motor height?

[rlguns]

I have a 17 foot stratos boat with a 90 johnson that has 140 heads and carbs on it.It replaced a 140 that went bad.Now my problem , its doing the same thing the 140 did.Only turns 4700 rpms and 42 mph , thats max!It does have a jack plate on it and is set about 1/2 to 3/4 inch from bottom of boat (the cavitation plate).Is it too low? By the way i tried a 21 pitch prop and a 19 and they both do exactly the same.Please help?

 

[rlguns]

Re: motor height?

Could someone please help me out on this please ?

 

[DJ]

Re: motor height?

What year is your 90/140? I never liked to turn crossflow v-4's much past what your getting.

 

[Dhadley]

Re: motor height?

At 4700 you are lugging the motor. That's not good. It can lead to coking and broken rings and/or beating out the wrist pins. <br /><br />Let's check a few things first. Make sure your tach is correct. I strongly recommend an OMC tach and make sure it's set on position 6. Have you tried raising the motor on the jackplate? (Watch the water pressure) What fuel? What's the timing set at? What spark plugs? Are the jets in the 140 carbs stock? Are the carbs clean? Are you sure it's running on all 4 at 4700?<br /><br />Don't be afraid to turn it up. It will actually live longer. We usually set up a crossflow to turn about 5800 with an average load. <br /><br />I really think we are goind to find the tach is not correct. If you figure the 21 pitch prop at 4700 RPM with your stock gear ratio, theoretical speed is right at 47 with no slip. If you figure 5500 - 5600 and 16 - 18% slip the numbers are pretty close.<br /><br />Good luck!

 

[rlguns]

Re: motor height?

The 90hp powerhead is an 84 model , the 140 carbs and heads are off of a 79'and are stock..Using 77jc4 plugs,and i dont know if the tach and timing is correct.It runs well no misses and is running on all 4.Water pressure is fine and im using 87 octane gas.I just think it may need raised because it runs at the same max rpms and speed as the 140 i just took off.It had a burnt piston which i see now could have been caused by the same problem i still have?I have tried a 21 pitch prop and a 19 pitch prop , and get the same results out of both.4700 and 42 mph max,it just will not go any faster.How fast should it go on a 16'9 stratos boat.I know the rpms should be mopre than 4700.Correct?Its about a 1/2 to 3/4 inch above the bottom of the boat.Should it be more?I just dont want to ruin another powerhead.

 

[Dhadley]

Re: motor height?

We need to get to a base line. What brand tach do you have? If it's an OMC tach make sure it's set on 6.<br /><br />Carbs are good -- fuel is good -- plugs are good. <br /><br />Things are lookng up. We'll get you going.<br /><br />Let us know. Good luck!

 

[rlguns]

Re: motor height?

It is an omc tach and it is set to 6.

 

[petryshyn]

Re: motor height?

As Dhadley says the tach can be out.<br />Even though its set on 6 doesn't assure you its accurate. Check to see if there is a calibration screw on the back. If so, it could have been messed with and is now out to lunch. Borrow another tach (hand held is easy) and compare to be sure.

 

[rlguns]

Re: motor height?

What about the speed should it not go faster than 42mph.I carried a gps out with me and it read the same so i know my speedometer is correct

 

[rlguns]

Re: motor height?

If i do raise my motor , How much do i need to try at first? An inch at a time , or what ?

 

[Dhadley]

Re: motor height?

Let's wait to raise the motor until we have established a baseline. Don't worry -- we'll get you more than 42, but we have to know where we are to determine where we are going. Like Schematic says, borrow another tach and lets get that fixed first. <br /><br />When setting up for performance we need several things you already have like a good boat, a GPS, a water pressure gauge and an accurate tach. Several props are a must (which you have a good start on). Then we make one change at a time.<br /><br />Good luck!

 

[rlguns]

Re: motor height?

Okay, So the first thing i need to do is get another tach and confirm that my original tach is correct and then let yall know the results?

 

[Dhadley]

Re: motor height?

Sounds good to me!<br /><br />Good luck!

 

[rlguns]

Re: motor height?

Can i put water hose on motor to check tach or does it need to be in water under a load?

 

[Dhadley]

Re: motor height?

I would check it out at WOT in the water. It's very possible that it's reading correctly on the bottom end but not at top end.<br /><br />Let us know!

 

[rlguns]

Re: motor height?

I have confirmed that the tach is correct .I even tried the 21 pitch prop and its exactly the same as the 19.4700 rpms and 42 mph is all it will do.I also checked my linkage and the carbs are opening all the way.I just cant figure it out.Still thinking about the motor heigth.PLEASE HELP!

 

[Dhadley]

Re: motor height?

How did you confirm that the tach is correct? It's very unusual to run props of two sizes and have the exact same RPM.<br /><br />Keep us informed.

 

[Dhadley]

Re: motor height?

Just a thought -- what about trim angle? When you get to 4700 is that at full trim out? Or all the way in? What's the difference in RPM from all the way in to full trim out?<br /><br />Let us know!

 

[rlguns]

Re: motor height?

4700 rpm is up and down is about 400-500 less.just cant seem to get rpms up.I ran up until it starts hopping , bad.

 

[SeaDawg]

Re: motor height?

Are both props aluminum? Are both 3 blades?