merc 100 will not run past 4000rpms

[joeb]

hey guys I have a 100 2+2 merc. 89 model on a 16 foot bow rider searay with a 19 pitch prop. IM have a problem , my motor will not run more then 4000rpms . I've had the carbs rebuilt by two different people, motor runs great other then it just doesn't run harder then 4000rpms any imput would greatly be appreciated. compression is great according to mechanics thanks, joe

 

[ob]

Re: merc 100 will not run past 4000rpms

Has motor ever run higher rpms on this boat with current prop set-up?If not,try changing out prop with one of reduced pitch.

 

[JB]

Re: merc 100 will not run past 4000rpms

Hi, Joe.<br /><br />Have you proven that your engine is running on all cylinders?<br /><br />Sounds like a 3 cylinder engine with a one cylinder compressor. They sound like they "run great", but just don't make full power.<br /><br />It could be that you are severely overpropped, but 19" sounds reasonable for a 16' boat and 100hp.<br /><br />What do your spark plugs look like after an attempt at WOT?<br /><br />Good luck.

 

[MartyTn]

Re: merc 100 will not run past 4000rpms

Hi Joe,<br /><br />Sounds like you might have some kind of engine problem. Could there maybe be a weak coil - or similar - if fuel and compression are good ?<br /><br />I also have an '89 Searay 160BROB with 100hp 2+2 Merc, and my motor has been giving me fits on cold starts, but my top end is 5200rpms with the 19 pitch prop. If the boat is trimmed poorly, the top rpms are around 4800 or so, but trimming it up gets it to 5200. Last year we switched from a 17 pitch that was a great ski prop but not a good cruiser. At 5200 rpms with the 19 pitch prop, our speedometer indicates about 42-43mph.<br /><br />Best of luck with that Merc.<br /> Marty

 

[joeb]

Re: merc 100 will not run past 4000rpms

hey guys thanks for the replies.. How would I check if there is a weak coil.. , doesn't this motor run on two cly. then the others kick in.. puzzled... whould this effect the spark going to the plugs.. all carbs are opening up when motor runs over 2500 rpms is that a indication that there is spark. any input is greatly appreciated. thanks again joe

 

[joeb]

Re: merc 100 will not run past 4000rpms

hey marty... sounds like the same boat... i'm only getting 35mph with trim.. i've had this boat for two years and it never ran more then 4000rpms. I've taking this to three different places and they all rebiuld carbs but makes no change..

 

[MartyTn]

Re: merc 100 will not run past 4000rpms

Hi Joe,<br /><br />I'm not one of the experts here, but JB who responded above is definitely one of the experts, and it would probably be a great idea to take a good look at the plugs after an attempt at WOT like JB recommended and then report the results back.<br /><br />I have the Merc manual and I've done a quick scan of the ignition system. There are 4 ignition coils - one per cylinder, but those coils are fed by the stator, the trigger coils and the switch box, and all the juice starts with the permenant magnets on the flywheel, and any of these components can go bad. After the ignition coils, a plug wire could also be bad. <br /><br />Have any of the mechanics done a full "Direct Voltage Adapter" (DVA) series of tests as called for in the manual? Some of the quick trouble-shooting tips from the manual say "Intermittant, weak or no spark at two plugs usually indicates a bad TRIGGER". "Intermittant, weak or no spark at all plugs usually indicates a bad STATOR or SWITCH BOX", and "Intermittant, weak or no spark at any one spark plug usually indicates a bad SPARK PLUG, COIL, or SWITCH BOX".<br /><br />We've owned our boat for nearly two years, and the switch box was replaced by the previous owner. I'm not totally positive, but I believe he said the symptoms were lack of rpm's at top end, but he got a mechanic to do the tests and narrowed it down to the switch box (or "brain"). I'll try to contact the previous owner to see if he can remember the exact symptoms and I'll report back if I can reach him.<br />Good luck,<br /> Marty

 

[joeb]

Re: merc 100 will not run past 4000rpms

hey marty thanks for the replies.. sea ray of greenville had this boat for 2mounths and chould not find the problem (2000.00) one boat place claimed they checked for spark.. .. REC.USA changed the lower unit. I know there's more stuff on this motor then any I had before.. maybe I'll take it to a brain surgen and let them get there lick too. thanks

 

[MartyTn]

Re: merc 100 will not run past 4000rpms

Hi Joe,<br /><br />I checked with the former owner of our SeaRay / Merc and he said when the Switch-Box "brain" went bad the rpms did drop at top end, but also the motor would quit quite a bit too and was hard to get re-started, so maybe the brain surgeon isn't needed, but who knows. Did they check for strength of spark on all cyls ? Also other, probably dumb questions, but were all the fuel filters changed, fuel-pump rebuilt, fuel line/bulb replaced and the vent checked to be clear? Also, when you said compression was good - what was it ? Ours is 110 on all 4 cyls, and I'm pretty sure that's a cold reading. <br /><br />The previous owner of our boat is a long-time Merc guy who has done a lot of his own maintenance for many years, and his response when I posed your symptoms was: "If 4K is all he can get out of it, it has to be one cylinder is not getting adaquate gas or fire". And I think that's what JB was saying in his response also. It will be interesting if you can see any difference in those plugs after attempted WOT. <br /> <br />Best of Luck,<br /> Marty

 

[joeb]

Re: merc 100 will not run past 4000rpms

Thanks marty for the reply.. What is the best way to check spark on this engine? doesn't it only idle with 2 cly. how does this system work? you also mentioned fuel pumps. How many does it have? The gas filter gets replace often. I just put new plugs in the boat will put it in the lake this weekend . I just hate working on it in the water I live on deap water, was hoping to try something before i did that but it's been nice and the kids are beating me up about it.. one more thing have you heard of 95 octane fuel? they sell this down here and a lot of people are using this in there boats . I'm going to try it what the hell. thanks guys again .

 

[joeb]

Re: merc 100 will not run past 4000rpms

Thanks marty for the reply.. What is the best way to check spark on this engine? doesn't it only idle with 2 cly. how does this system work? you also mentioned fuel pumps. How many does it have? The gas filter gets replace often. I just put new plugs in the boat will put it in the lake this weekend . I just hate working on it in the water I live on deap water, was hoping to try something before i did that but it's been nice and the kids are beating me up about it.. one more thing have you heard of 95 octane fuel? they sell this down here and a lot of people are using this in there boats . I'm going to try it what the hell. thanks guys again .