Welcome Guest - Sign Up today
Welcome Guest - Sign Up today
Store Links Mobile - Shop Now



Help Tip: If you have a question that has not been answered to your satisfaction in the archives, it is always best to start a new thread of your own. By starting your own thread, you will receive the maximum number of views by forum members.

Below are some additional forum policies in hopes of all iboats members will follow, Thank you.

1. Please do not reply to old topics or hijack existing topics. Old topics of a technical nature are like a library book, Please do not write in them.

2. Old topics should be considered archives and used for reference only. Please do not reply to them.

3. Do not take over someone elseís topic (aka hijack) with your own question, even if it is similar. If you have a question that has not been covered to your satisfaction in the archives, it is always best to start a new topic of your own.

4. If you have a question for the original poster (OP) and the topic is over 30 days old, send the OP a PM, he may not even visit the forums any longer, or may not notice your question in the old topic.

5. By starting your own topic, you will receive the maximum number of views by forum helpers that may not even notice your question when itís posted at the end of someone elseís topic. And those answers will be specific to your particular issue.

6. Please do not post to topics that have been inactive for more than 3 months unless you are the original poster. We have very active forums and any topic that remains inactive for that long should be considered "dead". It is especially confusing when there is an entirely new question posted to an old topic.

7. Posting at the end of any topic is considered to be hijacking the original posters topic which in turn subjects the topic to be closed if it continues to happen thus not making it fair to the original poster in the future had for some reason he/she needed to return for additional information or provide an update of the problem solved which is always welcomed within a reasonable amount of time frame.

8. Please note that you should see a red banner pop up near the bottom of each inactive topic asking you not to reply to old topics. The Red banner will read: Please note this topic has been inactive for 90 days. For the best results, please start a new topic.

Thank you all in advance for doing your part in helping iboats run a smooth ship.

Additional forum rules linked below.
See more
See less

7.5 Mercury Thunderbolt

  • Filter
  • Time
  • Show
Clear All
new posts

  • 7.5 Mercury Thunderbolt

    I purchased a 1973 7.5 HP Mercury Thunderbolt outboard enginge about a year ago. I have not tried it in the lake yet, but today I set it up in a small drum of water to test run it. My question is, should the water pump run all the time (no matter what speed)? It seems to be putting out a good steam of water, but when I slow it down to a low idle or to shift in either forward or reverse it just about stops flowing until I speed it up. Is this normal or a sign that my water pump may be going bad. Any help would greatly be appreciated.

  • #2
    Re: 7.5 Mercury Thunderbolt

    It should pump at idle, however in a small container somethings the water get agitated and they won't pump so try it in the lake or regular test tank.


    • #3
      Re: 7.5 Mercury Thunderbolt

      Laddies, thanks for the reply. I wondered if maybe the small drum (35 gal.) might have something to do with it.


      • #4
        Re: 7.5 Mercury Thunderbolt

        In a small container the exhaust sometimes aerates the water and the pump will pump it as the exhaust goes through easier than water will.


        • #5
          Re: 7.5 Mercury Thunderbolt

          Agree with the above plus a couple of thoughts for you.

          I would replace the water pump every couple of years, particularly if it sits up. Rubber vanes get stiff and this is cheap insurance.

          And there is a hole for idle relief exhaust just below your lower cowling in the rear that tends to plug up with exhaust debris/oil over time. When that hole is plugged, then too much exhaust goes out by the prop, & bubbles up into your water intake which is underneath your anticavitation plate. If you can squirt some WD40 or rod out this hole then it greatly helps this problem which is particularly noticeable when running in a barrel. When this hole is open as designed, you can see some exhaust coming out. Also, as said above, you want to have a water stream always coming out.

          Hope this helps and good luck, great little motors!


          • #6
            Re: 7.5 Mercury Thunderbolt

            Symptoms point to an old impeller. Especially since the motor's sat for a year since you bought it.

            Unless you know that the impeller was new or nearly new when you got it, it's likely that the blades are curled up on the "short" side of the pump and have taken a 'set', such that they won't straighten out when they get to the other side of the pump.

            Therefore most of the pumping efficiency is lost.

            I'd pull the lower unit and check it out, for peace of mind if not any other reason. Impellers are definitely cheap insurance compared to the cost of overhauling a fried powerhead or replacing a dead motor.

            IMHO, HTH...........ed


            • #7
              Sign up today
              Re: 7.5 Mercury Thunderbolt

              I have a thunderbolt I need help with please.