Measuring WOT RPM's - Will This Work?

Ever since I have owned my boat I have had a difficult time getting to the desired 5,800 RPM at WOT. My motor is a 1978 140 hp Johnson on a 1978, 18 foot Glastron SSV-188. Both the motor and the boat are in great condition. I am turning a 15 pitch SS Apollo prop. The best RPM's I have seen are about 5,480 during the cool spring months. As the weather warmed, and the air became more humid I lost a little RPM off my best, but I have noticed those lost RPM's starting to come back as the summer temps ease. Last Saturday I was about 5,380 RPM.

This is what I have done to get my RPM's up:

? Raised the motor 2 holes (also installed a water pressure gauge)
? Checked compression (all four are within a few pounds of 120 psi)
? Replaced the tach twice (current tach is a digital OMC)
? Installed unlimited RPM CDI power pack's from iBoats
? Rebuilt the entire fuel system including draining the tank, rebuilding the carb's (linkage & synch included), new fuel pump, replaced every hose inside and outside of the cowl, and installed a new water/fuel separating filter.
? Replaced throttle cables
? Installed Smart Tabs
? Annual decarb

Not everything noted above was done in a direct attempt to get the RPM's up, but all should be contributing to the performance I am looking for

The motor purrs like a kitten at low rpm's, jumps on a plane when the throttle is applied, easily pulls me up on a slalom ski, and tops out around 36-37 mph (GPS verified). I really can't think of anything to complain about in my performance at any of the RPM ranges.

I can't think of anything else to check on the motor, so I want to verify I am not hauling around extra water weight. When I bought the boat it came with a test wheel. I have never used a test wheel before, but I am thinking about installing it the next time I am at the lake, and then turning the throttle loose with the boat still strapped to the trailer. If the engine winds up near the 5,800 RPM mark with the test wheel installed, then I will guess that I am carrying extra water weight under my floor. If the motor will not get near 5,800 RPM, then I know to keep looking at the motor. Will running the motor at WOT using a test wheel with the boat still on trailer give me an accurate RPM reading?

Re: Measuring WOT RPM's - Will This Work?

you have not tried changing props. if every thing else is up to par, you have to change the gear ratio, which is the size and pitch of the prop. strongly suggest posting this in the Prop Forum.. the stainless prop is also harder for the motor to spin.

Re: Measuring WOT RPM's - Will This Work?

I wouldn't be so quick to assume it's a prop problem. A 15" prop on that boat with that motor should be getting plenty of rpm. As for the test wheel you have to understand how a test wheel works. A test wheel is designed to allow a given motor to reach a given rpm. It may or may not be 5800.

Look in the correct OE service manual for your motor. The part number and the expected rpm is listed. Again, it may or may not be 5800. If your motor reaches the listed rpm then we can assume the motor is OK.

Re: Measuring WOT RPM's - Will This Work?

tashasdaddy said:

you have not tried changing props.

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Actually I have tried several props, but I forgot to include that information in my original post. I had a 15 pitch Rapture, and both 15 pitch and 17 pitch Hustler props. I don't remember the exact RPM of each, but it was in the same neighborhood as what I am getting with my Apollo. The Rapture was eventually stolen off my boat, and I sold the Hustler's on Ebay.

Re: Measuring WOT RPM's - Will This Work?

Thanks Dhadley. It is obvious that I don't know what I am talking about and you do, but that is why I posted my question on the forum. Thanks for setting me straight.

I checked my manual (I have a Clymer manual) and found that the test wheel for my year and horse power motor is 386246, and the RPM for the test wheel is 4,900. I also checked the stamp on the test wheel that came with my boat, and it is also 386246. I guess the right thing to do is to pursue my original plan, except I should be looking for 4,900 rpm instead of 5,800 rpm. A lake trip is planned for tomorrow, but I will have a bunch of kids with me, and running a test wheel on the trailer may not be their idea of fun. I really think I can do this in 10-15 minutes, but that is an eternity for kids. More information will follow after I get a chance to spin the test wheel.

Re: Measuring WOT RPM's - Will This Work?

My SELOC manual states 5000 RPM WOT for that motor.

Re: Measuring WOT RPM's - Will This Work?

I believe that the test wheel is designed to keep back pressure and engine loading correct so that engine timing can be set in a tank. It is not designed to check horsepower at wide open throttle, just put enough load on the engine to stablize running conditions. That said, have you inspected your hull with a critical eye. As speed increases drag increases. Unless the area of the hull which is in the water while on plane is flat and straight you could be experiencing a drag increase, 'hook' just in front of the stern can limit top speed severely. Do you have a large fin on your anti-vent plate, they produce drag as well. If you want to go fast, you must have all of your horsepower and as little drag as possible. P.S. Do you have a large sun canopy?

Re: Measuring WOT RPM's - Will This Work?

Randybeall said:

If you want to go fast, you must have all of your horsepower and as little drag as possible. P.S. Do you have a large sun canopy?

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I am looking for 5,800 RPM and not speed, but if I can get to my desired RPM I suspect I will see a small increase in speed because the boat will be running more efficiently. My desire is efficiency, and a long life for my motor. The pros on this forum say 5,800 RPM is where I should be for efficiency and motor longevity. At my current RPM's, I am lugging my engine, and the motor will have a much shorter life if I can't find 500 extra RPM.

I do have a bimini top, but I notice very little change in RPM when the bimini is in the up position. I always keep a close eye on my tachometer whether the bimini top is up or down.

I am heading to the lake in a few hours so hopefully I will have more information on how the motor spun the test wheel later this evening.

Re: Measuring WOT RPM's - Will This Work?

Per the original specs on that motor 5000 rpms is the proper rpm for that motor. The range for that motor is 4500-5500. The fact that you are getting that indicates nothing is wrong with your motor. The advice of 5800 given to you was bad advice. You can probably run a 17 pitch prop on that and still be in the correct rpm range and gain a little speed. The 15 pitch will give you a better whole shot. You motor should be capable of acheving the correct rpm and it is, but in truth we seldom run them wide open. It does not lug the motor to run them less than wide open.

Re: Measuring WOT RPM's - Will This Work?

freddyray21 said:

The advice of 5800 given to you was bad advice.

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Freddy, I understand where you are coming from, and I know that I am within the recommended operating range. Most all the pros on this forum will tell you that 5,800 is the desired RPM on today's fuel. The manufactures recommended operating range was determined in 1978 when the gasoline was very different than what is available today. I appreciate that you are trying to help, but I am still searching for 5,800 RPM.

Re: Measuring WOT RPM's - Will This Work?

While at the lake this afternoon I tried spinning the test wheel with the boat on the trailer. To my surprise I could only get to about 2,800 RPM. The motor runs so good that I had myself convinced that I am carrying around water weight, and I expected to hit 4,900 RPM with the test wheel. As soon as I hit 2,800 RPM, the motor started to bog and then lost most of its RPM. I ended up putting my Apollo prop back on and hit my typical 5,300 RPM in the open water. What could be causing me to get only 2,800 RPM with the test wheel? Maybe no spark on a cylinder? If I am only running on 3 out 4 cylinders I would expect that it would be difficult to hit 5,300 RPM. Please share thoughts because something is wrong, and I am stumped.

Re: Measuring WOT RPM's - Will This Work?

search for 5800 if you want, but it's too much for that motor. IMO in this case the pros are wrong. The 50-75 hp and lower ran at that rpm and were supposed to. I can tell you your motor is running fine. I would imagine your test wheel in this case was made for that rpm limit. Not all test wheels are the same. They are rated for different rpm to simulate different load problems in the shop.

Re: Measuring WOT RPM's - Will This Work?

if you truly want 300-400 rpm go down one to two pitch in prop. Nothing else you do to your motor imo will give you that. Too much, but hey it's your motor.

Re: Measuring WOT RPM's - Will This Work?

If you got 2800 rpm with the test wheel and the engine bogs out there is something wrong. Get a spark tester and check all cylinders, there are spark detectors which you just touch to the wire while running and they will tell you the KV of the spark on that wire with surprising accuracy. Any big variation in spark would indicate a mixture problem in a cylinder. Maybe there is a bit of dirt or water in a carb which sucks up at full throttle dropping a cylinder which is hard to tell when driving the boat. Keep searching you will succeed. As an aside, I think freddyray is correct, I too find the 140 hp 78 johnson rated between 4500 and 5500. Perhaps the steel of the connecting rods has gotten stronger and you can run it long periods above the rating. Fuel has nothing to do with the rpm rating.

Re: Measuring WOT RPM's - Will This Work?

again I say there is nothing wrong with your motor. What rpm was the test wheel rated for? They are not all the same. They are rated for certain rpm's to simulate conditions at that rpm. If it is taching 5300 on the water nothing is wrong with the motor. NOTHING It would not tach that if it was dropping a cylinder. If you want 5800 go down a pitch in prop.