This interesting article was written by Bruce Wilson who has graciously allowed me to place it here. This article has an interesting point of view regarding the value of disabling the air pumps on a later model VFR. It was originally two (2) posts to the VFR List. It was very interesting to me and I expect to others.
I have a '96 VFR and my wife has a '91 VFR that was significantly faster in roll-ons and that NECESSITATED that I had to do something!
I purchased a Dynojet kit and installed it as per the recommendations and it performed like crap! To make a long story short, I was on the phone several times with Dynojet and into the carbs more times than I care to think about. The performance would get different but not better than stock which as I mentioned was not acceptable to me. I finally remembered something a friend and exceptional Honda mechanic had mentioned about the air pump system on the later VFR's. If you put on a pipe or jet kit, disable the air pumps. Back into the carbs one more time, everything setup as per Dynojet's instructions and I disabled the pumps. Took a test drive after a warm up and what a difference!
I went for a dyno run yesterday after these modifications and the results were significant. How about 3.5hp up @ 5K, 5hp up @ 6K, 8hp up @ 8.8K and at least 5hp better from 9.2 to 10.5K! Same dyno, same operator, temperature within 1.5 degrees and bone stock except for the kit and dismantling the air pumps operation. And the results against Michele's '91 VFR? In 3rd and 4th gear roll-ons we are now dead even, whereas she used to pull me 10-12 bike lengths easily and tease the heck out of me about it (yes, to save all the responses this is a fragile ego thing!) Plus, the throttle response is better at all rpm's. How much of the improvement is due to the kit and how much to the air pumps? I have no idea and I am not going back to find out.
But once you realize how the pumps work, drawing air from the airbox and shooting cold air into the exhaust right next to a very hot exhaust valve, can't imagine it's doing either end much good. The air pumps are real easy to block off, if anyone wants to know how just let me know, I'll respond on what to disconnect, plug, etc.
And someone asked Bruce and so here is his response.
And that's it! I see no need to take the pumps off and then try to figure out how to plug the lines at the exhaust ports, etc. If you notice the improvement that I did you will be very happy, if not then maybe as someone suggested my air pump(s) were defective. But I think it is interesting that the pumps on Michele's '91 had been plugged by someone before we bought the bike. Let me(us) know of your particular results.
- Remove the gas tank and the left and right middle fairings. Remove the screws holding the top lid on the airbox (as if you were going to change the air filter).
- With the lid and snorkel removed, look in the space between the airbox and the steering head. You will see two (2) hoses about 1 inch in diameter attached to the air box that is still attached to the carb plate. This is where the air pumps draw air. Pull these two (2) hoses off and plug the connectors attached to the airbox. Rubber plugs, electrical tape, duct tape whatever.
- Once they are plugged off, slide the hoses back over the airbox connectors to keep dirt out of the air pumps, to help hold your plug in place and keep them out of the choke mechanism for the front carbs.
- Follow these 1 inch hoses down from the airbox, one goes to an air pump on each side of the bike. Once you've identified them, you will see a small vacuum line running out of the back of each air pump that goes to the front cylinders manifolds (cylinder head) right below where the carbs mount to the head. This is by the way where you hook up for carb synchronization. The back cylinders just have small rubber plugs in place of these vacuum lines. You can pull the lines and replace them with a rubber plug like the back cylinders or simply kink the hose back on itself and lock it in that position with a zip-tie.
Bruce in Tuscon
'96 VFR750F
