I haven't posted much about my bike - today I did a baseline dyno run which is posted in the members garage. I was pleased to see that the bike peaked at 59 HP which is pretty much what we would expect with the few mods that I've done. My goal with this bike has always been to keep it as stock looking as possible - eventually it will get parked in the living room. When I first purchased it my main concern was to make sure the engine seals were ok - everything else appeared very clean. After extensive leak testing I determined everything was fine. I put on the Tyga pipes, reeds, DynoTek coils and HRC PGM. It's always run well, but has flat spot at about about 1/3 throttle (very rough approx, but it's clearly when on the needle). When I initially jetted I had added a 0.5mm shim under the needle. A week ago I removed the shims and to no surprise it just changed where the flat spot occurred, now it's at a little more than 1/3 throttle. Initially it was happening around 6k rpm, now its a little higher up closer to 8k rpm. The plot follows a similar curve as most of what I see posted, but obviously the bike isn't as modified as some of the other members.

The shop placed the air/fuel sensor in the right exhaust which is included in the posted plot (members garage). I feel good I've got the main jet right - could even drop it down a size. But mid range is just too lean - explains the flat spot. Since I don't have a box load of needles with different profiles.....I think I'm gonna just learn to live with as I'm not going racing, just maybe a track day here or there. However if I were to try to change it...I think I'll need to source some stock needles and mill flat spots (old school jetting). I soldered cables onto the mixture screws so I can adjust them on the fly. But since the flat spot is when I get on the needle, I haven't been able to work it out with the mixture screw.

I suspect more than one of you guys have been down this path have some ideas on how to work out the flat spot. But please don't tell me to go get an MC21 I like the MC28 look and don't need that extra 5 HP.
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MC28 SP

Sounds good, and good to hear you're both pleased with it, and realistic with your aims!

The simple answer to your flat spot, is an HRC jet kit! With those mods, there's no reason the motor won't take full throttle from 3000rpm to 12000rpm without a single hiccup!

It sounds like you've done everything else nicely, so you should finish it off properly. With the correct needles and air correction, you'll likely even pick up a horsepower or two too!
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Andy.
NSR-WORLD.COM

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Andy, you suggest the hrc jet kit but I've never seen one for the mc28 carbs. And from what I've read the mc21 hrc needles are longer. Well, if i can source one I'll do it.

Also, I'm running the oil injector. If hrc kit requires premix, then I'm gonna work with what I have. Although there should be no reason the hrc needles wont work with injector.

HRC made a jet kit for the '28 as well.

It works very well on a stock (none pre mixed) engine just as well as on a full race motor. Most of the '28's we de-restrict run a HRC jet kit.
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If I have to take the carbs off once more...

Sounds great.....now for the fun part.....trying to find one. If anyone has a kit they are willing to part with, please let me know.

You'll most likely need to scour Yahoo! Auctions in Japan, but be prepared to pay a handsome reward for one.

They do sometimes come up on the forum though, but very rarely.
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Andy.
NSR-WORLD.COM

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Well I've managed to an HRC MC28 hrc jet kit via various auctions. I picked up some jha kit AS25A needles too.

Before setting out to do the install, i have a few questions, I've searched the forum and haven't found answers to these.

Q1
I've noticed the jetting section provides base clip positions for the mc21 kit but provides numbers with several significant digits. Is it safe these are mm down from the top of the needle? I've also noticed that both the hrc and jha needles are quite a bit fatter, hence leaner, then the stock needles. All I'm really trying to do is remove the flat spot. Any comments on the difference in the needles is appreciated. ( stock vs hrc )

Q2
The needles appear to be the same length as the stock. Since i mainly sourced the kit from parts, does the mc28 kit require the longer main jet holder?

Q3
I'm running 155 mains in front and rear, so far bike runs great when I'm on the main jet so I'm inclined to leave them alone. Is there any reason to make the rear one size smaller or front one size bigger? I guess a plug chop would answer that...!

Q4
I'm a fan of making one change at a time. What effects do the y and x pieces have on jetting? Obviously they are the easiest to install. I'm okay dropping all the parts in at once, but would like to understand their influences.

Q5
I haven't plugged the pilot jets. I thought i read someplace that the mc28 didn't have them, but info seems conflicting. I confess when i rebuilt the carbs the first time they didn't stand out. If they do exist, I'm open to plugging them, will that drive me to a bigger or smaller main jet?

Like I mentioned above, I'm only trying to clean up a flat spot that is around 1/3 throttle. Other than that its spot on. But i also realize that what fixes the flat spot mat drive me to change some other settings to get it back right.

Q6
The needle holders came with two 4mm screws. I also saw these in a pic of a kit online. Based on prior observations, its not obvious these are any different then whats in there. Are they just extra?

Hmmm. I was really hoping someone would answer one or two on my questions.

This afternoon I decided to measure the tapers of the needles I've picked up over the past few weeks.

I discovered the foam in my micrometer box had pretty much crumbled and managed to attach to the tool. I didn't feel like cleaning it so i used a cheap dial caliper. I then plotted them. I'm pretty sure the tapers are constant so I also made a smoothed version. See the the attached for the plots.

The picture pretty much tells the story. The stock needles are pretty rich which I suspect could be the cause of my flat spot. I plan to start with the JHA A25A needle.

Probably not gonna do much on bike for next few weeks. Next weekend I will be in Austin for the motoGP.
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MC28 SP

I finally got around to installing my jet kit this morning with favorable results. I also answered some of my own questions above.

Q1: Clearly the thicker tapers run leaner. I assume Honda made the stock needles so thin to err on the conservative side and make the bikes run richer. I opted to use the jha A25A needles since the are the just slightly leaner then stock and not as lean as hrc.

Q2: Still haven't figured this one out, but i decided to set the needles both 3 positions down from the top as a starting point. This also makes them the same length as the stock needles. I decided to wait on the longer main jet holders . Mainly because I can change the needles without pulling the carbs and i figured that would be easiest and give me a starting point.

Q3: Based on the results, I will stick with 155 mains all around until I get more dyno data.

Q4: X and Y connectors clearly have smaller orifices which would tend to make the bike run richer and less dependent on the air solenoids. Not sure what leaving the center hose off on the air solenoids does, but suspect it allows the red X connector to depend on drawing ambient air to operate. Since the jet kit needles are thicker tapers, making the bike leaner, i figure that will counter the effect of the X and Y connectors.

Q5: I didn't plug the power jets (if they exist on these carbs), nor do I plan to based on the results.

Q6: The extra screws are for the throttle linkage to the slides. I didn't need to remove the screws so i didn't use them. I had my wife hold open the throttle while I surgically removed and replaced the needles.

Now for the results. Bike runs leaner and seems spot on. The air temp was about 75F and relative humidity around 72%. I forgot to record the air density. Fuel is 98 0ctane with no ethanol. Being that its lean i need to be careful if i go out on a cool crisp morning. Did a few runs down a nice straight and it clearly cleaned up the flatspot at 1/3 throttle. Pulls great from 3k through 12.5. Bike still idles great, but i now have a slight hesitation way down at 1/8 throttle. Having soldered cables onto the mixture screws with knobs at the end i was able to adjust the mixture. I was unable to clear it up from one turn out to three. It almost feels a little lean there so might try going in less then one turn when I go out in the morning. I should be able to work this out with the mixture, just need some time. Hoping i don't need to drop the needles because then I will need to install the longer hrc main jet holders. But...they to provide extra holes for better atomization so there might be the answer.

Next logical step is to get it back on the dyno. I will make some air/fuel mixture measurements at various rpms. Well that's all for now. I'm almost out of the 98 gas. Shop down the road has VP 110. Might try that. Probably overkill so might mix with a bit of super unleaded pump gas. Or maybe not!

I noted in the jetting section the installation states :

"The solenoid jets are still utilised in the Jha and HRC kits but now modify auxiliary airflow through the carb's. The Y joint is a direct replacement for the standard piece, with the straight side going to the solenoid closest to the frame. The + joint should have the rubber cap removed and the side with the jet left open to air. The 2 smaller joints go to the carb's, and the last joint goes to the pipe adjoining the Y joint."

I think this may be for the mc21 configuration. I found a paper online specifically for mc28 that says the rubber cap is to be left on, the jet end left to open air, one end to the Y joint and the other end to the carbs. I suspect this is because on the carb end there is another T fitting that splits and goes to both carbs. I'm wondering what others have done on the mc28. Perhaps there is an option to remove that small T and tie each carb to the + connector. This would indeed have a minor change in the airflow.

I am wondering if this is impacting my slow speed. Ironically today i was out riding and i was again unable to clear it up with the mixture screw.

I knew when i bought the bike I`d pretty much be on my own . More research is clearly needed on my part. The good thing is that i am almost there.

Well I have pretty much sorted things out and answered all of my questions above. I installed all of the fancy jet kit parts I've sourced from around the globe. After the major pieces were installed I noticed that the bike was running too rich at idle and I was unable to lean it out with the mixture screws - man am I happy I installed cables with knobs on the idle screws so I can adjust on the fly. Conclusion...the 38 pilot is just too rich for my location (sea level, really hot, air density ranging from 96 to 100%) so I sourced some 35 pilots from a Keihin jet supplier here in the US.

Overall the bikes runs great - it really needed the leaner needles, but still have a little tweaking to do with the clip position. If that doesn't do it I can always drop in the HRC B needle which you can see from my graphic in an earlier post is leaner then the JHA A25A. But I'm probably going to leave it where it is for now as I want to see how a cool morning changes it. I also think I will put it on the dyne and map out the air/fuel mixture to see where it's at. I'm certain I've picked up a few HP. The plugs when from flat black to a nice brown.

Andy commented earlier that the bike ought to be able to accelerate through the entire throttle range...I'm pretty much there. I was out riding this weekend and could pretty much leave it in a single gear and just roll the throttle as needed.

Here is the current carb config:

JHA A25A needle with both clips on 3rd (middle) position
155 Main Jets on both front and rear
35 pilots
HRC red X and black Y air solenoid correction jets
Tyga Airbox with stock filter
oil injector in place

Decided to run pump gas as it will be a challenge to carry a jug of race gas on long rides so using Shell 93 (R+M/2) with Lucas ethanol treatment and octane booster.

Yes I could keep tweaking this thing, but I'm not going racing, I just want it to be as rideable as it can be which I think it is.

Forget to mention one final update....i have since dropped the needles down one clip from middle leaning it out further. That cleared up the last bit.

I am running Shell 93 R+M/2 octane pump gas with a little bit of octane boost and ethanol treatment since we can't get ethanol free over here. I will put it on the dyno when time permits to see how things have shaped up.

I also sourced a NOS set of fork tubes which I will install when I get some time. The original tubes are more pitted than I prefer. So bad that one of the new seals is leaking.