1979 yz400 power?

my father in laws 1979 YZ 400 doesn't seem to ever come on to the pipe, it seems to have a very lacking power band with no top and hit at all. I am wondering if this is normal or if there could be something wrong with it? any help would be greatly appreciated!

Does it start easy, have you done a compression test?

Starts easy, haven't done a comp check, but compression feels good.

I had a '77 YZ400 and that was NOT lacking in power.  The hit was probably not as violent as even current 250 but I always had trouble keeping the front end down at the most inconvenient times.

Ya it doesn't feel right, it feels like there is no top end, runs only to where it should come onto the pipe but then signs off, and runs flat. Any ideas?

As was mentioned, check the compression.  You can always take the pipe off and peek into the cylinder and see if there are any obvious problems.  If something looks abnormal, take the head off for a good look.  If the bike sat for a long time in an unheated area, there could have been surface rust in the cylinder.  Even after breaking the piston loose, the rings and cylinder surface just don't seal good enough.

the motor was completely torn apart and reassemble to have new crank seals put in an new crank bearings with the new piston and rings everything looked top notch

at least according to the shop that did the work

Is the muffler plugged?

I don't think so but that's how it acts

I guess you can check the compression but with all new parts, it's probably OK.  I'd be suspicious of either intake or exhaust too.  First, run it without the silencer.  (I hope you live out in the country!)  Without the silencer, it should be REAL loud.  If it's not, the pipe may be plugged up.  An exhaust note on a plugged expansion chamber loses it's high frequency component of the sound.  Make sure the intake is clear.   Forget a rag in there?   Me?  No...Never done that.  I don't remember how to check timing on that motor but that may be suspect.

mine was never stronger than a good running modern 250,  the power band is not very broad  but was relatively smooth. never wanted to rip your arms out but would loose traction and smoke the rear tire easily.

Is one of the reed petals cracked or broken?

With above check reeds and clean pipe

i had a big bore yz490 that really was a big boar. It never screamed at all, just gobs of torque.

Also check midrange(needle) and hi-speed(main jet) jetting-too lean (or clogged up)makes it gutless and too rich makes it boggy.Find out what stock settings were and see what is in the carb now. Also correct float level is essential for the Yamaha to rip!Check that too!

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