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Technical Engine bucking on deceleration in gear

Discussion in 'The Hokey Ass Message Board' started by seb fontana, Nov 11, 2021.

  1. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,443

    seb fontana
    Member
    from ct

    SBF, lot of cam [242°], 10:1, Holley 1850 [vac sec], timing 18°initail-36°total in by 2200, rear gear 4.11. When decelerating in gear everything is fine until engine rpm goes below 2000. Bucks the most in fifth gear, the least in first. Does it more now than used to in the past as best I can remember. Any clue?
     
  2. sdluck
    Joined: Sep 19, 2006
    Posts: 2,883

    sdluck
    Member

    Is this a 302?242 @ 050?
     
  3. oldiron 440
    Joined: Dec 12, 2018
    Posts: 2,828

    oldiron 440
    Member

    Manual or automatic transmission?

    One thing you need to keep in mind is that cam will not make decent torque until
    2800 rpm....
     
  4. gene-koning
    Joined: Oct 28, 2016
    Posts: 2,838

    gene-koning
    Member

    5th gear under 2,000 rpm under decel? Maybe you should be down shifting a little sooner, or shove the clutch pedal in a bit sooner.
     

  5. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 4,000

    2OLD2FAST
    Member
    from illinois

    Interested to see what the consensus is on this .. Some I'm sure will say its timing ,others , too much cam Still others will say its too lean while others will say its too rich .....:confused:
     
    jaw22w, Blues4U and Flathead Dave like this.
  6. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,443

    seb fontana
    Member
    from ct

    289 +.030, 242° @ .050
     
    Last edited: Nov 11, 2021
  7. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,443

    seb fontana
    Member
    from ct

    Yes you are right, I had 5.14 gear in it. Now with 6sp it still 5.14 in 1 thru 4.
     
    gimpyshotrods likes this.
  8. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,927

    gimpyshotrods
    ALLIANCE MEMBER

    T56?

    I run that gear with my 6-speed.
     
  9. twenty8
    Joined: Apr 8, 2021
    Posts: 950

    twenty8
    Member

    My guess........with the cam you've got it's getting 'lopey' at lower revs, with enough torque so you can really feel it. I had an engine once that would actually chirp the tires idling along in smooth concrete carparks. Sounded awesome in the underground ones....:D
     
    seb fontana likes this.
  10. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,481

    Elcohaulic

    Disconnect the vacuum advance and turn up the initial timing to 15-20.
     
  11. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,927

    gimpyshotrods
    ALLIANCE MEMBER

    I suspect that the engine is lugging due to poor fuel delivery.

    Venturi that are too large for the demand leads to poor fuel metering. They need to be small enough to induce good velocity. If they are too large, fuel will not be drawn out.

    The carburetor is too big for the engine, and the cam is not fit for the usage pattern.

    I run a similar setup with an engine that is all of 200 inches. I don't have this problem, even with a decent cam, because I don't have venturi.
     
  12. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 4,000

    2OLD2FAST
    Member
    from illinois

    The only thing I've been able to do to " help" this problem is to be sure the throttle plates are as close to completely closed as they can be & still idle . As mentioned , this helps to keep the mixture velocity as high as possible ..
     
  13. I go with lugging too. Is this a new problem?
     
  14. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,443

    seb fontana
    Member
    from ct

    Relatively new, yes. Wasn't as harsh in the past. Carb is only a 600cfm with vacuum secondarys. Trans is Richmond with .76 6th.
     
  15. AHotRod
    Joined: Jul 27, 2001
    Posts: 11,651

    AHotRod
    Member

    :):D:rolleyes: .... you must be trying to pull-our-leg on this one.


    shorten-up the camshaft by 20* @ .050 and that issue will disappear
     
    jaw22w likes this.
  16. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,443

    seb fontana
    Member
    from ct

    I know but hate to disturb a 46 year veteran with a maybe problem, youth is not on my side:(.
    Here is something I thought of: maybe the secondarys are opening/closing some with the heavy vacuum of deceleration? Maybe the linkage that is supposed to keep them closed when primarys are closed needs adjusting; maybe not but is something to check..
     
  17. AHotRod
    Joined: Jul 27, 2001
    Posts: 11,651

    AHotRod
    Member

    OK .... then push the clutch in and down shift, you know you can't "lug" a small CID engine with that size of camshaft in it.
     
    427 sleeper likes this.
  18. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,443

    seb fontana
    Member
    from ct

    Well I am not convinced you can lug an engine decelerating but enough guys have said so.
     
  19. HOTRODNORSKIE
    Joined: Nov 29, 2011
    Posts: 128

    HOTRODNORSKIE
    ALLIANCE MEMBER

    Running a Isky E3 in my 292 Yblock does the same thing. it has a 3.80 rear gear so I run it in second at about 3000 rpm the little motor loves it . Anything below that i just press in the clutch.
     
  20. anthony myrick
    Joined: Sep 4, 2009
    Posts: 10,061

    anthony myrick
    Member

    Well my c40 chevy does the same if I don’t drop a gear soon enough.
     
    gimpyshotrods likes this.
  21. MAD MIKE
    Joined: Aug 1, 2009
    Posts: 702

    MAD MIKE
    Member
    from 94577

    Too slow of rpm for a given cam the engine will run poorly. Doesn't matter if the rpms are climbing or dropping. Get out of that sweet spot it's gonna run shoddy.
    Want the engine to run smooth, put it in the right gear for the road speed.

    There is truth to the ole tales of Ferrari tuneups.
    Up shift too soon, not enough revs, don't downshift when slowing, engine is gonna bog up and get carbon fouled.
    Run it in the right rpm range, it'll run clean.

    An engine decelerating in the correct rpm range sounds just as sweet as when winding up in gear.
     
    bobss396 likes this.
  22. 55blacktie
    Joined: Aug 21, 2020
    Posts: 688

    55blacktie

    I can't help but wonder what work, if any, has been done to the op's heads, and how much compression. You just can't throw in a big cam and expect an engine to run well. If he has good heads and compression, he should consider stepping up to a larger carburetor. I don't think he's looking to break any mpg records. Having a manual transmission and gears, he might want a carb with mechanical secondaries as well.
     
    seb fontana likes this.

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