KTM (penton) 400 - Mikuni conversion

[farmer58za]

Hokay!
The mikuni conversion project lives again! As mentioned earlier, I had a major problem getting a straight unobstructed passage from airbox to engine with the 38mm Mikuni.

The physical size of the mikuni means that there will be about a 15 degree angle difference between the open end of the stock manifold and the carb when everything is lined up nicely.

To deal with this, I will cut a tapered alloy pipe section which will fit inside the rubber between carb and manifold to help guide the gases through. This also means that the manifold is about 1" longer overall. Could help low end. The right solution is to manufacture a new manifold. I have already made one prototype which was close but not right yet.


On the back of the carb I have turned up a "velocity stack" which pokes right into the airbox so there is absolutely no doubt as to getting the air in.

Maybe we test tomorrow.

Regards

David
'75 KTM GS175
'75 KTM GS250
'75 KTM GS400

[Big Mac]

David,
When it comes to air boots between carb and airbox (or carb, velocity stack and airbox...whatever) check out all the options at //www.ozktm.com. The Ozzie boys have made up most ever variation imaginable and they are made of durable, yet thin and flexible rubber so that installing is manageable in the tiny work spaces the KTM engineers allowed around carb and box.

Their prices are right (around $20 last I checked)and doesn't cost much or take too long with Int'l mail, Rob and co. are helpful...you can even try a couple and send back the ones that don't fit.

Mac

[farmer58za]

Finally got the Mikuni jetting nearly right.
Bike starts first kick. Great throttle response off idle. Pulls like a bull up a long hill at 1/4 throttle. Accelerates up same hill at 1/2 throttle. Still a leetle rich on top, maybe at idle too.
But still a great result.

400 KTM (Mint) 38 Mikuni:
Pilot 40
Cutaway 2.0
Needle jet P6
Jet needle 6F9
Main 350
5000' above sea level



Regards

David
'75 KTM GS250
'75 KTM GS400

[farmer58za]

Hi Guys
An update on real life riding with the Mikuni. I went on our local off-road "Toy Run" yesterday in which we trail ride to schools to hand out toys to underpriviledged kids.
A lot of the trail was in dry, rocky conditions. The 400 pulled like a trials bike up rocky slopes. Like night and day compared to the old (worn) Bing.
I also got the chance to try out some real top speed performance. Again, I was not disappointed with the bike winding out to (seems like) warp speed. There are however two iffy spots in the jetting which I need to work on. Overall plug burns brick brown/red
Catch you later.



Regards

David
'75 KTM GS250
'75 KTM GS400

[John Ehrhart]

Just one question: Why on earth would anyone want MORE of anything, especially bottom end out of that engine? Mine would wheelie in fifth with stock gearing, and pull from idle with never a glitch--once I got it jetted (with the help of a son of JP.)

1969 Husqvarna 400 Cross
1974 Monark
1975 Jackpiner
1978 GS6 400
1981 250 MX

[farmer58za]

Why try to change? Easy - because it's there!! LOL
 
But seriously, if you read the thread, there is a lot of members who have tried the conversion with varying degrees of success. I don't doubt for a second that a stock (NEW), well set up Bing will give superb performance but I wanted to take the Mikuni conversion to it's  logical conclusion.




Regards

David
'75 KTM GS250
'75 KTM GS400

[farmer58za]

Hi All
It's been about 7 or 8 years since I posted on this topic. The KTM 400 was re-fitted with the (old and worn) Bing carb, for some reason I can't remember, and sent to the back of the lock-up.

Fast forward to 2014 and I now have a son-in-law who is also a keen dirt biker. We are going to drag the bike out, service it and put back the 38 Mikuni to ride in an up-coming vintage event.

Regards
David in Johannesburg

Regards

David
'75 KTM GS250
'75 KTM GS400