Honda-SOHC

SOHC.co.uk Forums => CB350/400 => Topic started by: Rirons on July 09, 2016, 08:48:45 PM

Title: 400/4 wont rev past 4000rpm when under load
Post by: Rirons on July 09, 2016, 08:48:45 PM

I have recently restored a 1976 400/4 when on its stand it revs well and seems to run cleanly but when on the road I can't get it past 4000 rpm

Has anyone got any ideas what to look for the timing has been checked and is ok

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: Green1 on July 09, 2016, 10:56:09 PM

I have had this in the past and later found the float hight to be wrong.
Have you had the carbs apart.

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: Rirons on July 11, 2016, 04:49:44 PM

yeah refurbed them this year, thought Id set them correctly maybe not, i will check them also thanks

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: Johnwebley on July 11, 2016, 05:50:45 PM

Quote from: Rirons on July 11, 2016, 04:49:44 PM

yeah refurbed them this year, thought Id set them correctly maybe not, i will check them also thanks

  the state of the plugs can tell alot,
what colour are they?

white?  or black,both are wrong,

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: Rirons on July 18, 2016, 12:35:22 PM

reset the float height using the correct method not the Haynes manual method which had been sued prior, they were miles out, also cleaned the plugs which were sooty, indicating a rich mixture, so weakend it by 1/4 turn, also set the timing, it runs much better but still a little slugging and also seems to have a flat spot, plugs are now a almond brown which looks ok according to Haynes manual. Just need to get it feeling like it has some power. I will check the compression.

Getting there slowly

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: gtmdriver on July 19, 2016, 07:40:35 PM

Don't forget that when you adjust the idle screws you are only changing the mixture at idle and perhaps just a little above that. The idle screw certainly won't be affecting the mixture at 4000 revs and above.

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: Spitfire on July 20, 2016, 07:13:23 PM

I don't know where this came from but it does show the relationship between the various carb jets/parts and throttle openings.

(http://i192.photobucket.com/albums/z244/DennisMcc/jetgraph-onpage.jpg)

Cheers

Dennis

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: Rirons on August 15, 2016, 12:38:40 PM

Bike it now running well, and although part of the reason for rough running is a little embarrassing  I thought I'd share it anyways.

Firstly I cocked up the valve timing so that didn't help, got away with it i.e. pistons and valves didn't clash, the float heights were miles out as i did them the haynes manual way. I had a noise which sounded like tappets but turned out to be the head gasket protruding into the cam chain tunnel, pushing the chain across and running hard against the cam sprocket, causing it to grab, now i though I'd put the gasket on the wrong way but the Ive checked and turning it over gives the same issues, it just a badly cut gasket, and its gen Honda, plus the cam chain was tensioned incorrectly. Now all is sorted and running well revs freely, also set the timing using strobe, so its spot on, now  has power with very little tappets/cam chain noise.

Thanks to all who pointed me in the right direction. Now its all about riding in this lovely English summer :(

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: mike the bike on August 15, 2016, 06:32:53 PM

Glad you've got it sorted.  There's nothing worse than a bike that's temperamental.   Anyone want a 650 Katana that needs a bit of fettling?   It's going on fleabay soon, I've had enough of it.

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: Laverda Dave on August 16, 2016, 01:43:08 PM

I have set my carb floats using the Haynes method although I haven't yet started the bike. Is there another secret method I should be using?
Dave

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: Chris400F on August 16, 2016, 02:05:45 PM

Have a look at this thread, it might help.
http://www.sohc.co.uk/index.php/topic,7617.msg48072.html#msg48072

Title: Re: 400/4 wont rev past 4000rpm when under load
Post by: Laverda Dave on August 16, 2016, 05:44:58 PM

Thanks Chris that's a helpful link especially about the throttle screw.