NC23 Top speed issue on cb400sf

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unkul79
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Re: NC23 Top speed issue on cb400sf

Post by unkul79 » Sun Apr 19, 2020 10:38 am

Also in another question are there 2 versions of the nc23 motor. As my case covers look different to the 400cbr versions i see a lot online. Did honda make 2 models with the same number that looked different?

magg
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Re: NC23 Top speed issue on cb400sf

Post by magg » Sun Apr 19, 2020 11:59 am

Not sure I would come to the same conclusions. Only conclusion I would draw is that the pods and current jetting is not suited to the airflow requirements of the engine. Pods on too slow, pods off too slow with added fuel issues.

Pods off performance limited by lack of fuel (lean jetting). Pods on performance limited by airflow through the pods. Looking closely at the picture of your bike, the pods do not look particularly big. What volume was the original airbox.

Might I suggest you return the airflow system to standard with an airbox, filter and jetting to establish a base for engine performance. Then if you want to fit pod filters you at least know what performance level you should expect.

Also keep in mind that pods filters that flow the same volume of air as the standard airbox and filter may not be possible or meet your aesthetics. In which case you may have to accept a trade off in performance.

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Re: NC23 Top speed issue on cb400sf

Post by unkul79 » Mon Apr 20, 2020 6:51 am

Cheers Magg

Yeah I think you are right with your conclusions. As said maybe I just have to wait it out and get a dyno session in when I can. This would help diagnose the situation right?

magg
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Re: NC23 Top speed issue on cb400sf

Post by magg » Mon Apr 20, 2020 9:09 am

Just to have a clear understanding, the bike had the pods when you acquired it and it has always run as you have described.

I would think you could get somewhat better performance from diy tuning before going to a dyno, given the current state of tune.

Forgive my ignorance, does the tank have a vacuum fuel tap and if so does fuel flow freely from the tank when a vacuum applied to the tap.

From the picture of your bike it does not appear too difficult (compared to a v4) to remove the float bowls and check jet sizes, at least the outer ones. Are they the same or similar in size to OEM.

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Re: NC23 Top speed issue on cb400sf

Post by unkul79 » Sat Apr 25, 2020 1:12 am

Hi Magg. Sorry for the delay replying. I've been busy at work all week.

Yeah pods were fitted since I've had it. And it's always ran the same.

Yeah the tank petcock is vacuum fed and works well.

I plan to remove the carbs for a good clean tomorrow and investigate what's in jet wise. I assume they are standard but I cant be sure. Do you know what standard would be?

Finally what does your gut think. Would you be going for a bigger main jet to start with, bearing in mind the symptoms I've already mentioned.

Thanks for all your advice

magg
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Re: NC23 Top speed issue on cb400sf

Post by magg » Sun Apr 26, 2020 2:44 am

STD main jet size I believe 110. If you assume the pods flow less air then it needs smaller jets than STD but will develop less topend although maybe better than you have now. If the jets are smaller than STD when you remove the pods it will run lean still have limited topend but may get there better. Too lean also not good for engine. You need to know what is currently fitted to determine which way to go.

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Re: NC23 Top speed issue on cb400sf

Post by unkul79 » Sun Apr 26, 2020 6:55 am

I've removed the carbs and pulled the main jets. They were as follows.

Cylinder 1. 95
Cylinder 2. 98
Cylinder 3. 98
Cylinder 4. 95

If you think Std is 110 maybe someone has fiddled in the past. I changed them all to 100 (before I'd seen your post) so maybe it wont make any difference. I'll be going for a test ride this morning and report back.

Cheers

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Re: NC23 Top speed issue on cb400sf

Post by unkul79 » Sun Apr 26, 2020 6:00 pm

Ok so I went for a ride and oh joy new problems came up. Bike started well (with choke) and idled ok o the drive. Let it warm up for 5 minutes and set off. After about 5 mins riding when everything was ok (general driving, no full throttle) all of a sudden it started to bog down. Went on for another minute or so as it got worse. Pulled over and it stalled. Struggled to restart so waited for a while scratching my head and tried again. Started but real rough. Added some choke and revs went right up so took off choke and bike ran ok(ish) for a couple of mins and then was sputtering. Feels like it's not running on all cylinders, but cant be sure as all pipes were still hot. Starting to get frustrated now !!! Managed to get home but any sort of throttle felt terrible and felt like it would stall. So after all that I didnt get to give it a good test!! As usual any advice?

Cheers

magg
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Re: NC23 Top speed issue on cb400sf

Post by magg » Mon Apr 27, 2020 1:22 pm

More research suggests the main jet sizes are as you have listed, 110 maybe for the CBR400. Pilot (slow) jet should #35. Problem sound like fuel starvation. Applying the choke adds more fuel. How did you confirm that fuel flows freely from the tank?

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Re: NC23 Top speed issue on cb400sf

Post by unkul79 » Wed May 06, 2020 3:48 pm

Hi Guys

Sorry for the delay in reply but not had any time for testing until today. Ive sorted the "misfiring/bogging down" in one simple step...Fill up the petrol tank (doh!!!). So back to my discoveries. Bike runs and pulls great. As it always has. What i notice is that if in neutral and i blip the throttle i see the rev counter go all the way upto 14k before the limitier hits. When riding it always feels like the limiter kicks in at around 10k. This is not on full throttle, in fact id say its more like 50% throttle. This happens in all gears (more or less, it seems to rev a bit higher in the lower gears). So i guess my next question is if the main jet is really for full throttle, could it still be a fuel issue. As i've mentioned it "feels" like im running out of revs? One other question. Does the carb use the same amount of fuel no matter which gear its in, or do higher gears need more fuel at equivalent throttle positions to lower gears?

Cheers


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