merc 80hp - similar to wrenchtrnr72

[FishinFool2002]

Hi, I had an earlier post about my boat's performance, but had never compared in and out of water. After reading wrenchtrnr72's post, decided to put muffs on in the back yard. Anyway, the motor runs great with the muffs on and throttles up fine. In the lake, with the throttle pushed all the way down, the motor will only go up to about 3100 RPMs. Is this really bad news? <br /><br />Boat is 16 ft glasstron bowrider

 

[achris]

Re: merc 80hp - similar to wrenchtrnr72

What's the pitch on the prop. Sounds like you could be WAY over propped (Very, very not good) or you have another problem in the engine, Ignition or fuel or timing adjustments. Has this combination of boat, engine and prop worked ok in the past? You should be getting around 5000 to 5500 WOT.<br /><br />Chris..........

 

[FishinFool2002]

Re: merc 80hp - similar to wrenchtrnr72

Just looked at prop....pitch is 17. Is that about right? Dont know what the combination was before. Bought used, had the mechanic check it, do some work, carbs rebuilt, impeller replaced, and a few other minor things before we bought it. Question, With the wrong pitch, would the engine still not run up to proper RPMs? As I said, in the lake, only gets up to 3100.

 

[FishinFool2002]

Re: merc 80hp - similar to wrenchtrnr72

What should the prop size/pitch be?

 

[achris]

Re: merc 80hp - similar to wrenchtrnr72

17" sounds about right for that boat size. (If I was 'propping' that's the size I'd start with anyway). If the prop is the wrong size, too big, then it will overload the engine and it wouldn't run up to the right rpm. If the prop is too small it 'light loads' the engine and it'll over rev.<br /><br />Was the mechanic a marine mechanic? Did he reset the carb float levels? If so, why, were they too high or too low? and how much? and were both done?<br /><br />Other than that, if you have a bit of engine knowledge, start with the basics. Check the compressions, then with the engine running on the flushes, pull each plug lead off in turn, using insulated pliers. The engine should loss about the same revs with each cylinder missing. If that all seems ok, then check the carbs are opening fully and they are both opening at exactly the same time (engine NOT running). If you have a timing light, take it all down the lake and check the maximum advance. NEVER do this on the flushers. An engine should not be run at more than idle on the flushers.<br /><br />Chris.............

 

[FishinFool2002]

Re: merc 80hp - similar to wrenchtrnr72

The mechanic that did the work was a marine mechanic. Also said compression tested good. I told him what was happening, he suggested a 20 or 21" prop without relooking anything. I'm tempted, because thats a fairly inexpensive fix. But I think I'm going to do the smart thing and take it to a different Marine shop. I dont know much about engines, but it just seems hard to beleive that it's a engine failure, when it runs up fine on muffs.

 

[emckelvy]

Re: merc 80hp - similar to wrenchtrnr72

A prop with MORE pitch won't make your motor run any faster. It's like putting larger diameter tires on your car. I.E., taller gearing.<br /><br />If you think you are overpropped, you would go DOWN in pitch to gain rpm. Approx. 100 rpm per inch of pitch. The next size down in your case would be a prop with 15" pitch.<br /><br />But before doing that, check the engine out thoroughly as suggested. Compression readings are critical and if they are more than 10psi off from each other, you may have ring problems. Running a carbon-cleaning fuel additive thru the gas might help in that case.<br /><br />HTH and G'luck.......ed

 

[achris]

Re: merc 80hp - similar to wrenchtrnr72

If the mechanic suggested a LARGER prop, find a new mechanic!

 

[Spidybot]

Re: merc 80hp - similar to wrenchtrnr72

The maths are true but something smells.<br /><br />You turn 3100 and should reach, say, 5100.<br /><br />One inch pitch equals 200 rpm, so to increase rpm at wot by 2000 you should go down 10" in pitch (!).<br /><br />This makes no sense. A 16' with 80 hp should not use a 7" pitch (don't think there is such a thing at all).<br /><br />Therefore, you'll have to look for other performance related problems. Remember that on muffs you should never exceed 1500 rpm, so this means testing at sea.<br /><br />Remove any excess load from the boat.<br />Check motor mount setup on transom<br />Check hull shape, cleanliness and overall state<br />Check fuel lines<br />Check throttle linkage to see, if wot at the lever really is wot at the engine<br />What speed do you get at 3100 rpm & wot?<br />Any chance the tach is faulty or misreading?<br />How does the motor sound? <br />How is accelleration ("hole shot")?<br /><br />For comparison I have a 16" Sea Ray with a 115 Merc that use 19" for skiing and 22" HighFive for cruising light load. Depending on use and load, I'd expect a 17" to be too high for you - more like a 15" as I used to have on a light 15" with a 50hp.

 

[achris]

Re: merc 80hp - similar to wrenchtrnr72

UU,<br /><br />Don't forget that your 115 has a different gear ratio to the 80 horse.<br /><br />Chris...........

 

[FishinFool2002]

Re: merc 80hp - similar to wrenchtrnr72

Update. The Marina called today and our boat is ready to be picked up!!! This time we took it to a different marina. Anyway, the first thing I asked was "how much".......It's only $103!!!! Wooohoooo. He started to go into telling me what the problem was, and said there was a problem with the lower carb linkage, but I was so excited that it wasnt gonna be 6 or 7 hundred, that I just told him we'd be there Monday morning to pick it up. They did test it with load. I'll get a better description of the fix, and post later. Thanks all.