Strange carb problem

[SouthRider]

Brian,

The stock "large motoplat" or enduro flywheel was a gargantuan thing, but it did help to smooth the 400 out in the woods. It was very large on a pretty small diameter crankshaft stub. It needed to be lapped in and loctited to the shaft every time it was removed (also check the torque after EVERY ride).

If you did not do this the bike would sometimes shear a mag key while simply trying to start the motor (if it popped back). It certainly would also shear a key in the woods under the right conditions. If your bike has the same size flywheel as your Berkie then it is a motocross model, not the "large" one.

The Bing carb made all of this worse. Installing a Lectron made the 400 much easier to start, and gave it a significant performance boost. Consider a 38mm for woods work, or a 40 if you want a Holy terror holeshot bike (please double check - are flat slide carbs AHRMA legal?). John is 100% correct about the carb screen - don't use any Bing without one. When they added that it corrected a myriad of problems.

Also consider the above suggestions of backing down the timing a bit & lowering the compression. The Penton 400 was a brute of a bike that typically was only ridden by large men that needed one or crazy folks like Carl Cranke. JP Morgen might be a good source for advice on tuning your 400.

No one in our shop EVER wanted to start much less ride one. I know of one bike that broke the ankles of 3 fools that dared to consider riding her.

With all that said you can make one ridable by doing ALL of the above. If you do that it will become an easy to start & ride high torque tractor that will never quit.

But I still can't figure out WHY you'd want to ride it......

Good Luck,


Clark

[SouthRider]

PS - Regarding the photo above - Why is the head off of a motorcycle engine in a chassis that hasn't been properly cleaned?

[firstturn]

Clark,
  Ernie didn't have the Late Honorable Mike Burgess as his teacher.  You might share with everyone your first experience at cleaning a bike when you worked for Mike.

Ron Carbaugh

[Big Mac]

I have a 77 MC5 400, stock except for an aftermarket GEM reed addition and minor jetting adjustments. Timing set per the book on the stock Motoplat, x-dimension per the book, good tight stock 38 Bing with floats set right.

Not sure what the fuss is about. Takes a strong boot, helps t be long legged, but always lights up after a good tickle, never kicks back, never gives fits or fouls plugs. It rides great in the woods and is not hard to hang on to, doesn't wear me out. When warm, it starts easy and I can kick it left footed without stepping off. Have had the flywheel off, just torqued it back down good and forgot about it. Probably more motor than someone less than 180# needs, but not the scary, fire-breathing arm-stretcher that guys are making it out to be. Set it up right, tune/jet right with a decent Bing, and have fun.

On the other hand, the '81 495 takes some getting used too.

[SouthRider]

Hi Ron,

Most have heard it before - they actually printed it in the last newsletter as part of my biography tribute to Mike. I don't want to bore people.

I was just poking some fun at Ernie since he was giving Brian such a hard time. Cleanliness IS next to Godliness isn't it?

Big Mac - in your post you mention that it helps to be long legged and 180+ pounds. Brian is neither of those (nor am I). We are just giving him suggestions on how to set the bike up for HIM to ride it.

The big flywheel helps a lot - but it does shake loose. An MC5 was the motocross model with the small flywheel. It did not come loose near as much. A reed valve setup would also make a big difference, and keep excess fuel from sloshing into the cylinder. They are probably harder to come by and install than the other mods mentioned. I was just mentioning the things we did back in the day that I personally know will make that bike easier to start & ride. We set up several that way in the mid to late 70's for SERA woods riders who loved the setup.

Thanx,


Clark



1979 SERA Louisiana State Champ

[brian kirby]

Clark,

Its definitely the small flywheel, its the same size as the one I took off the Berkshire. I will put a screen on the main jet first as this seems like the most likely suspect. I might also see if I can spin up some kind of press on weight for the flywheel. Some modern flywheel weights press on, so as long as it clears the mag cover I dont know why it would not work on this bike. As it is the bike is fine for MX, but it is not smooth, it is one of the hardest hitting bikes I have ridden.

Brian

'72 Berkshire