Backfire/misfire thru carb

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  • Beno


    Replies: 78
    Topics: 26
    #212047

    I was getting my old engine going and inspecting the reed valves and such, as I was working on it I caught a few backfires with exhaust coming out the carburator.
    this is something I’ve seen occasionally over the years and never thought much about it.. but then as I was thinking about the design of the reed valves …..

    the reed valves close to stop air/fuel/whatever to pressurize the crankcase.. how could misfire exhaust make it back thru the reed valves? wouldn’t they be closed?

    Perplexed???


    Ronaldu


    Replies: 4
    Topics: 0
    #212054

    Wires crossed? Wrong cylinder firing ? Maybe


    oldcollector

    US Member - 2 Years
    Replies: 21
    Topics: 9
    #212059

    This sounds like nothing more than a too lean a carburetor mixture adjustment,which causes a spiting back thru the carb.


    Beno


    Replies: 78
    Topics: 26
    #212114

    Ok yeah, I’ll check that.. and try opening up the low idle pin a 1/8th turn maybe?…. but the base question still perplexes me:

    if the reed valves close in order to stop pressurized gas, or pressurized air, or exhaust, or flame… how could there be a backfire thru the carb if these reed valves are intact and functioning properly??

    its like seeing a prisoner get out of his cell, run across the road. even tho the cell door is closed.
    ?


    need2fish

    US Member - 2 Years
    Replies: 224
    Topics: 13
    #212115

    To answer the reeds question, yes the reeds for the chamber on the downstroke (compression) will be closed. The reeds for the chamber on the upstroke will be open (inhalation). So, it if there is an ignition source, the fuel could ignite in the crankcase and blow back.

    Reeds can weaken and be ever-slightly open.

    1) Blowback could originate in the combustion chamber (at close to TDC) and make it past the rings as the reeds are open at the time just before the piston descends.
    2) What you are seeing is vaporized fuel, not combusted fuel when an engine sneezes from lean fuel-air mix.
    3) Reeds are bent
    4) Spark plug leads are reversed. This means the spark occurs while the exhaust ports are open.

    • This reply was modified 1 year, 2 months ago by need2fish.

    fleetwin

    US Member - 2 Years
    Replies: 4062
    Topics: 41
    #212165

    What year/brand/horsepower engine do you have?


    outbdnut2

    US Member - 2 Years
    Replies: 1618
    Topics: 83
    #212262

    On an old 4.2 HP Champion model 1J, there has always been a throttle setting a bit above idle where the carb can’t decide between the high and low speed jet where it repeatedly backfires. I just move the throttle a tad up or down and it goes away. All motors have a similar rough-running throttle spot, but that Champion is the only one I’ve seen repeatedly backfire.
    Dave


    Beno


    Replies: 78
    Topics: 26
    #212264

    140hp (.030over I believe) V4 OMC 1978


    fleetwin

    US Member - 2 Years
    Replies: 4062
    Topics: 41
    #212273

    OK, well the place to start on this engine is proper syncronization and linkage set up… The engine will never idle properly, and will sneeze and cough if its throttle plates are not fully closed at idle. Sounds like this engine has been rebuilt before, do you know its history? When was the last time it idled properly?


    billw

    US Member - 2 Years
    Replies: 1592
    Topics: 46
    #212343

    Those V4s will sneeze at the drop of a hat, if everything isn’t right, for sure. Fleetwin is right (as usual, of course) in starting with a careful link and synch. Next step in my book would be a thorough carb cleaning. IF those two things don’t cure the problem, I guess I would start looking for a leaking crank case sealing job, from the rebuild….

    I think the answer to the original, theoretical question, is that when there is a back fire event in the crank case, it takes a few fractions of a second for the reeds to close and stop the backfire, so you will see a bit of a blow-back out the carb. Be thankful for the reeds. If you had a rotary valve engine, you’d have a good chance of seeing flames come out.

    Long live American manufacturing!


    Beno


    Replies: 78
    Topics: 26
    #212371

    the engine clearly has had work, there is that dark gasket material allover the assembly seams. I got it as a “great running engine” some time ago

    I pulled the carbs and took the (square?) bowls off.. shockingly clean.

    I will get my video scope and try to make sure the butterflys are matched closed.


    fleetwin

    US Member - 2 Years
    Replies: 4062
    Topics: 41
    #212454

    OK, so the engine has run normally for you before, just started messing up now….
    With the carburetors installed, remove the black intake cover and have a look at the butterfly valves with the throttle at dead idle… Make sure the throttle cam is not touching the throttle roller though. Both butterflies should be completely closed. Feel free to post pictures of the throttle linkage when the engine is at dead idle. Go ahead and do a compression test as well…


    Beno


    Replies: 78
    Topics: 26
    #212460

    sorry, I meant that the engine was sold as “good running” to me. I have never run it before.

    \I have the black cover off, its hard to see because of the choke, but even with the choke open it’s down inside the boat (Sunbird/OMC L-Drive, jetboat) there is no Transom on this style OMC.

    the throttle linkage is definitely touching the butterfly controls at the lowest setting, I’ll get some pics tomorrow.


    fleetwin

    US Member - 2 Years
    Replies: 4062
    Topics: 41
    #212479

    Oh boy, you have got quite the set up there. Not OMC’s finest hour for sure. So, how did you mount the older 140hp in that cave? Or, did you just put the older 140hp powerhead onto the existing set up? Some pictures would help us understand just what you have/set up.
    Try loosening the screw to the roller, and move it away/off the cam at dead idle speed just to see if that makes any difference. Try to take a picture of the linkage before you make any changes, I realize it is tough to get down in there though….Take pictures of the black sealer you mentioned also.
    So, you have never had this engine running before, so we really don’t know what we have here. Try to do a compression test as well..


    Beno


    Replies: 78
    Topics: 26
    #212495

    lol.. no no. it’s all engineered like this from the factory.. years ago i pulled the 1997 115hp omc bubbleback and replaced it with the 1978 140hp bubbleback.

    it’s a straight bolt on swap. I just have to mix the fuel now since the oil injector never came with the buy (oil tank did though)

    my engine is white…
    These is not my engine pics attached but just examples of how a sunbird sizzler Ldrive is constructed

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