Mint 400

[Richard Terry]

I have a '75 400 that is frame #54 503 83007 and engine #5 5502242. From what I have read on here, the engines starting at 2808 had weights to tame down power delivery. I have a spare engine that is #6 5503136.

My question, is the weights heavier counterweights or added weight to the end of the crank? Would I be better off just exchanging the complete bottom end or changing the crankshaft?

Either way, I will be sending the head off to have the Carl Cranke mod performed on it.

[gooddirt]

I have a 400 heavy moto plat fly wheel for sale . From early 74 to 77 motors I have seen them run in every combo . The early 74 came with a light weight crank.... all depends on how  you like your power ?

[brian kirby]

The crank itself is heavier on the later engines. Even my '77 with the heavier crank is a brutally hard hitting engine.

Brian

[chicagojerry]

the weights were bolted directly on to the outboard side of both crank halves. that necessitated the cases to be different too. the heavier igntion flywheel will help some but that head mod helps smooth it out a lot and helps with starting. imo money (and not much) well spent.  chi jer

[brian kirby]

Who does the head mod now, JP Morgen?

Brian

[John Collard]

I've searched the posts and there's a reference to increasing the chamber volume to 45cc. Has anyone got details of the head mod. Would a suitable thickness decompression plate work.

John Collard

[Daniel P. McEntee]

I would be interested in knowing what the head modification is also. When I got my '77 MC5-400, I was warned about being carefull starting it and such. Once I got it running, I find it pretty easy to start, has never kicked back (yet!) and just running it up and down the street, I find it kinda "Husky" like in power band. This bike does have the Lectron carb, though.
  Thanks a lot,
  Dan McEntee

[brian kirby]

I'm sure its reducing the combustion chamber volume and reducing compression. I put extra base gaskets in mine, not the ideal way to lower compression, but I got it from 185+ psi down to 140psi. It started way better, had a softer hit, and still ripped on top like a top fuel dragster. I have a 38mm Lectron I need to try, I know they help.

Brian

[SouthRider]

The 45cc increase is probably not about lowering compression.

In the 70's some bikes were bumped up in class at the ISDT by making engine mods. This simple mod put a significantly lighter bike in a larger cc class. I don't remember the exact details - but I bet that is what the earlier post is about.

Don't forget to back off on the ignition timing a tiny bit to make them easier to start.

_____________________________________________________________________________________

"We the willing, led by the unknowing, are doing the impossible, for the ungrateful. We have done so much, for so long, with so little, that we are now qualified to do almost anything, with nothing."

1972 Penton Berkshire 100
1983 Husqvarna 250 XC
2011 Jayco 31.5 RLDS
2009 Chevy 2500 HD Duramax

[brian kirby]

They are talking about increasing the combustion chamber size to 45cc, not increasing displacement by 45cc.

Brian

[marsattacks]

I know nuthink but I'm about 102% sure that there are factory designed low compression heads available for the KTM 400cc engines of the 76-78 vintage.  That might help but I've only seen one for sale in about 6 years.

Mars Attacks!
72 Jackpiner
72 Jackpiner-in-a-box
74 Harescrambler
68 Planetary/Lunar Lander (very sadly, sold with Gamma death-ray to the Italians)
78 400 MC5 also sadly sold
79 space rock shox bike

[Richard Terry]

I have a complete, unassembled '76 engine. I will cc the head and compare it to my spare '75 engine when I get back to FL.

[Mike Lenz]

Penton 400cc Service Info- Augest 15, 1975;

we have been experimenting with the 400 and have come up with some better running, better mileage, better torque and smoother running machine for the woods and trail rider.  What we are doing is machining the head for better compression.

If you have facilities to have the head machined, it should be cut to 45cc.

With the remachined head the carburator should be set up as follows:
#283 Needle jet (needle all the way down)
#180 Main Jet (at sea level)

Sounds to me they are reducing the dome area?
The biggest thing you can do for the 74 and 75 bike is to put a 76 or 77 MC-5 pipe on it...or if it is a 76 or 77, put the 78 pipe on.  That one works even better.  If its still got too much hit use the thicker head gasket (called 1.5mm but measures new at about 2mm)or raise the cylinder as noted above. Yes, and backing the timing off a bit.  But Im interested in what this head mod was exactly.

[John Collard]

What's meant by "better compression". If you're machining the head I presume you're increasing the chamber volume which lowers the CR which makes sense. My only question is how is the head machined?

John Collard

[Jerry Birky]

Hi John, Here are some photos of how we do this modification here.