1987 Johnson 90

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jignem

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Mar 24, 2003
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I'm losing 600-700 rpm and about 4-6 mph at wide open throttle.The engine starts and runs a little rough.I replaced the fuel line and bulb with one made by OMC,replaced the spark plugs and fuel filter.I've been burning 93 octane from day one.Most of the engine parts are original other than the VRO pump which was replaced last spring along with the spark plug wires.Can anyone help?Any help would be greatly appreciated--Thanks,jignem.
 
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Dhadley

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Feb 4, 2001
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Re: 1987 Johnson 90

Welcome!<br /><br />Lets start with compression. We're always a bit concerned with combustion temps running 93 octane. <br /><br />What are your top rpm's when its running right?<br /><br />What spark plugs?<br /><br />And compression?
 
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jignem

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Re: 1987 Johnson 90

Dhadley-Thank you for replying.My top rpm's when running right are @ 5200-5300.The spark plugs I'm using are Champion L77JC4 and I just checked the compression 70-72 in each cylinder.Dhadley-you've got me thinking about the rpm's.I adjust the trim and Ive seen the rpm's higher but I don't run for more than maybe 15 seconds @ those rpm's.At full throttle I'm @ 4480 rpm's now.Thanks again,jignem.
 
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Dhadley

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16,978
Re: 1987 Johnson 90

Good choice on plugs. Little low on rpms. We'd like to see it around 5800 with an average load and trimmed.<br /><br />70-72 lbs compression really seems low but at least its all even. Might try another gauge just for comparison. <br /><br />If the heads havent been off for a couple of years I might think about pulling them for inspection and change the deflectors anyway.
 
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jignem

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Re: 1987 Johnson 90

Thanks for your help Dhadley.I will reply with results.Jignem
 
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