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Vacuum Questions

Discussion in '40- & 55-Series Tech' started by offcamber40, Nov 19, 2003.

  1. offcamber40

    offcamber40

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    After reading one of the recent timimg posts and the vacuum tech article, I bought up a vacuum gauge to try to help with my new weber install problems. Ive been working through most problem areas trying to eliminate errors.. drained the tank for old gas issues, new fuel filter.. etc. All componetnt are mounted to new gaskets on a fresh engine rebuild. I originally thought vacuum leaks were my problem, but got no fluctuation in running by spraying everything w/ carb cleaner. After hooking up the vac gauge, I am having a hard time figuring out what my problem is after comparing to the diagnostics in the vacuum tech table from this site. My vac reads as follows: I cant keep the motor running at a normal idle.. it slowly comes up and down and wants to die, I have to dial in the idle speed screw to keep it from dying.. Using my Dwell/Tach, the idle speed which it will stay running at is about 1300 rpm and the vac reading at that speed is a fairly steady 15.. If I blip the throttle, it goes to zero than out to 22, then back to 14/15. If I slowly let the engine die back to about 600-800 (near normal idle speed) while feathering the throttle to keep it running, I get a vac reading of about 10.. I dont know if this means I have a vacuum leak, or if vac is dying off as the motor sputters and pops at the low engine rpm? I figure it is probably a vac leak, as the hesitation and flat spots when depressing the throttle are not apparent when the engine speed is elivated with the electric choke activated. Any vacuum gurus out there have any advice? The chart in the tech article is nice, but doesnt tell me what the readings will do when you cant keep the engine at a normal idle/baseline. I guess I am trying to figure out if vacuum increases with rpm, or if it should remain fairly constant? Also, What type of gasket sealers do you guys use to seal threaded areas and gaskets in these areas (manifold, carb gaaskets, studs, etc).

    Thanks,

    Mike
     
  2. Rice

    Rice SILVER Star

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    Mike,

    What you're reading on the vacuum guage and reporting sounds correct. You will never get an accurate "at idle" vacuum reading if trying it by feathering the throttle. Opening the throttle opens the butterflies and vacuum in the engine is decreased. At the instant you let off the throttle there is plenty of piston speed but suddenly no air so vacuum spikes. Sadly, because every cam and timing combination leads to different vacuum levels there is no real way to know the exact vacuum you should be pulling and finding a vacuum leak this way (unless you have a huge hole in the intake) will be almost impossible.

    I'm doubting you have a fuel starvation issue because it's causing trouble at low RPM, not high ... so your filter, pump, etc. should be doing fine. Have you pulled the plugs to have a look at them? Just for fun try a couple tricks ... first, put in a fresh set of cheap plugs just in case the others got fouled. Then, pull off the vaccuum hose to your brake booster while the engine is running and let us know what happens. If producing the vaccuum leak makes the engine run better then you're way rich and are fouling plugs. If the engine dies off we go to plan 'B'.
     
  3. Degnol

    Degnol

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    I think you should be pulling about 15 inches at normal idle. Choking it masks the probable vacuum leak. Are you running a stock intake and exhaust? There was just a post about a vacuum leak, every thing was checked and replaced, and only when the intake exhaust was removed and separated did the guy see a hole, albeit small, burned in the heatriser on the underside if the intake.
    Do you have a fuel pressure regulator? And is it a 32/36 or a 38/38?
    Just guessing, but it sure sounds like a vacuum leak.
    GL :)
    Ed Long
     
  4. IDave

    IDave

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    I am certainly no "vacuum guru" but to answer your next to last question from my experience, yes, increased rpm increases vacuum. Also, although it is difficult for me to know, I wonder if you fall into the "late valve (vs. ignition) timing" catagory. Could your rebuilt engine have gotten the wrong cam or were the pushrods put in in the wrong order??
     
  5. offcamber40

    offcamber40

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    Im currently running a MAF header w/ the stock intake manifold. I have installed a weber 38/38 with a pressure regulator set to 3 psi. The cam is an RV grind from SOR. It is possible they sent me the wrong one, I though F and 2F cams were the same though? IDave, Im pretty sure the pushrods got piut back properly as I marked them. I have installed new lifers though.. is there a differnece in pushrods lengths, or are they the same? The previous owner pulled the stock exaust off, is there a section where there could be a leak from below tha manifiold? I thought units were completely seperate, just bolted together? Where would a cracked manifold show signs of failure? I did modify the Weber by adding large screws to utilize the stock heat riser below the carb? Would this pose a Vac problem? I think I am going to have to pull the whole thing apart and reseal it again.. what is the best sealer? This way I will eliminate the vac question and look for other issues. I hope its vacuum related, I really dont want to have to pull the cam, or re-check the timing gear locations (I KNOW I got that right).
     
  6. IDave

    IDave

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    The pushrods should be the same length, from what I know, but there is a wear pattern that develops that makes them "like" the same spot. I am stretching my area of knowledge here, but maybe you want new rods with a new cam. Seems to me I read something about that in the MAF catalog.

    Speaking of MAF: do you have the 6>1 headers? If you do, I can just about guarantee you have a leak. They have a lousy fit. If you don't seal around the manifold gasket you will get an intake/vacuum as well as an exhaust leak. I have had to double gasket with additional Permatex RTV copper high temp sealer (red stuff) to get mine to satisfactorally seal. Plus a few extra torques on the manifold bolts the first few months helped, too. Some people use half washers to fill in the uneven spots around the manifold studs, but I got away without doing that.

    :)
     
  7. dd113

    dd113

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    Really sounds like a Vac leak. One quick check: put you hand over the intake on the carb. Your hand should get sucked down and the motor should die very fast. if it does not die or sputters and dies slow you have a vac leak. Not 100% exact but good starting point.

    You should have a steady vac around 18-20. steady 15ish is OK but a bouncing at any point indicates vac leaks. Follw the link already provided. One thing, you mentioned a RV cam. I have no idea what the readings will be as the grind of the cam changes the vac readings; usually lower. MAF should know.

    You mentioned a Webber. The 2 plates are a very common leak point. Some of the alen head bolts that attatch the upper plate to the lower plate are just a "touch" too long causing the top plate to sit just a touch above the lower plate and the gasket will not make up the slack. If you decide to pull it apart check that. The carb cleaner trick is not all that effective. Try MAP gas or starting fluid. I use a gasket coating called aircraft adhesive or aircraft gasket maker. All it is is a shellack (SP?) liquid you brush on and it stays tacky for a long time. Indian head gasket shellack is the same thing. NAPA sells both.

    Unless you have a hole in it the intake will only leak at the head, carb or fittings on it. Disconnect the booster vac line and plug it just in case you have a small leak in the booster when doing the testing.

    this stuff is a PITA! :rolleyes:
     
  8. IDave

    IDave

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    Hey Dave, while we have your attention (not to hijack a post, but, hey), I have noticed that when I flick the accellerator I don't get the large spike down/spike up that is described. Just a 2 or 3 inch spike down and 1 or 2 up. Mean anything to you?
     
  9. dd113

    dd113

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    [quote author=IDave link=board=1;threadid=7767;start=msg65395#msg65395 date=1069292690]
    Hey Dave, while we have your attention (not to hijack a post, but, hey), I have noticed that when I flick the accellerator I don't get the large spike down/spike up that is described. Just a 2 or 3 inch spike down and 1 or 2 up. Mean anything to you?
    [/quote]

    What it means is that it is really fawked up and you should sell. :flipoff2: off the top of my head I would look at the carb primary or secondary and see it is opening correctly. Where are you drawing the vac from? I have not run into this before but if it runs OK I think it is a hardwear issue not a motor issue.

    I mentioned the RV cam and how it might lower vacume. I spoke with my machine shop about the RV cam and they said that, in general, a mild RV cam should not effect the vac much if at all. I would like to know if this is true.
     
  10. IDave

    IDave

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    What Vintage OffRoad price should I ask for my Fawked machine? I think it is worth $55000, at least :flipoff2:

    I'll look at the throttle valves this weekend, Dave. I am checking vacuum off the transfer case switch port on the manifold. Should that make any difference? Thanks.
     
  11. dd113

    dd113

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    Somthing does not sound right. If you rev it you should drop vac then gain back more vac than normal and eventually settle back to normal. If you have good compression, steady vac at ilde and it runs good then it almost has to be somthing else outside of the motor. I have no idea what. Hit PH with this.
     
  12. IDave

    IDave

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    You mean Charlie, right?