91 Johnson 140 - Missing at higher RPMs

I have owned this motor for 3 years and it has always run like a dream. Had the boat in the shop a few weeks ago to have a new trolling motor, GPS, & a new speedo pickup replaced. Nothing was to be done to the motor. Took it for a ride and had a new problem, seems awfully coincidental, but the problem occurred only after getting it back from the shop, not before. Anyway, here is the problem. Boat starts and idles fantastically, but when I hit the gas to get on plane, and as long as its running above about 4400RPMs, there is a definite "miss". You can hear the engine sound as though it has a miss, and you can feel it. However, at certain RPMs, above 4400, it can seem to smooth out, but generally, it misses. I am wondering if a VRO pump is about to go out, could it cause this. I am not sure if it is a spark problem or a fuel problem, but I get the impression its a fuel problem at high RPMs or when you give it alot of gas to get on plane. I have tried the usual first, replaced ALL fuel lines, spark plugs, fuel filter, water fuel sep, gas cap(actually I have 2 tanks, and it does this while running on either tank). I have had no trouble out of my VRO, "YET". I am at a loss here. Why is it doing this all of a sudden, is it just a coincidence that it happened after I got it back from the shop? I don't know what to do, can a dealer actually reproduce the problem in the shop or do they have to be able to take it for a ride? Can they get the motor up to that high of an RPM without the boat actually being in the water? <br /><br />Please let me know if I left anything out or if I could be clearer about anything.<br /><br />Thanks in advance!<br /><br />

Re: 91 Johnson 140 - Missing at higher RPMs

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Re: 91 Johnson 140 - Missing at higher RPMs

YOu need to do some troubleshooting and stop replacing parts. <br /><br />1. Do you have a shop manual?<br />2. What is the rpms at WOT?<br />3. Check for spark with a spark testor. All cylinders should fire with a bright blue snap and the spark should jump a 1/2" gap.<br />4. Check the VRO warning alarm though I doubt that the VRO is the cause.<br />5. What so the plugs look like? Do they all look the same?<br />6. Check compression & post results.<br />7. Run engine at night with cover off & look for leaking apark around plug wires & coils.<br />8. Try squeezing the primer bulb when it is missing.<br /><br />That should get you started. I doubt that bing in the shop is related. <br /><br />And as to the last question the shop cannot get your engine to WOT under load at the shop unless they have a large tank and a test prop.

Re: 91 Johnson 140 - Missing at higher RPMs

1. No, I do not have a shop manual.<br />2. I never ran the boat as high as it would go, I don't run the boat over 5,000RPMs, cruising normally at about 4400-4800RPMs.<br />3. Don't have a tester, added to the list.<br />4. VRO Alarm is working. Just wondered if the<br />pump is about to go out, would it not give enough fuel to the mixture when gassing it.<br />5. Plugs are new, the ones I just replaced(only 1 yr old) still looked new, and all looked the same.<br />6. 140lbs in 3 cyls, 135 in the other.<br />7. Will do.<br />8. I will try that, should the bulb stay hard when gassing the engine?<br /><br />Thanks for your reply. Trying to figure this out. Wish i knew how to test the timing too.

Re: 91 Johnson 140 - Missing at higher RPMs

I've got a 70hp Johnson that did the same thing when i got it back from the shop. all of a sudden i had an erratic spray from the carbs. kind of funny since it ran great before i took it in . anyway i took the carbs off and cleaned them really good and it runs really good now. If you want to check your spry pattern in your carbs, get a timing light, and whith it running look in the end of the carbs. you should have a nice fan spay. Good luck hope this helps

Re: 91 Johnson 140 - Missing at higher RPMs

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Re: 91 Johnson 140 - Missing at higher RPMs

*****UPDATE*****<br /><br />I have isolated the problem. It is the GPS Unit itself, which explains the coincidence of it running bad right after i got the boat back from the shop where it was having the GPS installed. Took the boat for a test run yesterday. Ran in it alternatively, with & without the GPS hooked up. The problem with the engine missing is only occuring when the GPS is on, turn it off, engine runs fine. Surely others have had this problem, if anyone has any ideas, i'd love to hear them. There has to be some fix that will correct the problem so I can use the motor and GPS at the same time, right????<br /><br />Awaiting your replys,<br />Pete

Re: 91 Johnson 140 - Missing at higher RPMs

What kind of GPS is it?

Re: 91 Johnson 140 - Missing at higher RPMs

Garmin 188C

Re: 91 Johnson 140 - Missing at higher RPMs

What spark plugs are you running?

Re: 91 Johnson 140 - Missing at higher RPMs

Champion QL78YC

Re: 91 Johnson 140 - Missing at higher RPMs

Thats the right ones. I was curious to see if you had the "Q" plugs or not. Interesting that the GPS messes up the ignition.

Re: 91 Johnson 140 - Missing at higher RPMs

C'mon guys, nobody has had this problem before??? No-one has a suggestion to get the GPS to stop interfereing with the way the motor runs?

Re: 91 Johnson 140 - Missing at higher RPMs

^ Just moving to the top, hoping to get some comments on the problem now that I have updated whats going on.<br /><br />Thanks

Re: 91 Johnson 140 - Missing at higher RPMs

Check the wiring for the GPS. Is it wired directly to the source battery(one that the motor is run off of)? If so make sure you have an isolator on the positive side. Just a guess.

Re: 91 Johnson 140 - Missing at higher RPMs

Sounds like you are somehow getting electromagnetic interference between your GPS and ignition system. Strange. This is most likely to come from your sonar system as this is generating electromagnetic pulses. You could try separating the sonar transducer wire from the others in the wiring loom and see if this helps. Check that you have good earth connnections on the GPS & motor. I'm not sure what SF174 means by an isolator on the +ve side?<br /><br />Otherwise you could talk to Garmin and see if they can offer any insights.

Re: 91 Johnson 140 - Missing at higher RPMs

If I were you I would one of two things:<br /><br />1. Take it back to whoever installed it and tell them what you told us about the GPS affecting the ignition system. They may have to call Garmin and see if they have heard of the problem before, but you didn't pay to have it installed so that you could troubleshoot and fix it..<br /><br />2. If you realy want to try and diagnose and fix this yourself, you can try posting this problem on the Electronics forum here on iboats, there are a lot of knowledgeable guys on there that really helped me out on some things when I was installing a RADAR system.