slows down at high speeds

fish3227 · Dec 12, 2003

my merc 200 is finally in running condition, i took it out and had just one problem with it. I would cruise at full throttle at 5,500 rpms for about 15 seconds, then it would suddenly slow down to 3,000 rpm. what could be causing this. I also notice that the hose on the flush port keeps expanding to the point of rupturing. it has ruptured on me two times, how can i prevenrt this?

rickdb1boat · Dec 12, 2003

Re: slows down at high speeds

Are you sure it's not overheating?

Elmer Fudge · Dec 12, 2003

Re: slows down at high speeds

Hey jeo,check your fuel lines, make sure they're tight at the fittings.check for clogged fuel pickup in the tank.Install a new diaphram kit in fuel pump.Clean out your carbs, a sticking float needle might just be the problem. get a wire coat hanger and run it up inside that hose and flush it out, some times insects can build there nests in there

rickdb1boat · Dec 12, 2003

Re: slows down at high speeds

I also notice that the hose on the flush port keeps expanding to the point of rupturing. it has ruptured on me two times, how can i prevenrt this?

That's what makes me wonder about the temp...

fish3227 · Dec 13, 2003

Re: slows down at high speeds

the temperature gauge reads hot, but i the engine doesnt seem to be overheating, its overheated on me be4 and its not the same. Also the horn isnt souding nor does the motor feel excessively hot, but the water coming out of the spitter is really hot

Elmer Fudge · Dec 13, 2003

Re: slows down at high speeds

Rick is on the right track,that motor is over heating or it's getting real close to it.something is clogging your cooling system.It seems whatever it is you did'nt get it out of there the last time that this problem occured. Are you getting a strong flow out the tell tale?

gss036 · Dec 13, 2003

Re: slows down at high speeds

JEO, I run a Merc 200 also and it seems to me I read some where that there is some kind of sensor that acts like a govenor and slows the engine speed down if it over heats. I don't have my manual handy right now and it has been a couple-three years since I have gone through it. Check with a Mechanic. There should be someone on this board that can answer that. TMD hasn't posted in awhile, don't know if he is still around, although he has his own site. This is a long string, maybe just do (themarinedoctor.com) and then go to the forum. http://www.themarinedoctor.com/cgi-bin/YaBB.pl/YaBB.pl?board=MM;action=messageindex;start=0

rickdb1boat · Dec 13, 2003

Re: slows down at high speeds

OMC motors(some) have the SLOW system, where if the is a overheat problem, oil or fuel problem, they drop down to the limp in mode. Don't know about that Merc. Probably does! But I still believe it's an overheating problem thats triggering the problem. Here's why:

I would cruise at full throttle at 5,500 rpms for about 15 seconds, then it would suddenly slow down to 3,000 rpm. what could be causing this. I also notice that the hose on the flush port keeps expanding to the point of rupturing. it has ruptured on me two times, how can i prevenrt this?

the temperature gauge reads hot, but the engine doesnt seem to be overheating

its overheated on me be4 and its not the same. Also the horn isnt sounding nor does the motor feel excessively hot, but the water coming out of the spitter is really hot

The flush port keeps expanding and bursting because it's overheating and has been for some time. 1) The temperature gauge reads hot, but the engine doesnt seem to be overheating.It IS hot. 2) Also, the horn isnt sounding nor does the motor feel excessively hot(Where??), but the water coming out of the spitter is really hot. It's hot because it's overheating and the warning horn or temp. sender is defective. 3) Check it out right now and don't run it again till you find the problem or you will have real serious problems, if not already!! Let us know what you find out, so that we may help you further. Not being nasty, just trying to keep from ruining your motor,

fish3227 · Dec 14, 2003

Re: slows down at high speeds

how can it overheat if water is running thriough it? the water comes out of the spitter just fine, regular flow

Elmer Fudge · Dec 14, 2003

Re: slows down at high speeds

Jeo, you said the temp gauge reads hot,and the tell tale is very hot,therefore i'm inclined to go along with Rick's conclusion that you are seriously close to over heating the motor,if you have'nt already done so. 1. Replace the check valve on the flush hose. 2. Replace the poppet valve.

andrewkafp · Dec 14, 2003

Re: slows down at high speeds

If the telltale is too hot to touch at high RPM, it could be overheating. If you have a pressure gauage, it should read around 15psi at 5000. When did you last change the impellor ?

fish3227 · Dec 16, 2003

Re: slows down at high speeds

it has a new impeller, but what i s a poppet valve? It seems to me there is either too much water pressure or too much air pressure or something like that. Why else would a hose explode like that?

Elmer Fudge · Dec 16, 2003

Re: slows down at high speeds

Jeo,you need to get a service manual before you attempt to do anything. The poppet valve is like a pressure relief valve it can be accessed from the back of the cylinder head next to the temp sender. If you're not quite sure how to go about doing this ,my advice is to get a marine tech to check it out. edit: Did this problem started after you replaced the impeller?

andrewkafp · Dec 16, 2003

Re: slows down at high speeds

Read Ricks comments... He's been around for a while and knows his stuff... Last thing you need is a cooked motor.

rickdb1boat · Dec 16, 2003

Re: slows down at high speeds

I would take the lower unit off again and check the impeller again. Maybe the key slipped out or was never installed correctly. Odds are it will cool at idle speed with a missing key. But under a load the impeller will spin on the shaft and not cool the engine. Were the vanes facing the correct rotation direction when you installed it? did the old impeller have ALL the vanes attached to it or was it torn up pretty bad? The reason I ask is because if it was torn up, some of the pieces could be blocking the cooling passages. Have a look again. I'm betting that's where your problem lies. My Johnson 28 HP was putting out a great stream, but was still overheating. It was due to a bad impeller. So just because it's peeing good, does not necessarily mean it's cooling properly. Also, check the thermostats to be sure they are operating properly. The engine should run around 140*-150*. Any higher and there is a problem that needs attention right away. Get a 140* theromelt stick from your dealer and rub it on the head. If it melts, it's overheatng.

Mark.24 · Dec 16, 2003

Re: slows down at high speeds

Sorry to jump on your thread Jeo! My problem might also throw light onto yours! (Maybe) I'm reading this with great interest, in relation to my own engine, Merc 70hp (1982), I-3, 9341608 (Belgium), Thunderbolt ign and it to operates fine, then drops to 3000 and then quits. Quick-Check shows the primer bulb is soft and the system requires 're-pressurising' but my engine/ boat I believe doesn't have a temp gauge or horn. squeeze primer bulb and your off again! I know its only limited information but I would like to act now before I can throw steaks onto it! Or does anybody think it maybe just fuel pump diaphragm/ check valve issue (fingers crossed) as I've read on these pages. Over Xmas I do have plans to install a temp gauge and maybe a water pressure gauge onto the block. Does the temp gauge attach into a thread just left of the #1 cyl? But where does pressure gauge go. I apologise Jeo

Mark

andrewkafp · Dec 17, 2003

Re: slows down at high speeds

Mark Does your '82 have 2 blue bands (Lt dk) and white "Mercury" in upper case with red "70"? Just trying to pinpoint (appearance v year) ?

Mark.24 · Dec 17, 2003

Re: slows down at high speeds

Unicorn, that image of the engine does look very much like mine. If its also of any help the firing order is 1-3-2, twin carbs and fuel pump is on stbd side near the #3 cyl BTW: I'm jealouse yours has pwr trim/tilt, thats next on my agenda!! I'm also gonna try & purchase some Thermomelt stick. Anyfurther details please ask Thanks for your help

andrewkafp · Dec 18, 2003

Re: slows down at high speeds

Mark That is not mine. Mine does not have PTT and is an '81 50HP But does go nicely...

slows down at high speeds

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