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Technical Compression check

Discussion in 'Traditional Hot Rods' started by Darin Younce, Jul 14, 2019.

  1. Darin Younce
    Joined: May 8, 2019
    Posts: 589

    Darin Younce

    I finally did a compression check on my 29a flathead. Lowest was slightly above 90 highest was about 105 . According to a few searches the compression is supposed to be 120. All sort of factors could contribute to the lower compression I suppose. The engine does not smoke so rings might be acceptable so that leaves other things such as valves not seating, cheap compression gauge , gauge hose fitting not tight enough ( of coarse that's wishful thinking) . Anywho I will add this . When I first got the truck it had a loud lifter I assume so I took plugs out and all were extremely sooty and they were brand new installed in previous month or so and ran very little. I adjusted carb and found it to be way out of adjustment .both screws were opened only 1/4 or so turn. I then put a small amount of Marvel mystery oil in each cylinder and the loud lifter stopped in a mile of driving. I then assume it was carbon buildup . So I am going to try one more larger dose of MMO and recheck compression . Otherwise I will probably do some internal work on engine later in fall. Oh I have dualed the exhaust this week and took it down the road. I ran a bit harder than previous and noticed it begins to run rough at higher speeds so I think the advance needs adjustment also checked points and they were a loose 16 thousands , I think they are suppose to be 14-16 thousands .I am going to adjust all this morn and see what goes on.
     
  2. Lost in the Fifties
    Joined: Feb 25, 2010
    Posts: 459

    Lost in the Fifties
    Member

    Was the engine at operating temperature when you took compression reading? 29A heads are 6.4:1 CR, 78-80cc chamber volume, so lower compression than 59A. 59A heads are 6.75:1 CR.
     
  3. Darin Younce
    Joined: May 8, 2019
    Posts: 589

    Darin Younce

    Well, started out warm , right after driving, but I had loaned my compression gauge to someone and they have yet to return it. I went to O'reillys and borrow theirs but after trying it ,I realized it did not have the check valve at the end of the hose, guess the last person lost it or it in in their cylinder getting hammered HA! anyway by the time I got back with one that worked I did not reinstall the plugs and recrank and warm it up so it ended up being way cooler but slightly warm . I am planning on retesting today at temp. below is a chart I copied off of Vanpelt's web site , it list the year,engine # , ci,hp,compression ratio and spark plug info . Even though the compression ratio is 6.4 as you stated, the compression at cranking speed is 120 .

    1932 18 221.0 85 5.50 (C.I. heads) 95 lbs Champion (7/8") 0.025"
    1933 40 221.0 85 6.30 (alum heads) 105 lbs Champion C-7 (18mm) 0.025"
    1934 40 221.0 85 6.30 (alum heads) 105 lbs Champion 7 (18mm) 0.025"
    1935 48 221.0 85 6.30 (alum heads) 105lbs Champion 7 (18mm) 0.025"
    1936 68 221.0 85 6.30 (alum heads) 105 lbs Champion 7 (18mm) 0.025"
    1937 77 221.0 85 6.12 (alum heads)
    7.50 (C.I. heads) 100 lbs Champion 7 (18mm) 0.025"
    1938 81A 221.0 85 6.12 all heads 100 lbs Champion H10 (14mm) 0.025"
    1939 91A 221.0 85 6.20 (C.I. heads) 100 lbs Champion H10 (14mm) 0.025"
    1939 99A 239.4 95 6.30 100 lbs Champion H10 (14mm) 0.025"
    1940 01A 221.0 85 6.20 100 lbs Champion H10 (14mm) 0.025"
    1940 09A 239.4 95 6.30 100 lbs Champion H10 (14mm) 0.025"
    1941 11A 221.0 90 6.20 100 lbs Champion H10 (14mm) 0.025"
    1941 19A 239.4 100 6.30 100 lbs Champion H10 (14mm) 0.025"
    1942 21A 221.0 90 6.20 100 lbs Champion H10 (14mm) 0.025"
    1942 29A 239.4 100 6.40 120 lbs Champion H10 (14mm) 0.025"
    1946 69A 239.4 100 6.75 120 lbs Champion H10 (14mm) 0.025"
    1946 69M 239.4 100 6.75 120 lbs Champion H10 (14mm) 0.025"
    1947 79A 239.4 100 6.75 120 lbs Champion H10 (14mm) 0.025"
    1947 79M 239.4 100 6.75 120 lbs Champion H10 (14mm) 0.025"
    1948 89A 239.4 100 6.75 120 lbs Champion H10 (14mm) 0.025"
    1948 89M 239.4 100 6.75 120 lbs Champion H10 (14mm) 0.025"
     
  4. Give it a Hillbilly tune Up. With it running Pour a quart of AFT or Risoline down the carb. get it warn and set it at a high idle and dribble the fluid down the carb. you will make a great cloud of smoke. what it does is lube the valve stems and guides.
     

  5. Darin Younce
    Joined: May 8, 2019
    Posts: 589

    Darin Younce

    Ok gotta admit I'm old and not as sharp some weeks as I once was but I was messing with dist and got to thinking, this dist is not what I had thought was on a 42 and up engine. First I thought perhaps some one had changed it with is pretty stupid so I started thinking what if my engine was not a 29a ? Well I did what I should have done before and looked at the rear of engine and right there it is 81A. DUH. I guess someone put different heads on it so my compression is better comparatively that I had thought but I just lost my fairytale extra 15 hp.
     
  6. Darin Younce
    Joined: May 8, 2019
    Posts: 589

    Darin Younce

    WELL I am a hillbilly from Watauga County NC originally so dadgumit I will do that.
     
  7. Darin Younce
    Joined: May 8, 2019
    Posts: 589

    Darin Younce

    Follow up . Took dist off reset points , ran down road got up to temp around 170ish , compression was same as cold . Since I have found our I have a 81a which the compression is supppse to be 100 I feel better being all were 90 to 105 . Byw ram better after setting point gap at 15 thousands.
     
    Old wolf likes this.

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