Installing DynoJet Jet Kit in the Honda 750 Nighthawk

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.

(You can click on the Thumbnail photo to see a larger version)
jetchange-037.jpg 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.
jetchange-001.jpg 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.
jetchange-002.jpg 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.
jetchange-003.jpg 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.
jetchange-004.jpg 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.
jetchange-006.jpg 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.
jetchange-008.jpg 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 nd the choke cables. An extra hand really comes in handy about now! Go slow, it is a tight fit.
jetchange-009.jpg Whew! The carburetor assembly removed!
jetchange-013.jpg 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.
jetchange-014.jpg This is the main jet.
jetchange-018.jpg 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.
jetchange-017.jpg 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.
jetchange-019.jpg This is the main well tube, it is nice and clean, no worries!
jetchange-020.jpg Now, with all four jets changed, put the bowls back on the assembly.
jetchange-021.jpg 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.
jetchange-034.jpg 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.
jetchange-032.jpg Slide assembly removed!
jetchange-026.jpg 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.
jetchange-030.jpg 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.
jetchange-027.jpg 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.
jetchange-029.jpg 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.
jetchange-026.jpg 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.
jetchange-033.jpg 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.
jetchange-035.jpg 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.
jetchange-036.jpg 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.
jetchange-011.jpg 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

14 Comments so far

  1. Byron Spreng on April 1st, 2009

    Hey Bob! I’m trying to install a Dynojet kit on my Honda 650 Nighthawk and I’m running into some problems. Do you think I could give you a call and ask you a couple of quick questions? Thanks. I’d really appreciate it.

  2. myles on June 8th, 2010

    i took apart my carb to clean, i dont know what my jets where set at or if my float level is right, i have a 91 nighthawk 750, how do i set the jets and the floats, please help me, i just got the bike and i cant ride it, thans

  3. admin on June 8th, 2010

    Not sure I understand the question about “setting the jets.” There is nothing to set on the jets. Unless you are changing them, just put them back in the carb. Now, if you are talking about setting the idle mixture screws, turn them in until they seat, but don’t tighten them down, you will damage them. Then back them out about 1 and 1/2 to 2 turns. That will give you a good starting point to start adjusting them once you have it back running.

    On the float levels, with the floats installed, turn the carb upside down and hold the base (where the bowl attaches) level. The adjust the tang on the float so that the float is level, parallel with the base of the carb. That will get you really close.

  4. wondering on December 5th, 2012

    Hi guy I had this guy out a dyno number 1 kit in my bike
    but it still sputters at med rpm’s,
    and has flat spot on takeoff,
    I dont think he balanced the carbs or cleaned them,
    sputters upto 4 grand then all of a sudden clear’s out.

  5. admin on December 6th, 2012

    Sounds like he didn’t do the job correctly or completely. Something is wrong for sure. Troubleshooting these problems can be broken down into three areas; Fire (Electrical/ignition), Fuel (carbs, jetting, fuel flow), and mechanical issues inside the motor. In my experience it is most often ignition, second is fuel, third is mechanical issues. You should investigate them all, staring with the ignition. After you have eliminated that as a cause, then move to the carbs. First thing is to determine if the sputtering is because the jetting is too lean or too rich. Have someone observe the exhaust from behind you when it is sputtering. If you are getting black smoke it is too rich, no smoke it is too lean. From there you can adjust as necessary. The idle screws will set the mixture at idle to very low RPM. The jet size will set he full throttle high RPM mixture. The metering needles will control the mid range RPMs. The metering needles are adjustable and they will greatly affect the mid range performance. Of course, you need to make sure the carbs are clean and in good repair first. Scan the web, there are a couple of articles on cleaning motorcycle carbs on Honda bikes.

    I have never had a problem with the “out of the box” settings on a DynoJet kit and I have jetted many bikes. There is probably something else wrong or the kit wasn’t installed correctly. Even if the guy had a dyno, something is wrong and if he ran it on the dyno he would have known it wasn’t right.

  6. craig on April 20th, 2013

    thanks for the great write up . i am going to try to perform this myself this weekend . i have the same bike . i do not know much about carbs but have done quite a bit of other types of repair work .
    i am a little nervous about the idle screw process . could you possibly send me a picture of where it is at ?
    it is very nice that guys like you help out people who are trying to learn this for themselves .

    thanks
    Craig

  7. admin on April 21st, 2013

    Craig,

    Thanks for the comments. Unfortunately I don’t have photographs of removing the mixture adjustment plugs and I don’t have that bike anymore either. I am sure that you can find it on the web somewhere. One thing that I will tell you, if it is plugged, don’t try to pry the plug out. I tried that one time and wound up breaking the carb’s casting. The way that works the best is to drill a small hole in the cap and put a small sheet metal screw in the cap. Then pull on the screw with a pair of pliers or channel lock.

    Ride Safe!

  8. Fadi on June 29th, 2013

    Hi Bob,
    I have a 91 Nighthawk with Dynojet installed, and 41T rear sprocket.
    Has 96,000 miles on it and still is quite a tool for mountain racing, just last Sunday it put to shame a bunch of European & Japanese crotch rockets:
    http://www.youtube.com/watch?v=2n2kvMxdeP0

    Do you have any idea what power is with dynojet is?
    Thanks
    Fadi

  9. admin on July 5th, 2013

    I never dyno tested the bike, but I would think that the horsepower was up about 5% over stock. That would be at the higher RPM ranges. Simply re-jetting the carbs is not going to give you a drastic horsepower increase. It may feel like a lot of increase as the bikes are very lean from the factory.

    I particularly liked that the bike would accelerate to the rev limiter in fifth gear, something it would not do before the jet change.

    Great video put together there!

    Ride Safe

  10. Mark on October 1st, 2013

    Hey man, nice presentation. Fast question, I’ve just purchased a V & H 4 to 1 set of pipes for my 98 NH 750. I’m assuming there is no stage 2 kit by Dynojet, all I’ve seen is Stage 1. Are you familiar? I intend to do some airbox changes as well. Thanks for your time and for putting this together – got a lot of knowledge from reading it.

  11. Anonymous on October 4th, 2013

    *I was very pleased to find this web-site.

  12. admin on October 5th, 2013

    Mark,
    I would think that if you modify the air box the stage one kit may be a bit lean, particularly on the top end. You can buy individual jets and metering needles for the carbs on line. Depending on how much you open up the air box a, one to three steps up on the main jet would be appropriate. This is where you get into “tuning” the fuel air ratio to get the best performance. You just want to be careful not to get the motor too lean and that can cause damage. You might change the exhaust, put the DynoJet kit in, and then try small changes to the air box and larger main jets to look for improvement in top end power.

    Ride Safe!

  13. Trevor Bredereck on September 26th, 2015

    Hi i have Cbr 900rr 97. I had the dynojet installed before i had shop supposedly REBUILT it i sence haven’t been back. The told me my bikes carbs were flooding out because of the dyno jet needels on one of the carbs. They asked if i still had the stock needles .they installed them put in 20# mains in middle two carbs and #18 in outer two. The bike ran good for about TEN days. And started stumbling on wot
    If i feather it it runs ok but hit the gas hard falls hard wants to go then stumbles. TANK clean. New filter petcock works properly air box clean plugs new
    When i installed the dyno needles two years ago we drilled out the vacuum slid as dynojet request. Now re installng Oem needle would that cause a problem? ? Since it was drilled for dynojets? I didtake carbsapart found a bad float needle. And float. Its spotless i did notice three of the boots to seal carbs to head were hard as rocks and cracked all the way through and more then one crack. Bad very bad.
    Is help???

  14. admin on September 26th, 2015

    I am calling a B-S on the folks that told you the problem was the needles from the DynoJet Kit. From your description of the condition of the carbs they didn’t know what they were talking about. Most people don’t understand how these Constant Velocity Cars work. Also, it sounds like the carbs have several problems that are not related to the DynoJet kit installation.

    Drilling out the hole in the bottom of the slide makes the slide respond faster to changes in vacuum, which will cause the mixture to be a bit too lean with the stock needles in the slide, especially when you slam the throttle open rather than rolling it on slowly. The DynoJet needles and jets need to go back into the carbs.

    Make sure the diaphragms on the slides are in good shape, no cracks or small holes, and they are still soft and flexible. Check each carb’s float settings, and make sure the needles and seats for the fuel shut off valves are not leaking. You can find the float adjustment procedure and dimensions in the shop manual for the bike or probably on the internet. I think I would just completely rebuild the carbs, with the DynoJet parts installed. Some of the DJ kits have different springs for the slides too, make sure those are correct. (My ZRX1200R has new springs)

    If the boots that attach the carbs to the head are cracked they should be replaced too, that would create a vacuum leak that will cause the motor to go lean.

    Let me know how you make out…..
    Bob

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