Top end performance

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jonathan
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Registriert: Fr Sep 13, 2019 12:11 am

Top end performance

Beitrag von jonathan »

Hope everyone's OK.

Went for a quick spin on the AX1 this afternoon, before the heaven's opened. Still not had much opportunity to put many miles on the bike but I noticed today that the engine didn't seem to 'run out' fully at the top end. By this I mean that the engine picks up nicely from idle, revs freely and cleanly until I'm in top gear and then seems to have a 'flat spot'. It's as if the ignition timing is slightly retarded. I didn't check to note exactly at what RPM this becomes apparent. Imagine going for a spirited blast, starting in first gear, going through the gears until you're hitting around 100KPH in top gear (on the flat). At this point I still have some left on the throttle, but when I increase the rpms it seems like it is 'bogging' slightly. I've changed the gearing on the bike to account for a larger wheel and tyre but this doesn't feel like a gearing issue. It's not really an issue as such, it just feels like the engine is leaving some top end performance in the garage.

Was wondering whether anyone had experimented with the main jet and/or airfilter and what sort of results you've had. The airbox is unmodified and I'm running a genuine Honda filter element, stock exhaust and standard jetting. No airleaks to my knowledge
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scrambler
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Registriert: Fr Sep 14, 2012 8:50 am

Re: NX250 top end performance

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jonathan hat geschrieben: So Nov 01, 2020 6:43 pm until I'm in top gear and then seems to have a 'flat spot' ... starting in first gear, going through the gears until you're hitting around 100KPH in top gear
You're not cranking the engine up enough :) . At 100 km/h in the top gear it turns just 6000 rpm. To get a (reasonably :roll: ) good acceleration you have to keep the engine speed between 8000 and 9000 rpm, just like a two-stroke. With 6000 rpm in 6th gear the acceleration of a shifting dune is all you get :) because with the standard gear ratio, the 6th gear is little more than an overdrive.

Therefore one has to shift down 1-2 gears to bring the engine back into its optimal range. The outside temperature also plays a role. The cooling water heats up very slowly at low temperatures, and then it runs slower than in summer.

NX250 gear ratio.jpg
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jonathan
Beiträge: 127
Registriert: Fr Sep 13, 2019 12:11 am

Re: Top end performance

Beitrag von jonathan »

hat geschrieben: Hallo Jonathan - wie ist deine AX-1 übersetzt (?)
Und wie schnell läuft sie auf der 'Geraden' - aufrecht sitzend, kein Wind;

mfG. Herbert
Hi Herbert...Ich habe noch keinen Höchstgeschwindigkeitslauf versucht, aber ich werde Sie wissen lassen, wann und wann.
jonathan
Beiträge: 127
Registriert: Fr Sep 13, 2019 12:11 am

Re: NX250 top end performance

Beitrag von jonathan »

scrambler hat geschrieben: Mo Nov 02, 2020 7:27 am
jonathan hat geschrieben: So Nov 01, 2020 6:43 pm until I'm in top gear and then seems to have a 'flat spot' ... starting in first gear, going through the gears until you're hitting around 100KPH in top gear
You're not cranking the engine up enough :) . At 100 km/h in the top gear it turns just 6000 rpm. To get a (reasonably :roll: ) good acceleration you have to keep the engine speed between 8000 and 9000 rpm, just like a two-stroke. With 6000 rpm in 6th gear the acceleration of a shifting dune is all you get :) because with the standard gear ratio, the 6th gear is little more than an overdrive.

Therefore one has to shift down 1-2 gears to bring the engine back into its optimal range. The outside temperature also plays a role. The cooling water heats up very slowly at low temperatures, and then it runs slower than in summer.
So it is effectively a gearing thing after all...I think I've got used to riding my CB 500 X :D This will pull through into 6th. and cruise with roughly 2000 RPM left to play with, but still feels pretty responsive when accelerating in top gear. So what you're saying is I shouldn't worry about nailing it everywhere? I think I can manage that ;)

It definitely felt like a fuelling or ignition advance thing, but I'll try 'staying in the band' next ride.

I'm in the process of tuning my old VW aircooled campervan at the moment. Upgraded to a 1776 cc engine with twin carbs, dumped the distributor and fitted a crank position sensor, trigger wheel, and Ford Edis 'wasted spark' module so I can use a programmable ECU to remap the ignition, based on the new engine specs....I think I must have fuelling and ignition advance on the brain!
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scrambler
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Beiträge: 8199
Registriert: Fr Sep 14, 2012 8:50 am

Re: NX250 top end performance

Beitrag von scrambler »

jonathan hat geschrieben: Mo Nov 02, 2020 8:22 pm It definitely felt like a fuelling or ignition advance thing, but I'll try 'staying in the band' next ride.
You could open the intake snorkel on the air filter housing a little, raise the carburetor needle one step, and install a 145 main jet. This brings a little more revving-up.

You could also try a CDI with more pre-ignition, which is offered by Ignitech for the NX250 viewtopic.php?f=4&t=472&p=3149&hilit=ignitech#p3149.

But the matter of the fact is - the NX250 lacks displacement and torque. An NX300 or NX350 with the same weight would be ideal.

In Brazil there is the XRE300 with a modified NX250 engine. A few years ago I was able to drive a test vehicle that BMW had bought for comparison (for the G310 development) viewtopic.php?f=11&t=1073&hilit=xre300#p8489. Unfortunately, road approval in Germany is not possible, otherwise I would of course have bought it.

Bild
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jonathan
Beiträge: 127
Registriert: Fr Sep 13, 2019 12:11 am

Re: Top end performance

Beitrag von jonathan »

It feels perky enough throughout the rev range but just seems to flatten out towards the top, but I should really put some more miles on it before drawing any conclusions. Honda Brazil have brought out some interesting bikes for their domestic market over the years. The NX350 Sahara was one of the models I was looking out for but you just don't see them in the UK. nice looking bike!
sahara.PNG
looks like a baby Transalp disguised as an RD03...21/17 wheel combo, similar output to the AX1
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