jhawley601
New member
Hey everyone, I really need help figuring this out. I just bought a 1983 Renken bow-rider with a 2.5L 120hp I/O 4 cylinder. I took it out for the first time on a smooth lake and the boat ran great! It would plane out without problem, ran about 35-40 MPH or so at 4200 RPM, Second time went out on a larger lake under windy conditions. The water was rough and the boat was bouncing around a bit. After running about 20 minutes, at about 3800 RPM it started to bog down. I could not get the engine to run above 2300 RPM. I backed out of the throttle to idle, then went back to WOT. It planed out and ran fine again about 3800 RPM until I had to slow down to cross a wake. When I slowed down it would not run above 2300 RPM again regardless of what I did. It feels like I am dragging something behind me or like I suddenly have to high pitched of a prop. With the engine out of gear or with the boat out of the water on muffs the engine will run and rev to over 4500 RPM without any problems. The person I bought it from told me that he replaced the coil, plugs, wires, cap and rotor button. Said he replaced the carb with a rebuilt one already because it sat up for awhile. OK fast forward 2 weeks and a few hundred dollars I am in the same boat (Pun intended) that is not going anywhere. Here's what I've done so far to try to fix the problem without success. I changed the fuel filter and rebuilt the carb again. Took out the plugs to check for problems. Plugs look good, not white and not wet. Just normal light carbon as a new normal plug would look. Gapped the plugs to .035 in. No change. Changed out the fuel line, took out the fuel tank, cleaned the tank and replaced with new 87 octane fuel. checked the filter again. Took it out on the water again and no positive change. Actually runs worse. Back firing through the carb, a huge pain to start and will not run well at idle in gear. (This concerns me because prior to this it idles fine and ran well in gear at low engine speed). SO.... Pulled the carb again. All new gaskets, no carbon or sludge build up anywhere I can find. The jets look great, the high speed valve is operational and clean, the float is adjusted perfectly and the choke is adjusted correctly, accelerator pump sprays two strong streams of fuel. Air fuel mixture valves are adjusted to about 2.5 turns adjusted while engine is warm and until the engine idle speed increases. Still back fires when throttle is opened. I do not have a timing light but I was able to adjust the timing by ear. Advanced the timing, now the engine starts beautifully and idles like it is supposed to. Runs great, until I put it in the water. Under load it still will not run over 2800 RPM (yeah I did get a few more RPM out of it). Had some spark knock but after retarding the timing a little the spark knock went away. I could probably use a higher octane fuel because it is more difficult to start again. But after all of this I am still not having any luck getting the engine above 3K RPM. I have run the engine without the motor cover and without the flame suppressor on without any change. I have changed the oil in the foot, upper and lower. Other then normal wear the oil looks normal. No major metal shavings. I am sorry for writing a novel here but I want to be sure that I have all bases covered as not to waste anyone's time. I have been told that I might have an exhaust flapper that might have melted? How would I find out if this is a problem or if I even have one? I have not been able to hear any difference in the exhaust tone. Does anyone have any suggestions of something I might have missed? I have decent mechanical skills from years of working on cars but for some reason this boat is about to get me.
