Dave Higgspeed wrote:

I don't know  Neutral the things I do for the good of all tripling kind.
Yet again I have fearlessly taken apart a perfectly good bike in the quest for knowledge to share with you all Exclamation  Laughing

As I told you all I took the purple KH400 out on boxing day up to Parbold Hill for a rideout.

The bike ran well al day, but as it was the first long run I had done on it, I realised that it was not that fast  Sad
I know it is a KH not an S3 but it just seemed to be lacking. It was certainly a lot slower than Simons, although that does have tuned barrels on it, and goes particularly well for a kh400.

So I decided to attack the bodged ignition on it. It had been converted to KH250 genny with a Boyer Bransden on it.

I had a spare Starfire unit, which as you know replaces the low and high speed windings on a KH400 and produces seriously fat sparks.
Fortunately, when I bought the green KH400 A3 (now sold BTW), I got a spare KH400 engine in bits.
I rummaged around and found a complete stator plate and flywheel...happy days!! Exclamation  Exclamation

So I removed the Boyer Bransden, and the 250 genny and fitted the KH400 stator plate. I had to completely rewire the coils, but the Starfire unit dropped nicely between the frame rails where the original KH400 used to be. I also had to remove the 3 phase universal reg/rec I had fitted and replace it with a 2 yellow wire universal reg/reg. Fortunately I had a brand new one still in the box which I had bought as a spare. They work perfectly on kh400's and replace both the regulator and rectifier. So... all in all a lot of rewiring  Rolling Eyes

It started first kick  Exclamation

I had also thought about replacing the huge KH400 airbox with an S3 one I had spare. On closer inspection I came to the conclusion that the KH400 airbox was probably better than the S3 one because of the larger capacity "sump", the problem with KH400 airboxes is not the box itself, but the lid. The lid has a nice large letterbox opening on the outside, in fact I just measured them. The KH400 outside letterbox is 73mm x 31mm giving an area of 22.63 suare cm's
The S3 airbox lid has an opening of 50mm x 30mm giving an area of 15 square cm's, so in fact the kh400 has the potential to flow more air .

The problem comes when you flip the KH lid over and you can see the problem straight away. It has a RIDICULOUSLY small letter box slit on the tin cover which smothers the inlet hole.  In fact it measures 83mm x 10 mm giving an area of only 8.3 square cm's, or to put it another way only 55% the size of the S3 airbox  Shocked  Shocked
No wonder people slag them off!  

So again ,without thought for my own personal safety and your thirst for knowledge in mind I attacked it with an angle grinder  Exclamation  Exclamation

The hidden treasure lies beneath. Now we have a much bigger hole at the top of the inlet passageway. 30mm x 72mm or if you like 21.6 square cm's or 44% more than an S3  Exclamation  Exclamation  Exclamation

Now we are getting somewhere.
Right, so I now rejetted the carbs to S3 ish kind of spec with 85 mains instead of 77.5 and 25 pilots instead of the stock puny 20's.

Put it back together and go for a ride.


It makes the clutch slip in every gear. Even after a thorough 10 minute ride to warm it up it still lets go at the clutch if I give it full throttle or go over 7000 rpm.
It NEVER slipped once when I rode it on boxing day ,and I DID cane it.

So there you have it a very unscientific explanation of why my KH400 now goes like a rocket.


   Is it the carbs and airbox mod or is it the 6 trillion volt spark it now gets off the Starfire unit ???????

Who knows......

Who cares.  Neutral

All I need now is a new clutch and uprated springs like the baby H2.

I will dyno it in the new year and see if my bum cheek dyno prediction is in fact correct. Exclamation