1987 Force 125 shudder/bucking at fast idle/mid RPM

[Bubba Blaster]

Hello there! I am brand new to the forum and excited to be here - this site has been a huge help with dealing with my Force outboards. I have a puzzling issue that I haven't been able to resolve, and don't have a lot of time or money to start throwing parts at it.

I just swapped out my burned up motor with this new-to-me powerhead (powerhead only, same lower unit). My issue is that the motor idles fine in/out of gear, but between fast idle and almost full throttle (RPM range between about 1500-3500 RPMs or so), the motor shudders or bucks badly the whole time you are in that RPM range. Once you give it enough throttle and get it out of that RPM range, it smooths out beautifully. In other words, it seems to only happen during the transition between the idle and main fuel circuits. When on one circuit or the other it's fine, it's in that transition that it does this. It's fine out of gear in this RPM range though.

I was able to get rid of the shudder or bucking when adjusting the idle mixture screws to just 1/2 out form lightly seated (I know that's bad - too lean). Which is weird, because the motor won't even idle that way - it dies after a few seconds at that setting. Otherwise at that setting it transitions through idle all the way to WOT very smoothly.

I put a timing light on all 4 cylinders while it was doing this and they all are firing consistently judging by the consistency of the light flashes. BUT, I know I have some weird spark issues/inconsistencies (see below).

So I am wondering - do I have a weak spark issue, or a carb issue? If weak spark, then why does it run fine at high speed? If dirty carbs, then it would have to be something I missed, as I took the carbs off, bowls off, idle screws out, and sprayed carb cleaner and blew air through every hole I could find. I did NOT take out the main jet (looked fine) or idle circuit tube, or welch plug. I didn't notice any blockages - I got carb cleaner out of all 3 idle ports in the carb throat when sprayed through the idle mixture screw hole too.

I checked the timing and it was at 29 at WOT static. In the water I checked it dynamically and it was about 31 - I retarded it to 28 dynamic just to see how it would behave and it seemed to help somewhat with the shuddering. I don't think I have a sheared key, as the 0 mark lines up when at TDC.

I'm sorry for the already long read, but please see below for what I've done and observed:

The motor looks lightly used and in great shape - came off an old boat from my area in OK. It looks like at some point someone did some tinkering with it but never got it running - throttle/gear linkages were not hooked up correctly, and they tried to do a spark plug thread repair, but shavings were still everywhere - even in the cylinder, but no scuffing on the cylinder walls and you could tell it was never started after the attempted repair. Otherwise looks like the motor was never used much, just been sitting most of it's life.

What I have done/observed:

- pulled head, inspected cylinders - look great, virtually no wear. Had head surfaced, new cylinder gasket
- compression is excellent - 155psi on all 4.
- 2 spark plug holes stripped (#2 and #4) - fixed with timeserts (someone did one before I got it and got the hole 2-3 degrees off angle, but is sealing OK)
- pulled carbs, blew all holes out with lots of carb cleaner and blew out with compressed air, reset float height, replaced with new float needles, and new gaskets (except the internal/external bowl nut gaskets - they looked fine, and the kit didn't come with the right ones anyway)
- a visual on the reeds checked out - they are all straight, closed, and shiny from what I can see.
- new NGK BUHX spark plugs
- new thermostat
- new fuel pump diaphragm
- new water pump
- fresh fuel
- timed static @ 29 (found it there, figured it was close enough). Later checked dynamic in the water @ WOT and played with it between 28 to 31 (for troubleshooting). Checked for 0 at TDC and it is right on.
- performed complete link and sync according to Frank's stickies and videos
- checked all wiring and nothing obviously broken
- I do NOT get enough spark at cranking speeds to trigger my timing light consistently - except on #4 wire. It did one time - enough for me to get a static timing reading. Also #3 tends to not fire for the first few seconds after start up, but then shortly kicks in (according to the timing light and the fact it runs rough until it kicks in). It stays firing as long as you have the motor running. I have not done a spark gap test.
- Motor starts right up and idles pretty well under load/no load. Also higher RPMs (3500+) is smooth under load/no load and has power like my '85 125 did, although there is the slightest stutter (like a single miss) every once in a while until you get to full WOT)
- As far as idle mixtures, it idles really well at 3/4 turn out, but pretty good (how you want a 2 stroke - a little rough) at 1-1/2, where I have it now. Anything past 2 to 2-1/2 it smokes more and doesn't want to start very well in the water. When fine-tuning during hole shots then anything 3/4 out to 3 turns out seems to perform the same - no hesitation or stumble

My thought is to swap out the stator and the entire ignition system from my old motor and see what happens (quickest/cheapest fix), and then completely redo the carbs (soak in parts cleaner, take out all plugs) if that doesn't do it. Thoughts?

 

[jerryjerry05]

The "miss" could be a trigger wire broken under the shrink wrap??
​The shuddering, bucking? sounds like it's sucking air??
​Can of starting fluid, run motor and spray the SF around the front and sides of the powerhead.
Lots of times when a blocks been changed the cyl. port covers suck air.
​1 backfire can blow the seals??

 

[Bubba Blaster]

I can definitely try that, although the motor hasn't been taken apart from what I see - the factory paint job looks all intact around the whole motor. And as far as backfiring, I haven't heard it backfire so far, so that's good!

It seems to be running too rich in that mid-range, as I can fix it completely by leaning out the idle mixture to just 1/2 a turn out from seated. But is that just compensating for weak spark? At the same time, why would it run fine at WOT? I guess I need to do a spark gap test.

 

[pnwboat]

Have you checked to see if the float level in the carburetor(s) is set correctly?

 

[Bubba Blaster]

Yes, I was very picky on that one - almost laser-straight horizontal to the bottom of the carb - just erring on just above horizontal (looking at the carb upside down). And then the fully open stop set to 1/16th from hitting the carb pickup/main jet.

Interestingly, I ran into a motorcycle guy today who thinks it's ignition related - says when you see this shudder problem within a certain RPM range, but runs fine otherwise, is normally ignition related. I don't know if this holds true for these outboards, but given that I don't get a strong enough spark during cranking speed to fire the timing light, except on #4 coil, it makes me wonder...

 

[jerryjerry05]

You check the trigger leads??
​They can look good and under the shrinkwrap be bad.
http://www.outboardignition.com/tips.asp​ here's a link to an ignition repair troubleshoot page.

 

[Bubba Blaster]

Excellent site, thank you! I will be looking into it. I also am thinking, just for time sake, to pull the stator and the whole ignition rack off my old motor, and put it on this one and seeing how it goes.

 

[Bubba Blaster]

Oh, and to answer your question, no I haven't pulled the shrink wrap off of them. I will check that.

 

[jerryjerry05]

You don't have to pull the wrap.
Just tug on the wire and see if it seperates from the connector.

​I replace them all anyway.

 

[Bubba Blaster]

Good thought. I did a check on that connector strip, and I do see one thin wire with a few strands of wire broken, but the majority still intact. There is another wire that feels too flexible under the heat shrink, so will pull that and look too. Also, one of the stator wires going to the left hand CDI box got smashed between the block and a connector, so rearranged the wires so it doesn't get pinched, but the wire is so flat, it may have effected the wire, even though the insulation isn't broken.

So my plan, for time's sake is to see if I get any change after fixing these questionable spots, and then if not, I'm going to pull everything off my 1985 125 motor and swap it out and see if that solves it. I'm at the point now where I have to get it going or make peace with waiting till next year.

 

[Bubba Blaster]

Update - I checked the trigger wires, and they look good. I am running out of time to troubleshoot, so I decided to swap out the whole ignition system from my burned out motor, which meant pulling not one, but two flywheels!

The current one was stubborn! I had to use a combination of a puller, heat, and a pry-bar. So now I have another question too:

I chipped the flywheel magnet in the process, as it rubbed on the stator while prying the flywheel off. The chip is about 1/2 inch wide, by 1/8 inch tall, by 1/8 deep, right on the edge of the magnet. Also wondering about the condition of the center armature around the hub of the flywheel - one spot looks a little squished, and another spot looks slightly mangled (plates not totally straight). I'll try to post pictures soon.

I could use the other flywheel, it's in better shape, but I imagine they are fine - but this is my first time pulling these. Side note: the spare flywheel just has a timing mark for 32 degrees BTDC, which I though that these are supposed to be timed at 30 dynamic? .


OK back to the root issue - correct me if I'm wrong, but I was thinking through things more, and I want to bounce this off of you all.

1. Since leaning out the idle mixture solves the mid-range shudder problem, one would think the carbs are dirty. BUT if the carbs are dirty, then that normally means something is plugged, and if something is plugged, then why does leaning out the mixture fix it? It should not be getting enough fuel as it is then! Yes I am assuming it would be a plugged fuel passage and not an air passage - just guessing by the way these carbs are designed (assuming the diagrams are correct). The fixed metered airway for the idle circuit supplies all three idle ports, so if an airway was plugged, or semi plugged, wouldn't it effect my base idle as well?

2. I suppose it could be a semi-plugged air passage on one carb, causing it to idle OK, but not when given more fuel and more load.

3. It could be a leaking internal bowl gasket, letting fuel seep into the main pickup only when enough vacuum is generated, like at mid RPM, which also could explain why it runs fine at hi-speed, as it would be running fast enough to even out.

4. Or it could be ignition - weak or inconsistent spark, causing issues at a certain throttle setting and load?

 

[Bubba Blaster]

No Title

Here's some images of the flywheel magnets and center armature.

 

[Nordin]

That chipped of the magnet would not affect the ignition system.

 

[jerryjerry05]

Flywheel's fine.
​Just make sure the glue holding the strip is ok.

 

[Bubba Blaster]

Thanks for the feedback! So I have some updates.

I completely replaced the entire ignition system on the motor (including stator and timing base) with the one off my old motor. SAME ISSUE - bad shudder at mid throttle - between 1500 and 3000 RPMs. Smoothes out fine above that, and it idles OK. So must be carb related. A couple of interesting things I did learn:

1. Under additional load (pulling a tube), it doesn't shudder as badly, and effected RPM range is less.

2. It's the top carb causing the issue! If I turn the mixture in to 3/4 out on the top carb, it smoothes out great. I can leave the bottom carb just about anywhere I want (3/4 - 2 turns out) without shudder if I leave the top carb there. It does start shuddering at around 2 turns or more. If I do the reverse and put the bottom carb in to 3/4 out, then I still can't put the top carb out anymore than 3/4 out or it shudders.

So I know I have a nick in two places on one of those carbs that I noticed when I was cleaning them. One nick is on one of the idle needles - it was such a small nick I didn't think it would cause an issue, but when I pull the needles it will be interesting to see if the one with the nick is the top carb. The other place was the bottom of the carb pick up tube, right where the internal bowl gasket seals. There's enough of a nick taken off of that sealing surface that if the gasket doesn't cover the very inside of that pick up tube, then fuel could leak into the main jet. If it was a small enough amount, I could see where it would be enough to cause an extra rich mixture at mid throttle, but not enough to effect it too badly at a higher RPM. It will also be interesting to see if this is the top carb.

I'll switch needles and see if the problem follows it. If not, then I bet it's a bad seal around the bottom pickup tube.

What would one use to fill in a nick inside the carb bowl, JB weld?

 

[Bubba Blaster]

No Title

Here's a picture of the nick in the bottom of the pick up tube - this is from when I was cleaning it previously, so I still don't know if this on that top carb that is causing the issue or not.

 

[pnwboat]

JB Weld might work if you can get to stay in place. There is another two component epoxy product called PC-7 that is more like clay and will not run as bad as JB Weld. Looks just like JB Weld just more like clay than a thick liquid like JB Weld. I've seen PC-7 in ACE Hardware stores.

I have to say though, I would think that the washer on the inside of the fuel bowl would prevent it from leaking. The nick doesn't appear to be 100% all the way across the sealing surface. But you never know.....

 

[Bubba Blaster]

Awesome, great suggestion. I assume either one holds up to sitting in gasoline after it cures?

Yeah, I agree, the gasket should be getting it, but I'll pull it and see how much wiggle room it has.

It's that, or a nicked needle (will know soon) or something still dirty in there I guess - that's all I can think of. Nothing looks worn out that I saw, but I didn't pull the welch plug, but I blew out with cleaner and air pretty good. Course I could have missed something.

 

[jerryjerry05]

That spot shouldn't be a problem.

Worked on the same motor, boat had twin 120's had a miss that just wouldn't go away.
Cleaned the carbs 4 times.
Even swapped the carb from one to the other and it a the carb for sure.
After cleaning the 4th time it just worked???
​Didn't see anything ??
W​hat ever was in there?? just went away??

 

[Bubba Blaster]

So I pulled the idle mixture screw and the bowl on the top carb causing the problem, and funny thing is that this isn't the carb that has the nick in the pickup or the idle screw. So it must be a blockage in the idle air circuit, unless it is a main jet issue? I forgot to check the jet size - it may be the wrong size maybe? Anyway to tell if it's worn out? I would think the bottom carb would have a worn jet too if that was the case, and that would cause the same issue as the top carb is, so I think it's probably that carb's idle air circuit or the wrong size jet - slim chance, but worth checking.

For Oklahoma - close to sea level, should be 094 bottom carb/101 top carb right? I see you can't get the standard jets anymore, but the optional ones are smaller sizes. I know the bottom carb is a 094, so the top should be a 101 I assume. If not, I'll check my old motor - they should be right.

It's running well enough and it is too late in the season to make it a big deal and take the whole carb off, so I'll be pumping a lot of carb cleaner and air through there and maybe pull the welch plug too if I've got a spare. Hopefully will do before for this week end's trip to the lake.