GB 400 - exhaust change + air box removal

[tmc98]

I have a GB and the exhaust pipe is a bit knackered, id rather replace it with a "shorty" (running standard headers and collector) but have heard rumours that it can cause engine and tuning issues? Also want to remove the air box but have heard the same about this - having a bit of difficulty finding decent answers for this - anyone care to shed some light on the issue? cheers!

[fridayflash]

ive owned a 500, and rode a few mates ones, and the best running one ever was my gf's 400, tail pipe
had been replaced with a fairly open mega, but retained original headers and collecter box
pilot and main jet changed to suit and thats it! i recommend leaving the airbox alone
those bikes love still air

[AllanB]

What he said. Muffler is a easy change and if needed the carb can be tweaked. Airbox mods will require a lot more skill to ensure it runs sweet.

[Canis G.]

Hey there,

Did the exaust and air filter mod on my GB400 and yes you need to re-jet the carb and also enlarge the holes in the slide so that it lifts up high enough to give you full throttle. If you don't the engine runs too lean and you risk OMG over-heating damage to the head and piston.

[Sable]

Keep the airbox, change the muffler. From memory you only need to change the jets, think it was maybe 1.5 sizes larger.

[avgas]

Runs fine with no exhaust on standard jets - run 5w50 oil to keep engine cool (10w30 is shit at air cooled motor cooling - too thick). Note loud exhaust (or no pipe) will make you tone deaf if you do lots of riding at 100kph.
You lose speed and HP if you open the pipe. So pick something with good adjustable backpressure for those times you don't want to sound cool. Supertrapps are my favourite as you can dial them up to whatever setting you prefer.

If you up the jets, fuel economy goes out the window. But as someone said earlier, megaphone with 1.5 jets runs sweet. Swap the filter (in the standard box) with a good oiled one, and change the plug to an iridium so that you get better response under low pressure.

Always retain the exhaust collector box, and never go to a 2-into-2 system. Every single one I have seen do this is left with an unbalanced mess. Not sure why - but by keeping the 2-into-1 collector the valves will thank you.

Swap the standard shocks with some KONI/IKONS or Ohlins if you have them. Preload the front forks with a 10-20mm spacer ($1 of old $0.20 pieces). This doubled the performance of the bike just through handling.

How many k's has engine done? While it is not required, simple things like rings and cam chain @ 100K can make a huge improvement.

[tmc98]

Thanks for the advice, looks like it'll just be the pipe then (dont want to get into too much strife), saw one with an oil cooler the other day - just curious would those have any real effect? also looking for a set of tyres (or just a rear one) anyone got any good recommendations? im abit flooded with the options available... Cheers and oh yeh the poor old girl has done 96 thou - still runs nice tho doesn't like idleing at lights tho - any clues?

[ducatilover]

Google is your friend.

Oil coolers work, that's why they're there.

Sport Demons/BT45s etc etc etc etc

[tmc98]

Yeh the question was more in relation to gb 400s than generic oil cooler - eg what benefits would a GB be likely to have from one...

[avgas]

They keep engine cooler (no shit) - but also another place for oil to leak out if not properly fitted. As the NC20 didn't come with one standard, it is safe to say if you keep good lightweight oil in it you will be fine without one. 5w50 I used had the best temp rating (DUH!) but problem with it is you need to change more often (5,000km) whereas 10w30 you could leave in for 10-20,000 no problems.

Your idle problem will just be idle screw - but if that doesn't work get some "Start ya bastard" ethanol and spray directly into air intake to give it a good clean also. You can even do this while the bike is running to clean all the way through that little mikuni.

I actually had a good run with the Dunlops TT/GT on the gb's. If your gonna get Sport Demons or BT's then get skinny ones. I also had a good run with shinko's on GB's which if your gonna do lots a wet day riding would be my pic (water tracks on BT's are tiny).

If your approaching 100,000k's......keep an ear open for cam chain noise.........its a motherfucker when that breaks. Usually around 130k mark.

[tmc98]

Thanks, just found a suitable and cheap Shinko (joy), I'd be keen to try the ethanol trick, so basically just use a squirter bottle to spray ethanol directly into the carb mouth while the bikes running? sorry about the simple questions but gota start somewhere. Any idea of the difficulty/expense of changing the cam chain? as a broken one certainly isn't on my fun list...

[ICE180]

I have done quite a bit to my GB

keep that air box as you need the still air for the carb to wprk propely
as soon as you take the air box off you will spend ages sorting out jetting /slide issues with them

The exhaust can be changed with out any mods to the jetting as My ran fine with the 45/165 with a straight trough baffled pipe from custrom chambers

have a look at my old thread for any ideas

http://www.kiwibiker.co.nz/forums/sh...ighlight=gb650