KTM (penton) 400 - Mikuni conversion

[farmer58za]

Glenn

I'm at 5000 ft above sea level.

David

[farmer58za]

Glenn

I'm at 5000 ft above sea level.

David

[farmer58za]

Just an update on the conversion.
Spent about 4-5 hours doing adjustments and fixed speed runs. This is where I am at present.

Mikuni 38mm
pilot jet 40
slide cut away 2.0
needle jet P6*  (*see note below)
jet needle 6DH4* (     "       )
Main jet 360-390* (      "       )

I couldn't get the carb to run clean at mid-range to top despite going down 30 numbers on the main. I ran the bike without aircleaner and no change. I then took off the "homemade" rubber carb manifold and
Pow! there was the power. Due to the angle and physical size of the carb, the rubber manifold bellys down, obstructing about 1/2" of the large mikuni bellmouth. This has a major effect on the airflow and I'm sure those needle jet/needle/main settings will change when I've sorted it out.
Has anyone modified the 400 airbox to fit a mikuni?

Regards
David

[john durrill]

David,
 how much can you turn down the bell on the mikuni befor you get into the enrichner air inlet? we had a the same problem with the 2 Lectrons we have on our 175's. we were able to turn down the Lectron bells to 1.968 inches just a little over 50 mm . if your mikuni bell is 63 mm thats 2.48 inches.. 13 mm would make a big difference in how much the boot would hang down. we were then able to use hot water and some muffler tubeing coupler's( 1 3/4 to 2 inch ? ) that we got at the auto parts store to stretch the stock boot out enough to slip over the Lectron bells.
 we are sending one of Als Lectron boots to you. it will take some time to get their but it may help.
here's a link with a picture of the engine , carb and air boot fitted before we put them in the frame on my bike.
http://briefcase.yahoo.com/bc/woolf_62000
hope this will help,
John

[farmer58za]

Thanks John
I've looked at the layout of my carb and manifild and it looks pretty tight with just about 3mm of clearance between carb and crankcases or exhaust. As the mikuni is about 1/2" longer than the Bing I should steal back about the same amount by cutting the manifold. This still doesn't guarantee a straight transition to the airbox. The solution here is probably to fabricate a new manifold with a bend that points the Mikuni down towards the airbox.

hmmm........time to phone our friendly piping contractor

David

[tomale]

David,
Did you ever get the jetting sorted out on your 400 with the mikuni? And what did you do to solve the clearance problem?
ON my 250 I had to cut 1/4 inch off the intake manifold but I should cut a bit more. I was able to use the stock airbox rubber. It fit fine on the carb. The bike runs great, it has taken me awhile to get the thing figured out and removing the carb each time is a pain but it has been worth it.
Thanks for the input,

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

[farmer58za]

Thom
I actually ran out of time before a planned ride so I didn't finish the Mikuni conversion and put the worn Bing back. I did, however turn up a new brass needle jet on my lathe (the old one had worn oval), drilled it the correct size and fitted a second hand (less worn) slide.

The net result is that the bike runs buckets better and fixed speed runs indicate good plug colour at 1/4, 1/2 and full throttle. Bike starts first kick.

The Mikuni conversion will follow, but right now I'm concentrating on a Bultaco Alpina and a BSA Bantam trials bike.

Regards
David

[farmer58za]

Hey Thom
Here's the old thread on the Mikuni conversion


Regards

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

[tomale]

Thanks David, Iwas looking at that the other day. But was hoping to see if you or someone else has done anything new on the subject. Ted has done some stuff on one of his bike and that was helpfull as well. How are you doing. The last time we talked you were still recovering from a hard crash at the reunion ride. I hope you are feeling better by now. I still look back with a smile when you and I sat and swapped stories.
Take care,

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
78' 400 MC5

[farmer58za]

Thom
I have not done any more work on the 400 conversion but I'll start again in a month or two. The 400 needs a clutch rebuild which must be done before I can look at the carb. I've finished the Bantam trials bike and still need to put the Bultaco together. At the same time I'm trying to assemble a Husky for my cousin. (phew!)

The major problem with the conversion was the obstruction of the mikuni bellmouth with the airboot which disrupted airflow. The hard facts are that the available space is limited and manifold modifications are likely to get it all to fit nicely.




Regards

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

[tomale]

I was concerned about that too, but I do not think I will have that much trouble. I installed a Gem reed on the bike and I actually ended up with more room for the carb. The carb now sits farther forward and a little bit farther up. clearing the center cases. When I put the Gem reed on with the Bing, I had to use a different air boot because it would no longer fit..... there was too much space between the bell housing and the flange on the air box. I ended up using an old boot from the 76 250. So I am fairly confident that I can work out the problems. In short order. Thanks again.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
78' 400 MC5

[farmer58za]

Whoo hoo!
Where'd you get that reed valve? I'm looking for one for my 250.
I reckon the 400 will be a stump puller with the reed and the Mik.
You may need to go richer on the needle jet when you fit a reed.

My shoulder is a bit tight after the RR fall but I'll survive. It was good meeting you too.

Regards

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

[tlanders]

Wow, the 400 Mikuni thread lives again!!! I did make the conversion to a 38MM Mikuni on my 400 for last years races. The carb ended up way too close to the air box and I know I had restricted air flow through the boot because of it. I am at work right now and do not have my final jetting to report. However, I was getting beat by a Penton 250 (Curt Kennelly is a great rider) and I am sure that the restricted air flow was part of the problem. The only good thing is that two Pentons came in 1st and 2nd nationally in the AHRMA +60 Intermediate division last year!!!

I am rebuilding this bike right now using a different pipe that gives me enough clearance over the carb and airbox to modify the airbox for a straight boot between airbox and carb. I am also going to use a bored out 38MM Mikuni to 39.5MM. They don't make the 40MM Mikuni any more or it is way too big to fit, I forget which. The Mikunis have better bottom end response because they have restricted air flow - the Bings breath better at top end. Solution, put on a bigger Mikuni than the Bing you are replacing. I'll report how the bikes runs after a couple races - and that is after my ribs heal up!!!!

Teddy

[farmer58za]

Teddy
Well done on your Ahrma placing. The 40mm Mikuni is huge and hasn't a chance of fitting on the stock setup. The 38 mm in my garage may have to go as part of a project 390 Husky which a friend is pestering me to sell to him. I will have to look for another.Lol

I do have a S&W (EI) 38mm flat slide carb which may work PROVIDED I can get jets or whatever they use to meter the fuel.



Regards

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

[tomale]

David, glad to hear that you are doing pretty well, With Mac's help we bought a 75 400 For an amazing price. I would not have done it at all but the price was right and it was only thirty minutes from home. I was wanting one for Vintage Ahrma stuff so we bid on it and... we got it. I got it home and started taking it apart to see just what we had and Wow. it had a Gem reed and the Piston was not cut for it. So I was replacing the rings on the other 400 so I thought that it should go on the 78--- which I did. I did have to change jets, I went up one on the Main and two on the Pilot. I had to go one notch on the needle as well. ON the 78 clearance is really not a problem. The bing with the Gem reed actually took up less room than the bing with the stock Manifold and the bing. So much so that I had to use a different air boot. With the Mikuni... being 16mm longer (according to Pro-flow specs) than the Bing, it should put me right back were it was in stock trim. Well at least I think so... we shall see[8D]
It will be awhile before the 75 400 is up and running. It is mostly complete and will need everything Painted but I am in need of a few things. They are available but you know how it goes. Got to wait for the cash. Thanks for all the help.
Ted if I remember right... the 40 mikuni is alot bigger and if my measurements are right... you would have a dickens of a time getting that to fit on a 75 400 even if you did have a Gem reed for it. so going with the bored out Mikuni sounds like a good idea. A bit spendy but should work...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
78' 400 MC5