel and vent lines need to be run behind the rail because there is no way to mount the air filters if the fuel lines are not run this way. Vent lines need to connect from below and the fuel lines from above. This is to prevent crowding the fuel line. In the picture, the reduced vent line from the tee is clear and runs about 1 feet towards the battery area to help dampen the slide flutter. I *think* I was able to confirm that it helps reduce slide flutter during testing with and without vent lines attached.
The old carbs required pumping the throttle about 5 times to prime the carbs with fuel before attempting to start it. On these carbs, you pull the choke on the carb rack and push the start button. I'm typically seeing ignition within the first 2 tries.
I had the carbs set initially to 2 turns on the mixture screws but found that I was getting a hanging idle (blip the throttle and the rpms don't come down as expected) because it was too lean. Bumped the mixture screw to 2.5 turns out (turn it down until it seats and unscrew)and the hanging idle is now just a slow drop after a throttle blip. 2.75-3 turns was still too lean so 3.5 turns should be perfect with the air filters attached and this was confirmed by another carb swapper.
Carbs are sourced from a California dismantler so I *think* my rack of carbs have specs for CA carbs. Main jet size: 112.5
ORing Shopping list (http://www.oringsusa.com/):
The K&N Pod filters have round mounting holes and the carbs have oval throats. Good thing the K&N filters are rubber and can be coaxed around the oval shape. Just be careful not to distort the air filters as you push them on. I found it helpful to remove the clamps and mount the filters by starting at the round part and pulling the air filter rubber over the oval part on top, then thread the clamps on afterward. Clearance is very tight so patience is required.
Picked up a heater hose from the local auto store and cut it into 2.5" straight sections to connect the new carbs to the engine (3" would be better to allow space for cuts). Since the carbs do not line up perfectly with the engine you must make some angled cuts on the connectors to get them to mate up with the carbs. My #2 connector was a little short and I will need to go back and make another one for it someday. I don't think it will leak, I just don't like how the connector is not fully seated on the carb spigot and engine.
Hooked up the throttle push cable (pull cable is not used) and finding that I've got too much slack to do WOT (wide open throttle). So I need to think about the throttle setup some more. I have a right hand euro switch coming that will have top exit cables and the lights off switch in case I have another charging issue in the future so I will worry about it later.
I'll need to make sure the throttle moves smoothly and doesn't hang up when I let go of the throttle. It should snap closed when I let go.
I managed to fire it up for the first time before finalizing things. The induction noise without airfilters are amazingly loud when the revs get above 3k rpm. The engine sounds different with these carbs. Sportier, faster reving and meaner.
I picked up some flexible clear yellow fuel line from mcmaster.com, it was a bummer that I had to buy the tubing I wanted in large quantity and worse that I needed 2 sizes. In the end, I used way more tube than I thought I would need after re-doing the tubing a few times.
The Mikuni carbs use a metric fuel line close to 5/16" but the cb750 uses 1/4" so I needed to find a reducing tee. Mcmaster had the 3/8 X 1/4 X 3/8 tee that was close enough to do the trick, so I bought 3, 1 for the fuel, 1 for the vent, and 1 for the breather system.
Fuel and vent lines need to be run behind the rail because there is no way to mount the air filters if the fuel lines are not run this way. Vent lines need to connect from below and the fuel lines from above. This is to prevent crowding the fuel line. In the picture, the reduced vent line from the tee is clear and runs about 1 feet towards the battery area to help dampen the slide flutter. I *think* I was able to confirm that it helps reduce slide flutter during testing with and without vent lines attached.The old carbs required pumping the throttle about 5 times to prime the carbs with fuel before attempting to start it. On these carbs, you pull the choke on the carb rack and push the start button. I'm typically seeing ignition within the first 2 tries.
I had the carbs set initially to 2 turns on the mixture screws but found that I was getting a hanging idle (blip the throttle and the rpms don't come down as expected) because it was too lean. Bumped the mixture screw to 2.5 turns out (turn it down until it seats and unscrew)and the hanging idle is now just a slow drop after a throttle blip. 2.75-3 turns was still too lean so 3.5 turns should be perfect with the air filters attached and this was confirmed by another carb swapper.
Carbs are sourced from a California dismantler so I *think* my rack of carbs have specs for CA carbs. Main jet size: 112.5
ORing Shopping list (http://www.oringsusa.com/):
The K&N Pod filters have round mounting holes and the carbs have oval throats. Good thing the K&N filters are rubber and can be coaxed around the oval shape. Just be careful not to distort the air filters as you push them on. I found it helpful to remove the clamps and mount the filters by starting at the round part and pulling the air filter rubber over the oval part on top, then thread the clamps on afterward. Clearance is very tight so patience is required.
That is really cool. Do you know what year bike these carbs came off of? Excellent write up.
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