KTM (penton) 400 - Mikuni conversion

[farmer58za]

Hi Guys
Just bought a KTM(Penton)400. It's a '76 model. The Bing carb is VERY badly worn and I was considering putting a 36 or 38 mm Mikuni on it (both of which I have in my garage). Has anyone on the list done this before? I'm looking for some jetting spec's.

Thanks in advance.

[Mark Annan]

Hi David,

I see you found the group.  Welcome aboard.  Some of you may have met David at the ISDT-RR.  He and his wife came all the from South Africa to join us in the celebration.  He is a vintage bike nut to the core :-)

Mark

[rob w]

I got a 400 not to long ago that had a 38mm Mikuni on it. The rest of the bike appeared to have been set-up nice at one time, although I can't testify if these carb settings are all that correct because it was not a runner. But at one time it was. I'll check it out and post them tomorrow.

[farmer58za]

Rob
Thanks for that. It's a start!
David

[rob w]

David, The Mikuni model number stamping is 38E (over) 9

Main Jet-    390
Pilot Jet-    30
Needle Jet-  166 (over) Q8
Jet needle- 6DH4 , clip, 3rd down from top
Slide-       3.0
Air Screw-   1 1/2 turns out

Good luck, I hope this helps.
Bob

[tlanders]

Both Bobby Lucas and Barry Higgins have told me that the Mikuni will inhibit the top end. It's great at low rpms, but falls off at the top end. I think it has to do with the width of the barrel where the slide is, I think the Bing is bigger allowing better breathing at top end. I have often wondered whether just increasing the size of the Mikuni might solve this. Use a 40mm Mikuni on the Mint 400 instead of the 38mm Bing. Any comments? I also get very frustrated with the Bings, they wear out so fast and the parts are very expensive.

Teddy

[firstturn]

Teddy,
  I haven't built a big engine Penton in years and always used a Bing, but I helped people change to Mikuni for the cost factor and ease of getting parts back then.  The main thing that I remember was I preferred to keep the 38 on the bike and make the changes on the chamber.  Since in my opinion(which has always been in question) I wanted the 38 for better low to midrange and the pipe(chamber) set up for more top end.  Since I felt it was easier to change pipes after testing than it was to work on Carburetors in the dirt.  My pipes back then we built for me by FMF and by only one of their builders.  Working with metal is a art.  So I guess you could say carburation is only part of the end result.  And remember Teddy this wonderful information is probably only worth what you paid for it.


Ron Carbaugh

[farmer58za]

Thanks Bob
I think I have all those jets in my Mikuni stuff. It'll be a great improvement over the Bing.

Does anyone such as Al Buehner sell a rubber boot to take the larger rear bellmouth of the Mikuni? Failing that I'll machine up a flange to fit the stock boot. I'll use a piece of radiator hose between the Mikuni and motor.

Teddy: Low end is OK for me. This bike has MUCHO top end!


Thanks guys
David

[tlanders]

I'm home now and took apart a 38mm Mikuni that came off a Mint 400. It has:
pilot 60
main 410
slide 2.5
needle 6FL14, clip on third notch out of 5
needle jet 166 JP6
The carb is stamped P9 (over) 7 (space) 1

Completely different than Bob's of course. The air boot problem is a problem but Barry Higgins says you cane build brackets to hold the metal MC5 airbox and it will line up better than the fiberglass one. He then uses some other manufacturers boot for a DelOrto carb.I forget whose, sorry.

I am currently struggling to get my 38mm Bing from flooding out during races. It failed me in Colorado making winning this years championship almost impossible to do with only one more race to go. I have rebuilt everything including replacing the float seat!!! It still loads up when I get hit by another bike or after a jump and then I put on the brakes very hard. I am going to change carbs this weekend and see if that solves it.

Good luck with the Mikuni,

Teddy

[tomale]

I did exchange my bing for a mikuni. It was the first time I have ever did anything like that but it workd out good. It is for my 250 but I sure like the way it works over the bing. I did get alot more low end. and I did lose some on the top end but, it was enough of a change down low that I had to add a tooth to the countershaft sprocket. I had it dialed in before I changed the sprocket and found that I was shifting alot more, because I was gaining revs alot faster than on the bing. tight corners are alot easier than before. With the change of both the carb and the gear ratio. I can keep up with some of those aweful honda's in fact they can not figure out why that old bike goes so fast. Top end is not everything. Another added plus is that it starts alot easier now than it ever did warm or cold.
The more I ride it with the mikuni the better I like it. I was able to use the stock boot from the air box to the carb, but I needed to make some changes to the intake manifold to get everything to work right. hope this is helpful.

Thom Green,I own and ride a 76 250 MC5 MX which I bought new.

[farmer58za]

Teddy
Does that mean you are going to try out the Mikuni?
David

[tlanders]

I might if I can find an MC5 airbox. I thought Thom's comments were very helpful in this discussion. Thanks.

Teddy

[farmer58za]

Teddy
I was just thinking as to why your Bing floods when subject to a impact. I noticed on mine that the holes in the posts that take the pivot pin for the float were substantially enlarged (+0.5mm). When subject to an impact, the pin/float/valve may move and allow a slug of fuel into the bowl and hence cause flooding.
check yours out.
My bing carb is so worn it's only good for a paperweight.

As for fitting the airbox boot, I'll machine an alloy pipe/flange that will fit over the Mikuni bellmouth with a clamp and be pop riveted onto the boot. I have a 3" alloy round bar that I'll turn up. It makes an incredible amount of shavings!

regards
David

[farmer58za]

Just an update on the Mikuni conversion

I got two sets of spec's, from Bob and Teddy.

Bob's spec's were waay too rich in the midrange (Q8 needle jet)bike would start easy but would go rich after 1/4 throttle.

I searched high and low to get a P6 needle jet as per Teddy's spec's. A friend lent me one and I fitted it and tested it tonight in the dark around our neighbourhood.

Bike starts first kick from cold or hot, pulls smoothly off idle and through the range.  No loading up at all. Power is very smooth although it doesn't have the kick of the Bing. All in all, a good basis for some fine tuning. On the face of it, Teddy's spec's are the closest.

I developed and made up a conical carb boot out of heavy duty truck tube. It's good for the present stage of the conversion.

If I get chance to do some fixed throttle runs this weekend, I might be able to post my final spec's next week.

Regards
David

[imported_n/a]

David, I am watching with great interest in your task.  

What is your elevation above sea level?  

Glenn