1978 cb400f2

[blackeagles]

I was doing a partial restoration of a cb400f2 but I have got a bit side tracked with a 750 f2, anyway I have taken the 400 out and it seems to 'go' ok but it does not want to rev above 6k. it has all stock size jets and the air screw is 2 turns out.

I am not keen on revving an old bike to high levels but am just interest what, if anything, I have screwed up!

any suggestions?

[Orcade-Ian]

Two very quick things spring to mind - is the choke mech allowing the butterflies to open fully, i.e. horizontal with choke off?  Not too easy to check without removing the air plenum.  Secondly have you got resistor plugs OR resistor plug caps fitted but not BOTH?  One or other is correct.
I was out on my 350/4 the other day to meet up with some visitors and the bike would go ok but not rev freely unless I very gently coaxed it.  Found out that I had caught the choke lever and moved it slightly with my gloved hand when I turned the fuel off to fill up.  Not so much of a problem on the 400 as there is a larger distance between the two.
Ian


Just thought about observing the choke butterflies but haven’t checked - with the air filter element removed can you use a mirror to look through the plenum into one or part of the intakes?  Dunno!

[blackeagles]

Two very quick things spring to mind - is the choke mech allowing the butterflies to open fully, i.e. horizontal with choke off?  Not too easy to check without removing the air plenum.  Secondly have you got resistor plugs OR resistor plug caps fitted but not BOTH?  One or other is correct.
I was out on my 350/4 the other day to meet up with some visitors and the bike would go ok but not rev freely unless I very gently coaxed it.  Found out that I had caught the choke lever and moved it slightly with my gloved hand when I turned the fuel off to fill up.  Not so much of a problem on the 400 as there is a larger distance between the two.
Ian


Just thought about observing the choke butterflies but haven’t checked - with the air filter element removed can you use a mirror to look through the plenum into one or part of the intakes?  Dunno!

ok, thanks for that.  I  will check the choke.  I did put a single plug cap on along with a new set of plugs but I have no idea if they have resistors in them - I will have a try at checking the type using the number printed on the plug.

I have since had a check and I replaced the d8ea with the same (d8es-l unobtainable) plug although I have managed t replace the cap with a resistor one whilst the other three are non-resistor but I guess this will not make any real difference top end rev wise?

[TrickyMicky]

The last time this happened to me was many years ago with a K2 750. A friend had volunteered to help me service it because he had purchased a new strobe timing light, so he did the ignition. Rode home through town it was fine, but open it up on the clear roads and it ran out of puff. It transpired that he saw the "T" mark when timing and thought that it stood for "Timing" but of course he set the ignition at Top Dead Centre which meant it was unbelievably retarded. Might be worth checking.

[blackeagles]

re-checked the timing and although it was a weeeeny bit out I have changed it and it's the same.

I should have said at the outset that this 'problem' only happens in top gear, running up throu the gears I can get it over 6k in each gear, in fact I have taken it to 7k (but I do not want to blow it up) but in top gear at around 40/45 mpg and with the throttle open it will belt up to 6k then no more!!

 I do not like revving the life out of it so 5/6k is my absolute max but I have to say I am interested in why it does this.

[Nurse Julie]

If everything is set as it should be, it may just need a very good hard ride. I ride mine regularly 8-9k RPM and at times just going into the red. These are sports bikes and were built to be ridden hard with high revs in each gear. Obviously, if you think your engine is past its best, I wouldn't advocate revving the nuts off it but one thing these engines absolutely hate is being ridden at low revs in the wrong gear (lugging) and being ridden like a touring bike. Ride it like a 2 stroke and she will thank you for it.

[hairygit]

That's normal! 6th gear is like an overdrive, it is there to reduce engine revs at speed. O)nce it's run in, you will be able to take it to the red in 5th, put it in 6th, but the slightest gradient or headwind you will need to drop back to 5th to maintain speed. The engine NEEDS revving, it's not like the 750 at all.

[Woodside]

These little hondas live off being revved .
Wasnt there a thread on these engines....may be yours julie.

About pre detination.
Alot off rebuilt engines completely fubard after 2000 miles .after being rebuilt
When built these bikes were designed to be ridden hard..and they were direct from the show room for many thousand miles
Now after a rebuild we are all abit to sensible and just dont ride them as originally intended not taking them above 4000 rpm.

The engine was designed to be revved hard..
That's why the red line is way round.

[mike the bike]

Speak for yourself.  Mine gets ridden hard, lots of revs and gear changing,  just as Soichiro intended.

[Nurse Julie]

These little hondas live off being revved .
Wasnt there a thread on these engines....may be yours julie.

About pre detination.
Alot off rebuilt engines completely fubard after 2000 miles .after being rebuilt
When built these bikes were designed to be ridden hard..and they were direct from the show room for many thousand miles
Now after a rebuild we are all abit to sensible and just dont ride them as originally intended not taking them above 4000 rpm.

The engine was designed to be revved hard..
That's why the red line is way round.

Yes Jason, my rebuild thread had some very interesting input from forum members about lugging engines and detonation and the damage that can be caused by riding these bikes far too gently. I played safe throughout the 1st 500 miles after the rebuild, not above 5k rpm, not lugging the engine and just generally taking it easy but, my engine had a huge amount of machine work so everything had to bed in properly. After 1000 miles or so, I was riding her like I stole her and she is an absolutely flying machine and just loves being thrashed.

[Northy]

Possible the carbs have gummed up and its fuel starvation......or the air filter blocked and its over fueling.

I'd check my plug colour and go from there. If it revs freely in neutral then I like to think the timing was Ok.   

[billywingnut]

Check that all four pipes are the same temperature, I had one running cooler than the other three which I cured by adjusting the balance screw.

[blackeagles]

thanks for the suggestions.

I do not know the condition of the engine - other than it seems to 'go' alright - so i'm not happy at pushing it into the red and, having spent my life with british and hd 'thumpers', I also can't get my head around such revs.  i'll just swan about on it until I get the 750 going and then think about its future.

[TrickyMicky]

You say that you do not want to damage the engine, but if you ride around at 45MPH in 6th gear, that is exactly what you will do! In my younger days you would have been know as a "Con-Rod Bender". When you say that you have occasionally taken it up to 7K revs, are you aware that you have not even reached the max. torque yet, which occurs at 7,500RPM. If you ride it in the 5,500 - 7,500 range you will be rewarded with a very pleasant, happy little Honda. I've owned mine for 31 years, and as I approach 73 years old I can tell you that the bike has fared a whole lot better than me! Use it as it was designed to be used.

[10tenmen]

I have sent my Carburettor set to Gerben in Holland for a full overhaul take a look at the pictures below of my slides and the residue on them god knows what has caused it Gerben said he has never seen anything like it in over 300 sets lol.