Home Forum Ask A Member Mercury KF5 occasional surge/bog

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    Hi there,

    I just got a 1949 mercury KF5 Super 5 running after sitting varnished for who knows how long. Cleaned the tank, fuel filter, and soaked and cleaned the carb thoroughly. Didn’t rebuild the carb at all, including leaving the old cork float as the float mechanism seemed simple enough to not warrant a float replacement. Good spark from both coils. Impeller is OK, working with Brian Wilcox for a replacement regardless. Pumped 105 gear grease in lower unit and got rid of old grease.

    Anyway Got it running reasonably quick after reassembly, tuned the high low carb screws to run well at throttle and low idle. Good warm stream of water coming out and powerhead not getting too warm to the touch. Running 24:1 non-ethanol. Compression is low for this motor but is even at ~75 on both cylinders. Plugs looked good, and regapped them to spec. Starting requires choke out for one pull even when motor is warm, and consistently starts on second pull with choke in.

    Question is, I still get a surge or bog down occasionally while idling or sitting throttled – maybe once every minutes or so without changing any variables. Is that just a symptom of dirty carboned up cylinders or whatever is causing the low compression as I feel good about the carb and fuel delivery? Just seems odd to me that the surge/bog happens randomly while variables are controlled.

    One thing to note is that I did crack the fuel line compression nut leading to the fuel shut off at the carb (I know, I know, thought I was being light handed). It leaks so little (maybe .1 oz every hour or so seemingly). Could that be letting the slightest air in once in a while causing the surge/bog? I’ve got a replacement line and nuts coming.

    Thanks in advance for any advise or questions!

    – Jon

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