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Thread: backfire through carburator

  1. #1
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    May 2009
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    Default backfire through carburator

    I have 79 Scout II 345 spreadbore thermoquad. Stock electronic ignition. I have installed new plugs, wires, dist. cap and rotor. Engine starts and idles fine. if you were to blip the throttle to kick up RPM its starts to run rough and pop through the carb. It will do this if you slowly rev the engine or if you accelerate quickly. If you try to drive, it has low power and will start to buck as RPM goes up. I have changed timing from 8 BTDC to 8 ATDC with no significant change. I swapped out Thermoquad for Quadrajet. Does anyone know the materials the timing chain and gears are manufactured? Has anyone had a vibration dampner turn? ANY ideas appreciated!

  2. #2
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    the timing set is gear to gear double check the wires are in the right order and that the cap isn't cracked or loose

  3. #3
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    I agree. Sounds like there are a couple of plug wires reversed, or possibly 5 and 7 are crossfiring. They need to be routed away from each other.

    1-8-4-3-6-5-7-2, 5 degrees BTDC to start.

    A sticking valve can also cause this problem, but I would eliminate the crossfire first.
    Allan E.
    Curmudgeon Extraordinaire
    Charter Member, Old Hippie IH Club
    Old fashioned binder freak

  4. #4
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    Default backfire through carburator

    Thats good information. My next question is how to locate #1 on the distributor cap and in what direction is the timing advance diaphram pointing when installed on the engine.

  5. #5
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    May 2009
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    I am a little rusty on automotive gas engines. We just removed the EGR valve with a hammer and chizel to find the EGR completely inoperable. We are in the process of fabricating a plate to eliminate the EGR. I will let you know the result. Is there an on-line source for service manual and parts manual information?

  6. #6
    Join Date
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    Aiken, SC
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    Service manual from www.binderbooks.com or any light line dealer. Pay what they ask. These books are worth FAR more than the asking price. They are factory manuals, the same ones that the dealers had, and the information in them is VERY comprehensive. Sites like this are good for suplement, but those manuals just plain rock. They sell used for almost as much as new on those rare occasions you can find them on ebay or wherever.

    EGR for an 85 suburban with 454.

    Time off number 8, not number 1. Repeat until memorized. If you time off number one, the idle timing is 50 degrees BTDC. The harmonic balancer didn't have room for a mark at number 1, so IH put it on number 8 instead, where the fat part goes.

    The rotor is pointed at number 8 when you have the timing mark at TDC on the number 8 compression stroke. Remember, it will also give you TDC on number 5, so you MUST BE SURE you are on the number 8 compression stroke. Pull the plug and stick your finger over the hole while somebody turns the crank pulley bolt (15/16") from underneath with a deep socket. You can put the wires wherever you want as long as they are in order and synchronized. This means you (1) center the vacuum diaphram so you have leeway to retard or advance as needed, (2) set up for number 8 TDC, (3) make note of where the rotor points, and (4) use that spot in the cap as the number 8 plug wire.

    The distributor rotates clockwise, so rotating the vac advance diaphram counterclockwise advances the timing.

    Ignore where it says (1) on the cap. That's for ford applications. Binderfolk figure it out the hard way.

    The IH engines do not have timing chains. You can not stretch one because it's not there. No intermeidate gear like in a Pontiac, either.

    The gears are very stout, like the Ford 300-6, which is similar in design to an IH in-line 6.

    If you remove the distributor, the trick is to insert it back into the oil pump. It can be re-inserted in ANY position, pointing to ANY plug wire, so you simply have to make sure. If you are not removing the distributor, you still can verify where the rotor is pointed in the same way. Chances are it's where it needs to be, but still, having that number 8 plug wire in the right spot is the key.
    Last edited by Allan E.; 05-31-2009 at 06:01 PM.
    Allan E.
    Curmudgeon Extraordinaire
    Charter Member, Old Hippie IH Club
    Old fashioned binder freak

  7. #7
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    Apr 2009
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    mississippi
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    Great information from Alan as usual. He's dead right about the manual's too. I shopped ebay and ended up buying new ones from binderbooks for about $110. They are so far ahead and advanced over one's like haynes and chilton it's not funny. Worth every cent!

  8. #8
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    Thank you for taking the time to respond! We did block off the EGR valve and that made no difference. So far we have installed plug, points, plug wires and Thermistat and Quadrajet. The original problem was backfiring through carburator. Not as noticeable at idle but gets worse as RPM goes up. The timing was at 8 BTDC. The truck came with a Thermoquad carb. We changed carb to Quadrajet, blocked EGR, and tried to adjust timing from 8 BTDC to 8 ATDC. The only thing that changes is the exhaust gets dark when timing retarded. all timing is done on #8 plug wire. The EGR parts were very caked with carbon. I belive my options are, vibration damper turned on rubber, distributor installed incorrectly, prestolite distributor components, valve mechanical problems burnt valves, stuck valves, carboned up valves not closing completely. I should add that this scout had been in storaged for 2 to 3 years before we purchased it. When we first got it running it did not have this problem. I am sure we have not driven the scout more then 50 miles. The carb was replaced because of difficulty geting set up correctly. Right now I am leaning toward a spun vibration damper or carboned up valve stems. Any advice for removing valve stem carbon with out disassembly? A good high speed run down the highway would work but I don't think it can get to high speed.

  9. #9
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    If your balancer is spun, you will be able to see it. The engine is externally balanced, but this would not cause you to be unable to reach higher rpms.

    What's your vacuum reading? Sounds like you have a clogged exhaust system. You could have carbon'd up the valves in that short of a duration, but I don't see that as likely to be the culprit under the circumstances.

    Vacuum readings and compression readings (fuel off, carb wide open, all plugs removed) will tell you a lot. Also, is your timing advancing with rpm? By 2000 rpm, you should be up in the 30 degree range.
    Allan E.
    Curmudgeon Extraordinaire
    Charter Member, Old Hippie IH Club
    Old fashioned binder freak

  10. #10
    Join Date
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    Bismarck North Dakota
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    Is the balancer marked so that if it is slipped you can see?

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