NC23 Top speed issue on cb400sf

[unkul79]

Hi All

Any advice is welcomed as im at a loss and i feel like i've tried all of the options. First a bit of background. I'm no engineer or mechanic but am a willing amateur and like to get my hands dirty. A couple of years ago i bought a project cb400sf on a J (92) reg. The bike was running and i had little knowledge at the time about the model. The plan was to Streetfigher/Cafe it which i've done and i love the look and where its heading (WIP image below if anyone is interested). However since i've had it it had no working clocks (speedo). I've added an aftermarket Koso that i'm happy with and i'm pretty sure i've got it all running as it should. Its been on the road since late last summer and this is where my issue started. It appears to Rev up to about 14000 revs (is this correct as im guessing the tach is working correctly from experience but without reference?) It pulls ok in all gears but the top speed is woeful. Regular cruising on town streets is all done in 5th/6th and the top speed is 65mph max meaning i can't really use it safely on a motorway. I cant understand why to top speed would be so low. It does have pod filters and an aftermarket exhaust but runs and idles pretty sweet. I'm not looking for ton up but id like to think that the engine should be able to pull to 80 even with my heavy ass on it. As mentioned i think it pulls well too, just runs out of revs really quickly. I haven't dyno'ed it yet due to lockdown (plan was to get it done in April). Does anyone have any ideas? Bit of extra info that might help inform you a bit. The engine is an n23 (image below). I have tried the resistor mod (although cannot be sure its been done correctly) as the wiring has been bastardised in the past (i connected 12v to the blue/orange with a 2k resistor). Any advice or ideas as it seems that the nc23 is an older model engine and not usual on the 400sf.

Thanks, Happy Easter and sorry for the long post.

Steve

https://photos.app.goo.gl/ucVkMP5aGSx8bN346

[magg]

If the "resistor mod" was not working the engine would not rev pass about 5000 rpm with out the std speedo. Blipping the throttle in neutral, the engine easily runs up to more than 10000 rpm? When you start off and run up through the gears using more than 10000 rpm what max speed do you achieve?

[unkul79]

Hi Magg

Thanks for the reply.

I can bounce of the rev limiter in all gears so as you said maybe the resistor mod is working. Speed wise im getting about 65/70(tops)mph in 6th gear. This doesn't seem right to me. Even if the bike had "lost" some horses over the years id still expect more performance than this. Would you agree?

[sa.racing]

Does the bike rev out in the lower gears? For example if you were to red line it in 2nd would it rev out to 14k and if so what sort of speed would you get? Like wise in 3rd

Sounds to me like a jetting issue, what did you do to the carbs after fitting the pod filters?

[unkul79]

Thanks for the reply. Yeah it will rev out in all gears. I'll check speeds today but all pretty low. The pods were done before I got it so haven't jetted yet (but cant be sure the previous owner hasnt messed). As mentioned I was planning to dynojet it this month. A few other things I'm considering are I'm pretty sure I can smell fuel. Would this indicate rich rather than lean? Also what if I restrict airflow in the pods, would this help as I was under the impression pods always leaned out the mix due to extra air getting into the carbs? Any ideas or thoughts are welcomed.

[sa.racing]

Pod filters can be an absolute nightmare to set up (speaking from experience as a dyno operator for many years) bikes are designed with air boxes for a reason pod filters can be made to work but it takes some fiddling, experience and all bikes respond differently,

As a rule to help you diagnose the issue, If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large, If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small,

Have a play with it and see what you can figure out and report back your findings

[unkul79]

Thanks again for the input.

So today ive done a bit of fiddling. As mentioned as i have the pods i was worried about it being lean. However i could smell petrol which i thought meant probably a rich mix. So i thought (rightly or wrongly) if i restricted some of the air flow into the carb that would maybe help. I added a perforated stainless disc inside the pod. I was wrong. The bike run much worse with the less air flow. Idled OK but anything from about 4k was awful. Really spluttery and rough. The revs would climb but were not smooth at all. I even noticed some loud pops from the exhaust as i rolled off (unburned fuel maybe?) . This all makes my gut tell me too rich. Could this be the case after adding pods? I think i may be getting in over my head and the carbs may need to come off and be handled by someone with more knowledge than me, but no where is open atm (obvs). Any other thoughts that may help?

Also i did an approx speed test up to the limiter. before i messed with the pod. Pretty smooth all the way.

1st - 20
2nd - 35
3rd - 45
4th - 50
5th - 55
6th - 65

Seems really odd?!?!?

Thanks

[magg]

Take the pods off. A easy significant change that hopefully will help point you in a direction.

[sa.racing]

If your reving out to the limiter in those gears but only getting those speeds have you looked at the gearing your running is it stock how many teeth front and rear?

If the gearing is stock it does sound to me like it could be a rich mixture, way I see it you have two options the previous owner could have fitted the pod filters assumed it needed re jetting and fitted massive main jets, pull the carbs have a look what's in there and cross reference it with stock to see what your working with, write everything down and have a play with it if it doesn't work you can always put it back to how it was, you can still get parts online have a play with it what have you got to loose, second option the previois owner fitted the filters and left the carbs stock in which case you need to make a decision which way to go, we've got nothing but time on our hands at the moment get some jets and have a play with it.

Where abouts are you? If your still not comfortable playing with the fueling I know Dynotech in Basingstoke are still working but adhering to the social distancing, can't recommend them enough for dyno work they really know their stuff

[unkul79]

Hi Thanks for the replies, and sorry for not posting but i haven't had any time until yesterday to test.

OK so here goes. Yesterday i removed the pods and and went for a quick ride. Almost immediately the bike was rough (3/4k up) but still pulled okish. After after a mile or so i turned around to head home. I was comfortable that the test had proved that the bike was now much to lean. As i headed back (almost all uphill) the bike started to lose power and eventually stalled. It then wouldn't restart. A few tries covering the air intakes of the carbs with my snood made it fire up but then as soon a throttle was added it stalled again. So i had to walk home collect the removed pods and head back to add them on. Once i got back i put the pods on and the bike restarted after a few tries. I haven't checked the carb jet sizes but am i right in thinking that as making the mix leaner or richer are both having negative effects then the mix must be about right?

So this is my conclusion so far. I think that as the bike seems to run well throughout the range in all gears with the pods on i think it isn't too lean or too rich. Without some sort of diagnostic testing equipment i can be sure but thats my gut feeling. So back to the original problem. Why would the bike be so slow? Can you guys point me in the direction of what approx speeds i should be expecting from this motor across the gears please?

Also thanks for the nod towards Dynotech, however im up in Manchester.

Maybe im at the point where i really need to test with some proper diagnostic equipment, i'm just a bit frustrated that i cant get any closer to a solution myself.

Thanks again guys