California Emission Late Model 750 91-2003

Started by Bob H, Jun 24, 2023, 11:30 AM

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Bob H

We shall begin with the disclaimer that this is for off road use.
It is odd that for over 10 years Honda made a 750 with a ton of emission garbage only in California, yet the same 91-2003 Nighthawk 750 sold in other states was the same bike without it. So we have a direct comparison "on" and "gone".

When I re-jetted my carbs and cleaned them again, I decided to dig into the emission stuff.

When vehicle makers started to attempt compliance with emission rules, the carbureted vehicles in use at the time became very complicated.

At first I tried to identify the various emission components, and what they did. It took hours, but I got through the other side of the complexity and achieved clarity. Things started to become simple. As Albert Einstein says:

"Simple can be harder than complex: You have to work hard to get your thinking clean to make it simple. But it's worth it in the end because once you get there, you can move mountains."

― Steve Jobs
"For the simplicity on this side of complexity, I wouldn't give you a fig. But for the simplicity on the other side of complexity, for that I would give you anything I have."

― Oliver Wendell Holmes Sr.

I hit the wall of complexity, learned what the various parts did.
On the other side of that complexity, it finally became simple.

1993 Nighthawk 750

Bob H

The California NH 750 has the same carbs as the other 49 states (and same as the 84-86 NH 700S) with the exception of slightly smaller main jets and these vacuum hose connections that goes to a rat-nest of tubes that are visible in the previous photo, far left.

I pulled the hoses and placed 1/4 inch rubber caps

Those tubes connected to all this garbage. The only thing you need to keep is the vent to gas tank, cut the hose and let it hang down a bit so air can vent into the tank as gas is consumed.

1993 Nighthawk 750

Bob H

Even modern fuel injected motorcycles often have a "Pair" valve, which is a Pulsed Air Intake system.
This puts air into the departing exhaust gas and has zero effect on how the bike runs.

To keep it simple, you can just cap the "air coming in" to the exhaust.

Many engines have aftermarket "block off plates" available that serve this function, but not for vintage bikes like mine.

Initially I was concerned that I needed something stronger than rubber, but found that this is a common solution for various other car and motorcycle engines as alternative to block off plates.

Honestly I could have left this "PAIR" system in place, it doesn't hurt anything.
But this rat nest is so interconnected - the PAIR system pulls air from the pile of canisters and vacuum lines with various control valves.

Once I figured out what was simply needed to make the engine run (49 state) I decided to get rid of all of it.

PAIR = Pulsed Secondary Air Injection. I believe the "Pulse" refers to exhaust stroke on the engine. The rhythmic pulsing causes a reed valve to move, and it draws outside air in to the exhaust for more complete burning of exhaust gas leaving the engine.
I believe "secondary" refers to a source of air in addition to the burnt exhaust gas.
1993 Nighthawk 750

Bob H

All combustion engines have a breather, and this is the 49 state version

Top view of 49 state version. Breather on right, fuel line from gas tank petcock on left.

Basically you can achieve that 49 state version by retaining the tubes that blow crankcase pressure to the air intake, nothing wrong with that.

This other stuff can be removed (California diagram below)

California diagram below. I pulled all of these items.

1993 Nighthawk 750

Bob H

I did a LOT of research, and the following video was the best reference that I found to explain this topic

This guy specializes in Honda Shadow, but it was the closest description to similar late-model California Nighthawk 750 emissions that I could find.

I don't agree with tossing the factory air filter, they are going for a certain appearance.

3 minutes in he makes a great comment about the need to VENT the carburetor float bowls.

The 49 state NH 750 achieves this by the far left and right tubes in this photo (arrows):

The fuel line is the one in the center.

Unlike the 49 state model, the California carbs have longer vent hoses that have clamps at the ends because the hoses connect to the rat nest of emission components. I cut mine to appear as close as possible to the 49 state vent tubes, and everything has been working fine (3 months so far)

In the video above, his carb vent hoses (3 minutes in) also have clamps at the end of them, because on the Honda Shadow they also connect to various emission components. He says to cut the tubes, but you have to have some venting (you can't cap them off - or the float level of gas in the carb will be affected).
1993 Nighthawk 750

Bob H

Gaining access to the carbs is difficult. I have struggled with the process a few times.
On this journey, I decided to loosen the rear inner fender (plastic) so I could move the battery box / Air box further back. You remove these 2 bolts that go through the plastic inner fender.

The forward part of that inner fender "snaps" around the frame. You can push it down about an inch, which frees it up to move back some. This made a lot of difference, as well as lube the inner carb boots (I used tire lube paste, you can use a silicone based product, I could have used the "syl-glyde" that is approved for rubber brake caliper seals)

1993 Nighthawk 750


Dropping the rear fender onto the tyre is the only way that I have found to get enough room to push the airbox all the way back.  With that done, pulling or installing the carbs isn't really a difficult job at all.
1984 Honda Nighthawk 700S
2012 Honda NC700X
2005 Vespa GT200
1982 Yamaha Maxim 550 (sold)
2006 BMW R850R (sold)
1981 Honda CX500B (sold)