maxing out at 5K rpm....but speed is low

bob johnson

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I have a Lowe roughneck 12 degree Mod V...I had a 115 ocean runner on it and even with level flotation pods( meaning some drag!!) I still got about 38 MPH with a 12.75 X 15 aluminum prop. well that motor blew and I tore the boat apart because the foam absorbed so much water, and I installed a 1999 150 hp Johnson. I removed the center console and built a small side console and installed it on the front deck.... the motor ran great when I bought it. It was a long shaft so I pulled the power head and put it on a 1997 175 hp faststrike midsection. That mid section was wearing a 15 X 19 pitch aluminum prop in good shape. yesterday I took the boat out for some trial runs... and I only got up to about 35 mph at 5000 rpm.....WOT. the boat weighs right at 2000 lbs rigged and me and a friend were in it for an extra 360 lbs plus a 65 lb lab..... so am I close? I have a SS and an aluminum 15 X17 prop to try....but I am thinking ill only get up to about 5200- 5300 maybe with that pitch... I would have though I could run easily into the 40s at 5000 rpm... with a Big V6.... as the boat sits now the flotation pods are off.....I am wanting to get a baseline of WOT before I reinstall the pods...... Id hate to think I need to go down to a 15 pitch to wind this outboard up enough to get to 5600-5700 rpm.... WOT....which is where id like to set this motor up at.....thinking ill seldom run that RPM, because the boat is a duck hunting rig. does anyone think the motor isn't running up to snuff... compression was 90 psi even all 6 cylinders... I think I checked spark before I bought it... would it wind to 5K rpm on 5 cylinders???


heres a photo when I pulled out of the river last night

345zjnb.jpg


thanks


bob
 

Bondo

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Ayuh,.... What's the chances that the lower unit ya swapped to is of much taller gears,..??
 

bob johnson

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Ayuh,.... What's the chances that the lower unit ya swapped to is of much taller gears,..??

there might be some chance.... the lower unit I am using.. comes from a 1997 175 hp FAST STRIKE 20".... the power head came off a 1999 Xtra(25") long shaft 150 Johnson ocean pro( didn't know they went from ocean runner to ocean pro in JOHNSON garb!!) I ran into that before..I think you might be onto something . I think the 25" shafts have different gears......hmmmm...what to do......if so... can I solve this with props?......or do I need a gear change?? is there a source to tell me gear ratio in various lower untis over the span of my years?

bob
 

bob johnson

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Looking through the GL Maraine catalog, it looks like the V6 of the years in question( type O) lower unit had the same gear ratio for 20" and 25" lower units.......... my lower unit is a 1997 fast strike 175hp , 20" V6 unit and it has the prop it came with....because I put a 1999 150 hp power head on that mid section and lower unit.....shouldn't matter.... they all used the same 1.86:1 gear ratio.

bob
 

SkiDad

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It's a little hard to follow your post, but sounds like an engine issue - even if the gear ratio was taller the speed should still be much faster than 35. I average 37 mph with a 15 pitch and 2:1 ratio when I had a 1.73 ratio i was even faster but I like the lower gears for water skiing.

why did you change the mid leg - did you want a 25" leg ? how is your hole shot ? Could the lower unit be too low ??
 

jakedaawg

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5k is a little low for that motor....

I'd start with the simple stuff first cylinder drop test to make sure youre hitting on all six...

Mark the hubs to see if you're begining to slip the hub a bit...Is it a cupped prop?

Verify engine hight, I see you have a jack plate or is that just a tilt plate? If it's a jack plate was it all the way down or is it known to creep down? Doesn't look like you had a nosecone and a low water scoop so be careful if it is a jack plate.

Were you trimmed out? Having moved from a center console to a forward helm can surely effect trim. You may have more riding in the water now even if you were trimmed out.

I follow some of the restoration threads on here and it seems that when guys rebuild their boats they beef them up quite a bit at times. Any chance you added a bunch of weight? All those little things can add up to a whole bunch of weight in a hurry. That translates into a whole bunch of slow....

To gain RPM you can also decrease diameter which helps maintain speed but allows hole shot to suffer.
 

bob johnson

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It's a little hard to follow your post, but sounds like an engine issue - even if the gear ratio was taller the speed should still be much faster than 35. I average 37 mph with a 15 pitch and 2:1 ratio when I had a 1.73 ratio i was even faster but I like the lower gears for water skiing.

why did you change the mid leg - did you want a 25" leg ? how is your hole shot ? Could the lower unit be too low ??

the boat has a 20" transom, I found a good deal on a long shaft ocean pro 150... and I had a midsection and lower unit sitting at home that was 20"...so I bought the 150 ocean pro just for the power head! the motor is mounted so that in the full down position the plate on the leg is about even with the bottom of the hull..... so I get full use of the jack plate for when I go in shallow water...... as for the boat the rebuild was mainly to remove water soaked foam and to move the console for more open space. I did add the extra motor weight to the stern(about 60 lbs) as well as about 3/4 of a 4 x 8 sheet of 1/4" thick aluminum to the transom( sandwiched). that was to stiffen the transom...I has flotation pods welded onto the factory skin and they pulled and distorted the sheet some and . this time I am installing removeable flotation pods attached to hinges and lenco trim tab actuators. havent installed them yet. because I want to get a baseline performance with the boat motor combo...

bob
 

bob johnson

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5k is a little low for that motor....

I'd start with the simple stuff first cylinder drop test to make sure youre hitting on all six...

Mark the hubs to see if you're begining to slip the hub a bit...Is it a cupped prop?

Verify engine hight, I see you have a jack plate or is that just a tilt plate? If it's a jack plate was it all the way down or is it known to creep down? Doesn't look like you had a nosecone and a low water scoop so be careful if it is a jack plate.

Were you trimmed out? Having moved from a center console to a forward helm can surely effect trim. You may have more riding in the water now even if you were trimmed out.

I follow some of the restoration threads on here and it seems that when guys rebuild their boats they beef them up quite a bit at times. Any chance you added a bunch of weight? All those little things can add up to a whole bunch of weight in a hurry. That translates into a whole bunch of slow....

To gain RPM you can also decrease diameter which helps maintain speed but allows hole shot to suffer.

Ill look up what it is to do a drop test...... its kinda tough in cold weather and I really need a calm day..... that is one of my suspects.....verifying if the motor is in fact running as it should , before I mess with props. But it is hard to tell on a 6 cylinder

bob
 

jakedaawg

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bob johnson search cyl drop test in the J/E forum, I am pretty sure you have been there before;)

Basically you want to run the boat under a light load and briefly pull off and place near ground each spark lead, one at a time and note difference in operation. This tells you if in fact each cylinder is contributing. If you were to pull off a lead and not notice a change that would tell you that specific cylinder has an issue and is not firing, compressing, getting fuel,....It doesn't tell you what is wrong, just that something is wrong.

One misfiring cylinder could lead to the drop in RPM you are experiencing.

Has this been your motor on a different boat? Has it ever run properly for you? You stated a comp test I recall, have you performed an open air gap spark check?

I would tend to lean towards the helm being moved and or a little to much weight added during a resto/mod causing the boat to ride different in the water being the issue. Since the motor was never originally propped for this boat we do need to verify that the motor is 100% before going much further except I would check for a starting to spin hub now, its easier than hanging over the back of the boat on cold water pulling leads for the drop test.

Another thing to think about is you went from a lighter v-4 at 5800rpm to a heavier v-6 at 5k RPM. This will also change the ride and the amount of boat in the water when trimmed, this all effects top speed.

Its always nice to work ewith you on your issues because you answer questions asked of you and you also ask good questions. Sorry you're having issue but I am sure you can sort it out.
 

bob johnson

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jake, I don't have a HUB...that would be nice to troubleshoot in the yard!!! I was thinking to test spark at speed, Id have to install a timing light or a spark gap tester one at a time and run up to speed and have someone check while I drive.. then stop and switch to the next plug!! I guess I could get a pair of insulated tongs and try to pull each plug wire at speed , if I was going 3000 rpm...id think id be able to hear a change or not... I can tell you I don't feel comfortable going much faster and hanging off the back of the boat on a motor with no cover and trying to pull spark plug wires!!!!!!!!!!!!!!!!!!! the water temps sunday on the river was 35 degrees!!!!!!!!!!!
 

ondarvr

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You don't pull them off at speed. Run it wide open and record the speed and RPM's, stop the motor, pull a wire and ground it, start the motor and run it wide open, record the speed and RPM's again, stop the motor and move to the next plug. Compare the results from each run. You may be able to feel the difference too, but not always.

You do have a hub, but that's not the issue.
 
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bob johnson

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You don't pull them off at speed. Run it wide open and record the speed and RPM's, stop the motor, pull a wire and ground it, start the motor and run it wide open, record the speed and RPM's again, stop the motor and move to the next plug. Compare the results from each run. You may be able to feel the difference too, but not always.

You do have a hub, but that's not the issue.

I was thinking the "hub" was a test wheel...to give the motor some back pressure while running in a tank or on muffs..
 

ondarvr

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The rubber hub in the prop, but if it was slipping your RPM's would skyrocket.
 

bob johnson

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so the plan now is to drag the rig to the marina near where I hunt... go out in the am on the water and when I pull the boat drop it off at the shop there. They will install a brand new water pump kit.... and then they will put the motor on their dyno and check all the electronics for firing, as well as anything else they notice... they have a test wheel they use and they will also tell me the hp the motor is putting out... I really hate dropping the LU on these bigger motors.... it is a totally pain gathering up all the wires and hoses that go into the front of the motor and making them fit without pinching when I install the lower covers....since I have over size battery cables, a water hose for pressure and two extra wires for temp senders....its TIGHT!!! and you can pinch off hoses and wires not knowing when you install those lower covers...as well as making sure the housing gaskets are sealed right.......I don't think it is possible to disconnect the shift rod without taking the lower covers off

bob
 

shaw520

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90 psi seems way too low-no power???
My original thoughts exactly,.... compression should be up in the 120-140 range,...got any idea how many hrs on this power head,... maybe time for a top end rebuild.
 

ondarvr

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Don't get hung up on compression numbers, gauges can read very different from one another, as long as they're all close you're typically OK.

Knowing your gauge helps.
 

bob johnson

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so the marina called and they are done with the boat. They put it on the dyno and took "pressure" readings from the device that hooks to the prop shaft I guess... and while it doesn't give HP readings it gives readings of pressure that are compared to reading of a factory fresh motor. the readings are identical to a brand new motor!!! they have a range where they say the readings are still "good", but mine are the exact same #s as a new motor....so the compression is just fine.... the motor is running great on all cylinders, and they are all very equal..... so good news.... he did tell me someone had disconnected the QUICK START feature...which I didn't know about...but I don't need it anyway...as when its cold I am usually playing with the throttle up and down to get it running, but not choke it out....its a fine line when its 10 degrees out...to start the motor, and keep it running those first few minutes.... so now its time for me to take it out on the river and try a few props....
 

Sea Rider

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Now that you finally know that motor is healthy, check if sits at the sweeet transom height, if so, go for a prop maximization to pull wot rev as usually loaded betweeen middle to max wot rpm range factory stated.

BTW, this is the sweet motor/transom height as well for your combo, follow guideliness on last post and will be fine boating happily shortly.

http://forums.iboats.com/forum/gener...different-prop


Happy Boating
 
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