70hp Mercury, bad idle

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David R

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Aug 21, 2001
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61
I have a 1988 3 cylinder 70hp Mercury, that idles really badly. I'm new to Mercuries, so I'm not sure what is normal.<br /><br />When cold starting I have to keep the rpms up to about 2500-3000 and play with enrichment at the same time. If don't do this it catches and dies over-and-over. After about 2 minutes or a little more I'm able to ease the idle back to about 1800. When I go for forward or reverse its about 800 to 1000 rpm. There seems to be a lot of kicking and sputtering, and I have to rev the engine in gear to keep from stalling. If the engine has been running 20 to 30 minutes, at cruising speed and then it is brought down to idle, it really wants to kick and starts missing.<br /><br />As far as cruising, its got lots of power and the boat planes out very fast.<br /><br />Any ideas, suggestions?<br /><br />David
 
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Trent

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Nov 17, 2001
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3,333
Re: 70hp Mercury, bad idle

Its time to clean the carbs and put a kit into it.<br />Dont put that engine into gear at those RPM you will tear up the lower unit, and dont idle it over 1500 RPM unless you like to see rods comming thru the block.<br /><br />I have this this engine also and it purr's like a kitten. However I removed the oil injection and premix my fuel.
 
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David R

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Re: 70hp Mercury, bad idle

Thanks for the reply.<br /><br />Actually it won't let me go in to gear above idle. The idle-up lever has to be all the way down (home) before I can shift. So its kinda-like, idle down, quick in to gear, rev up again and don't hit the dock or another boat.<br /><br />What is it that makes it kick and miss after running for awhile? The engine kind-of chugs as it revs up then clears. Is it going rich or lean?<br /><br />Thanks again<br /><br />David
 
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Trent

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Nov 17, 2001
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3,333
Re: 70hp Mercury, bad idle

Sounds like your describing "lean sneezing". If it were me I would pull all three carbs and clean real well and install kits in all the carbs. You need to warm it up and adjust the idle mixture screws with it in gear and in the water (not on the hose).
 
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David R

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Aug 21, 2001
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61
Re: 70hp Mercury, bad idle

I'm fine with rebuilding the carbs. The scary part for me is, is it possible to put the carbs back on without having to redo the linkage and timing again? :eek: <br /><br />Thanks<br /><br />David
 
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David R

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Aug 21, 2001
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61
Re: 70hp Mercury, bad idle

Thanks Trent<br /><br />I'll use the boat this weekend. Then pull the carbs off and bring them home to rebuild. Keep my fingers crossed that I can put them back on again without having to reset everything.<br /><br />Thanks again<br /><br />David
 
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petryshyn

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Oct 3, 2001
Messages
2,851
Re: 70hp Mercury, bad idle

Davidr<br /><br />Before overhauling the carbs, take it for a good run with the engine cover off. If it runs and idles OK, look for:<br /><br />>fuel or water leaks on engine<br />>sagging upper motor mounts, allowing exhaust to enter engine compartment<br /><br />there are 2 designs for upper mounts. the one with the bellows seal is prone to exhaust leaks..... :)
 
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David R

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Aug 21, 2001
Messages
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Re: 70hp Mercury, bad idle

schematic, thanks for the reply. This message is sinking so fast I didn't know you had added to it.<br /><br />The cover is one of those 2 and 1/2 piece designs. Specially engineered to damage knuckles.<br /><br />There does seem to be alot of places for air to get in around the covers. I'll try it to be sure, it is an old engine.<br /><br />Best<br /><br />David
 
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ahm

Cadet
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Jun 26, 2002
Messages
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Re: 70hp Mercury, bad idle

I have a similar problem with a Merc 80 (same engine basically). Is it easy enought to re-balance the carbs after a strip down and clean? What kit does Trent refer to ?<br /><br />Thanks in anticipation.
 
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David R

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Aug 21, 2001
Messages
61
Re: 70hp Mercury, bad idle

I passed the motor over to the mechanic at my marina. There just seemed to be to many conflicting symptoms. They had a go at it, reworked the fuel pump, adjusted the timing and tweaked the carbs. They report it as running fine. The real test will come next weekend. A normal trip to the cabin takes 20 to 30 minutes. Previously this kind of trip would have caused the engine to stumble and miss coming down to idle. At Idle it would feel like it was about to stall.<br /><br />As far as carb balancing, mechanically I would expect its just a matter of the linkage settings. Everyone WOT at the same time and everyone closed at the same time. Probably the simplest way to do this would be to do one carb off, rebuild, then back on, do the next one. This way the timing is not likely to move. The manual says 1 1/2 turns out from seated on the mixture screws, for 50 & 60hp Mercuries. My selco manual says to go to some other chapter for the bigger engines, but there is nothing there.<br /><br />I have read there is another way, but that requires measuring carb flow. I haven't seen any indications that this is being done near me. All the mixture screws are set the same when ever I've checked.<br /><br />David
 
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David R

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Aug 21, 2001
Messages
61
Re: 70hp Mercury, bad idle

Follow-up information<br /><br />I did a trip out and back this weekend to the family cabin. The engine starts much easier than before, but it is still tempermental when starting cold. I can feel it running on two cylinders and stumbling to three after 30 seconds or so. The cold idle rpms are much easier to control. The rpms don't swing between stalling and over-revving in a heart beat.<br /><br />The engine is much better behaved after a long run, than before. Now it runs rough, but doesn't feel like its going to die.<br /><br />David
 
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