High rpms @ start-up

[Tripp Halbert]

• start it up and motor begins reaching high rpms, sometimes coughing out carb #2 and dying. sometimes it keeps going but i shut it down after about 4-5 seconds. i believe its running lean because the butterflys are closed and the linkage is relaxed during neutral and seems to rotate correctly as you shift to WOT. compression check showed 130 on all. spark test good blue colors. exhaust smokes after running it also.​
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  could this possibly be an electronic problem? or is carbs more likely? Where do i start?​

 

[Bosunsmate]

Carbs, sounds like a blocked passage way.
got a model#?

 

[Tripp Halbert]

Bosunsmate, that's sounds about right. I will pull them tonight and go through them

 

[Bosunsmate]

Bosunsmate, that's sounds about right. I will pull them tonight and go through them

Its the tiny holes that can can cause this, and they are easiest to get blocked as well so just be sure you check that every little one can have carb cleaner or compressed air blown through or everyone on this site will be saying do it again!

 

[Tripp Halbert]

Haha okay I will do my best man! Thank you for the detailed feedback too

 

[alldodge]

Hello all. Just bought a 1995 mercury 115hp motor. (2+2 series I believe). I haven't been able to see the motor run long enough to troubleshoot because as soon at it fires it begins soaring to high rpms where I just shut it off. Butterflys are all closed. Linkage looks all good and where an idling motor should be. Its obviously running really lean somehow... also it coughs out carb #2 only. After it has been shut off I have seen the exhaust fuming (unburnt fuel right?). So what can I do now ,without running the engine, to start narrowing things down? I don't want to damage it by having to crank it a bunch of times for tests. Are these 2+2 carbs much different then most standard carbs?

No expert here but an engine needs air, fuel and spark to run. The more of the three it gets the faster it runs. I think there is a linkage or carb issue and the butter flies appear closed but probably are not. Maybe remove all linkages and use something to hold them closed and make sure the idle screw is all the way out

 

[Tripp Halbert]

I will check those items in a bit.

hmm well if i screw the idle screws all the way out (only the top two carbs have them anyways), will that lean the system out a bit? should a big difference be noticeable?

 

[alldodge]

I will check those items in a bit.

hmm well if i screw the idle screws all the way out (only the top two carbs have them anyways), will that lean the system out a bit? should a big difference be noticeable?

Don't worry about leaning an engine out during idle or revving up in neutral. If it is a bit lean there should be no damage due to no load. What your wanting to find out is if something is keeping the throttle plates open some. Think of it it like a lawn mower. The difference between a lawn mower engine idling and high rpm is about 1/16 to 1/8 inch. Make sure you don't have a vacuum hose off or a bad leak.

The engine should start to choke out with the throttle plates all closed down tight

 

[achris]

Take the spark plugs out, put a timing light on #1, crank the engine and watch the timing. It should be around 0 degrees (TDC). The more advanced it is, the faster the engine will run. Check that the carb throttle plates are fully closed. Outboard carbs have a small hole in the throttle plate that allows the right amount of air into the engine for idle. Engine speed (idle) is then controlled by the timing, but those throttle plates need to be fully closed.

One thing to check on those engines is the accelerator pump. If (somehow) it's leaking and bypassing fuel, you could end up with the same problem. (just pinch off the fuel lines running to it.)

Chris.........

 

[Tripp Halbert]

Okay holy moly, FML. I found my problem and it seems very bad. a butterfly bolt is missing, hopefully it shot out and not into the engine....The butterfly seems warped also...what do I do guys. Is it safe to run this motor if it's possible there a screw in the cylinder?

 

[boobie]

Got a model # ??

 

[Tripp Halbert]

It's a mercury. I couldn't get any responses on the mercury forum so I sswitched to hear as I've always got good quick feedback for my johnsons. It's a 1995 merc 115hp serial OG109040

 

[achris]

Okay holy moly, FML. I found my problem and it seems very bad. a butterfly bolt is missing, hopefully it shot out and not into the engine....The butterfly seems warped also...what do I do guys. Is it safe to run this motor if it's possible there a screw in the cylinder?

DO NOT run the engine in that condition. You need to replace the missing screw. At a guess, I'd say someone was 'servicing' the carb, removed the screw (which you shouldn't do unless you are replacing the plate or the throttle shaft) and didn't peen it over when they replaced it... :facepalm:

You can buy a new screw. Loosen the other screw slightly and run the new one down until it's just touching. Jiggle the plate and snug one screw, then remove the other and put the Loctite on it, run it down and tighten it, remove the other screw and do the same (Loctite)... I also like to peen the end of the screw if you can reach it.

For the screw the part number is 10-F10155, but be warned, they are very expensive $1.80....

Remember to set the carbs up properly when you put it back on...

As for where the screw went.... Anybody's guess... Could still be in the engine (and unless you do a full strip you won't know)... Could have been 'digested' by the engine and shot out the exhaust, could have fallen out and into the air box (I assume you have checked that one)...

Chris.........

 

[Tripp Halbert]

Okay yea I will get parts tommorow. I am freaking out at the idea that missing screw is in the engine somewhere. Would this engine even fire if a screw flew in at some point in the distant past. It's been years since it has been fully operational and the OP said it all of a sudden had a super high idle and he never found a solution. Sorry for the long response but I'm having an anxiety attack haha

 

[achris]

If the screw fell out while the engine was running, then it's likely gone right through...

You have 2 options. Put a new screw in and run it....
Strip the engine and maybe find the screw, maybe not.... THEN put a new screw in and run the engine...

Ok... personal opinion... That is a very small screw. If it was small enough to make it past the reed plates (and that's somewhere you might like to look) it's probably small (and light) enough to make it through the transfer ports and exhaust port.... That's just my opinion and how I would deal with it.... May be get a borescope and have a look at the piston crown for that cylinder. You MIGHT see a screw impression on the crown... You might not too....

Chris.........

 

[Tripp Halbert]

Chris, you must be pretty experienced because you were right. The screw is wedged in one of the reed valves man!!! I cannot believe it!! The reed is stuck open so that explains the extreme rpms too! This is the craziest thing ever! What should I use to get this thing out? What happens if it falls through?

 

[achris]

Check the other screw still in the throttle plate. If it's steel, use a magnet. If it's not, just a pick up tool. If it hasn't already gone through the reeds, it's unlikely to...

 

[Tripp Halbert]

Gotcha. I will pick some stuff up to do this. Thanks for the good info as well.

 

[Tripp Halbert]

archis. i am a lucky man i suppose. your are correct. i went searching for it and there it was stuck in the flap of the reed plate. it was holding the reed flap open about a 1/4". I got it out along with the little lock washer. I checked the other screw on the butterfly plate and it was loose, i backed it out with ease.

Should i order two new ones or just set these in Locktite like they should have been in the first place. My question is really if these screws let go, is the reason more likely being the threads or the fact that someone didnt put some adhesive on them when setting them??

 

[achris]

You mentioned earlier that you thought the plate was bent. Is that still the case, or have you determined that it's flat? If the other screw was loose, I'd be pulling the other carbs off and checking them all... Replace with new (and Loctite), all loose screws...

Chris....