Hi all,
We have a problem with the Yanmar 1GM in our 25D. It starts fine, idles smooth, runs fine under load (in gear) up to 2500 rpm. Advancing the throttle past that point the engine will move up to about 3200 rpm, then back down to about 2400 rpm, back up, back down, cycling back and forth about every 20 seconds. It sounds smooth, not missing, not popping, no black smoke from the exhaust, it just waivers up and down by itself, without touching the throttle. Moving the throttle back to 2500 rpm or below, it runs fine again, rpms stay steady.
All fuel filters were changed and fuel lines were bled at the end of last season. Prior to that, we didn't have the surging rpm problem. Thinking we might have old or bad fuel, we ran a line from a spare tank with fresh fuel directly to the first fuel filter, but the problem remained. We're thinking the problem must be somewhere in the fuel delivery, but can't figure out what else to check. Any suggestions what to look into, fuel system or anything else, will be appreciated.
Chuck K.
Yanmar 1GM high end rpm surge
Moderator: bobdugan
reply
Hello Chuck K. A couple of years ago I had about the same problem with my 1gm and went through all you are going through and more. There were two main problems: The 1gm develops rated horsepower at 3400 rpm and my engine tach would never indicate more than 2800 rpm. The engine also had the same surge problem you describe. Eventually I was able to talk by phone with a gentleman at Yanmar technical services who really knew his stuff and was a great help. The 1gm has a max fuel delivery adjustment and by making a slight adjustment here fixed the surge problem. H e told me that if I opened it up too much the engine would smoke at top end but wouldn't hurt anything. When the tachometer read 2800 rpm the engine speed was actually 3400 rpm. The tach gage was faulty. Chuck K. your problem might not be the same and I wanted to relate this story only as food for thought that you ask for. I'm definatly not a marine mechanic