Tach goes high after letting off throttle

jerkputter

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My 1999 50 HP Mercury 2 stroke stopped firing on me. I found the stator was defective so replaced it ( 16 amp red ) and regulator and while I had the flywheel off I just bought a new trigger. I don't think this engine has ever ran this good. It idles perfect and starts on 1st try, not like in the past. Now when I am running and watching the tach it shows 5200 RPM's at full throttle which is great. The problem now is when I let off throttle the tach goes to 6500 RPM's for a few seconds. It never did this before and probably is no big deal but wondering what might be causing this. Thanks for any tips on this. Tom
 

Texasmark

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What is your battery voltage when the engine is between 1000 and 1500 rpm?
 

Texasmark

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If your battery is supposed to be charged and this is just maintenance voltage while the engine is running, you aren't currently running a BOOM BOOM box, that's not enough. Needs to be up around 14.5 with a regulator, 16-16 ? on smaller engines without regulators. First thought is take your battery to WW or somewhere with a load tester and have it load tested. It needs to have at least 11V across the tester with at least 150 amperes load. If it flunks, get a new one. Then clean up your connections between the battery and engine. We'll go from there.
 

jerkputter

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I was thinking 14.5 volts was correct. I may have found a ground wire that is not making good contact. The battery is new ( installed 6 weeks ago ) and always started this boat great. It spins the engine fast, at least it did before it stopped firing on me I am just not sure about the charging now. I will get back with you on what I find on the ground problem. Tom
 
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jerkputter

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If I were to install an Amp gauge on the console of this boat what wires would I connect to make it work ? I don't want to burn up this new stator and rectifier. It would be nice to make sure this stator is generating amps before I get stuck in a storm again. Not much fun when 10 miles from dock and boat would not fire. The stator fixed that part but now I got to make sure it is indeed charging the battery. No Boom box for me. I go to the lake to relax and sure don't need music :) Update tomorrow after I go to my shop where boat is parked.
 
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Texasmark

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50 hp alternator probably is a black 9 amp stator. An automotive ammeter would work. I'd place it in series with the large red wire coming from the rect/reg module. Just remove the lug from the starting solenoid ⅜" input terminal (where most engines connect not sure about yours) put it on the ammeter, and connect a short 12 awg wire back to the terminal on the solenoid. Most DMMs have a 10A dc plug but with the tiny leads of the meter, you aren't going to get 10A through them reliably. They are like 30 awg and 10 amps needs more like at least 16 awg of any length.
 

jerkputter

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Thanks, the manual start 50hp engines are indeed 9 amp charging systems but mine is 16 amp as it is electric start. I will install the amp gauge as you mentioned. Then I would know for sure if I am not charging when driving down the lake :) I could just install a volt meter instead but will decide later. It is hot in my shop today even with all four doors open. I am off to work on it again. Tom
 

jerkputter

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Charging like its suppose to now that I got the ground wire connected in the right place. It was not making good contact and now all is good. I have not put it in the water yet so not sure about the tach problem. Lets hope the grounding problem will solve that also. I will know next week when I put it back in and post results. Thanks for the help. Tom
 
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jerkputter

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Update > I took the boat to the water yesterday. The first few times I opened it wide open it worked perfect. Topped out at 5200 rpm's and tach fell like it was supposed to when letting off throttle. Went about three miles and shut it off for a few minutes. Started back as soon as I hit the key, idle was perfect. I then opened it up and tach went to 6000 rpms and then fell to 5000 while still wide open throttle. Let off and tach shot up to 7000 rpm's and then gently fell back. I run it for a short while at 5000 rpm's and then when I let back on throttle tach started to go high again and then fell like it was supposed to. Each time I would let off throttle the tach would look like it was going to jump high and then settle to where it should be. Its no big deal but it never did this until I replaced all these new parts. The engine does not sound like it is over speeding or anything but I don't like the way this tach is acting. The stator is now charging 14.1 volts at wide open throttle so maybe I will just leave it be. Tom
 

Texasmark

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Not all that bad (letting it be) of an idea unless you are a perfectionist....I am a lot of times on certain things. How old is your tach? You could have some internal binding that is intermittent, like a bearing that is low on lube (illustrative example only). I never tore an electronic tach apart so I don't know what's inside, but something has to support the shaft upon which the needle is mounted and since there is rotational friction of some sort, it is a possibility that the lube mechanism, whatever it is, has dried out if an old tach......may just be a coincidence that the timing with the replacement parts caused the occurrence. Course the replacement parts could have put the tach into an operational mode/position that it wasn't in until you changed the input via the new parts outputs.
 

jerkputter

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The Tach is 17 years old and I am beginning to think it might be going on the blink. I am only going to keep this boat in the water for a few more days as our lake is dropping. Would not mind having one with an hour meter as this engine as less than 50 hours on the new power head. Now where do I find a single tach or one with an hour meter that would work. I would need to measure the diameter across so it would bolt right in but I am almost sure it is the standard 3 inch. Its easy to replace . Any suggestions on where I can buy one that would work ? The hour meter is really not that important but I really do think I need a new tach. Tom
 
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Texasmark

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The hour meter would be a separate instrument powered by something that gets 12v when the engine control switch (ignition) is in the ignition position.......probably your tach 12v and ground would be the best place since it gets powered in the ign. position.....red wire, purple stripe, or purple wire in semi-current (at least) systems.....my 2002 used that color code. Mercury makes the tach and other instruments. Most of their engines of the sort use the 6P tach made for 12 pole alternators. Some aftermarket have a switch on the back that you put in the 6P position for your Merc.........all as I recall, subject to error but I think I'm onto it.
 

jerkputter

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Texasmark, today I drove the boat many miles. I used 12 gals of gas I do know that.
The tach never missed a beat and worked perfect like before the stator change. I noticed that if I trim the engine while running say 4000 rpm's the tach will jump to 6500 rpm's and then fall when I let off the trim switch. I never paid any attention to that before and not even sure why I tried it today but it would move the tach each time I hit the trim switch while running it.

I normally just put the engine all the way down but today I was just messing around to see if it changed the performance any by doing the trim thing. Anyway I think I will just leave everything as is. It is charging fine and tach working back like before. I am planning on taking this boat out in a few weeks anyway as the lake level is way down.

I don't think this boat has ever run this good since I owned it so I assume the stator was going south before it finally died. Thanks for the help and if anything else happens I will let you know. Tom
 

Texasmark

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My guess is that your 12v supply line or ground to the control box, has some unexpected resistance. Since you never noticed it before, maybe it's a dirty connection.

My take on it would be that you load either of those lines....the one causing the problem with the extra current flowing from it .........just hit me, it's the 12v from the Ignition switch. because ground for the trim relays is in the engine where the relays are located.

The 12v switched is supplying power to your tach and trim relays. When you energize the trim relay you run extra current through the wire and generate heat at the bad joint (wherever it is). This decrease in current (V=IR) isn't noticed by the tach as it is increasing as you trim out. When you release the trim button that current stops, the current in the line drops so the voltage dropped across the bad connection drops and more of the battery voltage is available for the tach to cause a momentary spike.

If that isn't it then I have no idea. Glad things are improving.
 

jerkputter

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I just noticed your post Texasmark. Didn't even see page two. You may be on to something. I put gas in the Pontoon this morning and noticed it didn't start as good as before. I also noticed the tach going crazy when trying to get it going. Where you think I should be looking for the dirty connection because I know it is still not right? The tach should not have went high today when I was trying to get it running. First time I have seen this and like I said it didn't want to stay running like in the past. It would not idle to begin with but finally when I did get it running it would idle fine. After I got it running I just touched the trim and again the tach went crazy. Tomorrow I have an old Air Force friend I was stationed with back in the late 60's going for a ride with my family and I sure hope it runs ok. After this venture I think I am taking it out for the summer. I have a shop to put it in and will see if I can trace down the problem. I sure don't want to burn up these new parts. Which control box are you referring to ? More updates after running it tomorrow so wish me luck. Tom
 

jerkputter

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Here is a picture of the engine before all these parts were changed and the new power head was installed. I think I have all the grounds like it was before powerhead and ignition parts were installed. If you see anything I need to check please feel free to let me know and again THANKS. Tom
 

Texasmark

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Great. Thanks for the feedback. Your Avatar brings back memories!
 

jerkputter

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TexasMark, I assume you were in SAC like I was ? I was in from 1966 -70 long long time ago. Just had an old B52 crew chief pay us a visit after 48 years. It was great seeing him again as he and I had lots of old stories to tell and laugh about. :) Even though the B52 model's that were at our bases are long gone it was still good to share stores with him this past weekend. The only 52's that are left are the H models as most of the others have been scrapped. Best plane ever and they are still flying :)
 
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