400 cc hysterical engine

[dirtbike]

I have started my 400, and even though it runs fine, I find it very snappy and temperamental. I have weights on the crank (see pic), and an outer rotor motoplat. Despite that It's not like my friends goregous 420, or like my excellent maico 400.

I would like to smoothen the power curve out a bit. I have a Mikuni 36 mm carb. What do you think that'll do instead of the Bing 38.

[Larry Perkins]

That will help it a tad but I think no matter what you do it will hit more than the two bikes mentioned.  The other two bikes are pretty much tractor power with their strong suits being low to mid range power.  The 400 Penton had some hit and that is just part of its characteristics.  If you add a bit of base gasket and lower the compression just a tad it will help some too.

Larry P

[Kip Kern]

Dirt

I have a 76 GS 400 with stock enduro exhaust, 38mm Bing, weighted crank and a trials Motoplat ( larger than normal flywheel).  My bike has no hit and pulls like a tractor.  One thing you can do is when installing the cylinder and measuring the deck height, the piston manufacturer gives a measurement from say .055 to .075 from the top of the cylinder down to the top ring.  Set it tall at the max measurement and this will tame her down a bit!  Have fun!

[wfopete]

Depending on your riding conditions/gearing, one tooth up on the countershaft sprocket is a quick & low cost method to take some hit away. If the C/S is not feasable, I would go with a thicker base gasket(s)to alter the port height & timing.

Pete Petrick
175 Jackpiner
Slow but Good

[brian kirby]

My MC5 has the stock 38mm Bing and flywheel, its one of the hardest hitting difficult to ride bikes I have ever ridden. That thing put me on the ground more last season than in the previous five years. Its fine in motocross, but for hare scrambles/woods I decided half way through the season that I simply do not have the talent/skill required to control that monster on trails. It was a very humbling experience and by the time the Reunion Ride came around I didnt even want to ride it but I had to for the Team competition.

I would really like to try one of those ''trials'' heavy flywheels to see how much they help because I suspect the primary cause of the 400s abrupt power delivery is not enough flywheel weight.

Brian

[checkcrew]

Brian,

the power to weight ratio on your bike is most deffently in favor of the 400 motor, straight line speeds are a thrill but getting between the trees is quite a handfull,
 
i have watched your videos of the RR severall times and can feel the tension you have in the tights to keep the motor at bay,

if you think the weighted flywheel motors are a handfull you should ride one of the 74 1/2 Mint 400 that have no weights on the crank shaft and a 1mm smaller and lighter piston, talk about a rush !!

those early 400 motors need the thicker base gasket and the big,big flywheel that Kip describes, but they are also prone to braking the flywheel side of the crank off so it's a catch 22 situation, you really get a lesson in throttle control on the early motor !!

i know when Speedy reads this he will just say Lecktron and your problems are solved - LOL -

OK, hope this helps some,

regards,

Mike Gallagher, NJ.

[dirtbike]

I have the X measure set at 1,5 mm (0,06"). I don't want more since I then would risk having detonations, I belive.

I have also the impression from over the years that a worn engine or a lower compression due to leakage between the rings and sleeve (bad break in??) also could produce such a characteristic.
I have honed the cylinder but the cylinder and piston where almost like new. So I decided to reuse them, just honed it a little. Maybe that take some bottom away and make it hit harder?

Anyway, my maico buddies claims that a 36 mm Mikuni will improve the tractability and smoothness, just giving very little away on top. So I will try it on my 400.

[Larry Perkins]

Mike,

When I was young I raced one of those early 400's and they were a rush.  I raced mainly moto with it and had come off racing 100's and 125's then right to Open Ex.  It was like a Huge 125 and I had not yet learned the art of short shifting.  It was probably scarry to watch but for me it was just a constant adreneline rush.  I remember racing it in West Texas Grand Prixs and having it pegged on double track as fast as it would go and wishing there was more.  To be young and senseless again...Sigh.

Larry P

[Bill Campbell]

DB I think I have said this to you before. I have a 76 400 that i believe to be fairly tame.The major difference is the 81 350 pipe I am running. Not period perfect but great for the bike. Then again I can never use any bikes full power. As the previous posters have said its a different bike to your others and the best thing is to enjoy it for what it is. Cheers Bill

[dirtbike]

Bill, you probably have, it's just this thing with memory and age...

My bike is a 250 1980 from the beginning, so I have the 250 pipe on it but it has way more volume than the old mint 400 pipe. A friend said that it will be much more tame than with the slim older pipe. I figured that with my pipe, the weights and an outer rotor it would be fairly controlable. But it hits very much harder than my other bikes. I'll ride it when spring comes and we'll see. If it's ok, i won't do anything, but if it's unridable, i'll fix it.  

[SouthRider]

You really should try to find a large motoplat - you don't have to find a trials model (they are probably rarer than hens teeth), but the woods versions of these bikes came with a large motoplat that had the same crank taper as the small one (should be much easier to find). It made a huge difference.

Because of the small taper shaft and large flywheel mass - If you do get one make sure that after setting the timing you lap it in to the shaft, and then use a drop of red loctite on the crank (not the flywheel nut). What ever you do use retarded ignition timing, as if you set it too hot it will become very easy to stall in the woods and shear the mag key. Hot timing also makes them a bear to start.

I've seen them shear keys & break ankles while just kick starting the bike on fools who tried to bump the timing too much.

The secret to setting one of these bad boys up for the woods is  do ALL OF THE ABOVE recommendations on this thread.

Lower deck height.
Retarded ignition timing.
Large flywheel.
Jet it just a tad rich.

And yes, a Lectron will make a world of difference. Much smoother powerband.

And even if you do all of the above - Brian you are just too small for a bike like this (as am I). These bikes were best ridden by big men (think Frank Gallo).


Clark

[dirtbike]

Large motoplat?
Exactly what motoplat is that? I have a slightly larger stator with a riveted steel rotor for maicos. Do these exists for KTM as well?
Or do you mean the regular outer rotor (GS) ignition that I already have?

[Kip Kern]

Mine has a stock 38mm Bing, raised deck height, and the trials motoplat set at 2.4mmBTDC and works fantastic, always starts on the first kick warm seated on the bike (left leg) and 2 kicks cold in impound or at Ohio ISDTRR 1 kick on day one starting line! Pulls like a tractor, climbs anything;)

[dirtbike]

Have only started it twice but it seem to come to life on the very first stroke, so that is likely no problem.
I have the timing 1,8 mm BTDC.

Thanks for your input, I will see how it feels when riding, we have deep snow now. I will try the Mikuni anyway and, if needed I'll change the deck height as well.

The Lectron is not common over here, never seen one so I would preffer to try something that I can discuss with shops and friends out by the tracks.

[454MRW]

There are at least 2 versions of the heavier 400 flywheel that I am aware of. Here are a couple of pics that I grabbed from current auctions on eBay for the heavier KTM Flywheels. Mike



Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
75 Can Am 175 TNT & 77 250 Black Widow
1976-78 RM & 77-79 PE Suzuki's
74 CR250M 07 CR125R & CR150R