Backpressure??????? Help please!

[Gratefull Beginner]

Hi All.

My XJ650 trike has now been set up for the k&N style air filters, but it splutters at 4-5000 rpm, engine then runs ok after that. I've heard that it could be caused by back pressure (either not enough or too much) from the exhausts. Can anyone help with an explanation or design tips for making new exhausts. (Currently it is 4 into 2, with 2 universal silencers fitted. Pipes are about 4ft long, not including the downpipes.)

Thanks
Lea

[contail]

had a look in my book. some bikes have a mid range dip. when filters/ pipes are changed this creates a mid range hole. (nothing there). i cured mine by changing the jets.
i have k&n type filters.NOT K&N. apparently theres a big difference. k&ns do there job properly( slightly restrict air intake) the ones i bought just stop stones and birds being sucked in.
the difference: k&ns have a filter. the others are basicly mesh.

my bike still has a slight flat spot, however its a lot better since re jetting. i run a 4-1 straight through. sometimes i put baffle in. doesnt seem to change performance, just pleases neighbours. i hope this is some help.



ps. i richened my mixture by 1 whole turn of the mixture screws. ive got a better pull from standstill.

right then im off to work. got to change the threaded screws and vac the filters.

[Gratefull Beginner]

had a look in my book. some bikes have a mid range dip. when filters/ pipes are changed this creates a mid range hole. (nothing there). i cured mine by changing the jets..
That sounds like the problem. I tried putting bigger jets in but the engine just wouldn't pull and the plugs got badly sooted up within the 3 miles I did on it. At the same time I raised the needles by one "slot" (about a mm). Think I might have gone too big on the jets. I went from 118 to 130 (probably too much!). Am I right in thinking that increasing the size of the jets is a bit hit and miss? And is it a good idea to use a colour tune each time you increase the jet size to set the mixture correctly?

i have k&n type filters.NOT K&N. apparently theres a big difference. k&ns do there job properly( slightly restrict air intake) the ones i bought just stop stones and birds being sucked in.
the difference: k&ns have a filter. the others are basicly mesh.
I was thinking of using the K&N spray on the mesh filters I have on, would this help in restricting the air flow?

Sorry if this is a bit "noddy" but only just getting my head around carbs, so any carb experts with advice much appreciated! :D

Lea

[dracken1]

main jets effect mainly 3/4 to full throttle.
the needle position is 1/2 to 3/4
idle mixure from tick over to about 2500 rpm
of course these positions overlap each other

there are also the air bleed jets

a miss at 4500 won't be cured by the mixture screws as at those revs that circuit is no longer in use.

you have made quite a jump in main jets.
try leaving the needles were they are and going down a few sizes on the mains.

some very expensive equipment is used to set up the carbs in the design stage. so yes it can be hit or miss for mere mortals
:D

[contail]

i agree. mine was trial and error. even the proper place with dyno didnt get it wright.
just to explain what was done to my carbs altogher. main air jets had to blanked off due to un restrictive filters( at dyno shop). main jets are a lot bigger, which is why i lowered the needle.(dont think they had the size we wanted. and then i turned my mixture screw out 1 turn.( on the gxrs the mixture screw controls flow of mixed air and fuel, not fuel or air. then i turned my tickover down. hey presto it runs like a dream. i just wish i had lubed the screws that hold float chamber and diaphram cap. today i am going to the common job of hacksawing a slot in the screws. again.
any way i hope you get it sorted. :D