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I'm trying to debug some intermittent behavior with my outboard. I've only had the boat about 2 weeks, taken it out 4 times. 2 of those times it ran absolutely perfect and did whatever I asked of it, it would happily run at 5300rpm or better. 2 times, including yesterday, it ran like crap.
When it is running bad the symptoms are some difficulty cold starting and then it will cough/sputter/stumble at higher rpms. I could sometimes hear what I'd describe as a little 'cough' or 'sputter' from the carbs, and rpms will fall 200-300, then it will return to the original rpm. When it gets bad though it doesn't recover without reducing the throttle. Sometimes it would go to 5000 before doing it, sometimes 4800, sometimes only the high 3000's. When it happens, opening the throttle further makes it worse. Slowing down and the slowly bumping the throttle up to a cruising rpm helps some, but it eventually stumbles again. I have 2 6 gallon fuel tanks, doesn't seem to matter which one I use, and both have have fresh gas. It uses pre-mix oil, the VRO2 has been bypassed since the original owner, according to the guy I bought it from (2nd owner who had it 9 years).
Based on the fact that it isn't consistent and that it sometimes runs good, my first thoughts were maybe something faulty with the fuel line from tank to motor, letting air in. The fittings that attach to the tank and motor have some play and don't fit super tight, I'm not sure if they are supposed to? Maybe trash in the fuel filter? Worst case I'm thinking maybe the carbs need clean or an ignition component is in its death throes.
Other ideas based on the behavior? I'm trying to start with the simple and easy.
Thanks!
I have had similar issues before with a 2 stroke motor.....very frustrating! I found the problem was gunk/varnish in the carbs from sitting for the winter without draining the carbs. After taking apart the carbs, cleaning them and reassembling the engine ran great!
I thought about trying to inspect the carbs and maybe clean them, if I could do so without having to replace a bunch of gaskets and stuff that costs money . I'm a little afraid of messing up the linkages and sycnronization between the carbs if I disassemble it.
I've rebuilt a quadrajet on a car and the carb on a little 1.2hp outboard, but never something with multiple carbs.
Am in the midst of the same problem with my 1990 Evinrude 48 SPL, also running on pre-mix. I don't have a tach, so can't report RPMs but I can say it has happened at least once or twice over most of the RPM range. I drop it to idle and gently rev it in neutral to "clear" the problem. It stumbles badly but has never stalled. I think my problem is more intermittant than yours 69Boo. I just had the carbs cleaned and rebuilt at the local shop. Overall the motor runs much better since, but this pesky little problem happened before, and continues to happen occasionally at least, after the carb cleaning/rebuilding.
Well, I found a possible culprit, and I can only hope it's this simple. I noticed some fuel residue and a small puddle of fuel in the engine case so I started inspecting fuel hoses and such. I noticed on the bottom of the airbox a little vacuum tube that was unattached, it apparently goes on a nipple sticking out of the bottom of the airbox, and attaches on the intake manifold. That also appears to be the source of the fuel residue. I reattached it and clamped it.
I know from my days building a '69 chevelle with a Quadrajet carb that a vacuum leak can cause all kinds of headaches and be tough to track down. I hope to test it out soon and find that I've solved the problem!
I have been fighting demons for over 2 years with my 80hp Merc. I get one problem fixed and another pops up. But age has caused most of that. Fuel lines, dirt, etc.
But have you checked the fuel pump. I dont know how it workes on your exact motor but it was bad and causing all kinds of problem on mine. Then another time i had a small amount of rubber inside the bowl of one carb. It caused problems at high speed but not at low. Check the fuel pump in you havent allready. Joe
I havn't checked the fuel pump or done anything in-depth yet. I took it out again yesterday morning. At first I thought I had fixed the problem with the vacuum tube, it was running great at 5200 rpms, then it started acting up again. It would still cruise fine around 4800-5000, but above that would start dropping rpms and stumbling. I checked the fuel hose bulb when it happened. It wasn't collapsed, but it wasn't fully pumped up either, not like it is just after I prime it before starting the engine. I'm not sure how 'hard' it should be? I tried squeezing it, but it was hard to tell if that was making a difference or not because the stumbling is so intermittent.
Another thing I noticed, I still have fuel collecting under the engine and it is NOT coming from the vacuum tube I fixed, it is coming from the bottom of the air box. Is it normal for fuel to be collecting in the air box?
. I'm a little afraid of messing up the linkages and sycnronization between the carbs if I disassemble it.
