Cheap exhaust and induction modifications for a D

Having recently bought a MY06 Duke II Ltd Edition and perusing various articles about modifying them, I have carried out a few extremely simple, and more importantly for me, extremely cheap modifications.

I removed the solenoid bleed valve set up on the right side of the engine,

blocking the hole in the top of the carb,

and removing the gauze filter from the small link pipe and fitting it to the section of tube exiting from the side of the carb which vents the atmospheric chamber in the carb.

The carb diagram below shows that this pipe is the atmospheric chamber vent pipe and should not be blocked as it would act as a damper on the movement of the slide.

Item 18 is the atmospheric chamber which needs to be vented, the sketch is NOT a Duke II carb, but is a Mikuni CV carb, so the principles are the same.

I have left on the air bleed system into the exhaust as it makes no difference to the power of the engine, or fuelling.

I have removed the top cover off the air box.

Replaced the 165 main jet with a 170 main jet.

Changed the 47 idle jet with a 47.5 idle jet (the idle jet change requires the removal of the carb float chamber).

Set the mixture screw 2.5 turns out from fully in.

Remove the top plastic cover off the carb (2 screws) and remove the plastic slider/spring/needle.

Cut 4 coils off the slider spring within the carb.

Drilled out the holes either side of the needle hole in the plastic slider in the carb to 3mm.

DO NOT DRILL THE MIDDLE HOLE, THAT'S FOR THE NEEDLE.

Moved the circlip on the std needle to the 4th one down from the top (std is 3rd one down).

Drilled off the 3/16" diameter pop rivets from the end caps on the original exhaust silencers, (use/buy a cobalt 5mm drill bit to make it easier) and swapped the left cap with the right cap, so the bent outlet pipe in the middle of the std end cap sits inside the perf tube in the silencer.

You have to remove the silencers from the bike to do this, but once complete looks totally standard.

I haven't had chance to do some fuel mixture checks yet, i.e. engine cut at full, load and check plug colour, (which was the method I have previously used on my 1972 Mexico with twin 40's some 22 years ago, and that produced no more power when I eventually went on a dyno) but the bike pulls very well, and the 3,500rpm hesitation on steady throttle which was apparent on the std bike has completely gone.

The jetting is a combination of the recommended settings from KTM when installing the Akropovics and the screen mesh air filter cover, and the settings used on the conversion covered on this web site.

http://www.teamincomplete.com/Projec...keiiintro.html

One section references carb settings, one section describes an exhaust rebuild, but mentions a halfway house mod for the exhaust, which is what I have done.

Total cost.

£7.00 for jets.

£1.50 for stainless pop rivets.

Plus a little of my time.

It certainly seems to have improved the pick up of the bike, it's a little noisier, but not much, it looks completely std, and as I have said, it's cheap.

It may well be running slightly rich, but that would not be as detrimental to engine life as running lean would be.

On competition crossflow fords the air fuel ratio was always run richer than the 'correct' 14.7:1 stoichiometric ratio used for maximum economy at about 12.5:1, to give maximum power and keep the piston crowns cool, so I will not be concerned if its running a little over the stoichiometric setting.

I don't think I will bother with a dyno session, as ultimate power figures have no interest to me, and the mechanics of a single cylinder carburetted engine with one main jet, one idle jet, one carburettor should be fairly straightforward to get running safely if not at maximum power.

If you want to strip and rebuild the silencers then this chaps web site gives a nice clear explanation of what to do, although I think 1.25"dia perf tube would give better results than the 1.5" dia he used.

He mentions that the bike feels a bit flatter at the bottom end, and I believe is probably due to too large bore exhaust.

http://tonysleep.co.uk/dukepipe

Cheers

MTR