1980 80hp - hard misfire

[mcurcio1989]

It is a 80hp 4 cylinder 2 stroke. I finally got this thing running and it ran good for 20 minutes and then developed a misfire. It runs fairly good at idle and will rev up and sounds strong in neutral but under load it has a hard and consistent misfire. It feels like I'm hitting a large rock or something with the prop - the engine even noticeably kicks.

Compression is a consistent 120 on all cylinders. I put an inductive timing light on each one of the wires to check for a consistent spark while operating and nothing looked amiss. I cleaned the carbs just to make sure that something didn't come dislodged and clog up a passage giving a lean misfire-didn't see anything and that didnt help. I tried unplugging and grounding each wire at a time and trying to run with each cylinder dead to see if that would isolate it and that didn't really seem very indicative either. If anything it may have pointed to the 4 cylinder but not strongly. The last thing I did was put an IR gun on each cylinder to see if any were running cooler. It had sat for a bit by the time I grabbed the gun 1, 2, and 3 were at 107?F and 4 was at 99?F so that seems to point to number 4. I'm going to try swapping coils around tonight to see if that temperature pattern will follow that coil. DO any of you have any other ideas? It is just such an aggressive misfire it seems like there should be more of a smoking gun than a slightly cool cylinder.

I kind of want to rule out fuel and timing as the cause as it ran good for twenty minutes and now it is doing this (hot or cold) all the time, without either one of those being changed. To me that sounds like an electrical component that went bad but I don't want to get tunnel vision.

 

[mcurcio1989]

Re: 1980 80hp - hard misfire

so the number four cylinder did not change in temp when I put a different coil on. Actually under operating everything was a fairly cool 95-100 degrees and then things warmed up when shut off. So I do not think that was indicating a misfire. I am really stumped here and this is frustrating to say the least !

 

[Yesterday]

Re: 1980 80hp - hard misfire

Hmmm no expert but I would think that an occasional misfire wouldn't kick that hard, in fact on one of four cylinders you'd barely notice it. Now if "all" were misfiring at the same instant...

How is your wiring? is it suffering the notorious Old Merc wire rot blues?

 

[sutor623]

Re: 1980 80hp - hard misfire

I had a terrible misfire after the motor would warm up on an '86 115 hp. Ran good cold, developed the misfire after it got warm. Motor would not plane the boat out. Wound up being the stator (and possibly trigger but we just replaced both) if you inbox me your cell #i can text you the vid of the motor doing it so you can see if it is similar to your issue. It took us a long time to isolate the issue.

 

[sutor623]

Re: 1980 80hp - hard misfire

One other thing these motors don't have thermostats so it takes a really long time to warm up to ideal operating temp on 40degree waterhose water.

 

[mcurcio1989]

Re: 1980 80hp - hard misfire

One of the wires was bad and I replaced it the rest look good and aren't cracked. This does it warm cold whenever. The first time it did it I had put it in the water and ran it for about 20 minutes all of the way through the throttle range and it ran great. Then after being underway at idle speed for a minute or two I went to hit the gas and felt it bucking. The motor is on a pontoon boat in the water. It only does this under load if I was running it off of a hose I wouldn't have the slightest clue anything was amiss

This isn't really an occasional misfire it is basically every revolution maybe every other but it is fairly consistent.

 

[Yesterday]

Re: 1980 80hp - hard misfire

The stator is really only there as part of the charging circuit and not related to ignition, so it's really a non-issue. I would locate the wires from the trigger coil to the switch and check then closely. It sure sounds like your issue lies between the trigger and the switch box, and if i's not the wiring, it's one of the two otherwise. A serial number can help provide a year & wiring diagram.

In another thread, someone suggested running it in the dark - looking for arcing amongst the wires. Also, does your motor have a distributor? It's helpful to know which ignition system we're dealing with, hence most people post the S/N to determine year etc.

 

[mcurcio1989]

Re: 1980 80hp - hard misfire

5769356 is my S/N, I can try running it in the dark tonight and I think I will also probably pull all the wires out to and from the switchbox and check for damage. This is a capacitor discharge ignition so no dizzy.

I'm going to do what I can before the 4th and I think if checking and repairing any wires doesn't fix the problem then it may be put on hold for two weeks (I'll prly just buy a new switchbox and trigger but they won't make it in time). I have a spare (smaller) motor I can throw on the boat to get me through the 4th. I just really don't want to as then I cannot diagnose this one.

 

[sutor623]

Re: 1980 80hp - hard misfire

The stator is really only there as part of the charging circuit and not related to ignition, so it's really a non-issue.

Actually this is an inaccurate statement. If the "charge" that you speak of is not making it back around, due to a bad stator, you can have erratic idle as well as constant misfire. Trust me I have run into this several times.

 

[sutor623]

Re: 1980 80hp - hard misfire

Is this the 4 cylinder? Either way you need to look at the CDI troubleshooting section on this motor, and make or buy a DVA meter. Without this you will start literally throwing parts at this motor. Switchbox is very easy to diagnose with DVA meter, as is stator and trigger.

 

[mcurcio1989]

Re: 1980 80hp - hard misfire

It is a 4 cylinder, i'm going to try and get a video of this tonight but I cannot make any promises as I have some other things I need to get to also. I'll def start looking at the CDI troubleshooting section but I don't know how good that is in my manual. I'll prly trying making the DVA meter tool as I don't feel like buying one.

 

[sutor623]

Re: 1980 80hp - hard misfire

Did you get the video that I texted you? Does it look similar to what you have going on?

 

[Yesterday]

Re: 1980 80hp - hard misfire

Actually this is an inaccurate statement. If the "charge" that you speak of is not making it back around, due to a bad stator, you can have erratic idle as well as constant misfire. Trust me I have run into this several times.

That's interesting. Mine runs fine w/o the stator hooked up at all, (decently charged battery) thus is can't "make it back around." Perhaps if it was dead shorting and driving the battery voltage down it could cause issues, but then I would expect to see a whole lot of heat & smoke coming from it.

 

[mcurcio1989]

Re: 1980 80hp - hard misfire

^I don't want to get off subject here but in my experience a bad stator or rectifier can put out incorrect voltage and spikes (since it isn't rectifying the AC current) which can definitely mess with your ignition. Often times you can diagnose this by unplugging the rectifier / stator and running off of a battery. All the stator and rectifier do, in engines that do not get a trigger signal from the stator, is charge your battery so they aren't needed but can mess with ignition. I would guess that what sutor is saying is that in a capacitor discharge ignition that runs purely off of the battery (not all do) if your stator dies it could drain the battery and then your going to start having a weak erratic spark but I don't know how that would ever happen (since you couldn't start it with a dead battery and the ignition has very little drain on the battery) unless it was a pull start which then probably wouldn't be the kind of CDI that requires a battery anyways.

Anyways here is a video I took last night to try and show the misfire. I guess this would be a good time to point out that this is on a homemade tiki bar themed pontoons boat that has 4 toons so it handles like a barge - That may exacerbate the misifire.

 

[sutor623]

Re: 1980 80hp - hard misfire

That's interesting. Mine runs fine w/o the stator hooked up at all, (decently charged battery) thus is can't "make it back around." Perhaps if it was dead shorting and driving the battery voltage down it could cause issues, but then I would expect to see a whole lot of heat & smoke coming from it.

Yea there are tons of different ignition systems out there. I'm not sure if a stator would cause this issue on this motor, but a simple DVA meter ad CDI troubleshooting checklist would definitely isolate this issue. Switch boxes are known culprits in these motors as well.

EDIT- actually I just clarified. This motor can most definitely misfire with a bad stator, period.

 

[sutor623]

Re: 1980 80hp - hard misfire

ISSUU - CDI Electronics Troublbshooting Guide - 2012 by CDI Electronics You need to look at page 82. If you build a dva meter you can find the issue. Look man I know what I'm talking about you have my number I'm off today. Call me man we got this. I'm going to drop a 50 evinrude on my buddies glastron right now actually, an check out a Suzuki 150 for my newish pontoon!!

 

[Yesterday]

Re: 1980 80hp - hard misfire

Wow! That's quite a nasty kick! I'm just thinking out loud here but watching that would make me wonder if perhaps it's more of a mechanical thing where the load is causing an issue to surface, which wouldn't otherwise. My train of thought here is the electronics shouldn''t be affected by physical (mechanical) load.

 

[mcurcio1989]

Re: 1980 80hp - hard misfire

Part of me wonders if it is a mechanical thing as well but I haven't heard any grinding and the motor does not make excessive noise when turned over by hand. I also pulled the prop off and put it in gear and gave some gas to try and see if it was something in the lower unit and it made no odd noise so if it is mechanical it def is only under load and not just in gear. I lost the thrust washer in the process which sucks. I am definitely going to go through the manual that sutor linked because more often than not a misfire is ignition related. I am pulling the off of the boat tonight so I may check a couple things but probably won't be able to get to do much on this until two weeks after the 4th.