400/4 Hot Idle Problem

[andut]

I’m having hot idle problems with my 400/4, and I’m not sure what to do next !  Any help or suggestions would be gratefully received !

The bike starts from cold ok with a bit of choke and then quickly settles at a 1200rpm idle off choke.  Pulls ok and will return to idle whilst still warming up, but after 10 miles or so when hot it just won’t idle and will die when trying to open the throttle from idle or low engine speed.  Above 2500rpm, all is fine and it pulls well, it’s just that 1200 – 2500 making it a pain in traffic and at junctions.   

I’m sure it has to be carb related, and I thought I’d found the problem with float heights at 24mm, but after resetting floats to 21mm it is still stalling when hot.  Float chambers are clean of any debris, Idle jets are clear and the fact that it revs fine when >2500 suggests the main jet / needle circuits and tank to carbs fuel delivery are fine and I think it would rule out ign (new plugs and caps with Boyer ign at std timing)

I have ranged the air screw settings from 1.5 turns to 3 turns in ½ turn steps and it affects idle to main jet transition as you’d expect, but it still cuts when hot – I’m thinking about dropping the float height another 2mm to make sure the idle jet is sat in fuel – maybe a bit of vapourisation when hot or a slightly porous float ?

I’m running K&N filters with a 0.82 main jet and needles raised a notch and plugs look spot on – I know from air filters are notorious for tuning games  - would they really cause such a dramatic running problem ?

Any ideas or suggestions ?

Many thanks in advance !!

Andy

[Erwin83]

Hi Andy,

Hope you found the setting by now? If so, could you share what you did? I'm facing the same situation with my 400f. Everything is fine, except for hot idle.

Thanks.
Erwin

[Erwin83]

By the way, i get a dry fauled plug so it would read rich.
Carbs are rebuild, in sync, standard settings.
Floatbowl level too high? So increase float level to 24? (Sits at 21-22mm now)

[philgresty]

valve clearences too tight?
disconnect the hose from the crankcase breather ontop of the engine and try that. I had a 550 taht would build up crankcase pressure and die after a short ride

Phil

[K2-K6]

Are you saying you have increased the amount of fuel contained in the float bowl? I can't quite visualise which way up you are saying.

Are you sure you've no air leaks between carb and engine? As the symptoms you describe seem to indicate lean at idle to give you a stall and flat spot.

Something quick and easy to try, put some cotton trainer socks over each filter and run it to see if it changes the characteristics you are getting.