Low compression pistons

Discussion in 'Engine, Fuel and Exhaust' started by mrwizz, Dec 4, 2015.

  1. mrwizz

    mrwizz Member Contributor

    I just recently bought a 1973 FX. I was told by the seller that his Father whom he got the bike from, had installed low compression pistons for easier kick start. My question is..what kind of compression should I be getting if this is true. I am getting 73psi both front and rear cold and about the same wet.
    This bike fires right up first or second kick after a couple primes. Doesn't seem to smoke cold, idles nice and do not notice any odd eng noise. Haven't ridden it yet, need to do some brake work and check out the sticky clutch plates and all the other stuff that needs to be done to a bike that's been sitting.

    Attached Files:

  2. jamesearl

    jamesearl Junior Member Contributor

    what a cool bike
  3. Breeze3at

    Breeze3at Well-Known Member

    My Clymer manual shows that std. pistons for that era to be 8:1 for FLH models, and 7.25:1 for FL models. Engine compression should be 90 psi or better, with less than 10 psi variance between. Your figures are low, but it's old and if it's been sitting for a while, it may get better with some miles.
    Nice bike, enjoy it like it is.
  4. tourbox

    tourbox Senior Member

    After I rebuilt my '73 with 8:5/1 pistons I had 135F and 130R. I agree with Breeze3at about being low.
  5. dolt

    dolt Senior Member Contributor

    90psi might be an acceptable service limit but I would expect a bit more from a "tight" 74" motor; like 100-120. The fact that there is no difference between the two cylinders is healthy sign. Mileage and wear should also be taken into account.

    There are compression calculators that you can use to compare your actual with calculated which will also help confirm the condition of the top end. Those calculators allow to correct for elevation; the closer to sea level, the higher the CCP. You also need to hold the throttle wide open when testing to get an accurate reading; did you do that? If the carburetor is a CV, you will have to remove the carb from the rubber boot since the slide is activated by vacuum. If the carb is an S&S or Mikuni, the slide is activated via the cable and holding the throttle open and the grip will work. If you didn't do this, your 73psi reading is wrong and low.:s
  6. Jack Klarich

    Jack Klarich Expert Member

    A shovel head motor will run quite well with that low of compression, how ever I would do a top end job on it just to freshen it up, these older FL models some came from the factory with no valve seals, I would fix that. You can also change the compression ratio with different thickness base and head gaskets
  7. mrwizz

    mrwizz Member Contributor

    Thanks all for the reply's, I do realize that the compression is pretty low... just didn't know what to expect if it does have low compression pistons. It does have a S&S carb and i did test with the thottle open so it should be fairly accurate.
    I think at this point I'm going to do all the maintenance that needs to be done and put a few miles on it (which may not be till spring here in Mich). I'm sure I will also find other things that will need attention.
    For now I'm trying not to throw too much money at it. My 20yro son has his eye on it for his first bike and I would like to keep it within his price range.