Register now to get rid of these ads!

Technical 340 cam tuning

Discussion in 'The Hokey Ass Message Board' started by mcmopar, Apr 28, 2020.

  1. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,549

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    I finally got a day to play with my truck, and here is what I did. I started at the timing, and checked tdc with a stop and that was right on. Next I went to timing while running. That is still at 12 and 35 all in at 2100. Idle is at 750 rpm. I went to the plugs, and they were looking pretty white, like on the lean side. On to the carb, first thing I noticed was I believe that I had the accelerator pumps set to tight. I changed jets from 71 to a 72 on the primary, and it was 78 to a 79 on the secondary. Im sure people will say I should have went up 2 sizes, but I didn't so there. If it needs more I will go up more. I believe I adjust the idle screws on each corner to get a little more, but it seems good now.

    While sitting still it now hits the rev limiter. I have only driven it once today and it did hit the rev limiter in first gear. When I shut it off it has shut off so far, so I believe I am getting closer. if not raining next Wednesday I will take it to the race track and see how it goes.

    Thanks for all the input.

    Tony
     
  2. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,549

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    I went to the Out of site drags, and was hopeful for a 11 sec run, well that didnt happen. I kept getting slower and slower no matter what i did. I believe it is the fuel pump, and i will be swapping that out today, and will try to run on Wednesday at Rockfalls. I did have alot of good people giving me ideas, and trying to help, and I thank them.

    Tony
     
  3. Detonation or spark knock can result from ignition timing problems. Too much advance, too much timing under heavy engine load, sticking mechanical and/or vacuum advance mechanisms, etc. Lower grade fuels and high engine temperatures will also magnify the problem.

    Dieseling or engine run-on is usually caused by other, less controllable, sources of ignition. High cylinder temps or carbon hot-spots in the combustion chamber can get hot enough to ignite the air/fuel mixture at the wrong time, with no ignition spark present. But there still needs to be a combustible air/fuel mix getting into the cylinders for this to occur.

    This is why it's critical that the throttle plates are not open too far at idle. Many carbs use a idle stop solenoid to allow the throttle to close more completely when the ignition is shut off. It's also helpful if you can adjust the idle without so much throttle opening that the engine begins pulling fuel thru the main nozzles. On a modified engine with a radical cam it's not unusual to look down the carb at idle and see fuel dripping out of the main nozzles onto the throttle plates. This gives the engine an unwanted air/fuel mix even after the ignition is shut off. With enough heat in the combustion chamber it will try to keep running, at least for a short period of time. Ideally, when you shut off the ignition you want no fuel and little to no air to enter the combustion chamber. The engine can't diesel without them.
     
  4. I'm sure you'll get it figured sooner then later. Oddly enough , shortly after talking with you in the staging lanes, Paul Zielsdorf told me he is having the same problem with his impalla. He was sure it was ignition related, and changed the whole system. It helped, nutrient cuts the problem.
     
  5. What kind of fuel pressure are you making? Will it make keep making fuel pressure on the big end in top gear? This is where a fuel pressure gauge is very handy. But, a big but, NHRA will not allow a mechanical fuel pressure gauge in the car without an isolator. When I was bracket racing, I had mine installed on the cowl.


    Sent from my iPhone using The H.A.M.B. mobile app
     
  6. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,549

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    I am not sure of pressure on top end but at idle it is at 7 to 8 psi. I can rev to 6800 and do a burn out to 6800, but when under load is where the problem is. I come out of the hole good until i shift then it bogs down and builds but not to 6800, maybe 6000. I had a edelbrock high flow pump on it but now i have a stock one. I have a pressure gauge but cant see it when running. I will be modifying it so i can see it on the big end.

    Tony
     
    Truckdoctor Andy likes this.
  7. stubbsrodandcustom
    Joined: Dec 28, 2010
    Posts: 1,221

    stubbsrodandcustom
    Member
    from Spring tx

    500 lift cam in a small block with big duration, and you expect vacuum? Be lucky anything close to 10 in your combo. Ditch the power brakes for sure, go manual brakes and problems solved. SHould be a hell of a runner.
     
  8. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,549

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    I got a chance to try it out, and still the same thing i lose power when i shift and cant get it back up in rpm range. Fuel pump was not the problem. I have to do some more looking, hope i find the problem soon this is getting old.

    Tony
     
  9. Joe H
    Joined: Feb 10, 2008
    Posts: 901

    Joe H
    Member

    Pump may not be the problem, but volume could be. Is the fuel line big enough? Any tight corners? Is the pick-up tube in the right place? Is the fuel system vented properly?
    Above all else, what is the fuel pressure doing through the whole rpm range?
     
    Truckdoctor Andy likes this.
  10. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,549

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    I have 3/8 line all the way with no tight bends. This is the same setup that i was able to pull 6500 rpms last year. I now have a fuel pressure gauge so i can see it. I will try to make a few pulls today.

    Tony
     
  11. Kan Kustom
    Joined: Jul 20, 2009
    Posts: 2,553

    Kan Kustom
    Member

    Just marking to watch discussion. Thanks everyone.
     
  12. mcmopar
    Joined: Nov 12, 2012
    Posts: 1,549

    mcmopar
    ALLIANCE MEMBER
    from Strum, wi

    Just took it out to stretch its wheels, and found that i have 3 1/2 psi of fuel at 5800, couldnt get past that. I then took the gas cap off just to check for venting, and no change. I know i should have more fuel pressure, but this is with a stock pump, and i do have a high flow. Running out of things to check, but that means im getting closer.

    Tony
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.