Nc21 carb setup and stuff

[Vfr400KM]

So i had to get rid of the standard exhaust off of an nc21 i bought because it was rotten and i now have a stainless system with a micron end can (which has no baffles only the perforated tube and the sound dampening material)
This obviously has thrown the bikes standard setup out of the window and it now has a huge hesitation between 6 and 7,500 rpm and then it pops and spits flames after 9,000 rpm. What do i need to do to the carbs and the jetting to fix this?

[Neosophist]

what do you have in there at the moment?

For the mid-range flat spot you can try some 0.5 or 1.0mm washer under the needle to richen the mid-range up.. the popping could be too rich on the mains...

a dyno run would help...

[SevenThreeSeven]

I wish I had a dollar for every time I've read a post promising that some bit of carb tuning will eliminate "the mid-range flat spot" (which isn't possible due to the fact that there's nothing you can do about the simple, dumb, 2-dimensional, uncompensated ignition advance curve built into the stock CDI box)

[Vfr400KM]

Neo the engine and carbs are all standard

[Neosophist]

I wish I had a dollar for every time I've read a post promising that some bit of carb tuning will eliminate "the mid-range flat spot" (which isn't possible due to the fact that there's nothing you can do about the simple, dumb, 2-dimensional, uncompensated ignition advance curve built into the stock CDI box)

While it may be true that you can only tune within the limits of the carbs and the ignition unit this shouldn't be used as a catch-all to say that its impossible to improve a bikes response by carb tuning as it still may be possible to improve within the boundaries you have set by the bike.

Some flat spotting is probably inevitable but in your case espeically it sounds like youve gone from an ok running bike to somethign terrible just by changing the exhaust system.

I forgot to mention in the original post to check the new system for leaks, an air leak into the ehxuast can cause flames.

a proper powercheck at a dyno will tell you how its fuelling across the rev range and give some insight into if you need to change jetting to suit the exhaust system.

Compared to a modern fi bike with space-ship style ecu to control things you are quite limited but you should still be able to get something acceptable with proper adjustment of carbs, it might not be like a modern bike but it shouldnt fall on its face to the point its unrideable, even with an aftermarket sytstem.

[Vfr400KM]

Excellent thankyou, i will get it on a dyno and then pay someone else for the pleasure of repeatedly pulling the carbs on and off

[magg]

Agree with neo that carb tuning should be able to improve/solve your problem although an open muffler could be more challenging. Do your neighbours/police enjoy the noise from the non-baffled muffler. Paying someone to remove/refit the carbs might be unnecessarily expensive given one main jet change may fix the top end and the change of needle position can be done with carbs in place.

[Neosophist]

Before you jump the gun a bit make sure the bike is all ready for a dyno... electrical issues like charging system problems can mimik fuel issues and cause issues (bad earths will do this too)

go through the bike and make sure everything is ok, including fuel tank vaccum hose.

Once your sure everythings as good as it can be go get a power run and youll get a fuel / air graph too.. you can look at this to see how the bike is currently running and go from there, by all means post it up for some advice, bit of trial and error / educated guesses should have it running well.

as magg has said, a lot of the aftermarket jdm cans were desgined to be used with stock airbox / jetting... maybe a single size increase.

since the nc21/24 used a collectorbox design a modified exhaust might not be as strightfoward but you should be able to get something acceptable with the right advice.