High thrust 9.9 L/U interchange

[ChipG3Eagle]

I own a 2002 Yamaha T9.9ELRA High Thrust motor. I purchased this motor used and was told that it came off a sail boat. It came with the "standard issue" high thrust prop. (I believe it was a 11 3/4 x 9 1/4 or something very close to that.)
I purchased this motor to use on my new G3 Eagle 165 on 10hp limit lakes in the area. With the original prop I was only able to turn 2200 rpm at WOT, not sure if that is normal or not for a high thrust with that prop, and got about 7-8 mph by GPS.
In an effort to get the motor to it's WOT rpm range of 5500 rmp I replaced the original prop with a "normal" small engine prop, a 9 x 9 3/4, as a test. I am now able to turn about 2500-2600 rpm at WOT and get about 7-7.5 mph by GPS. I have considered going to about a 7 pitch in an effort to further increase my rpm and put less stress on the motor. My only concern is that I will most likely lose even more speed due to the low gear reduction and lower pitch. Can anyone give me any advice on using these high thrust motors on smaller boats? I knew it would not be fast, but I don't want to hurt the engine by running it at low rpm while at WOT.
My last ditch effort to remedy all this is to find a standard long shaft lower unit and swap it with the high thrust lower unit to get back to the higher gear reduction. I don't know of any way to determine what the interchange possibilities of this are. Does anyone know if the standard long shaft 9.9 lower units will interchange with the high thrust lower units? I really think that I would be happier with the standard gear reduction for this boat. My loaded tournament weight is less that 1500 lbs total.
Any help or suggestions would be greatly appreciated. Thanks.
Malvin

[99yam40]

My guess is that your motor is sick and can not produce the power needed to turn the prop properly under load.
how are you getting the RPMs?

Leakdown test the cylinders , check for spark at problem RPM , check timing at idle and WOT( timing light should show any loss of spark also). check fuel pressure/vacuum to make sure supply is OK.
The clean carbs and adjust as the manual calls for

But you are asking a lot from a 9.9

Here is the only performance info I could find on the T9.9
http://www.yamahaoutboards.com/sites...sa74ea.tmp.pdf

[ChipG3Eagle]

99yam40 - Thanks for the quick reply. You may be on to something with the motor having an issue. It starts and runs great, purrs like a kitten, but even if I trim the motor up while running I'm not seeing an increase in rpm. By the way, to answer your question about how I'm reading the rpm, I am using the factory installed gauge that was already wired into the existing harness.
I'm not sure that I'm familiar enough to perform all those checks you mentioned, but I did clean the carb and fuel filter after rigging the motor and before taking the boat out for the first time. I also replaced the sparkplugs and lower unit oil, and changed the engine oil.
I purchased the boat new with the installed fuel tank and fuel/water separator. I installed a new fuel line and primer bulb when I rigged the motor. I also had to manufacture cables for the starter and ground since the previous owner did not provide them with the motor. I used 6 ga. automotive cables and they seem to work fine.
I noticed in the performance data that you cited that the hull weight on the test platform was around 600 lbs plus the two people and gear, which is quite a bit less than what I am looking at. Do you think that the weight I'm trying to push would keep the motor from being able to turn the prop and achieve WOT?
Also, do you know if the possibility exists of swapping the lower unit with that of a standard long shaft 9.9, once I get the motor lined-out? From what little I've researched it, they appear similiar.
Thanks again for your help.

[99yam40]

Is the tach set properly for your 4 stroke motor?
I'm Not sure how many pulses the 9.9 has, but you need to find out and make sure the tach is set properly.
Probably can be swapped with higher geared lower, but that will just put more strain on motor.

You need to find out why you cannot get the RPMs up

9.9 may not plane a boat that heavy, and if it will not get the hull out of the water you have a lot of drag

[boats.net]

Set tach for 6 pole setting, T9.9 rpm range is 4000-5000 rpm's at full throttle.

[ChipG3Eagle]

Thanks for all your help and I will work on these suggestions and let you know how things turn out!

[ChipG3Eagle]

Here's an update on your suggestions...

I checked the tach and you were correct. It was not set for 6 pole, it was set for 12. So, presumably I was only reading half the actual rpm. I took the boat out for a test run with the tach setting corrected, and sure enough, it was turning 5200 - 5250 rpm at WOT! I feel much better now.
My next question is, should I try a prop with a little more pitch to bring the max rpm down a little? The boat is getting about 7 mph or a little better, (with 2 people and 2 labradors!), and seems as though it is very close to planing. I'm sure I would need about 9 or 10 mph to plane it with two people and gear. It actually sounds as though it's cavitating a little when I go beyond 4500 rpm, and I don't see much speed increase beyond that rpm.
Just curious what your thoughts are. And thanks again for all the helpful information. You guys are lifesavers!

[99yam40]

which prop are you running on it now?

[ChipG3Eagle]

I am currently running the 9 x 9 3/4 and getting about 5250 rpm at WOT. That gives me a gps top end of around 7.2 mph. Although I'm not seeing any significant speed increase from about 4500 rpm up to WOT, and it vibrates a little and sounds as if it's cavitating a little beyond 4500 rpm. The cavitation plate is set level with bottom of transom. The bow walks a little acting as if it trying to climb up on pad.

[99yam40]

put your other prop back on and see what RPM you get with it and if the cavitation seems to go away or at least changes.
Is the speed the same at 4500 and 5200?

Do you think the prop is sucking air above 4500?

Make sure the weight distribution is good and the motor angle is set properly.
Too much up front will cause it to plow
Then try running it without all the weight in it to see how that makes it act.

A flat bottom is easier to plane than a semi-v, and I am not sure what yours is.
Sorry but that is all I can think of

[ChipG3Eagle]

99yam40 -
It may be a few days before I can swap the props back and run it. We are in the middle of a terrible heat wave experiencing record temps of 107-109 degrees... I had the bright idea of fishing yesterday and it was nearly unbearable.

[99yam40]

I know Heat plus humidity is tough on a person.
I was out wed morning down here on the Texas coast in East Matagorda Bay and at 8am I had sweat running down my arms dripping off my elbows while just casting and reeling out of the boat. Picked up 2 - 23 " redfish and lost 2 others. Must have had 20 or so blow ups on top water.
tide was still up about 2 foot above normal all the way over here from the storm that hit Florida I guess

[tizzel57]

standard lower unit

The lower units are a direct swap, I also have a F9.9 ELRA I did the swap the other way standard to high thrust,I sent you a PM, I have a like new about 10HR'S of use standard lower unit,I also have the items needed to go from the standard 22" to25"X long.
Tizzel57