Late 70's model Mercury 90 HP I/L 6 cyl

jaysmgr18

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Apr 25, 2007
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I recieved this motor along with a boat from a friend of mine. The problem is that I cannot get it to keep running. Will start with full choke but dies when you come off the throttle. New carbs, fuel pump, plugs, wires etc. A mechanic my friend was using had it running fine on a portable gas tank in the tank, but never could get it to act right in the water. would not run. Trying to decide whether to fool with it or scrap it for a mid 90's rebuilt Mariner 100 HP. Any Ideas, Please let me know. Thanks.
 

Clams Canino

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Re: Late 70's model Mercury 90 HP I/L 6 cyl

Fool with it.... something easy is probably amiss. If it passes a compression test - then it's definately an easy fix.

-W
 

jaysmgr18

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Re: Late 70's model Mercury 90 HP I/L 6 cyl

Does anyone have any ideas on what might be the problem. I dont know a whole lot about outboards and any advice or starting point would be greatly appreciated. Thanks.
 

jimmbo

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Re: Late 70's model Mercury 90 HP I/L 6 cyl

You say it ran fine in a test tank on a portable fuel tank but dies on the boat. Try it in the boat with a portable tank. If it runs fine, that would tell you the boats fuel system has issues.
 

jaysmgr18

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Re: Late 70's model Mercury 90 HP I/L 6 cyl

I'll give it a try this weekend. Thanks for the advice. I was reading some of the other posts and was wondering if it could be some thing like that. My friend said it was purrin like a kitten in the tank but when he took it to the lake.... Notta. wouldn't stay runnin. I can get it cranked here in the backyard on muffs but only at high RPM. One other thing... the bulb does not stay tight. I don't think that is normal, but this is really my first boat to really work on. I'll let you know what happens with the portable gas tank.
 

jaysmgr18

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Re: Late 70's model Mercury 90 HP I/L 6 cyl

O.K. got her running with new portable tank and new hoses. At first would still only run at high rpm but then I was able to spray some B-12 in the carbs and work it down to about 1000 rpm's. It was still running rough so I made some slight Carb adjustments and now it seems fine. Only thing is that with the low RPM's the water outlet seems to be losing pressure. Not a good stream coming out. Fine with a little throttle. But sputtering at idle. Any advice?
 

tashasdaddy

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51,019
Re: Late 70's model Mercury 90 HP I/L 6 cyl

if it's been sitting a while, i'd go ahead and change the impeller. to me it's almost worth it to let the shop change it. not hard just a real pia.
 

jaysmgr18

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Re: Late 70's model Mercury 90 HP I/L 6 cyl

Thanks for the advice. I was wondering what kind of undertaking that would be. Going to take it today to have them do a once over to check everything out. I just feel better knowing that at least its running. Thanks again.
 

jaysmgr18

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Re: Late 70's model Mercury 90 HP I/L 6 cyl

O.K. Got it back from the shop. everything was fine. 120 compression in all 6 cyl. good fire. took it to the lake and would not stay running. would crank and run with choke on, but was very rough running and then dies at an idle. :confused: any one have any ideas where to go from here. Thanks for all the help.
 

Bob_VT

Moderator & Unofficial iBoats Historian
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26,019
Re: Late 70's model Mercury 90 HP I/L 6 cyl

Who did the carbs? Was the fuel pump done? Were you told these things or did you actually have the proof and first hand knowledge??

Link and sinc is proable next on the list.
 

jaysmgr18

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Re: Late 70's model Mercury 90 HP I/L 6 cyl

Thanks for your help. Certified Mercury mechanic did the work. All three Carbs redone and fuel pump. Have personal knowledge of the work being done. What exactly do you mean by link and sink? Outboard illiterate. Sorry!!
 
Last edited:

rwoods

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Sep 26, 2002
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Re: Late 70's model Mercury 90 HP I/L 6 cyl

Thanks for your help. Certified Mercury mechanic did the work. All three Carbs redone and fuel pump. Have personal knowledge of the work being done. What exactly do you mean by link and sink? Outboard illiterate. Sorry!!

had the same problem with mine it was two years to get it fixed the carbs was dirty and needed ajustment runs great now
 
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