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Thread: Hesitation

  1. #1
    Join Date
    Oct 2009
    Posts
    29

    Default Hesitation

    Sorry if this has been a used up topic. I haven't been around these forums for years. Here is some back ground.
    I have had this scout II for over 10 years. 5 years ago I put a 392 in it. It had a square bore holley and a presto light dist. It had a hesitation then but had low compression and some other problems so I accepted it.
    So I had this engine rebuilt. Seems like they did a good job. Put in a COMP cam with a 260 duration. Small RV according to the shop. I put on a new Holley truck avenger carb, DUI distributor and an old set of headers on 2 1/2 in exhaust.
    Since I knew how the presto light worked I broke the engine in with it. The engine hesitated with the presto light. I removed the vacuum advance on the DUI and installed the block off plate.
    Ok. Now comes the part where I cannot except the hesitation. I have adjusted timing from 5 ATDC to 20 BTDC and hesitation is the same. I called Davis Unified and they sent me a new dyna module because I guess they go out. Changed it and still hesitation.
    The carb is a Holley truck avenger. I can't think of the size but I know the cfm was for an engine the size of a 392. I have adjusted the idle mix screws from completely closed to 3 turns out and still hesitation. I changed to accelerator squirter from a 28 to 35 and still hesitation. Adjusted the floats over and over.
    I checked the fuel pressure. Added then removed another regulator. Pump is a pusher mounted about a foot from the tank with a regulator on it. Pressure is ok.
    Thought it could be the black canister. Hooked it up. (Impressive fire ball)
    Thought it could be a leak in the brake booster. 180 bucks later, not it.
    I am about ready to load my shot gun with slugs and unleash on my beloved scout.
    The hesitation is just off the line. A little throttle. If I leave the throttle there the truck dies. If I feather the gas or romp on it, it lives.
    Thanks for lookin.

  2. #2
    Join Date
    Apr 2009
    Location
    San Luis Obispo, California
    Posts
    283

    Default

    I have seen that carb on a IH345 i was considering buying.. i think it is a 650? not sure (also there could be more than one model for all i know)
    -72 Scout2 - SBC350 with RV camshaft, 4 speed manual Trans, D20 xfer case with CV drive shafts.
    -Dana 44 Detroit locker rear.
    -Dana 44 (out of a 75) Rebuilt /w trutrac limited slip frnt



    ~Conquest is not in our principles. It is inconsistent with our government.

    ~T.J.

  3. #3
    Join Date
    Oct 2009
    Posts
    29

    Default

    I am pretty sure mine is the 670cfm. They also have a 470 and 770 i think. I really don't think its the carb. I am looking into a manifold leak or possibly installing an egr valve. As of right now there is no smog on the engine. Maybe it needs it??

  4. #4

    Default

    The egr is the first thing you remove and plate off to make the engine run better. It could be a vacuum leak, the intake manifold probably has a few threaded ports in it you don't need. an old way to check for leaks is with a can of brake cleaner. spray it in the area you think is sucking air and if it is the motor will rev for a second.

  5. #5
    Join Date
    Oct 2009
    Posts
    29

    Default

    Well no vacuum leaks. I did backfire out the carb this morning dang it. Backfire out the carb can only be two things. Timing and fuel starvation? Timing is fine. Accel pump is squirting with throttle and the float is fine. Fuel pressure was ok. I am lost.

  6. #6
    Join Date
    Feb 2009
    Location
    Aiken, SC
    Posts
    2,423

    Default

    What is vacuum at idle/rpms?
    What happens to vacuum when you rev from idle?
    What happens to timing when you rev from idle?
    Is there play in the linkage to the accelerator pump? Should be a tiny bit.
    What happens when you disconnect the vac advance and block it off? Where is the timing when you block off the vac advance, and does it affect the off idle accelleration?

    5 degrees ATDC could cause hesitation. I always shoot for 3 degrees BTDC plus a degree for every 1200 ft above sea level, 650 rpm. 20 degrees at idle is too much unless you have fuel injection controlling the timing. Some of the EFI controllers will run the timing up that high at idle, but that's because they "can."
    Allan E.
    Curmudgeon Extraordinaire
    Charter Member, Old Hippie IH Club
    Old fashioned binder freak

  7. #7
    Join Date
    Oct 2009
    Posts
    29

    Default

    Quote Originally Posted by Allan E. View Post
    What is vacuum at idle/rpms?
    What happens to vacuum when you rev from idle?
    What happens to timing when you rev from idle?
    Is there play in the linkage to the accelerator pump? Should be a tiny bit.
    What happens when you disconnect the vac advance and block it off? Where is the timing when you block off the vac advance, and does it affect the off idle accelleration?

    5 degrees ATDC could cause hesitation. I always shoot for 3 degrees BTDC plus a degree for every 1200 ft above sea level, 650 rpm. 20 degrees at idle is too much unless you have fuel injection controlling the timing. Some of the EFI controllers will run the timing up that high at idle, but that's because they "can."
    Vacuum at idle is 14psi in neutral. (i don't remember what it was in drive)
    Vacuum drops rev-ing from idle, goes a little over 14 coming back and then steady at idle.
    My timing is set at 8 degrees BTDC steady at idle and rises to a little over 20 rev-ing. Seems to track fine. I am at around 2000 to 2200 feet.
    My idle is 750 in neutral and about 600 in gear.
    There is no play in the accel pump linkage. I adjusted it to give it play then back to just take it out.
    I removed the vacuum advance and used a block off plate provided by Davis Unified. The DUI is too big with the vacuum advance installed and I don't have the water neck that goes to the drivers side.
    I appreciate the help.

  8. #8
    Join Date
    Feb 2009
    Location
    Aiken, SC
    Posts
    2,423

    Default

    I suggest you drop the neutral idle to about 650, reset timing probably not needed, should end up around 6 degrees. Also, you might want to try leaning out the idle mixture, see if you can get a higher vacuum reading at idle. I believe it should be closer to 16 at that altitude. You might be running a bit rich at idle.

    Accelerator pump clearance should be about 1/32". Zero clearance not good, can cause rich idle and inadequate fuel squirt off idle. Excess clearance not good, can cause late squirt off idle.

    I would try it with the vac advance module disconnected and the carb port blocked off. You also might want to hook the vac advance to manifold vacuum AFTER you set the timing, since the timing will change when vacuum is applied.

    You did set the timing with the vac advance disconnected, right? If you have a blown power valve, it will apply vacuum at idle, which means your timing is retarded for normal operation. The vac advance in a smogged vehicle is supposed to be disabled at idle, so if you set it at a normal setting with vac applied, it's wrong. A blown power valve will do that.
    Allan E.
    Curmudgeon Extraordinaire
    Charter Member, Old Hippie IH Club
    Old fashioned binder freak

  9. #9

    Default

    from what I gather from his earlier posts, he doesn't have a vac advance on the distributer. If this is true, don't you want to advance timing a little more to compensate?

  10. #10
    Join Date
    Feb 2009
    Location
    Aiken, SC
    Posts
    2,423

    Default

    Quote Originally Posted by scoutguy66 View Post
    from what I gather from his earlier posts, he doesn't have a vac advance on the distributer. If this is true, don't you want to advance timing a little more to compensate?
    Yes, about 7 more degrees would be where I would start, but I'd do it at 650 rpm, best lean idle.
    Allan E.
    Curmudgeon Extraordinaire
    Charter Member, Old Hippie IH Club
    Old fashioned binder freak

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