9.8 2-stroke Loss of thrust at higher RPMs

[fortunate2001]

Well, was cruising around last weekend in the inflatable with no problem, then on the way in, I lost most of the power / thrust. At idle or a little above, seems normal, but at any higher RPM than that, there is no increase in thrust. I swapped the propeller with another and had the same issue. I think that means it is not a 'spun hub' (I think that's what its called).

I have read that the 'coupler' may be next, but everything I found on this seems to apply to larger engines.

Any advice on next troubleshooting steps?

 

[Sea Rider]

How old is that OB ? does it bogs down, misses, accelerates through middle to max throttle with little or no trust whatsoever ? Is the second prop in good shape, visually doesn't count, must be 100% operative.

Happy Boating

 

[fortunate2001]

Sea Rider, thanks for the reply.

It's a 2000 9.8B2. Never showed any of the things you mentioned before. Since the problem, no bogging down, but yes, it accelerates from middle to high RPM with no increase in power.

I pulled the second prop off an older 5HP nissan that was working fine when it went into storage a few years back. I didn't test the prop before swapping it in.

 

[pvanv]

Sea Rider, thanks for the reply.

It's a 2000 9.8B2. Never showed any of the things you mentioned before. Since the problem, no bogging down, but yes, it accelerates from middle to high RPM with no increase in power.

I pulled the second prop off an older 5HP nissan that was working fine when it went into storage a few years back. I didn't test the prop before swapping it in.

Not possible. The prop for the 4/5/6 will not fir on the 8/9.8. Period. You probably installed a bad prop.

 

[Sea Rider]

Fortunate,

If OB pushes fine at idle/fast idle speed, doesn't bog down or misses at middle to max wot range, revs high but don't have propulsion looks as having already a spun hub. Props goes bad with time, use, abuse, worst if hitting a underwater object at speed. You're looking at a 16 year oldie prop. What's the current application and size that 9.8 is powering ?

Before venturing into fingering my OB would install a brand new same prop pitch if water performance with current load was excellent. If not go one less pitch to pull revs up. Props are cheap, go for one, check Iboats.

Happy Boating

 

[fortunate2001]

Not possible. The prop for the 4/5/6 will not fir on the 8/9.8. Period. You probably installed a bad prop.

It is a much smaller prop, not a replacement, and probably a bad tester. The only thing that matched is the 12 splines... I'm going to re-test that original prop.

 

[TGuy]

Once you put your original prop back on, mark a line on the prop and hub. See if they still match up after you test the motor. If the hub is "spun", the lines won't match up.

 

[fortunate2001]

Fortunate,

If OB pushes fine at idle/fast idle speed, doesn't bog down or misses at middle to max wot range, revs high but don't have propulsion looks as having already a spun hub. Props goes bad with time, use, abuse, worst if hitting a underwater object at speed. You're looking at a 16 year oldie prop. What's the current application and size that 9.8 is powering ?

Before venturing into fingering my OB would install a brand new same prop pitch if water performance with current load was excellent. If not go one less pitch to pull revs up. Props are cheap, go for one, check Iboats.

Happy Boating

The boat is an inflatable. Air floor. The whole thing weighs only about 50 pounds. The day it had the issue I had a full load of passengers, though. And you're right, the prop is old, probably was just the last straw.

So, I marked the prop and put it back on, took a spin, and then removed to examine. The marks no longer lined up, spun. So I bought a new prop... Hopefully that is the only problem...

 

[Sea Rider]

If lines doesn't align, you have a spun hub. Is not same dialing a prop pitch that works well with say 1-2 boaters and same with 3-4 up, that's if wanting to take the max HP out of that nice portable OB for each case. Should check that new prop max wot revs with an induction tach while lightly loaded, ideal is to run at max wot range with current load.

Happy Boating

 

[fortunate2001]

New prop installed and working, dinghy is back in service.

New issue- a medium high pitched grinding noise while underway. Doesn't happen in neutral or reverse, and in forward, starts making the noise steadily at about 1/4 throttle and up...

 

[pvanv]

Is the correct thrust bushing installed on the prop shaft? Considering the incorrect information (there is NO WAY that a prop for a Nissan 5 hp motor could ever possibly fit on that motor), I am skeptical that the work is being done in a competent manner.

 

[Sea Rider]

New prop installed and working, dinghy is back in service.

New issue- a medium high pitched grinding noise while underway. Doesn't happen in neutral or reverse, and in forward, starts making the noise steadily at about 1/4 throttle and up...

Were you having same issue on previous prop ? If not, it's a prop issue. 5 & 9.8 are 2 different props that will not ever spline match.

Happy Boating

 

[fortunate2001]

I am skeptical that the work is being done in a competent manner.

Haha, me, too. That's why I'm here...

I never removed the thrust holder when I swapped out the prop. Took out the cotter pin, removed prop nut, and washer, slid prop off. Did the reverse with new prop

 

[fortunate2001]

Were you having same issue on previous prop ? If not, it's a prop issue. 5 & 9.8 are 2 different props that will not ever spline match.

Happy Boating

I don't recall ever hearing the 'grinding' noise before, so I think it is related to the new prop, or I messed something up when swapping. My next step is pull the new prop and see if I can find any wear spots where it might be making contact.

Maybe not important at this point, but I'm curious. The 5HP prop is a smaller in all respects, but the splines seemed to be the same. I had tried just a quick test (<30 sec) to see if the prop shaft was spinning properly. The result was inconclusive, seemed like the 5HP prop had the same issue, maybe it's spun as well- its very old. I'll see if I can upload a couple pics, the darker prop is the 8.5x9, the lighter is off the 5HP.

.....Wouldn't upload, but here is a link to the pics: https://1drv.ms/f/s!Aurb1rQcCYtwrVGYh1htAcLzqmv2

 

[Sea Rider]

Sorry missed the point,, did you purchased a brand new 5 or 9.8 prop and with which pitch number stamped on body ?

If with 5 probably has too much back and forth play, do both props have same lenght bodies ? Onc prop is correctly adjusted tight with nut, there should be a 2 mm back and forth play, but that's normal.

Happy Boating

 

[fortunate2001]

I bought a new prop for the 9.8, it is identical to the old, 8.5 diam and 9 pitch. I tightened finger tight and got the pin in, there is about 1 mm of play.

 

[pvanv]

Again: There is NO WAY a Tohatsu/Nissan 4/5/6 hp prop will ever fit on a Tohatsu/Nissan 8/9.8. Period.
What kind of <30 second test did you do to see if the prop shaft was spinning correctly?
The 9.8 prop should be sandwiched between a thrust bushing and washer, and the nut should align with the cotter hole in the prop shaft.

 

[Sea Rider]

If prop nut is adjusted tight by finger torque that could be the culprit, advance to next nut cut and insert cotter pin on hole. Can place a piece of wood to lock blade against lower antivav plate for the job. Once done check if sound has finally cleared.

Happy Boating