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

[jerryjerry05]

Jets don't wear out..
​They can corrode and the hole enlarge.

 

[Bubba Blaster]

Hey All, I confirmed the top carb main jet is .1015" and the bottom is .094", so that should be good, and they don't look corroded or worn out.

I am focusing on the top carb's air passage for the idle circuit and getting it cleaned, as I think I have it isolated to this. I'm planning on taking out this Saturday, so will let you all know if my cleaning efforts get whatever is restricting that airway.

 

[jerryjerry05]

Try a fuel system flush.
​I use, Chevron with Techron fuel system cleaner.
​I mix the whole 8oz.bottle(or what ever's in it) with 1 gal. reg. gas NO ETHANOL.
​Add the right amount of TCW-3 for the fuel and cleaner and 1 extra ounce of oil..

Unhook the regular fuel line and hook up the flush.
​Start and run 10-15 mi n.
​Then let it set for 1 hr.
Run again for 10--15 min.
Then let set 1 hr.

This will get rid of some deposits.

Mercury's Powertune or OMC Engine Tuner.
​This help remove built up carbon.
​You can mix this with gas and run it through the same way.

Just remember to add oil for the amount of additives you put in the fuel.

​The tiny passages in the carb? if the bowl a dirty??
Then they need to be cleaned.
​The welch plug needs to be removed to clean that out.

 

[Bubba Blaster]

jerryjerry05, thanks, great suggestion.

Like I mentioned earlier, I sprayed all sorts of cleaners in all the effected passaged I could see, particularly the air passage for the idle, let it sit, then blew out with air, and did it again. Brought it out to the lake and it seems a little better, but basically the same.

It is running well enough to use, as I don't run it in that range, and outside of that range it runs fine. I hate to leave an issue open ended, but time is my enemy for the rest of this season and I will have to call it good for now. This winter or next spring I'll take the carb off, probably both, and actually soak them in a cleaner bath, remove welch plugs, etc. I'm pretty sure it's just a dirty top carb idle circuit.

Thanks to all for their time and input - I'll start a new thread next year if it still gives me issues.

 

[jerryjerry05]

I worked on a 94 120 Force.
It had a miss that just wouldn't go away.
After hours of troubleshooting.
​I even took the carb from his second 120(twin120's) and swapped it over.

​The CARB was the problem!!!

I cleaned it 3-4 times.
Each time, same results

I didn't have a repair kit so I couldn't take the welch plug out.

​The 4th or 5th time I cleaned it.
It started and ran like a champ!!

​I did nothing different the 1st time or 3rd time or the 5th time I cleaned it.

I must have dislodged whatever was blocking the passage???

Just make sure there is a small particle filter between the carb and the pump.
​Fram G-2 G-3 G-12 one of these will do the trick.

 

[Bubba Blaster]

So it's been a season since updating this thread but wanted to follow up on my shudder/running rough at mid throttle issue.

I ran the boat the rest of the season without issue, just ignoring that shudder in that lower-mid RPM range, and I noticed the more load I gave the boat and the longer I ran it the less it would shudder, to the point it almost went away by the end of a day's run pulling the kids around on the tube.

So I take it out this year and before running it for the first time I inspect the plugs - they are the surface gap style NGK BUHX plugs, which are correct for this motor, and were brand new last season. I haven't run it this year yet so from last year's run #1 and #3 are really clean, with only the center electrodes black. The other two are pretty black - overly rich, not glazed. Top of pistons in all 4 cylinders look the same and look normal - black carbon covered but not sooty or glazed. How do I interpret this? Top cylinders for each carb too lean and the bottom pair are too rich? How do you fix this or is this normal?

I ran the motor in a tub of water at idle for a few minutes as a first test run for the season and then pulled the plugs - they are all nicely wet as you would expect for a 2-stroke at idle, so I know all cylinders are getting fuel. I'm getting strong spark on all 4 plugs and the motor starts right up and idles great. And again WOT is great and super smooth.

I've heard on these Force 4 cylinder models that the design of the intakes were poor and caused the lower cylinder for each carb to run richer than the other, but by this much? Is this OK? I attached pictures - As you can see, left to right, #1 and #3 are clean and #2 and #4 look like they are running a little rich (You can see #2 has a Timesert on it - came out with the plug this time, lol):


This leads to the all important question, which is going back to the point of this thread - I've got both carb's idle screws set at 1-1/4 out and I've confirmed the main jets are correct for my location. Other than that low-mid RPM range shudder, it idles and runs perfectly. I can leave it there and deal with the shudder, or I can put the top carb idle screw at 3/4 out and get rid of the shudder. I'm a little scared to run inside of 1 turn out on these. I could try it with new plugs and then check them after a short run. Thoughts?

I have burned up these motors before by doing WOT (5500) with no load and a 13 pitch prop (for towing) with under-jetted mains and idle screw at 3/4 out. Learning from my mistake I've replaced the main jets on this one to be the correct size and I won't do WOT on a 13 pitch prop with zero load again! But can I get away with 3/4 out to get rid of that shudder, especially with those two plugs so clean?

 

[jerryjerry05]

Updated from last year??? The post is 5 years old?

You setting the carbs in the water and in gear??

Do the starting fluid test, spray starting fluid around the intake side of the motor, under the flywheel and down, under the packs and the coils. If this makes any difference in the way it runs??? Then its sucking air.
Lots of times the port covers gaskets blow out with 1 backfire or a cough

 

[Bubba Blaster]

Oh my goodness, no way - how's it been that long? I could have sworn I dealt with this last year...well sorry to raise such an old thread, lol!

To answer your question, no I adjusted while out of gear, which is where I got the 1-1/4 turns out. I can try it in gear but I've tried multiple settings while in gear and it still doesn't get rid of that shudder while in that high idle to full WOT transition. To be clear, this shudder/bucking at low-mid RPMs is ONLY happening when I'm in the water in gear. Shudder goes away at 3/4 turns out if I adjust top carb only. I'm using the carbs from my older 1985 125 engine that I burned up - wondering if I should have just repaired the ones that came with this 1987 engine - they looked identical except for the needles and seats and main jets (top and bottom were .094 whereas now top is .1015 and bottom is .094).

I just had a thought - my 1985 engine was running without this shudder with top carb main jet size at .094, whereas this one I've got running at .1015, which is correct for my location from what I can tell. This top carb is the only one I need to adjust to 3/4 out to get rid of that mid-RPM shudder. Could I be too rich in the mains for that top carb? With that said, keep in mind that the 1985 engine that was set with this .094 main jet on the top carb was also the one I melted a cylinder on, so I don't know!

 

[Jiggz]

Since it is #1 and #3 that are firing correctly with the correct mixture, I would suggest checking the recirc system. It would seem the bottom cylinders fed from each carb (#2 and #4) are getting too rich due to non-recircing of unburned fuel. Sometimes this is caused by clogged recirc hoses or stuck check valves on the elbow connectors. Check your manual on how to do maintenance on these hoses and elbows.

 

[Bubba Blaster]

I'll give it a try - of course my manual has to disappear on me. Will order another one and update you.

On the flip side I checked my old motor for that burned cylinder and it was #3, which would have been using the bottom carb with the .094 jet. SO if none of this tweaking works, I'm planning on doing the following to see if I can get the shudder out of it:

* Replace plugs with Champion ULV18s (running NGK BUHX now).
* Bring the idle mixture down to 7/8ths out and see if I can get away with it.
* Put the .094 main jet back in the top carb since that's what was in my old motor that ran without the shudder (although it burned up in the end, lol) and then see if I can fatten up the idle back out to 1+ turns out.

 

[jerryjerry05]

Do the starting fluid test!!!

 

[Bubba Blaster]

Do the starting fluid test!!!

Yes, sorry, was planning to do that just forgot to mention. In the meantime I did take apart the recirc plates top and bottom. Some crud in the screens but nothing blocking and the reeds are straight/flat. Will clean and make sure I can blow through all passages then put back together.

 

[Jiggz]

Be mindful of the elbows, T's and straight tube connectors on each recirc hose. The T and straight connectors do have check valves and sometimes the cause is not clogged but stuck check valve ball. Carb/Choke cleaner can sometimes unstuck the ball valves. Test by blowing through it to make sure it is not stuck in close position.

No need to dismount the connectors instead just disconnect the hoses to check for proper operation.

 

[Bubba Blaster]

Thanks. This '87 appears to be the reed type - the Clymers doesn't mention check valves in the elbows like it does for the other years/hp - do you think there might still be a check valve to check? Definitely not on the plates, but maybe the block?

 

[jerryjerry05]

The older 87 doesn't have recirc hoses like the newer blocks have.
The bottom of the block/front has a hose barb that goes to the plenum, it can clog and cause problems.

 

[Jiggz]

Thanks. This '87 appears to be the reed type - the Clymers doesn't mention check valves in the elbows like it does for the other years/hp - do you think there might still be a check valve to check? Definitely not on the plates, but maybe the block?

I see. I had an 88 and presumed it has the same type of fuel recirc. Anyways, follow Jerry with his recommendation. The plugs fouling on bottom cylinders like yours is a perfect example of a clog or malfunctioning fuel recirc system. You would have to trace it all to make sure. In fact, it would not surprise me this was the cause of the stuttering to start with.

 

[Bubba Blaster]

Looks like I've got the newer block (1258V7B) - two hoses for two dog bone-shaped top and bottom recirc plates. The hoses look to feed the intake ports directly as I can only blow into them as I move the piston to certain positions. Will get some brake clean/compressed air in there and make sure I have good flow.

What about the reed stops for the recirc system here? I looked in the Clymers and there is no specification for how much gap there should be to allow the reeds to open. Any ideas? Some of them are different then the others, but they are all set with very little gap.

One other thought as I wait for new gaskets (yeah I know, I could have made some) - about the shudder, I noticed the Clymers said that the 1985's had a top and bottom carb main jet of .094, whereas for '87 it is .101 for top only. The parts diagrams don't mention this so I have this jetted for .101 per my elevation, which is 1200 ft above sea level. I have 1985 carbs (1258V5A) on a 1987 powerhead so which one should I use? I find it interesting that the top carb is the one that I can tune to 3/4 out on the idle circuit and get rid of the shudder, whereas the bottom carb doesn't cause shudder or take it away no matter how much you adjust. Wonder if my main jet is too fat for that mid RPM transition...

You're right Jiggz, the manual says that if the recirc system isn't working properly, it could show the kind of symptom I'm seeing. I hope that's all it is, but I'm not seeing anything glaring after taking it all apart, so will see. Hopefully can get it together this week end and test for a Labor Day cruise.

 

[Jiggz]

Well, if the carbs are 1985, then for your elevation of 1200 the main jet is supposed to be 0.094 (0.0937 to be exact). Have you tried this jet size to see if it makes a difference? And also since your elevation is not that far from 1500 ft, a jet size of 0.092 might also work.
Although, base on the spark plugs analysis, #1 and #3 seems to be firing well with the correct mixture. I say wait until you put it back together after cleaning the fuel recirc system and if the shudder is still there then t/shoot the carbs by experimenting with different size jets.

 

[The Force power]

View attachment 369155View attachment 369156

Looks like I've got the newer block (1258V7B) - two hoses for two dog bone-shaped top and bottom recirc plates. The hoses look to feed the intake ports directly as I can only blow into them as I move the piston to certain positions. Will get some brake clean/compressed air in there and make sure I have good flow.

What about the reed stops for the recirc system here? I looked in the Clymers and there is no specification for how much gap there should be to allow the reeds to open. Any ideas? Some of them are different then the others, but they are all set with very little gap.

One other thought as I wait for new gaskets (yeah I know, I could have made some) - about the shudder, I noticed the Clymers said that the 1985's had a top and bottom carb main jet of .094, whereas for '87 it is .101 for top only. The parts diagrams don't mention this so I have this jetted for .101 per my elevation, which is 1200 ft above sea level. I have 1985 carbs (1258V5A) on a 1987 powerhead so which one should I use? I find it interesting that the top carb is the one that I can tune to 3/4 out on the idle circuit and get rid of the shudder, whereas the bottom carb doesn

You're right Jiggz, the manual says that if the recirc system isn't working properly, it could show the kind of symptom I'm seeing. I hope that's all it is, but I'm not seeing anything glaring after taking it all apart, so will see. Hopefully can get it together this week end and test for a Labor Day c

error

 

[Bubba Blaster]

Thank you! My thoughts exactly, just wasn't sure whether to jet for the block or jet for for the carbs.