Topic: Rough as a Badger's BUm...

[seabird]

Here's the problem(s)

the '75 FourF now starts and runs on the first prod from the K/S, but it will not idle consistently or slowly - It will only start and run with 95 - 100% choke.
Lotsa noise from the top end, but using the old screwdriver in the ear and against the cylinder head, no really nasty banging noises to be heard - therefore IMHO it's prolly valve clearances sonmewhat out.

If I reduce the choke more than half-way, the engine struggles and dies
All four pots are firing, but the header pipe on #1 runs way hotter than the remaining three
Static timing seems OK, but I can't balance the carbs (won't idle, see)and the vacuum gauges show all four equal but no increase in vacuum when I open the throttle.

My feeling is that the carbs are running really lean, hence the hot #1 header pipe and the inability to idle.

Comments, suggestions gladly welcomed

Terry
(Seabird)

[supersport_CB400F]

Take the carbs off clean them again and then take the to the petrol station then get the compressor you use for bowing up tyres on them if you don’t own on yourself…they could still have crap in them.

[seabird]

See?  I knew someone would come up with the good advice!  Even though I hate removing the carbs (and hate 'em even more when I have to refit them) I'll go blast them through - although I feel tempted to take them back to the company which cleaned them up orignally and tell them to get it right this time!

Thanks Bifta!

Terry
(Seabird)

[KB02]

yeah, It sounds like your idle jets are still blocked. You should have removable slow jets on the bike. Just drain a bowl, pull the spring and drop the bowl, and then you should be able to just unscrew the jets and pull them out for inspection. Use a can of carb cleaner and try to spray right through them. You won't get much, but you should get at least a little bit of a good spray coming through.

[GroovieGhoulie]

Agree, check the slow jets.  I just readjusted my valves and on Hondaman's suggestion, set them an extra thousandth out. .0025 to .003 versus the factory .002.  A little clatter when cold, but it's supposed to perform better.

Does it run if you crank the throttle open and drop the choke?  If so, it's definitely the jets.

[TwoTired]

I would drop the bowl on number 4, at least, to examine the carb inards, and clean the idle jet.  If blocked but, it looks otherwise clean inside, then do the same to #2 and #3.

However, if it looks bad inside then take off the bank and do a thorough cleaning.  I'd also ask why they are so dirty inside from the outfit that rebuilt them for you.

Reinstallation will be a bit easier if you completely remove the rear band clamps.

Cheers,

[tsflstb]

I just fought this battle with help from this site.  My 400 had cooler exhaust pipes on #2 and #4 cylinders.  #4 turned out to be a loose plug cap.  #2 was the slow jet, even though I was 100% positive my carbs couldn't be dirty.  Just a little piece of varnish from sitting too long I guess.

To remove/reinstall the carbs, I use a bunch of silicone spray on all the boots and snorkels.  I then loosen the air filter housing and just move it back a couple of inches to disengage the airbox.  I can then pop that off the carbs and take it out the right side.  Trying to remove the air filter/battery box is like a damn rubix cube to me.  The only way I ever removed it completely was by first removing the rear wheel.