Honda-SOHC

SOHC.co.uk Forums => CB350/400 => Topic started by: scifi on May 03, 2026, 09:56:39 AM

Title: cb400-four speed.
Post by: scifi on May 03, 2026, 09:56:39 AM: Hi All, I am a bit underwelmed by my 400-four's top speed. I thought they were capable of 100mph laps of the TT, .
I have a local one and a half mile straight road, and find that it is difficult to get more than 65 mph from the 400 either in 5th or 6th gear.
Is this as much as they do, because of the Sit Up and Beg riding position..?

I Also don't like the six speed box.. Need to change 1st to 2nd, to 3rd, to 4th, just to get up to 30mph in the village. My old BSA could do all that in 2nd gear.

I am sure my old CB200 was quicker.... Any thoughts..
Title: Re: cb400-four speed.
Post by: K2-K6 on May 03, 2026, 10:17:30 AM: The CB200 is much more of a grunty little motor and tuned more for that torque in low range, to quite good effect.

The 400 really needs to be revved out to get the higher end performance. Generally low torque but productive high rev performance. Youd probably need to ring it right out to red line in forth before even thinking about fith if wanting to see how fast it'll go. It just won't reward early upchanged ratios with much in the way of ultimate pace.

You've got to let the four sing its head off for extraction of absolutely speed.
Title: Re: cb400-four speed.
Post by: Johnwebley on May 03, 2026, 10:17:43 AM: Never had a CB400,

remember these are early 70s bikes,

Your bike sounds like it needs a going over,

I think head on the tank,bum in the air it could do a ton,

The engine has been raced,enlarged to 460 by Mochek,

They were quick,

Sure you're not confusing it with the CBR400 Baby blade,or the VFR400?

Totally different bikes

Sent from my SM-A546E using Tapatalk
Title: Re: cb400-four speed.
Post by: Mikep328 on May 03, 2026, 10:19:37 AM: IF you are going through the gears/shifting at, say 9k+ RPM and having trouble getting to 65MPH, the bike is clearly not running properly. My totally stock 400F will easily reach 90+ on a level road while riding in the normal position. Never tried it in a "race position" but 100MPH seems possible in a crouch.
Title: Re: cb400-four speed.
Post by: SteveD CB500K0 on May 03, 2026, 11:44:06 AM: My roommate at Uni (1977) had a speeding ticket at 104mph on his std 400/4 on the M23.

Must have been the downhill bit.

He was really chuffed

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Title: Re: cb400-four speed.
Post by: Nurse Julie on May 03, 2026, 12:39:39 PM: It's definitely not running right, or it has a problem, as these are little pocket rockets when running on top form and ridden hard as that's what they were designed for. You do need to use the gears a lot, that's the nature of the bike, they need high revs, they absolutely hate being lugged in the wrong gear.
Title: Re: cb400-four speed.
Post by: McCabe-Thiele (Ted) on May 03, 2026, 01:18:51 PM: My 400 will easily reach 70 mph, I expect it would readily reach 90 mph plus if it were legal in the UK.

As Julie says you need to rev them through the gears, pretty sure mine would do 60 mph in fourth gear with two to spare.
Title: Re: cb400-four speed.
Post by: scifi on May 03, 2026, 02:17:43 PM: Thanks for the replies..
I'm putting two and two together here...
I am unable to get the seat to open, as the lock is stuck. Also the plugs are a bit sooty.
So maybe my air cleaners need cleaning, as it is most likely running too rich.

Thanks for all the replies so far..
Title: Re: cb400-four speed.
Post by: McCabe-Thiele (Ted) on May 03, 2026, 02:39:23 PM: Figures from 1974 road test on CB400F.
Top speed 98-104 mph in favourable conditions.
6th 98-104 mph
5th 90 -95 mph
4th 82 mph
3rd 68 mph
2nd 52 mph
1st 33 mph

mph/1000 rpm in top 11.
speedo error 4 mph fast at true 70 mph.
Reprinted in Classic Motorcycle mechanics Feb 2010.
Title: Re: cb400-four speed.
Post by: K2-K6 on May 03, 2026, 03:52:30 PM: Quote from: scifi on May 03, 2026, 02:17:43 PM
Thanks for the replies..
I'm putting two and two together here...
I am unable to get the seat to open, as the lock is stuck. Also the plugs are a bit sooty.
So maybe my air cleaners need cleaning, as it is most likely running too rich.

Thanks for all the replies so far..

When contemporary, a few friends completed the "I left something under the seat" routine, that to restrict the air intake. Notably a pair of waterproof overtrouser that his bike shop removed for him when he put it in for service because of the poor running :)

He was suitably embarrassed about that one ;D
Title: Re: cb400-four speed.
Post by: Mikep328 on May 03, 2026, 05:46:02 PM: Yep! Good point!! I had something similar happen one day last year when the tool box cover slid back to obstruct the intake. Same symptoms - power went away under heavy load.
Title: Re: cb400-four speed.
Post by: TrickyMicky on May 03, 2026, 06:16:24 PM: Sorry Scifi, but 4th gear at 30MPH and you are what we used to call a 'con-rod bender'. Firstly, to get the seat open, use a 1/4DR 10mm extended socket on an extension bar, carefully work it up either side of the lock and locate the two nuts which hold the piece which fits into the lock. Check for obstructions to the air intake. Put a new set of plugs in. check ignition timing and valve clearances, plus of course oil level. Once the engine has reached running temperature, ride away changing up at about 6500 RPM in 1st and second, then for the remaining gears, wind it up to a minimum of 9500RPM. you should easily feel the change in acceleration as it reaches 7500RPM. If you rev it to at least 9500, when you change up, just barely feather the clutch lever, and only take the tension off the throttle cable. This will give you a very rapid gearchange leaving the engine revving at about 7000 in the next gear giving unbroken acceleration, that's why it's got a six speed box. PLEASE!!! do noit be guided by the road test figures that were bandied about in its day, there was a lot of backhanders from manufacturers to journalists for gleaming reports. 1ooMPH TT laps??? never in a million years, Malcolm Uphill only just managed that on a "carefully prepared" Triumph Bonneville in 1969. Outright speed testing conducted my the magazines of the day usually involved inflating the tyres really hard, brakes were backed off so no binding, the rear chain was cleaned and then lubed with something like 3 in 1 oil, and the rider would be wearing a skin tight leather one pice suit, lying flat on the tank with his left hand holding the left front fork. You met the nicest people on a Honda!!
Title: Re: cb400-four speed.
Post by: Laverda Dave on May 03, 2026, 09:39:11 PM: 65mph😲. Something very wrong somewhere! You do have to rev them and they do thrive on revs.
The 400/4 should just top the ton unless of course it's a yellow one that some people may say were good for 130mph+ even against a force 10 headwind although I couldn't possibly comment......
Title: Re: cb400-four speed.
Post by: Yetanotherbike on May 03, 2026, 10:15:45 PM: Speaking from experience from my previous four 400/4's and presently restoring a 400/4, I'm with Laverda Dave all the way. My present 400/4 will be yellow for the reasons Dave has explained.
Title: Re: cb400-four speed.
Post by: scifi on May 04, 2026, 08:33:18 AM: Thanks Tricky Mick for the seat info. I managed to undo the lock and open the seat. The air Filter looks brand new, so I will have to investigate further.
If the weather holds today I will try some further tests. I have compared the speedo with a Sat-Nav and they are within 2 mph of each other.

Mine is a Red bike, but I have several litres of JCB Yellow Paint, if all else fails. . . !
Title: Re: cb400-four speed.
Post by: Mikep328 on May 04, 2026, 10:07:45 AM: The red ones were far more reliable; the additional speed from the yellow ones compromised engine integrity!

OK...that's not true of course...
Title: Re: cb400-four speed.
Post by: K2-K6 on May 04, 2026, 12:53:39 PM: Quote from: scifi on May 04, 2026, 08:33:18 AM
Thanks Tricky Mick for the seat info. I managed to undo the lock and open the seat. The air Filter looks brand new, so I will have to investigate further.
If the weather holds today I will try some further tests. I have compared the speedo with a Sat-Nav and they are within 2 mph of each other.

Mine is a Red bike, but I have several litres of JCB Yellow Paint, if all else fails. . . !

Something that may need double checking, some years ago I had a honda that had stood for a while, that air filter looked new, but just would not flow properly, giving rich mixture and "capped" power output.
Whether it had been damp and possibly compromised, or something else that I couldn't readily explain, the result was clear and when swapped it was fine.

Tenuous link, but a possibility to consider.
Title: Re: cb400-four speed.
Post by: Athame57 on May 04, 2026, 05:23:37 PM: Quote from: Mikep328 on May 04, 2026, 10:07:45 AM
The red ones were far more reliable; the additional speed from the yellow ones compromised engine integrity!

OK...that's not true of course...

But is true that the yellow ones are faster, after all Dave told us so! ;D
Title: Re: cb400-four speed.
Post by: exvalvesetdabbler on May 04, 2026, 06:05:56 PM: Mine is red and an indicated 110 was regularly achievable in the 80s with my girlfriend on the bach. It was dead flat where we lived, long straight roads and no traffic.

Not sure if I would be brave enough now. Too many other idiots on the road.

The local bobby only had a hilman advenger so the best he could do was tell my dad in the pub on a Friday darts night.

That was more of a deterrent than being brought before the magistrate.

Dave
Title: Re: cb400-four speed.
Post by: scifi on May 04, 2026, 09:50:04 PM: Well after looking at the air filter beneath the seat, I removed the air cleaner top lid and went for another test ride.
This time 70 came up after about 1/4 mile straight, with more to come.
I found that the air intake was masked by the seat, as the following photo may show..
The bit between the screwdrivers is the only bit not obscured by the seat....

[attach=1]
Title: Re: cb400-four speed.
Post by: Bryanj on May 04, 2026, 09:58:29 PM: Something not fitting right there
Title: Re: cb400-four speed.
Post by: TrickyMicky on May 04, 2026, 11:16:01 PM: If you have the standard seat, are all the rubber spacers fitted as per the photo? These keep the seat base clear of the air intake. If the seat has been allowewd to settle too low, this could also be why the lock jammed.
Title: Re: cb400-four speed.
Post by: scifi on May 05, 2026, 11:14:01 AM: Thanks Tricky Micky, my seat does not have those 4 rubbers, or anywhere to put them. So it looks as if I will have to run with the filter uncovered.
Got a new seat lock on order, from China, should be here by end of the month.

(http://)
Title: Re: cb400-four speed.
Post by: McCabe-Thiele (Ted) on May 05, 2026, 12:41:46 PM: My 400 has a type of plastic seat base not steel.

I don't know if it's original or not.
Title: Re: cb400-four speed.
Post by: TrickyMicky on May 05, 2026, 02:07:14 PM: Original ones are steel, Ted. Even the new reproduction ones from DSS have a steel base.
Title: Re: cb400-four speed.
Post by: McCabe-Thiele (Ted) on May 05, 2026, 08:52:50 PM: Quote from: TrickyMicky on May 05, 2026, 02:07:14 PM
Original ones are steel, Ted. Even the new reproduction ones from DSS have a steel base.

On the upside it will not rot away, it has the rubber buffers, the seat hinge and catch work so I assumed it was original. 👍
Title: Re: cb400-four speed.
Post by: Bryanj on May 05, 2026, 10:32:48 PM: DS first "pattern" seats were plastic based and tended to rock a bit front to back, i have an unused one somewhere for a 500
Title: Re: cb400-four speed.
Post by: Laverda Dave on May 05, 2026, 11:18:37 PM: If you fit a pattern seat from DSS make sure the hinge and seat lock brackets are in the correct place, mine were miles out to the extent the front of the seat was so far forward when I fitted the hinges it fouled on the back of the fuel tank with big rucks in the cover. I sent the first seat back and the replacement still needed the seat lock and the hinge plate modifying to elongate the mounting holes by 8mm to fit properly.
To be fair to DSS though he accepted the original seat back despite me adding the Honda logo on the back using a stencil so I can't complain and he was very fair about it.
Title: Re: cb400-four speed.
Post by: scifi on May 24, 2026, 11:03:10 AM: Thanks everyone for your replies. I have just received a new seat lock from China ( £9 delivered.).
With the top removed from the air filter box, the 400-4 now goes faster than I will ever need.
Title: Re: cb400-four speed.
Post by: Mikep328 on May 24, 2026, 11:48:11 AM: "With the top removed from the air filter box, the 400-4 now goes faster than I will ever need."

Heck my very first motorcycle, a 125cc Kawasaki two stroke with probably 12-13HP could go faster than I ever really needed! :)

But somehow I still managed to "need" 160+ HP Ducati sport bikes, etc... But I'm working my way back down with my 65HP V7 850!!