NC23 Top speed issue on cb400sf

[magg]

Fuel usage dependent on throttle opening but lower gears use unless you do not change up. Higher gears use most fuel as max continual revs for max speed requires max power. Simple explanation use main jets big enough to reach max revs in any gear although max revs in top gear is usually not reached with a road bike. In your case if the bike is now running cleanly and consistently through the rev range then try larger main jets, 120 perhaps and set what happens. It will either go faster, slowing or no change and from there you can decide bigger, smaller or just right. A dyno would give the same result but more quickly as exhaust o2 sensing will tell you immediately which way to change the jetting, it just costs money.

[Neosophist]

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

dont bounce a bike off the limited with no load on the engine, you risk doing all sorts of damage... also since there is no load on the engine you will find it revs up easier.

as magg has suggested get any competent dyno operator to test it, a decent fuel air graph will show you exactly whats going on, the operator will also be able to advise you on what to change to make it work better if anything.

the other option which might be cheaper is to convert it back to factory airbox and induction system, clean and adjust the carbs and set them up per service manual settings... these bikes run pretty good from stock and theres enough guides on how to tune the carbs on the forum, a lot of tinkeres dont really know what they are doing and automatically think pods are going to be better, where in reality the stock airbox is designed to deliver a consistent stream of air at specific volumes for the bike... some bikes can run very poorly unless set up extremely well with open / pod filters due to engine characterists and how the air flows when the bike is at speed... ive seen all sorts of weird things over the years

[sa.racing]

From what you describe, I would say your running lean on the main jet, try going up to something like 116/118 and see what it does to the bike, trial and error is the only way your going to figue out whats going on

As has been said if you can return it back to stock air box and jetting you are starting from a known good set up however pod filters can be made to work perfectly well and are often used for asthetical or design reasons and not performance, we have nothing but time on our hands at the moment have a play around with it and see what you can learn, bottom line if you can get it somewhere near it will ride better in the mean time and cost you a lot less later on to fine tune on a dyno.

[unkul79]

Thanks guys. So I upped to 110. Bike feels much better through the rev range (smooth and powerful) but still has the same issue at about 10k(ish) where it feels like the rev limiter being hit. Sort or wobbly though, not so much a brb brb brb brb brb brb brb sound of a smooth limiter but more like an uneven br br brb brb brb brrrrb br br brrrb brrrb br br type of sound. (Sorry for the poor explanation). I'm tempted to put in 120 main jets. But not sure tbh as it seems a big jump, what would you guys do?

Cheers

[sa.racing]

So you've gone up the jet sizes and the bike feels better? You've just answered your own question just keep going up in size until the bike rides and revs clean right through to the red line,

Depending on what sizes you have like I said above I'd look at around a 116-118 but 120 will work for the purpose of elimination try them and see how the bike feels

[unkul79]

So today I've had a bit of time to ride and play around a bit. Still waiting on the local dynos opening so atm it's still just guess work. What I've noticed today is is that I hit the "limiter" independent of throttle position. By that I mean I can creep up to, and hit, the 10kish limiter with virtually no throttle (about 1/5th open). I'n my mind this tells me that if I get to the limiter and then open the throttle fully I would be adding more fuel, however this makes no difference. Does this sound right? Also the bike pulls easily uptown the limiter and then just blips. It almost feels that there aren't enough gears and that if I had 7th or 8th my issues would be sorted!?! Any ideas guys?

[magg]

You keep referring to the "limiter" at 10,000rpm. Can you describe this "limiter", the rpm just stops increasing, the engine misfires or the acceleration rate is dramatically reduced. I am be wrong but the CB400Sf with NC42 type engines made max power at 10,500 rpm red line at 12,500. Your engine cases may be stamped NC23 but an NC23 model is a CBR400. What is the model number stamped on the ID plate on the steering head. What speed are you reaching in 6th at what indicated rpm. Are you sure that the replacement speedo/tacho are giving accurate info. How fast do you want to go. Few road bikes will reach max power rpm in top gear. The gearing may have been changed to improve the low speed acceleration and thus the engine now runs out of revs at high speed. Does it accelerate through the lower gears very quickly.

[unkul79]

Ok sorry for the delay n reply Magg. Family life got in the way a bit. To answer your questions:

1.Can you describe this "limiter"

Much as you would expect tbh. I hit that rpm (indicated by the tach) and i get the "BR BR BR BR BR BR BR" limiter sound. The rmps dont go higher after this. Accelleration doesnt continue either.

2. What is the model number stamped on the ID plate on the steering head.

I cant see this (make it out) however after a bit more research on here im inclined to thing its actually an NC31 engine viewtopic.php?t=2477

3.What speed are you reaching in 6th

Approx 60mph dependent on wind etc

4.Are you sure that the replacement speedo/tacho are giving accurate info

No for revs but it feels right based on engine note and experience. Speed is correct, verified with gps.

5.How fast do you want to go.

Just want to be able to do 70 without the bike revving out. Im not looking for speed per se as im not looking at doing loads of miles or racing. Just want to know that if i need to get on the motorway i can safely.

6. Does it accelerate through the lower gears very quickly.

yes, cruising at 30/40 is 5th and 6th (mainly 6th).

In addition to this i found this website to give me some info (really useful tbh) https://www.motorbikecatalog.com/moto/1 ... _four.html. Shows what i should/could expect. Interestingly the speed v rpm chart show my speeds/rpm match up at approx 5500rpm. Which screams resistor mod to me, however as mentioned the bike feels/sounds like its revving much higher. Is there a different/manual way to measure rpm?

Cheers

[CMSMJ1]

Two things come to mind when reading this. (apart from standard airboxes save all manner of issues)

1: Have you had the carbs off the bike? Does it have restrictors fitted? - There is no way a bike that ought to produce 50bhp cannot go faster than 70mph..especially if you say you are revving out...which leads to

2: get us a video of it please - I cannot believe that the bike can pull 10k revs and not pull more than 20mph in first.

3: So what are the sprocket sizes? If it is 14 x 55 or something mad - then that is your issue

A left field option - as this has had an engine replacement and it is not 100% from the same model - shoot em down if I miss it - but in the NC30 and NC35 - both the same motor give or take - there is a difference in the tacho adn pickups - you canotnuse NC30 tacho on an NC35 etc. you must change the pickups.
So - this has an aftermarket set of clocks? It might have been setup for the OEM pickups - but you have another motor - and maybe, this is where you shot me, different pickups? It runs, as the std clocks are missing, but reports 5.5k revs (and restrictor) as 11k revs,

That's a bit out there - but it's either restricted in carbs - hence no power or it is not revving out as it says - as mechanically, if the motor turns at 10k revs, them in 6th, it is going faster than 70mph