392 V8 Dies under 1000RPM

Discussion in 'General IH Tech' started by AkioOhtori, May 15, 2012.

  1. AkioOhtori

    AkioOhtori Farmall Cub

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    First: I would like to apologize. Searching through the forums I have seen a number of these threads, but haven't really found my answer.

    My setup:
    1970 IH Scout 800A 4x4
    IH 392 V8
    Edelbrock 1406 carb (new)
    Pertronix Ignitor II (new) w/ Ignitor coil
    Also new: fuel system (pump, lines, filter, switcher, and tank), valve cover gaskets, thermostat, battery, spark plugs (Autolite 85s), etc... :hammer:

    I am trying to get my Scout road worthy. Sadly, when trying to adjust my idle screw on my carb I noticed that the car doesn't really like to run under 1000RPM warm, 1200 cold. Anything less and it begins to sputter and eventually die. When running the throttle seems responsive and the engine roars pretty well. Didn't seem to have any problems driving (though I've never taken it out of first...). It probably needs an oil change, but I can't imagine that being the problem.

    Searching through the forums, I have found the following possiiblites:

    Vacuum leak: Most of the vacuum hoses are new. People seem to recommend pulling off hoses but I can't really say which I should/ should not pull and where to test. Total newb. :( I did just replace the valve cover gaskets, and while I have been REALLY conservative with tightening those, I think they are tight enough (but not too tight). I used a rubber seal w/o any gasket epoxy or that sort of thing.

    Running to rich/ lean: This is actually my primary guess for the moment. The exhaust smells "gassy" to me, even when the engine is at temp. My understanding is the Edelbrock carbs don't need adjustment out of the box though. Furthermore I have literally no idea how to adjust this. (Always willing to learn!) The manual that came with it isn't super helpful, but it is possible I don't know what I am looking for.

    Not enough fuel: Brand new pump (Carter M6959). You can view the fuel flowing through the filter and it does seem not great. I don't really know what that is supposed to look like though. Is it possible the 1406 is just too thirsty for the pump?

    I could post a video later if that would help?
     
  2. john breeze

    john breeze Farmall Cub

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    what is the timing set at. that might help
     
  3. Doc Stewart

    Doc Stewart Content Team Staff Member Moderator

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    Weak fuel pump would give problems at speed not at idle.

    Really sounds like a vacuum leak. Valve cover gaskets would not be the problem. Get the engine warm and use Brake Clean or even WD40 to spray around the carb base and any vacuum connections and along vacuum hoses. Be slow and observe the engine's reaction.

    Edelbrock carbs definitely CAN use tuning right out of the box. See their website for details. Not difficult.
     
  4. scoutboy74

    scoutboy74 Y-Block King

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    Verify timing and correct routing of plug wires first. Timing adjustments with a timing light are done off #8 which is the rear most cylinder on the passenger bank (LHD). Timing will be difficult to adjust accurately at engine idle speeds above 800 revs. There's virtually no aftermarket carb in existence that doesn't require at least some minor tuning for operating elevation and optimum balance of performance and economy.
     
  5. AkioOhtori

    AkioOhtori Farmall Cub

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    I have verified the plug routing and it is all OK. I had a mechanic friend out to veirfy the timing, but we ended up not as it was running well and had good throttle response (and was smoking quite a bit thanks to some leaky valve cover gaskets). I can see about checking it anyway. What are the challenges of checking timing over 800rpm?

    The idea of spraying with carb cleaner is when you spray the vacuum leak the engine rpm changes, yes? I will certaily check that before getting all wild and crazy.

    With playing with the carb I am concerened more than anything about screwing something up haha. If I don't find a vacuum leak I will man up and see what I can do.
     
  6. scoutboy74

    scoutboy74 Y-Block King

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    Base timing for a 392 should be no more than 7 degrees of advance (BTDC). This is to be verified on a warm engine with curb idle around 600 rpms. The reason for the low speed is to ensure that mechanical advance is not in play which will effect your reading. Your seat of the pants feel is a factor, but it is far from being the only determining one for verifying that things are as they should be. The key to successful diagnosis and repair is eliminating variables in a methodical fashion, starting from the easiest and working out from there. You make one minor adjustment, followed by testing to see what effect that has on the issue. Then you make another. One thing you want to avoid is the shotgun approach. By that I mean changing fifty things at once hoping one piece of mud will stick to the wall. Maybe you get lucky and maybe you don't. If you do get lucky, which one out of elevendybillion was the cure? You have no idea. Plus it can get rather spendy in a hurry.
     
  7. IH64Scout

    IH64Scout Dreams of Cub Cadets

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    Before you do anything get the timing set. I run it usually around 5 degrees. I've had luck around 10 too but I wouldn't take it that high until all the problems are fixed. If its still high after id look for a vacuum leak. Now by not touching the carb out of the box, you did at least adjust the idle screw right?
     
  8. fredsterra

    fredsterra Y-Block King

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    Where do you have your dist. advance ported to? I have a 1406 on my 345 / 392 non EGR intake. And I found it better to hose to manifold vac. vs. carb.
     
  9. Jeff Jamison

    Jeff Jamison Dreams of Cub Cadets

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    There are 4 screws you need to adjust to start with on the carb,you have the fast idle adjustment for when its cold,the regular idle adjustment that is by where the throttle cable is hooked up,and the air mixture screws on the front base of the carb(I think these are you problems)These should be out about 1 1/2 turns to start with,start it up and see what you have,turn the idle adjust ment down some,keep going if it will let you.Now back to the front,turn one at a time until it smothes out,and rpm increases,keep going until it starts to slow down then turn back in to where it was the highest rpm,do the same for the other screw.Now you should be able to bring the idle down more.
    To check for a vacumme lleak,take the air cleaner off,and hold a rag in the carb to block off all air,it should kill the engine,if it does not,you have a vacumme leak
    Jeff
     
  10. IH64Scout

    IH64Scout Dreams of Cub Cadets

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    That'd be a pretty huge vacuum leak if it stays running with the carb blocked
     
  11. AkioOhtori

    AkioOhtori Farmall Cub

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    I don't have a timing light and couldn't borrow one for about a week. That said, If I was looking at buying one should I do adjustable or basic. ~$30 difference but I don't know what will actually be useful.

    Yes, I have played with both idle screws.
     
  12. AkioOhtori

    AkioOhtori Farmall Cub

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    I have the vacuum advance on the left but may try moving it to the right. No inspections in Oklahoma. The EGR is on the manifold. Thats the only vacuum hose that hasn't been replaced so I'll have to check it.
     
  13. fredsterra

    fredsterra Y-Block King

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  14. TBAKPhi22

    TBAKPhi22 Binder Driver

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    This sounds like an Impala I had years ago with a 350. I put an edelbrock manifold on I got from my friend that he took off his camaro. Could never get it to run well under 1000 rpm. Finally figured out the intake was warped. Tried putting it on twice with new gaskets each time, but still had a vacuum leak between the heads and the intake runners. Ended up putting the old cast iron 2bbl manifold on and it ran like a top.
    Like someone else said, take some wd-40 or carb cleaner and spray around the whole intake to see if it will speed up the engine. If so, definitely a vacuum leak.
    Also, I have a 600cfm Edelbrock on my '73 with a cam, and didnt even touch the carb except for the idle jets. They are pretty damn good right out of the box. Same thing with my camaro years ago... messed with the jets and rods, but came back to the ones it came with. Ran the best that way.
     
  15. Will Marsh

    Will Marsh Moderator Staff Member Moderator

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    The EGR could be your vacuum leak, in the valve if not in the hose. All the TVS's should either have ALL the hoses hooked up or NONE of them.

    In starting an engine with a new carb, I disconnect and plug EVERY vacuum connection with the exception of the vac choke pulloff if it has one. The Edelbrock doesn't. Set the timing and idle speed, then hook up the hoses one by one. It eliminates any unnecessary problems in the initial setup, and identifies any problems as I hook them back up.

    The vacuum advance on the distributor needs to be hooked to the right side port on the Edelbrock. If you hook it to the other side you're just masking a problem you should fix.

    Every 392 I've had pings at more than 5 degrees initial advance at 5000 ft of altitude on regular gas if everything else is right. Set the initial at 0 and make the rest of your adjustments. Then if you want to play with timing advance it a couple degrees at a time until it pings under load then back it off to the last setting.

    HTH,

    Will Marsh
     
  16. AkioOhtori

    AkioOhtori Farmall Cub

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    Thank you everyone for the responses! I went out last night and tried checking for vacuum and adjusting the timing. Here is what I came up with:

    Timing: It was set to ~25 degrees advance best I can tell. Yikes! Now that was at 1000RPM, so the vacuum advance was probably in play to some extent. I need to play with it more, but I can get it to run ~600 (but no lower) at about 10 degrees. Any lower and it dies. Thinking about it, I probably need to adjust the idle screw and it would run lower (duh!) but I didn't think of that at the time. (Again, total newb.)

    Vacuum leak: There may have been a small vacuum leak on the carb. Spraying carb cleaner around the gasket the RPM did surge a bit when I hit the front. I tightened down the top bolts and that seemed to help. I'll need to double check this.

    I took it out for a drive after all of this and it actually ran pretty well (aside from the steering is still all messed up). That said, I made a "small" mistake in my wire routing and sort of let some magic smoke out. :oops:

    After I get that junk sorted I'll double check my vacuum leak and see if I can get my timing to 5 or 7 degrees.
     
  17. scoutboy74

    scoutboy74 Y-Block King

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    Timing was off by a country mile! That'll do it. Good thing you didn't attempt a severe load with it set there. That's a great way to blow a hole in the top of a piston. If you can get it to hit on all eight and keep on hitting fairly smoothly at 600 revs, you're pretty dang close. Be careful about tightening your carb bolts. If they are noticeably loose, that's one thing. If they're already snug, they can be over-snugged and you don't want to go there. The thing about spraying vapors near your carb, is its easy for them to be sucked through the top of the carb making you think you found a vacuum leak. You need to be precise with the direction of your spray and be sure the top of the carb is shielded so this can't happen.
     
  18. AkioOhtori

    AkioOhtori Farmall Cub

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    Goooooooood point. I'll try that once I fix my screw up...
     

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