Will a blown motor get 6000 rpm?

[spacecoyote]

I have a 1989 Force 125 on a 16'7" Aggressor bass boat. The motor was supposedly rebuilt by the previous owner, but there are some telltale signs that it could have been a hack job. That aside, the motor seems to run smooth. I had the carbs rebuilt, so I'm pretty sure there is no problem with the motor getting fuel. The problem is that when operating at 5500 rpm, I'm only going 25mph. I can't even get the boat trimmed, it just plows. I can open the throttle even more and get up to 6500rpm, but I don't keep it that high. Even then, the boat won't get up on it's pad.
My question is, would a motor with bad compression or a missing cylinder, even get 6500rpm? Is there something else, motor-wise, I should check? I am trying to rule out the motor before I consider waterlogged foam, prop selection, or transom height. It just seems that even with a heavy boat or too little prop I should be getting better than 25mph.

 

[pnwboat]

Re: Will a blown motor get 6000 rpm?

I would also consider confirming that the tach is reading correctly. That motor requires a 20 pole tach. Not very common these days.

 

[spacecoyote]

Re: Will a blown motor get 6000 rpm?

I considered that possibility. The tach reads 1200 at idle, which seems about right. At wot, the motor certainly sounds like 6500 rpm. But, assuming the tach is right, would a motor with low compression be able to rev this high? In other words, wouldn't a blown motor, or one with lower compression in one cylinder (by more than 10%) run roughly, and not be able to achieve high rpms?

 

[milehighboater]

Re: Will a blown motor get 6000 rpm?

Yes a motor on only some of the cylinders can reach that high with a prop pitched down or just simply too small of a prop. I have seen guys loose engine power and instead of finding the cause they will just put a different prop on the motor to make it run at the correct rpm at wot. Now if the prop is the correct size and pitch then there are some other things you need to start looking at. I suggest going to Autozone picking up a 20 dollar compression tester and just finding out for sure instead of guessing. There are some good threads on how to do a proper compression test.

 

[crablegs]

Re: Will a blown motor get 6000 rpm?

Sounds like prop to me

 

[spacecoyote]

Re: Will a blown motor get 6000 rpm?

I have actually done 2 tests in the last week. I did one myself, with the throttle wide open, which, I assume must be in the neutral position. I was dismayed to find low readings on all the cylinders, with #1 being 60psi, and the rest around 80. I began to start the process over, then noticed my top carb shooting fuel out of the overflow. Not wanting to risk an explosion, I stopped. The next day, I took the boat to the guy that rebuilt my carbs. He cleared the carb, and we did another compression test. The mechanic this time did not open the throttle. The results: #1-132, #2-125, #3-122, #4-110
Which results am I to believe. I know that 132psi - 110psi. = too much. Would a decarbing be a good place to start?

 

[jerryjerry05]

Re: Will a blown motor get 6000 rpm?

Get a GOOD comp tester.One that screws into the plug hole.Remove all the plugs and open the throttle all the way.Then do the comp test.

If it idles good and you can get it to do 6000 RPM.What size prop do you have? It sounds like your hub has spun.
Post the results.J

 

[jaybirddog11]

Re: Will a blown motor get 6000 rpm?

I have actually done 2 tests in the last week. I did one myself, with the throttle wide open, which, I assume must be in the neutral position. I was dismayed to find low readings on all the cylinders, with #1 being 60psi, and the rest around 80. I began to start the process over, then noticed my top carb shooting fuel out of the overflow. Not wanting to risk an explosion, I stopped. The next day, I took the boat to the guy that rebuilt my carbs. He cleared the carb, and we did another compression test. The mechanic this time did not open the throttle. The results: #1-132, #2-125, #3-122, #4-110
Which results am I to believe. I know that 132psi - 110psi. = too much. Would a decarbing be a good place to start?

A decarb wont hurt. Try making sure your head bolts are torqued correctly. If all appears the same after a decarb and head bolt tightening, you may consider pulling the head. Your head gasket could be bad. I would tend to agree with previous posts that your prop is undersized or your tach is not correct.

 

[cougar1985]

Re: Will a blown motor get 6000 rpm?

sounds like you might have a spun prop hub.a decarb would not hurt atall.use a permanent marker or scribing tool and put a mark on the rubber hub and one on the prop body adjacent to the hub,take it for a spin then check the prop ,if the marks dont line up anymore your hub is toast.this is assuming you dont have a super low pitch prop on there by accident .

 

[spacecoyote]

Re: Will a blown motor get 6000 rpm?

Would a spun hub be evident by looking? I've tried to spin the hub while in gear, and it turns over the motor. I removed the prop, and everything appeared fine. I will try the test you recommended, thanks.
But, to stay with the thread, if I am truly getting 6000+ rpm, is the motor useable, assuming I can take care of the problem if it is not motor related? What I am saying is, I do not wish to spend much more time and effort on this motor. If it truly requires being rebuilt, then I will sell as is. But if I can get a few months, even a year or two out of it by decarbing and taking the other inexpensive steps mentioned, I will be happy.

 

[Frank Acampora]

Re: Will a blown motor get 6000 rpm?

With 130 PSI and reaching 6500 RPM, the engine is fine. Do not decarb, it is not a cure-all, and may only cause more problems.

Look elsewhere for the low speed, ie. engine height on the transom, bad prop, but first use a GPS to verify speed. If you have a pitot speedometer, check to see that the probe is not kicked up and giving an artificial low reading.

 

[spacecoyote]

Re: Will a blown motor get 6000 rpm?

So Frank,
Am I to understand that a motor with the highest psi of 132 and the lowest of 110 is ok? I thought (from reading this forum) that the difference between any two should be less than 15%. Does it just mean that the motor won't perform at full capacity, but is still usable?
Could you clear up something about compression for me. It goes back to my original question about getting high rpm from a low comp. engine. With all other factors being equal (prop, mounting height, hull, etc.) will a motor with high compression at 5500 have more power than a motor with low compression at 5500? Or does 5500 mean the same thing either way? Does compression have an effect on rpm, or is it the link between rpm and the driveshaft? Does that make any sense?
I am weighing the boat today. This may provide more clues. Assuming the boat is overweight, how much additional weight would it take to be a suspect in my boats slowness?
Thanks for all of your help everyone. I'm glad you jumped in Frank. I've read many of your posts and find them very imformative.