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1998 merc 115hp won't run ovet 4000 @ wot

C

cabledoctor

New member
Hi all I have a 1998 merc 115 that was running awesome a couple weeks ago, but now it won't go above 4000 rpm @ WOT. Any ideas? SN 0g663741 I have tried cleaning carbs rebuilding fuel pump draining gas, refilling with 93 octane, new filters new fuel line & female quick connects. All with no change anything I may have missed? Also spark is good on all 4 &compression is 140ish on all 4 any help would be great.
Thanks
 
Re: 1998 merc 115 won't run ovet 4000 @ wot

(1)Marine growth on hull???? (2)Remove the cover and run it wide open on the water....then shut it down and immediately use your hand to feel the temperature of each spark plug or cylinder...if it's not running on all cylinders, you will be definitely be able to tell.
 
Re: 1998 merc 115 won't run ovet 4000 @ wot

I may have found it, the link for the oil pump came loose & was making full oil mix all the time. haven't tried in water yet, but runs better in the barrel
 
Re: 1998 merc 115 won't run ovet 4000 @ wot

Bobbyc thank you for advice all plugs seem equal temp aftet running high rpm in barrel only went to about 3000 but all seems ok, Couldn't go out in buzzards bat today cause 30=40 mph wind & 6-8ft seas no way I could run full throttle in that in a 19ft boat
 
Re: 1998 merc 115 won't run ovet 4000 @ wot

I was able to get out on the water yesterday, Ran @ wot still only 4000 rpm. I found #3 plug cooler than other 3 hooked a timing light to the plug lead & don't see it breaking down at all changed plug, no change all plugs look the same (a little wet with gas) Did comp check this morning & all about 140. Any ideas? Going nuts with this, Please help
 
Re: 1998 merc 115 won't run ovet 4000 @ wot

Check #3 carb and linkage and synchronization...What RPM did it run at when working properly???? Try running with number 3 wire disconnected and grounded to see if #3 is contributing at all. Running it at night with the cover off might be useful to detect any visible spark leakage
 
Re: 1998 merc 115 won't run ovet 4000 @ wot

I did link & sync pulled carb off again all looked clean float level was a tiny bit high adjusted that no change. I found that if I spray gas into the throat of 3 it picks up. I also found that if I choke # 3 @ idle it floods do the same with # 4 it picks up
 
Re: 1998 merc 115 won't run ovet 4000 @ wot

By design of this 2+2 Yamaha based motor, #3 and 4 cylinders are meant to receive only enough gas to lubricate their cylinders below 1800 RPM...they have a fixed idle jet which only allows a very small amount of fuel to enter the cylinders until 1800 RPM is reached. Choking either one of the carburetors at idle should result in that cylinder coming to life and a subsequent increase in RPM. At 4000 RPM, spraying fuel into the carburetors should result in a over rich mixture and a drop in RPM if everything is working properly. From what you have said, there should be a difference in color between the #3 and #4 plugs...that might reveal the problem. In this one carburetor per cylinder setup, if one cylinder is running lean, it is also running lean on oil and could result in damage.
 
Re: 1998 merc 115 won't run ovet 4000 @ wot

We got it! I had a fresh set of eyes look at #3 carb & he found a tiny piece of crap in the main jet. Now it runs like new. Thanks for your help
 
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