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1987 70 hp etlh acceleration rough

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kentrx1

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I have a 1987 2 stroke 70 hp Yamaha outboard that sat several years before I got it. Compression 130,130, 125. I cleaned carbs well getting all jets open. Put new fuel pump and fuel lines on. Replaced water pump. It cranks and idles great. It runs very smooth from 2/3 idle up, and if backed down once it accelerates it runs smooth at about half throttle up. I set the carbs according to manual (I think 1 5/8–but don’t have book in front of me now) and have tried tuning carbs about 1/2 round in each direction with no change in idle or performance noted. Have tried 2 new sets of plugs gapped to proper specs with no difference. My problem is a rough acceleration up to point pontoon planes—-or as much as a pontoon planes. Engine almost sounds like it is skipping. I have a very flat pitch prop (11) so even thou it is a big pontoon should push it ok. If I gun the engine to 2/3 on takeoff it does fine, but I am hoping someone can clue me in on what I have missed. I did not buy carb kits—-as they are pretty pricy and my gaskets all appeared fine—so I realize that may be an issue. Appreciate the Yamaha outboard gurus pointing me in right direction!
 
check the primary and secondry resistance on your ignition coils and compare our readings with cdi electronics chart.
 
primary between the 2 wires ,secondry between spark plug wire and the wire that comes into the coil the power pack wire plugs into it , sorry i do not have the colours some one will chime in .
 
Tegweni said:
primary between the 2 wires ,secondry between spark plug wire and the wire that comes into the coil the power pack wire plugs into it , sorry i do not have the colours some one will chime in .
Click to expand...

i checked the colours on a 1984 the blk/whitestrip wire and black wire are your primary wires and black/white stripe wire and spark plug wire are your secondry wires
 
Ok....I took all three off and used two different meters- one analog and one digital. The primary readings when I connected black and black white wires all gene me a complete circuit—-same reading as if I had connected probe ends. Secondary did the opposite—-no reading at all. I tried various ohm scales and still got the same results. What the heck am I doing wrong?
 
Have had to work way too much this summer but had a chance to play with pontoon again this past weekend. Everything I have checked has seemed fine so far and I pulled plugs and they seem to be all burning identical. I guess I am back to I must have not gotten carbs cleaned good. One question. Where does idle circuit get over ridden in acceleratio—- or does it? If my idle jets were somehow still a little clogged could that explain why when I am 2/3 throttle or better it runs really smooth? Only other clue is when I go to crank it for first time I must have cold start throttle nearly wide open to get it to hit but runs and cranks fine after that. It needs gas really open to crank after it had been sitting overnight
 
Ok. New season and playing with my project again. I rebuilt the carbs, boiled out in ultrasonic, and no difference. I checked compression and all 3 cylinders between 105 and 115, and that is with a weak battery. Cranked and put in gear tied up to dock and pulled one plug off at a time. Could not tell much difference. Ohmed coils and all 3 showed about same resistance but was not range that cdi electronics book showed. I have pickedup some extra coils to experiment with. Boat still runs exactly like it did last year. Cranks fine. Idles fine. Accelerated rough until I get to about 3/4 throttle then smooths out. Thinking perhaps to look at timing again and perhaps oil blend as I am using the Yamaha precision blend system. Not smoking much and plugs not fouling so I am hesitant to mess with it. When I pulled plugs from last year bottom plug was a little blacked than other two. Put new set in and ran about an hour this year and plugs all look same so far. My gut still says carbs but I went thru them with a fine tooth comb three times and all jets crystal clean. Any ideas appreciated!
 
Noticed a slight surging at mid throttle now. Not a strong surge, just enough no notice. Seems to disappear once throttle opened up above 75 percent
 
After chasing this for a year I think I finally found my main issue. Linkage was not adjusted correctly and throttle was not opening but halfway. The linkage with timing was advancing timing too early. Hope this helps someone else!
 
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