Rpms

[waterboy84]

Ok so i been having this problem that i have never had before and i just cant seem to figure the issue out. I will explain issue in full detail on the syptoms and what the motor does.

So the motor will start up perfect and will idle perfect.
When i take off and run the boat at wot ill will then drop back to idle slowly and then shift to neutral. When i do this the rpms will raise up bouncing around and i will hear this rattling noise i guess you could say.
Now if i run the boat at wot and slowly drop back to idle and then let it sit in idle for about 30 seconds or so and then shift to neutral it will idle perfect.
Rpms wont raise or bounce around and you dont hear that rattling noise.
However last time i was out on my boat and i heard this noise it sounded like it was coming from either the leg or lower unit.
4 days ago i drained the oil in lower unit to see what the oil looked like and to check the drain plug magnet. Oil looked good. No water. No milky color. Oil looked just like it did when i put new oil in it a month ago.
The magnet had hardly any shavings on it.
I took the shavings that where on there and rubbed them between my fingers and didnt get poked.
I know some shaving will appear on magnet. In my case there was hardly any and there were no big chucks or big shavings.
However i have noticed this. When i drop throtle back to idle from wot. The back of my boat and motor will get hit by the waves i created and when this happens the 3 exhaust port holes get submerged.
Is it ok for those exhaust ports to get submerged.
When my boat is just sitting in the water those exhaust ports sit about 6-8 inchs above water line.
Anyhow. I just got timing set perfect. I took carbs apart to clean them. I put new fuel filter. New fuel lines. It have a new rectifier. A new switchbox. I got airmixture on carbs set at 11/8 turn from setted.
Could this issue be due to the waves hitting back of motor and the three port holes getting submerged.
The motor i have is a 95 120hp force.
The motor runs great overall. Only issue is when dropping back to idle and into neutral rpms go crazy and i get the rattling noise.
Any info pointing torwards possible issues would be greatly appreciated. Thank you

 

[pnwboat]

The exhaust ports getting submerged from the waves after a sudden drop in speed should have no affect on the motor.

Not sure if this is what you're experiencing, but I had a 1995ish 120HP motor with after market composite reeds that I had a similar symptom as yours. Started great, ran great purred like a kitten. However, if you immediately moved the throttle back to neutral after running the motor at or near WOT for few minutes, the RPMs would climb up to about 2000 and a rattling noise could be heard. If I hit the choke for a split second, the motor almost instantly would drop down to normal RPMs and the rattle instantly stopped. I suspect that it was pre-ignition, or detonation probably from too lean a mixture. This motor had a set of 1998 120HP carburetors on it. I looked closely at the carburetors and I noticed that the idle circuit on the 1998 carbs were slightly different from the 1997 and earlier carbs. It also had smaller main jets than the 1997 and earlier carbs. I tried clean the carbs several times, put larger main jets, adjusting idle mixture screws, running premium unleaded gas...... all kinds of things to no avail. Finally, I put a set of 1996 120 HP carburetors on it, and that solved the problem. I don't know if the after market composite reeds had anything to do with it. I seen comments that you have to increase the main jet size etc, etc.

Don't get me wrong, I'm not suggesting that you change the carburetors on your motor, but I suspect that what you might be experiencing is too lean a mixture. Try hitting the choke for a split second to see if this has any affect. The mixture screws on your set up at 1-1/8th turn out should be in the ball park. Make sure the fuel pump (fuel pump diaphragm) is in good working order. Is your fuel filter the see-through type, and installed between the fuel pump and carburetors? If so, then your fuel filter should stay at least 3/4 full once under way and no bubbles. Also, the carburetor float level should be checked to make sure they're not set too low.

 

[waterboy84]

Pnw . thanks for that info. Im gonna take my boat out asap and try the choking thing you mention.
So let me ask you this. If pushing in the choking drops rpms and the rattling stops do i replace the jets i have now with bigger jets or are the reeds the issue.

 

[waterboy84]

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[waterboy84]

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[waterboy84]

Above are some pics i posted of fuel diaphram. Do they look ok from what you can see in the pic

 

[SkiDad]

Interesting, i have stock reeds and carbs and have the same issue from time to time - it does''t really bother me. I just shut my engine off.

The issue i always fight is getting it to start 1st try when warm - always does the 2nd try.

 

[jerryjerry05]

I believe the knock you experience is something like a spark knock or pre-detonation. Not comin from the lower.
The problem can be from the carbs not synchronized, the timing off, floats set wrong, trigger firing wrong, linkage not set right.
The towershaft not setup right.
Do a compression and spark test. Check the timing.
Check the reeds.
Does the squeezie get hard?
Test the primer for excess fuel leaking out.
Change the diaphragm. It's stretched but not bad(yet).

The holes on the back, don't worry about them.
Water splashing in won't hurt.

 

[pnwboat]

One other thing I forgot to mention. If you haven't already done so, check your water pump impeller and thermostat.

 

[waterboy84]

To check impeller the lower unit must come off correct.

 

[waterboy84]

I have a question for all the force experts that read this.
I was looking at the carbs on this motor with my brain going crazy trying to figure out what could be causeing the rpms to climb when put into neutral and the rattling noise that pnw boat and jerryjerry mention is possible pre- ignition or detonation.
I looked over carbs very closely and noticed something.
If carbs aint synchronize proper. Im mean like on point to open and close exact same time will this cause crazy rpms and that rattling noise i hear.
Heres what i noticed.
My boat of course is parked in my garage and is sitting on trailer in neutral.
I noticed with the boat in neutral one carburetor was slightly opened while the other carb was completely closed.
I noticed that when idle stop screw was adjusted to set idle only the bottom carb was opening and closing when adjusting idle stop screw. The top carb stayed complety closed due to there was alittle slack/play with the tie bar the connects the 2 carbs together to open and close.
Anyhow i fixed that issue and now there is no slack or play in the tie bar.
Now sense i fixed that both carbs with boat on trailer in neutral sit exactly the same.
Wit

My question is. With one carb being slightly open and the other one completly closed could this be my possible issue with crazy rpms and the rattling noise.
Thanks for the info

 

[pnwboat]

Yes, to check the impeller you have to take the lower unit off. If it's been 2 or 3 years or more since this has been done, it's probably a good idea to replace it anyways. The reason I say check your impeller is because when you go to idle right after running at WOT, the motor is the hottest at this time. The water pump is pumping the least amount of water through the block at idle speed and the motor is still pretty hot from running at high speed. Takes a minute or two for the water temp to start coming down. I see the temperature spike after running the motor hard and going to idle on my temperature gauge.

It's possible that the carbs being out of sync may affect it, however, the only way to be sure is to take your boat out on the water.

 

[waterboy84]

Pnw boat. I got a question. You mention in this conversation we had about putting bigger jets in the carburetor. The question is do you think the jets that are in the carb now are just bad and need new ones of the same exact ones thats in the carb now.
Heres the thing. The rpms never climbed when i would put boat into neutral. Thiss issue just started.
However when i mention about i noticed carbs wasnt syncd proper. I fixed that issue but it still looks like one carb isnt right. Im gonna look into that more.
After i sync the carbs and got them to open and close at the exact same timing i took my boat out to the lake and i was pretty happy about how it performed.
The rattling noise i mentioned that i keep hearing was gone and rpms for the most part stayed at 1100rpms. However sometimes they would climb and sometimes when shift into neutral the would stay at 1100rpms.
So that tells me carbs not being synced properly is deffenently the issue.
Anyhow just wanted to know sense i never i had rpm climbing issue if just replacing the jets with the exact same one might help the issue.

 

[jerryjerry05]

The jets don't go bad
​They can become clogged or corroded but don't go bad unless someone tries to open them up.

Is there tension on the throttle cable????
The fluctuation could be the throttle not closing all the way.


I thought the knocking might be from the carbs not synked

 

[pnwboat]

Like Jerry said, the jets typically don't go bad unless they have some physical damage, or some crud clogs them up. I doubt that replacing the jets with the same would have any affect.

I didn't notice but did you ever change the fuel pump diaphragm?

 

[waterboy84]

No i havnt changed the fuel pump diaphram. I was going to but after posting the pics of the fuel diaphram its been said that fuel diapham looked ok.

 

[waterboy84]

Jerryjerry thanks for that info.
Yes. I was looking over my motor very closely and notice with the boat just sitting on the trailer in the neutral postion that the bottom carb was slighty opened amd the top carb was completely shut.
Due to the fact the was alittle slack/play in the tie bar. So i got the slack/play fix and both carbs operate exact same time.
However i took boat for test run and it ran so much better. No rattling noise and rpms were much much better but still had alittle bit of a climb. I did what pnwboat mention and pushed in on choke and rpms would drop to normal operation.
Also jerryjerry about only getting 4500rpms and boat only running 30-35mph. I did what you mentioned and played with the trim alittle and i got my boat up to 40mph and rpms got up to almost 5000rpms.
However if the lake i was on was bigger to contiune going im pretty sure i could have gotten alittle more speed and alittle more rpms.
How does the carbs create that rattling noise. That is wired carbs can create a noise like that.

 

[pnwboat]

The carburetors control the fuel mixture. The rattling noise is detonation or pre-ignition due to the fuel mixture being too lean. The fuel mixture is igniting in the combustion chamber in an uncontrolled manner, rather than a smooth controlled process starting at the spark plug itself.

You mentioned that pushing in on the choke dropped the RPM's down to normal. That probably means that you might still be too lean. That's why I asked about the fuel pump diaphragm. Might be a good idea to replace anything that has to do with fuel delivery.

 

[jerryjerry05]

If your getting 5000 or think you can get a bit more??
​Then the load in the boat might be too much?
The "normal" is 1-2 normal sized people.
1/2 tank of fuel and coast guard equipment.
Coolers and dogs and chunky people just drop the rpm's down.
If you can't squeeze anymore than 5000 then maybe a re-prop is in order?

My boat with 3-4 people and 2 coolers full of ice/bait/drinks, 8-9 rods and 2 tackle boxes and CG equipment, 40gal. fuel, I can get about 5200 on a smooth ocean.
That's about 33=34mph. trimmed up.

 

[waterboy84]

Just wanting to post a update on my boat.
I took my boat out to the ocean waters. The gulf of mexico waters for memorial day.
First ill say trying to boat out in those waters sucked that day. Seas were at 1-2 feet and water was a bit on the rough side.
It was kind of odd becuz i sat at the boat ramp for about a hour waiting to see if it was gonna rain cuz off in the distance looked nasty and you could see the rain falling. Anyhow hole time im siting at the ramp i noticed nobody was putting boats in the water. Everybody was coming back to dock to take there boat out of the water. So once i got my boat in the water and was heading out to the island. I noticed there was not one boat at the island. That was alittle odd becuz this island is packed on holidays. So packed they sheriff boat cruises around the island patroling it. Anyhow yea not one boat.
So i looked over my boat and said that explains why no boats are at the island.
The water was shallow shallow shallow.
Dummy me didnt check boating conditions until after the fact.
So anyhow the little bit of boating i could do i took advantage of. For a hour and a half stright i ran the motor. And in that period of time she ran like a beauty. Couldnt ask for something to run any better.
I even played with the trim like jerryjerry mentioned and i will have to say i got such a smooth ride compared to not knowing the trim would create such a lot smooter ride. Before my wife would yell at me telling me to slow down becuz i hit a wave to hard. I didnt hear that one time out of her mouth yesterday. So jerryjerry thanks for the info about the trim.
I think i found a new toy.
Thanks for all the info to all the experts that helped me with knowledge. I have to say. I have learned alot. Thats again. Maybe one day will see each other on the waters.