STRANGE - Blowing distributor cap off

Discussion in 'General IH Tech' started by rponder, Dec 6, 2007.


  1. rponder

    rponder Farmall Cub

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    OK, here's a really weird one. I've got a Prestolite distributor on my 345. Ever so often when I turn the key and start to crank it over I'll hear a big "pop". The pop is from the distributor cap being blown explosively off the top of the distributor. What the heck is causing that? Has anyone ever experienced this one? I've never seen anything like it. It's rare, I never know when it's going to happen. One time it even blew the rotor and the dust cap up off the distributor body.

    Thanks - Robert
     
  2. Dennis Bernth

    Dennis Bernth Moderator Staff Member Moderator

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    I've heard of it before, IIRC it was caused by a bad diaphragm in the vacuum advance allowing gas vapors into the distributor on shutdown. Then when you start it, the first spark ignites the gas vapors and blows the cap off. Try unhooking your vacuum advance, I would bet that cures it since that is the only way the fuel system might cross over into the distributor.
     
  3. George Womack

    George Womack High Wheeler

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    Dennis nailed it. I've heard of this problem, too. Check your vacuum control diaphragm--I'll bet it's bad.
     
  4. Architectdave

    Architectdave High Wheeler

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    It it wasnt bad I bet the detonation in the cap fubared it pretty nice.....:rockon:
     
  5. darrin

    darrin Farmall Cub

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    i saw that happen myself i was with binderbird and his did it a couple times until he changed vacum advance out
     
  6. jordandoc

    jordandoc Binder Driver

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    Had the same problem - it was a bad vacuum advance.

    Dan
     
  7. Nathan Kussmann

    Nathan Kussmann Farmall Cub

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    Seen that also. Dennis hit it on the head. Previously I've had that happen to me too, PO had wrong rotor hitting inside of cap, blowing off cap while breaking hold down tabs.


    Nathan
     
  8. lectrick

    lectrick Farmall Cub

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    how could the wrong rotor break the hold down tabs? the rotor is on the inside of the distributor cap.... if it had interference with the outside wall, you would have a real hard time mounting the cap....
     
  9. Nathan Kussmann

    Nathan Kussmann Farmall Cub

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    Since my head isn't big enough to be inside the cap, I'm not sure. To me it *looked* like upper contact of rotor was hitting on the inside of each cylinder contact of cap although outer tang of rotor contact was also a very tight tolerance to inner wall of cap. The impact on the prestolite cap was enough to break off a lower (in my opinion weak spring type hold down) as cap was partially ejected. In fact I've seen that twice in 14 years wrenching. I even took a pic, I'll have to dig it up.... 345 engine in my Scout was incomplete. I got it as a non-running project. Rotor was on distributor, correct cap I purchased I installed. After switching out incorrect rotor and back to parts store yet again for anaother cap, it ran great. I haven't had any problems since..


    Nathan
     
  10. lectrick

    lectrick Farmall Cub

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    hmm sounds like you just had to match the cap and rotor? I guess a dizzy ID tag would help a lot here, but if ya fixed it, great...

    I'm running a '79 345sv with a '73 points dizzy with a pertronix kit in it... guess my point is, just 'cuz ya got the engine somewhere doesn't mean the dizzy's original. ID the dizzy and check here or somewhere with IH knowledge for part #'s, ya can't just get 'em at the counter by telling 'em yer year/make/model. They mostly don't know what it is to start with (cuz most of 'em are younger than your truck) and it's likely you don't have the stock dizzy too if it's seen another owner and a few miles.

    Case in point, I was looking for a base gasket for my thermoquad... both the napa and the autozone sites showed me a 2 barrel baseplate for my '79 scout...

    my understanding is that '79s and '80s only came with the 4 barrel.
     
  11. scoutboy74

    scoutboy74 Lives in an IH Dealership

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    Amen to that. The 345 in my '74 SII should have a Holley points dizzy in it, but some p.o. stabbed a Delco dizzy in it's place. No biggy. It works fine. Even better with the Crane Cams electronic igntion module.:tt1: It was a bit of a head scratcher though when I first discovered there were no pictures of my dizzy in the 73/74 SII manual.:confused:
     
  12. Dennis Bernth

    Dennis Bernth Moderator Staff Member Moderator

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    The Delco is probably the best out of all the distributor options as far as holding up, parts availability, and easy to work on. Which Crane ignition do you have, the XR-1? I've got a Delco and the XR-1, but I haven't put the two together yet....
     
  13. scoutboy74

    scoutboy74 Lives in an IH Dealership

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    Yeah Dennis, I believe it is the XR-1. I like it. As a matter or fact, I got it on your recommendation several months back.:cool2:
     
  14. Dennis Bernth

    Dennis Bernth Moderator Staff Member Moderator

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    Well, I'm glad it's working for you! I never put mine on yet. Did the Mopar pickup conversion thing on the yellow soft top, but I think the Delco will be going on the next one I get running, whatever that may be ;) I know one thing, you won't be blowing off the distributor cap on a Delco distributor, not in one piece anyway!
     

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