Installing the Dynojet Kit on '92-'03 Honda 750 Nighthawk
Submitted by Bob Cheek (April 2007)

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I decided some months ago to change the jetting on my '93 750 Nighthawk. From the factory the bike is very lean for emissions, and I felt that I could gain some improvement in the smoothness and performance by rejetting the carburetors. I looked at several of the modifications to the carbs such as putting washers under the metering needles, and of course just changing the main jets, but based on past experience with the Dynojet Kits for other bikes, I decided to go with the Dynojet Kit for the Nighthawk.

The Dynojet Kit accomplishes three things. It changes the main metering jet, it changes the metering needle, and third it changes the vacuum source for the slide in the CV Carburetor. The vacuum holes in the slide are enlarged to give the slide more vacuum signal. This works in concert with the kit's jets and metering needle to provide better throttle response and horsepower. My expectations for increases in horsepower were not great, because, looking at the specifications on the cam timing on this engine I was not expecting a great increase in performance. It just isn't there for a engine with no overlap on the cam timing! But, anyway, here we go

Step one, at least for me, clean up the workbench! You will need a clean space to put the carburetor assembly on to install the kit. For me, a new project always involves cleaning up from the last project!

This is what we're going to do: remove the carburetor assembly, install the Dynojet Kit, and reinstall the carburetor assembly, and then do the fun part, road test the bike! So let's get started with the removal.

Remove the seat, side covers, and the fuel tank. To remove the fuel tank, remove the bolt from the rear of the tank and stick a block of wood under it to hold the tank up while you remove the fuel and vacuum lines from the petcock assembly.
Remove the fuel line and the vacuum line. Use a pair of needle nose pliers to open the clips and slip them back off the tubing. I replace these clamps with screw type clamps when I re-assemble the bike.

Remove the fuse holder by snapping it loose from the rear fender. Insert a small flat blade screwdriver under the fuse holder where it attaches to the fender and release the latch.

Remove the battery and the bolts securing the air box. The air box is secured with two 10 mm bolts on the lower right and left and one on the top back under the seat area. Snap the edges of the the of the fender off the frame rails so the air box can be moved backwards.

Loosen (but do not remove) the 4 screws on the clamps that hold the ducts from the air box to the carburetor intakes. Get them loose enough that the ducts can be removed. In the same fashion loosen the screws holding the carburetors to the engine.

 

Use a 10mm open end wrench to loosen the two throttle cables. By turning the top several turns you can slide the cable ends out of the bracket without completely removing the nuts. Use a phillips screw driver to remove the choke cable clamp. Likewise, just loosen the cable clamp enough to get the cable out then put the screw back in. Don't worry about detaching the ends of the cables, we'll get to that in a minute.

Now, carefully slide the ducts off the carbs, and move the air box back as far as you can to give you clearance to get the carbs out. The air box won't move back very far, but you can get it back far enough to get the carburetor assembly loose.

Grab the carburetor assembly from both ends and rock it up and down and pull it back away from the engine until it comes loose.
Gently work the carb assembly out of the right side of the bike. You may find removing the large vent line from the camshaft cover will help this process. Work the assembly about half way out so that you can tilt it up and remove the ends of the throttle cables and the choke cables. An extra hand really comes in handy about now! Go slow, it is a tight fit.
Whew! The carburetor assembly removed!

First, lets do the main jets.

Place your assembly on the work bench an start by removing the fuel bowls one by one. There are four screws on each. Remove the screws, then hold the assembly over a container to catch any fuel left in the bowls when you crack the bowl off the carb. Place the bowls to the side, but make sure you put them back on the same carburetor. Three of the bowls are the same, but one has the drain screw on the opposite side.

This is the main jet.
Find a screwdriver that fits tight in the slot and remove the main jet. They can get pretty tight, but go easy, as these are brass parts and are very soft. There are two main jets supplied with the Dynojet Kit, one for engines with modified/performance exhaust and air intakes, and one for stock. Consult the instructions that came with the kit and select the proper jet.
If the whole main well tube comes loose, you may have to hold the well tube with a wrench and take the jet out. I took mine out anyway, this was a 13 year old bike, everything looked pretty clean in the fuel bowl, but I wanted to see if there was any dirt/gunk in the main well.
This is the main well tube, it is nice and clean, no worries!

Now, with all four jets changed, put the bowls back on the assembly.

Now to the metering needles and slide modifications. Flip the assembly over and start by removing the covers. As you take the last screw out, be mindful there is a spring under there. Carefully remove the cover and spring and place them to the side.
The black part here is the diaphragm for the CV slide. This is very thin so be very careful removing it. Note the small "loop" on upper left hand side. Starting there, gently unseat the diaphragm from the carburetor body and remove the slide assembly.

Slide assembly removed!

Remove the needle. Use a 8mm or 5/16 nut driver to reach in and depress and turn the holder. There is a small spring under the holder that holds the needle down, press down and turn the holder to release it from the tabs on the side of the slide. it will spring up and can be removed.

Here's a tip. Put a small piece of tape inside the nut driver and wrap it around the outside and stick it to the tool. This will make it fit tighter over the holder so you can remove and install it without it dropping down inside the slide.

Remove the holder and the metering needle.

Drill the vacuum holes in the slide. Flip the diaphragm up (or down depending how you are looking at the slide) so that you can set the slide on the table.

There are TWO drill bits supplied with the Dynojet Kit. Consult the directions. The slide is drilled with the 7/64" bit! MAKE SURE YOU GET THE CORRECT DRILL BIT. In my case, there were two holes in the bottom of the slide, one was already 7/64, so I drilled out the smaller one to 7/64. Carefully remove the metal bits. Stick your finger down inside the slide and make sure there are no burrs or loose metal cuttings inside the slide. If there is a burr on the inside where the hole was drilled, remove it with a small flat blade screwdriver. Clean everything up after drilling.

Assemble the E clip and washer to the end of the new Dynojet metering needle. Follow the directions in your kit for the correct position of the clip and washer. In my case, it was to be put in the second groove from the top. If there were any other spacers in the original assembly, reuse them in the same position.

Insert the needle into the slide and install the holder. This is where the bit of tape on the nut driver comes in handy. Keep the slide as close to vertical as possible to keep the free floating washer in place until you can install the holder. Keep the holder as square as possible when assembling it so that the spring on the end will fit over the washer on the metering needle and not knock it off the needle. Press the holder in and twist it until it locks in place.

 

Re-install the slide assembly, making sure the needle goes down into the main well. You may have to use your finger to align the needle with the main well, in this photograph the needle is not aligned and the slide will not seat all the way down.

 

Before you seat the slide, rotate the tab on the diaphragm to line up with the small slot in the top of the carb body. (Circle in photograph) Insert the spring and install the cover. There is a small notch in the cover that should align with the tab on the diaphragm. (Square in photograph)

Do all four carbs in this same way.

Some Dynojet kits have replacement springs also, if yours comes with springs use them here.

Ok, you're almost there. Reverse the process. Install the carb assembly back in the bike. Put it halfway back in and attach the ends of the two throttle cables and the choke cable. It is not very easy to get the cables back in the grooves, but take your time and it can be done. Again, a extra set of hands really helps here. Make sure you have the correct cables, the pull cable on the throttle is slightly longer that the push cable so it makes it difficult to switch them, but probably not impossible! Check this carefully. I always like to to have a little extra filtering of the fuel, so I spliced in an extra fuel filter in the supply line from the tank.
I noticed that I had the cracking sealer between the ducts and the air box that I have seen on the Nighthawk forum. I'll reseal this after I have everything back together.
   
   

There is one thing that I couldn't cover here, that is the idle air adjustments. These carbs come from the factory with the idle air adjustments sealed. There are two methods used. A limiter cap on the adjustment, or the adjustment is recessed into the body of the carb and a plug is pressed in to cover the adjustment. My bike had the limiter caps, however they had already been removed. The Dynojet kit comes with a drill bit to remove the plugs if your carb is so equipped. If it has limiter caps you can break the stops off the caps or remove the caps, which ever is easier.

In either case, set the screws back to their original position when you re-assemble the carb assembly. I screwed the idle air screws in all the way and counted the turns. Then before I put the assembly back on the bike, I set them back to that original position, which in my case was 2 turns. That would be a good starting point.

How did it perform?

It was in the 40's the day I did this project. Heck, it was April in the Atlanta area and it was supposed to be in the 70's, but we got a late blast of winter and here I was in the garage changing jets instead of riding! So, I didn't get a good long ride to check it out and it was a bit cool to get the motor fully warmed up. But, I did get in a few miles and I was pleased with the change. The higher RPM power was up and throttle response was better, particularly above 5000 RPM. I got on it hard off the line and was surprised to hit the rev limiter very quickly. When we get some better weather I'll try it out some more and see what it looks like!

Questions? Email me at wb4eje@yahoo.com

Bob Cheek

 

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