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400 Four detonation

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K2-K6:
Alright,  I know I " go on about it" but been looking at why I feel detonation can easily occur on this engine given the right / wrong conditions.

As others pointed out you normally associate lean mixture with something like this,  which is not obviously the case if jetted properly,  you'd have thought?
A characteristic of slide carbs is that you can create it with how you ride,  even if setup is correct.
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Above should be a dyno trace that Jensen has kindly shared of his 400 Four running in 4th gear with throttle rolled open in that one gear to build the power curve you see in top traces.

The really interesting bit is the lower trace,  left scale is mixture with rpm along the bottom. It's obvious from that peak leading up to 4000rpm that it's running toward an over lean condition. Complete burn is considered to be 14.7 parts air to 1 part fuel. Most engines won't get that close. During acceleration you'd expect closer to 12 to 1 to make it move,  with cruising state in between the two.
It's not a wrong setting on here,  just characteristic of running from low rpm to high in one gear which is needed for the dyno plot.

What it does is the vacuum the engine draws at low rpm with throttle wide open cannot match the flow the carb venturi is making,  resulting in loss of vac on the main jet,  and so less fuel than ideal is pulled from float chambers ( it's this bit that PD carbs fill in with accelerator pump,  clever a?)  you can't jet your way out of it as the carb and main jet is almost fully open. It's really a failure of slide carbs used in this way and what would be termed "lugging " the motor in high gear.

In addition,  the ignition advance reaches max around 3000rpm and quite early for full load application ( more suited to cruising on light throttle to give good economy)  so in that lean peak area you've also got probably too much advance for a short period.

Put those conditions together,  perhaps riding along into a headwind,  fith or sixth gear,  trying to keep up speed at lowered rpm and you've got classic detonation. Riders with experience would recognise it's not pulling properly,  but if the circumstances are held like that without the engine accelerating through that, it will most likely detonate. It'll only take a few minutes like that to start the destruction of the pistons and rings.

You can't jet it differently,  it's just how you ride it. They need to be revved if you want them to go.
You've either got to back down the throttle or drop gears and spin it to higher revs.

ka-ja:
Even when relatively a new bike (got mine in 1979) you had to use them above 5000/6000rpm to gain decent performance.

Nurse Julie:
That's really interesting Nigel. So, going back to the mess on my pistons / rings on my engine strip thread, it would make sense to me how my engine got in such a state. If a PO had not ridden it as the bike likes to be ridden for optimum air / fuel mixture ie, high revs and the engine 'pulling' a majority of the time then it makes total sense how the damage had occurred. I have said many times that the CB400/4 likes to be ridden like a 2 stroke, high revs and minimal lugging and I can see how riding it for long periods in the wrong gear / too lower revs can start a chain of events leading to damage.
Can I ask you to put a copy of this on my engine thread as I think it is a very good explanation of how the chain of events could have lead to the damage on mine.

K2-K6:
Transferred to your post Julie, it's a bit awkward for cut and paste on the tablet I use but think it's done the job ok.

There's something with these engines as they are so refined and benign ticking over and just tootling around that it convinces some that they don't like revs,  but they were built to do just that,  as you point out. Honda did a remarkable job in combining such a range of response in them,  and such tough little things to survive the effects you had in yours.

Larger engines will generally have enough torque to pull it through sub optimal sections like this,  but the smaller one's have to be driven much more like a small racer really. Properly oiled they don't have any real problems but people convince themselves that revs are damaging.

Nigel.

taysidedragon:
I'd never really thought that the way I ride would affect the fuel/air ratio. Very enlightening, thanks.  :)

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