400 compression?

[brian kirby]

I've been working on a '77 MC5 400 aquired out of California. The bike was supposedly built by Bobby Lucas in '96 and had few hours since the rebuild. Kicking it through it felt really tight, it felt like it had a lot of compression, I have to jump up in the air and put all of my measly 145lbs on the lever to even get it to move. Reading here I knew the KTM engines of this era had high compression ratios. Well, after riding it around a bit I decided to put my compression gauge on it and it has 190 psi! I am a little concerned that even with full race fuel this is too high. Should I check the head for carbon, order some thicker base gaskets or with full race fuel will this be OK?

Brian

'73 Berkshire

[john durrill]

Brian,
 I would get it down to 160 or less. Use some solder through the spark plug hole to see what the piston to head gap is. Could have too thin a head gasket? Head could have been milled? Could be carbon . If the combustion chamber is clean and the head gasket the right thickness stack about .020 more in base gaskets.
With 160 psi it will still water your eyes when it comes on the pipe []chuckle chuckle and lot less chance of seizing on long straights.
my 2 cents ( worth only .5 mills really )
John D.

[gooddirt]

I had a 400 snap the kick starter , had to take the spark plug half way out to do a test start.

[brian kirby]

John,

160ish is where I thought it should be too. Just by looking at the gaskets from the outside both the head and base gaskets appear to be very thick but I really need to take the top end apart and see what I have. It could be carbon, but as I said this engine is only supposed to have around 10 hours since a total rebuild and I believe it, its very tight.

I noticed it has a spot for a compression release on the back of the cylinder, maybe I should look into getting one?

Brian

'73 Berkshire

[slvrbrdfxr]

Brian,
I agree with John that you need to lower the compression a bit. I'd say to pull the cylinder head and take a look at the "X" dimension and ensure it's within limits. Check the top of the cylinder and the required dimension should be stamped into the top fin. If it's not stamped then go with .7mm(.028-.030"). Vary the base gasket thickness to set it as near as possible to .030" which is the upper end of the limit. The reason for this is because once you ride the bike a few times and retorque the top end the dimension will most likely shrink due to compressing the base gasket more than the initial setting. The compression release designed for the rear of these cylinders are hard to find but can't hurt to make a post and see if someone has one to spare.
Dave McCullough

[brian kirby]

Thanks Dave. I'll do that, but it will have to wait till after this weekend. I'm riding the bike in a CC race Saturday and Sunday so I'll just have to get the best race fuel I can find and hope for the best.

Brian

'73 Berkshire

[tofriedel]

Brian,

Based on the information I have, there were 4 different base gaskets available for the 400 - .2, .3, .5, & .75mm thick.  Head gasket was
1mm thick.  As Dave suggested, check the "X" dimension.

Tony

[tomale]

most of the big bore maico's use a compression release so you might find a source that makes them through Northwest Maico or another maico source....It will make it alot easier to start and it is alot easier on kickstarters too....what you need to be careful about is making sure that the compression release does not stick too far into the cylinder, especially when the release is in use....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[Rain Man]

Brian, how does the bike start once its been warmed up ??  My 400 is a real bear to start when cold. And I mean its cold up here in Maine in the fall.
 It starts 1st kick the rest of the day though...

Raymond
 Down East Pentons

[pketchum]

"Baby Brian" Kirby weighs in a 145 lbs; have you been working out lately?  I look forward to seeing you again.  Are you bringing the rest of your 'kin with 'ya?

Bobby Lucas is currently building my '80 KTM 250 engine right now and it should be ready for Diamond Don's.  I'm picking up a '79 Husky from Dan McEntee that weekend too.  For my wife - it will certainly be a surprise!


Phil

[john durrill]

Brian,
I have always used my " weight " not my leg to spin the engine. Even the 125 Sachs engines I use that technique. Pull the clutch lever in and get the kicker at about 2 o'clock , then Come off the ground with a slight bend at the knee and use my weight to do the work. It works for me chuckle chuckle.
 The compression release would be  nice to have and extend the life of all the parts involved. I think it would be worth the search to find one.
 We talked  to Bobby several times when we did the 175 engine. He does not set one up with that much compression normally. Check the head to piston clearance with some solder and look the head over. It sounds like the previous owner may have had the head milled ? or asked for the extra compression when the engine was rebuilt.
 John D.

[Mike Lenz]

If you set up a 400 engine to specs it will have about 190 lbs of compression after break in every time.  They will run for years that way for MX...on even 93 octane. However proper jetting is crucial.  However if any road riding or desert just put another thicker base gasket under the cylinder.  It will lower the compression and let it rev out a bit more by moving up the port timing...and start easier! Thats my 2 cents.