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Author Topic: backfire through carburetors?  (Read 31988 times)

NorthernCat

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backfire through carburetors?
« on: November 19, 2011, 02:24:42 am »
Hey folks what is the general cause of backfires through the carburetors on these old two strokes?

I thought my timing was spot on...rebuilt tillotson hds....i'm at a loss why it won't run...rebuilt crank, new seals.  :-\

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    Yamikid

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    Re: backfire through carburetors?
    « Reply #1 on: November 19, 2011, 02:50:48 am »
    I am not a Cat person but a little more info would be nice, will it stay running at idol? or is backfiring even trying to start? Or is it while increasing RPM?  There are a lot of reasons an engine will back fire it is not always timing. For any engine to run proper  even I have to keep in mind these simple things Air, Fuel (Quantity), and timing (on time) It will run.  Just my two cents   Doug
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    Rainman56

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    Re: backfire through carburetors?
    « Reply #2 on: November 19, 2011, 02:52:01 am »
    Crank out of phase?
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    NorthernCat

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    Re: backfire through carburetors?
    « Reply #3 on: November 19, 2011, 03:08:28 am »
    it will idle somewhat with the throttle up, but go much more the pto side backfires out carb and the motor will usually quit. Has twin tillotson hd 14a's on it linked with a brass pull rod and both carbs have grose jets installed. (wondering if they have something to do with it). crank was rebuilt by the best shop in my area so i doubt it is out of phase. It didn't really backfire out carbs before when I was trying to convert to dual mikunis.

    engine is a 760 JLO mod free air with twin megaphones with baffles.

    mikek5510

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    Re: backfire through carburetors?
    « Reply #4 on: November 19, 2011, 03:46:01 am »
    lean

    2009-c-miller

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    Re: backfire through carburetors?
    « Reply #5 on: November 19, 2011, 03:56:54 am »
    lean
    Are both carbs opening the same?  Sync'd.

    NorthernCat

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    Re: backfire through carburetors?
    « Reply #6 on: November 19, 2011, 04:01:13 am »
    yeah they are synced, they have to open at the same time, they are linked with a brass rod, its setup just like my other 760 and that works perfectly. i think i better recheck my timing?


    snojet68

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    Re: backfire through carburetors?
    « Reply #7 on: November 19, 2011, 04:32:21 am »
    yup i would re- ck timming and just as important ck the advance on flywheel, i had 399 jlo that was back firing because it woudnt advance once it started. i played with timming untill i figured it must be something else so i ended up pulling flywheel and sure enough it was sticking had to clean with brake clean,compressed air then lightly lube. only other thing i can think of is maybe a bad condensor. good luck
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    NorthernCat

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    Re: backfire through carburetors?
    « Reply #8 on: November 19, 2011, 04:41:56 am »
    hey guys thanks i will re check timing and leaks around the carb flanges.

    you guys don't think the grose jets will cause trouble like this do you??? I had a set and figured I'd use them because I've heard how wonderful they are.

    JLOpower

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    Re: backfire through carburetors?
    « Reply #9 on: November 19, 2011, 09:56:00 am »
    Gotta say it . Recheck the basics, including inspecting the exhaust for restrictions and testing the case for leaks. As previously mentioned, check the advance mechinism and operation of the points. I would also remind you to make sure the points fit the mounting correctly, sit flat and don't hit surrounding parts. Then I would start swapping parts side to side to see if you can isolate the offending component. Plugs, wires, coils, carbs and mounts.
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    MERCDIVER

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    Re: backfire through carburetors?
    « Reply #10 on: November 19, 2011, 11:19:59 am »
    good chance that the side that is backfiring, the points are not grounding properly and firing at the wrong time
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    NorthernCat

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    Re: backfire through carburetors?
    « Reply #11 on: November 19, 2011, 02:35:06 pm »
    Hi guys, ok I will be digging...wish I had some form of heated shop though, it gets old working in less than ideal temps!

    beverlynsteve

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    Re: backfire through carburetors?
    « Reply #12 on: November 19, 2011, 02:36:24 pm »
    Isn't that what the living room is for?  I need to put the decals on my sled.  I was thinking of bringing it thru the slider and into the living room.  (Haven't shared that thought with the wife yet)

    NorthernCat

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    Re: backfire through carburetors?
    « Reply #13 on: November 19, 2011, 03:05:15 pm »
     ;D

    strainer

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    Re: backfire through carburetors?
    « Reply #14 on: November 19, 2011, 03:45:43 pm »
    Going to say lean carbs also. New sleds will backfire through the carbs until they are warmed up. Modulate the choke and see if it improves. If so then not necessarily the grose jets but too high pop off pressure will cause lean. You could also fatten up the low jets and even the high. If turning them out improves the situation again at least you'll know what it is.