Topic: pulsing sound under load 4-6.5 k rpm

[kajtek]

Hi all,

bike is a cb400f

i have something i cant quite diagnose, under load between around 4-6.5 k rpm i get a pulsing sound from the exhaust. it could be happening at higher revs but hard to tell. seems to be more prevalent when the bike warms up.

dont know when it started... but im sure it wasnt there before. Carbs very recently cleaned (properly), checked the carb rubbers visually and no air leaks, fitted electro ignition the other day and timed perfectly. only thing still to do is valve clearences. havent done carb sync either, though that only makes a difference at idle right? stock airbox stock filter (though i could do with a new filter, 8-9000 miles?)

so my last consideration is a possible air leak at the exhaust manifolds. one of the pipes has blued where the others havent leading me to think this. there isnt an obvious soot deposit near it though. i can smell exhaust a bit on my bike jeans. come to think of it, i noticed the pulsing in the period after a took my exhaust off/on to paint the collector.

please note exhaust is a marving 4-1.

thanks for any advice!

ashley

[iron_worker]

Would you describe it as surging? Surging under steady load is generally a lean characteristic.

If it wasn't doing it before I would suspect an air leak. Have you checked for air leaks other than visually? Using ether or some type of flammable spray around all the carb rubbers and listen for a change in idle. If the idle changes that means the engine is drawing the spray in through somewhere it likely shouldn't be (ie an air leak).

IW

[kajtek]

Hi, no its not surging, i cant actually feel anything, just hear the pulsing. Kinda sounds like when u ride behind another 4 cylinder bike and their revs r slightly diff and u get a pulsing sound.

I will definately give it a go to double check for air leaks at the carbs, is there any way to diagnose a leak at the exhaust side?

[matt mattison]

Pull the plugs, check deposits. Maybe you could identify the offending cylinder that way.

[kajtek]

Pull the plugs, check deposits. Maybe you could identify the offending cylinder that way.

Good call, will have a look tonight, although i suspect if its any than its no2 as the pipe has blued while the others havent.