Compression

[RoadGlideRob]

Fired it for the first time today (one year in the making) and its running kind of rough.  Dont have at timing light yet but I did check the compression and I"m readin 75 lbs on both cylinders.  Is that right?  seems awful low.

Thanks

OH yeah,  Nothing sweeter than working on something for a year and finally hearing it go..  wish it was smoother though.

[96flhpi]

100lbs and up with no more than 10% diiference b/w cyls according to my 78 & 1/2 to 84 manual.  Try putting a capfull of oil in cyls through spark plug hole - if comp goes up ya got ring issues, if not ya got valve issues. 

Of course, if your scoot is older than '78 double check.  I may be wrong cause I'm only going by my manual - 75lbs may be acceptable for older bikes.  Someone w/ more knowledge will chime in sooner or later.

Good luck.

[RoadGlideRob]

Hmmm.  RTFM, what a great idea 

Just checked the clymer manual i have and it says 90 PSI or better.  So I'm low.  Just had the heads done and while it was running we saw no smoke. 

[96flhpi]

'Taint my idea - like I said just going by what the manual says.

BTW, can't figure out what RTFM means.

[ramcr913]

Set your timing before you do anything. Then you might want to check the compression on another engine, to verify the compression tester is reading OK. I had one that read low. According to it, my bike and truck and another bike all needed valve jobs.
Good luck.

[FSG]

....   can't figure out what RTFM means.

   is pretty close 

[RoadGlideRob]

RTFM=Read The Fu#%ng Manual

[96flhpi]

Ah, I see.  Yeah, you should RTFM

[massheadcase1]

Thats with the throttle plates open right?

[RoadGlideRob]

Yep,  wide open throttle

[gryphon]

Low compression, runs rough. Are the pushrods are adjusted correctly. If you're running hydraulics, did you allow them to bleed down. If adding a little oil doesn't raise the comp then I'd double check the pushrods. Heck, I'd double check 'em anyway.

[monty101]

You know if it hasn't got enough compression it may not fire sometimes. I had that problem once and when I checked the compression by cranking the motor it had 100+ lbs but when it started it up it wouldn't hit on that cylinder. I finally pulled the head and the seat was mushroomed. It would seal off when the starter turned it over and show compression but once it started and the valve got to moving at high speed it wouldn't stay sealed long enough to fire on that cylinder. I put some oil in it and check the rings.

It will need rings if the oil seals it up . When you put the rings in I'd go ahead and and check the valves and seats out too. Hasting rings are inexpensive and pulling the jugs will also give you a chance to check out the rod bearings too. Its a sunday afternoon job. shouldn't cost over 30 bucks or so i figure.