Catus Magnus
Cadet
- Joined
- Mar 31, 2013
- Messages
- 16
Got a new-to-me boat w/1988 Merc 115 inline six. It runs badly, barely turning 4000rpm, for the first few minutes on the lake. I want to sort this out. Here, in detail, are the symptoms & what I've done so far:
1. Starts alright
2. Upon pulling away from dock it runs a little rough, but doesn't die if I've let it idle for a couple minutes first
3. Past no wake – advance throttle - it’s slow to plane. Cannot get on plane without giving full/close to full throttle, and then at WOT it barely turns 4K RPM (this is just me in the boat)
4. Then after a few minutes – this is inconsistent - gets better and smoother, and will turn around 5k
5. Then, few minutes later, it completely clears and leaps forward, very smooth, vastly more responsive – I can pull way back to part throttle and maintain mid-30s at around 4000rpm, with great acceleration from that point. At full throttle it’ll turn about 5800-5900 rpm and hit 49mph on speedo & GPS (speedo is apparently accurate). After this, it runs fine for the rest of the day. After clearing up, it’ll plane quickly, and is very responsive. Huge diff.
I’ve had it out three times. Second time, before going, I started it in the driveway, and sprayed some seafoam into carbs. Spraying into #2/middle carb had no effect - engine did not stumble as it did when spraying into the other two carbs. I went around behind motor and yanked plug wire from #3, no effect on engine. Did same to nos. 2 and 5 and got zapped each time, and motor ran rougher, as expected. Took it out to the lake… ran crappy for first ten fifteen minutes, then woke up and ran fine rest of the trip.
Next day - pulled plugs. They all looked about the same. Started in driveway before leaving & checked spark with spare plug on each cylinder while engine running, and to my satisfaction it had spark in all six. Took it to the lake, thinking it cured… and at first it ran crappy, as related above, and after a few minutes cleared up & ran fine. I’m stumped. Intermittent spark? something temperature-related? What to replace? Or should I just disconnect/reconnect every wire on ignition circuit, making sure they've all got good contact? Or carb/float problem?
I’ll be grateful for any guidance...
1. Starts alright
2. Upon pulling away from dock it runs a little rough, but doesn't die if I've let it idle for a couple minutes first
3. Past no wake – advance throttle - it’s slow to plane. Cannot get on plane without giving full/close to full throttle, and then at WOT it barely turns 4K RPM (this is just me in the boat)
4. Then after a few minutes – this is inconsistent - gets better and smoother, and will turn around 5k
5. Then, few minutes later, it completely clears and leaps forward, very smooth, vastly more responsive – I can pull way back to part throttle and maintain mid-30s at around 4000rpm, with great acceleration from that point. At full throttle it’ll turn about 5800-5900 rpm and hit 49mph on speedo & GPS (speedo is apparently accurate). After this, it runs fine for the rest of the day. After clearing up, it’ll plane quickly, and is very responsive. Huge diff.
I’ve had it out three times. Second time, before going, I started it in the driveway, and sprayed some seafoam into carbs. Spraying into #2/middle carb had no effect - engine did not stumble as it did when spraying into the other two carbs. I went around behind motor and yanked plug wire from #3, no effect on engine. Did same to nos. 2 and 5 and got zapped each time, and motor ran rougher, as expected. Took it out to the lake… ran crappy for first ten fifteen minutes, then woke up and ran fine rest of the trip.
Next day - pulled plugs. They all looked about the same. Started in driveway before leaving & checked spark with spare plug on each cylinder while engine running, and to my satisfaction it had spark in all six. Took it to the lake, thinking it cured… and at first it ran crappy, as related above, and after a few minutes cleared up & ran fine. I’m stumped. Intermittent spark? something temperature-related? What to replace? Or should I just disconnect/reconnect every wire on ignition circuit, making sure they've all got good contact? Or carb/float problem?
I’ll be grateful for any guidance...
