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.
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
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.
Rob
Thanks for that. It's a start!
David
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
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
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
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
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
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.
Teddy
Does that mean you are going to try out the Mikuni?
David
I might if I can find an MC5 airbox. I thought Thom's comments were very helpful in this discussion. Thanks.
Teddy
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
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
David, I am watching with great interest in your task.
What is your elevation above sea level?
Glenn
Glenn
I'm at 5000 ft above sea level.
David
Glenn
I'm at 5000 ft above sea level.
David
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
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
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
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.
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
Hey Thom
Here's the old thread on the Mikuni conversion
Regards
David
'75 KTM GS175
'75 KTM GS250
'75 KTM GS400
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
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
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
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
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
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
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
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
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
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
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
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
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
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