1975 70 hp Johnson...Take off problem...Update and more information

[cvlngineer]

I took the boat out again today with 2 other people aboard. Here are some more symptoms.<br /><br />I tried out two other props at the boat ramp and chose the better of the two. I let the motor warm up and idled out about 200 yards from the ramp. I then gave it gas and it bogged down. I backed out of it and eased it up gradually, and it accelerated little by little, until I reached a point in the throttle, the sweet spot, and it took off like a striped ape. It ran fine until I stopped to throw a couple of lures under some birds, and we got a nice trout. I then started the motor up again, and had to go through the same procedure again. Once I have enough forward momentum, the motor takes off fine. There is a point in the throttle, where it transitions from a lugging sound to revving fine, and you can hear the motor make the transition, like you flipped a switch or something. Before it hits the sweet spot, it feels like you are trying to take off with your pickup in 3rd gear, having to ease into it very gradually (and of course burning up your clutch (c; ) <br /><br />I checked this morning, and all cylinders are getting fire. I also checked compression and all are right at 120 psi. None of the plugs were fouled. I am not sure how to check the carb synchronization or timing.<br /><br />Another interesting thing is that my top speed dropped very little with the added people and gear weight. By myself, armed with only a fishing rod and tackle box, I topped out at 32.2 mph. With the other 2 people, a big ice chest with ice and a few fish, and a smaller ice chest with ice and drinks, my top speed was 31.2 mph. This is most likely due to the other prop I was running. The one I used yesterday is pretty well worn out, and has been relegated to the position of spare.<br /><br />If you have any ideas as to what could be the cause of this mess, please help.<br /><br />Thanks,<br />cvlngineer

 

[ob]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

Engine not running,check to see that the carb linkage roller is aligned exactly on center with the scribe line on the fiber throttle cam and just touching when controls are in neutral.This setting dictates at which point the carb flaps begine to open in relation to the ignition timing.Also check to see that all three carb flaps are closed fully and equally in neutral.Next , check to see if all three carb flaps are opened fully horizontal and equally at full throttle position.You may need assistant to spin prop while shifting controls.Also at full throttle position check to see that the timer base is against its rubber stop providing these settings have not been moved.<br /><br />What dia and pitch prop are you running and what type and size boat is this 70 on?

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

ob,<br /><br />Thank you very much. I will check these things in the morning.<br /><br />The prop is 13 3/8 x 19 and it is on an 18 ft home built flat bottom boat. The hull weighs about 650 lbs.<br /><br />

<br /><br />Does this sound like a problem with the carbs? I just read a post that I wish I had read before I took it out today, it said to try and push the ignition key in when it is bogging down during acceleration, activating the choke. I haven't tried this, but I have tried everything else, like cussing, bouncing up and down, holding one leg in the air, etc.<br /><br />Thanks,<br />cvlngineer

 

[ob]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

That too is a possibility and if your engine picks up after bumping primer button repeatedly it is usually a sign of a fuel delivery problem.ie weak fuel pump or carb floats out of adjustment.However if your engine reaches full rpms at WOT it is not likely a fuel pump problem.Could be that the ignition timing is off as well but my first post just gives you a starting point.Bumping the primer and getting engine to pick-up in an out of hole situation could also be compensating for carb linkage out of adjustment.

 

[ob]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

Have you tried a 17" pitch prop on that rig? 13 3/8 X 19 seems a bit steep for that boat and may result in the symptoms you are experiencing.I'm thinking that a 15" pitch will better suit your 70 pushing that 18 footer.

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

ob,<br /><br />Yea, I had a 17 pich on it yesterday, and tried a 15 and this 19 today, and the motor does the same thing. It did seem to perform better today with the 19. I need to go to Corpus tomorrow and pick up a tach, that will cut out a bunch of guesswork on the right prop. I got the motor from a place on Flour Bluff Drive, D & J Marine. I could take it over there and let him look at it, but I am not sure if it is worth dragging it all the way back over there if it is something simple. I will probably let this guy here look at it, if I can't figure out what it is.<br /><br />Thanks,<br />cvlngineer

 

[ob]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

ob,<br /><br />Thanks, I really appreciate your help!<br /><br />cvlngineer

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

Okay,<br /><br />Here is the latest update.<br /><br />I didn't make it to Corpus today, but I was able to find some parts here in Kingsville.<br /><br />I was looking when I opened up the carb cover, and noticed that the bottom carb had a little more fuel around the opening than the others. I had nothing better to do, so I rebuilt all 3 carbs, though they were in fairly decent shape. The needle valve on the lower cylinder had a tiny ring on the seat, as did the middle cylinder, though not quite as bad. The top cylinder's needle valve looked good, but I replaced it anyway. All of the needle valves were the old style, and I replaced them with the new ones with clips. All of the jets were clear, but the float on the bottom cylinder was a little high (with the carb upside-down). I adjusted all floats to where they were level (parallel) with the rims of their respective bowls, cleaned them out, put new gaskets on them, reassembled and installed them back on the motor. I haven't started it up yet, so I am still hoping I did everything right.<br /><br />Another thing that I noticed was that the bottom carb's (throttle plate?) didn't seem to open quite as far as the other two, by just a tiny bit. When I reassembled everything, I turned the linkage bar (not the choke linkage) around and tried it the other way. Now they seem to all open the same amount. Is one leg of the linkage bar a little longer than the other? Or did I just imagine this?<br /><br />My next question is this:<br /><br />On the cam that moves the throttle plates in the carbs, there are two marks, one says "start" and the other one is about 1/8 inch below it, but it is unmarked. Which one of these marks should the roller for the throttle be centered on?<br /><br />Thanks,<br />cvlngineer

 

[rickdb1boat]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

cvlngineer<br />Looks like between OB and the Carb. rebuild, you should be doing fine. Your last question about the marks on the cam, the answer would be right in between the 2 lines is where you want it and just lightly touching. You're getting very good advice already!!

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

rickdb1boat,<br /><br />Thanks, that is where it is now, right between the two lines. I doubt that rebuilding the carbs is going to totally fix the problem, that would be waaaayyy too easy (c; I can sure hope, though!<br /><br />Thanks,<br />cvlngineer

 

[ob]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

Bare in mind that these cam marks are of original factory set-up and the exact setting is not set in stone.Notwithstanding roller wear or actuating linkage this setting can be played with a bit and waterchecked for performance.Familiarize yourself with what is needed to make slight adjustments and you can do it while on the water.

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

ob,<br /><br />Thanks, I fiddled around with the cam adjustment today, just to see how to adjust it. It looks like the only way is to either tighten or loosen the throttle cable, and then remove the bolt on the cam and rotate it. Is this the preferred method, or is there another way to do it?<br /><br />cvlngineer

 

[ob]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

Don't have a diagram handy for your year model ,but on my later model 70 the roller has an adjustable plate with a lock screw.Yours may very well adjust at the cam itself.

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

ob,<br /><br />On mine, the cam's push-rod is threaded, and the cam has a fitting where you can rotate the cam to adjust it, but it makes pretty large adjustments. The roller does have an adjustable plate and lock screw as well. <br /><br />I got up this morning to get the boat hooked up and ready to take it to Corpus and take you up on your offer, but it was foggy and raining,so I decided to stay in the garage and play with the carbs. I am going to start it up tomorrow to make sure it still runs. <br /><br />I don't know if I fixed the bogging problem or not, but I found two questionable needle valves, and a carb float that was a little too high. I don't know if it was high enough to starve the cylinder for fuel or not, but it is possible, there isn't much room in the carb bowl for a lot of movement. I suppose all of these things added together could have posed a problem.<br /><br />cvlngineer

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

Before I start this thing up today, I would like to confirm that the timing is set properly. I see some numbers (a scale) on top of the flywheel, and an arrow to line them up on. How do I check the timing at idle and then at WOT? What values should I get? TDC @ idle and 17 deg @ WOT? Just a guess, I haven't checked yet.<br /><br />cvlngineer

 

[Walker]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

I didn't see your original post so these questions may have already been asked. Did you put in new spark plug? What plug and what gap? I'd go with 821 Champions (L77JC4) gapped at .030

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

Walker,<br /><br />I haven't changed the plugs, yet. I did take a look at them, and they looked to be in good shape. I need to get a new set just for peace of mind, if nothing else.<br /><br />Thanks,<br />cvlngineer

 

[cvlngineer]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

Here is the status thus far.....<br /><br />I took my boat over to let ob look at it. He adjusted the throttle linkage and we ran the motor. He noticed that the motor was not getting warm, and this made me remember that the guy that put the motor together for me said that he pulled the thermostat out, that, in his opinion, here in south Texas, a thermostat wasn't needed. ob advised me to put a thermostat in, and to get some new ignition coils, as two of the coils had cracked covers on them.<br /><br />I went by the place and talked to the the guy that built the motor, and told him what had been found. He didn't have any new coils, but he had two used coils that looked to be in very good condition. He gave me 4 coils, and apologized for his oversight. I got home, replaced the cracked coils, and installed the new thermostat. Now, I am just waiting for the gasket sealer to dry. <br /><br />ob, <br /><br />Thank you very much for your help and hospitality. I will keep you posted on how the motor runs. If the weather is fairly decent tomorrow, I will probably take it down to the bay for a test run. Here is a link to my website, it is basically a journal detailing how I built the boat. I put it up to help other people who were building.<br /><br /> http://fillet-n-release.webhop.net <br /><br />cvlngineer

 

[ob]

Re: 1975 70 hp Johnson...Take off problem...Update and more information

cvlngineer,here's a print for a 1980 70 hp johnson.After looking at yours today i feel confident that this is the same T/stat housing and pressure relief set-up on your motor.<br /><br />#23,#101,& #78 are in order from left to right ,the pressure relief valve spring,the pressure relief valve ,& the relief valve grommet.<br /><br />The T/stat is #37 and its gasket is # 100 .<br /><br />Are all of these parts in your assy? <br /><br />Drawing