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Thread: Help!! Mercruser 7.4 sputtering issues

  1. #1
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    Default Help!! Mercruser 7.4 sputtering issues

    I have a 1999 7.4 with carb on my boat, starts fine runs good at all rpms from Whitter out to about Blackstone then it starts sputtering (losing rmp running rough) does not die but limps along roughly at idle for awhile then can run it back up for a few minutes before it sputters again. First I thought it was the fuel filter, replaced no change, then I figured it must be bad gas so I drained the tank and ran it dry, replaced filter again filled with new gas and tested yesterday same results. I am stumped. I read other thread and have good compression so I do not think that is the issue. Any ideas? Anyone that knows motors want to go out and help troubleshoot?

    Thanks

    Ken

  2. #2
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    I had a similar issue not to long ago. I found I had a pluged jet in my carb. It slowly got worse and finally would sputter under load at all time. I pulled my carb apart and blew out the jets with compressed air. Problem solved.

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    warm it up and tie it off on the dock and run in gear and see if it happens again. I'm thinking fuel starved either kinked or flattened line .
    On the other hand it could be heat related to the ignition. How long is the run before it starts acting up? (I need to go to Whittier)

  4. #4
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    Quote Originally Posted by retiredinhomer View Post
    warm it up and tie it off on the dock and run in gear and see if it happens again. I'm thinking fuel starved either kinked or flattened line .
    On the other hand it could be heat related to the ignition. How long is the run before it starts acting up? (I need to go to Whittier)
    I think it is about 40 to 50 minutes before the problems start.

  5. #5
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    Branr1, thanks for the suggestion. I will check out the jets, I thought if they were plugged I would have the problem from the start.It is worth a try.

  6. #6
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    I fought the same issue on a 350 in a bayliner. Tuned out to be carb ice. I have run many 350s in various boats but that one in particular had SERIOUS carb icing issues! Whats your engine temp run at?

  7. #7
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    Runs at around 165. Still having the problem. Installed new fresh water pump and fuel pump after impeller failure. Just replaced. Cap, rotor, plugs and coil added sea foam and going out again for another test run in the next few days, who knows maybe I will be lucky this time.

  8. #8
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    Make sure the mechanical advance on the dist. is not stuck. It can be checked with a timing light.

  9. #9
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    Based on your description I would guess you're pulling a vacuum on your fuel tank. Make sure it's venting properly.

    Did the new ignition parts fix the problem?

  10. #10
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    Not sure if new ignition parts fixed the problem or not, hauled it to Whitter for another test and as I upped the throttle just past the breakwater it started to backfire so I turned around,I don't want to find out much it will cost to get towed back even for a short distance. It could have been from the carb cleaner or the sea foam I put in the tank, not sure as it seems to run fine on the muffs. Paying a mechanic to come look at it (again) on Wednesday. I have been reading other forums and will check the anti siphon valve and the fuel tank pickup screen as they seem to be mentioned frequently. I really hope I get the problems solved as I would really like to get my family out to enjoy the sound and stop wondering why I bought something that just seems to cost money and time that we have yet to enjoy.

  11. #11
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    Had a similar issue with my new Yamaha 115 a couple of years back-ran on muffs fine, at all throttle positions under load, etc... then for "no" reason it would choke & sputter/cough for a while then run back up, but usually only after hitting waves or wakes. FINALLY noticed that the primer bulb connected to the boat tank would be collapsed when this happened-we were about 1/2 way to seldovia this time! Wife was Very unhappy, so I switched out to one of my spare portable tanks and it ran fine on those. Tried tank drain, new gas, sea foam, all the stuff you are trying. It took me the rest of that summer to find the little screen under the tank shutoff valve that had-I don't know-cotton/paper towel clogging it all up. Probably from when I replaced the fuel sending unit the year before. I used a compressor to blow BACK through the fuel line, hear the bubbles, and thought the problem was not there, but sure enough, that little ball of crud would slosh around the tank and eventually get sucked back in & clog the darn screen! Removed that sucker and put in a racor filter assy, and have never had that problem again... Just a thought. Good Luck

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