propeller is too big? must read!!!!

[quantumman29]

hello everyone,
as many of you have known for a few months ive been having some problems with my 1985 115 tower of power,
the original problem was the engine would run fine then die and stall 20 minutes after running,
the engine has since been worked on,heres what has been done:
all 3 carbs cleaned and rebuilt,new lower switch box installed,new water jacket,new impeller,fuel pump rebuilt,new plugs,and total link and sync.....

the ol girl starts up like shes brand new,better then ever,idles sweet,takes off like a rockett runs great for 20 mins then dies,she does start right back up again but will only get to 1500 rpm and thats it........let her cool down for an hour in the shade,she runs like a dream again.....for 20 minutes only!

heres the kick in the ***:
the engine is on a 17.5 ft. glasstream bass boat, I put a stainless steel prop on that is a 23 inch pitched prop, its 3 times heavier then the 17 inch pitch aluminum prop that was originally on the engine,

I am 100% sure that the cause of the engine stalling is caused by overheat due to the 23 inch pitch heavy stainless steel prop creating too much torque on the engine.....
also, the engine will not hit more then 4800 rpm with wot with the stainless prop.

I AM PUTTING THE ORIGINAL ALUMINUM ENGINE PROP BACK ON THE ENGINE AND CALLING IT A DONE DEAL......NO MORE FANCY UPGRADES FOR ME!!!!!

can anyone agree with my discovery that this is the cause of the stalling? and if so what is the exact reason why the 23 inch prop cant be used? and why it causes the overheating?

23 inch stainless steel prop for sale, private message me if you want it,,,

mike

 

[roncoop75]

Re: propeller is too big? must read!!!!

I'm guessing high speed stator. It gets hot, opens/shorts internally, this kills spark and consequently the engine. You start it up hot and it runs up to 1500 rpm on the low speed stator but no higher until the high speed section of the stator cools.

That's my theory and I'm sticking to it. Well, until someone comes in here and shoots me down.

 

[asm_]

Re: propeller is too big? must read!!!!

That's my theory and I'm sticking to it. Well, until someone comes in here and shoots me down.

Sure. I'll take a shot.

Your theory sounded good, but according to my manual high speed coil doesn't kick in until 3000RPM.

 

[Ski'nBlind]

Re: propeller is too big? must read!!!!

I won't disagree the manual says 3000rpm, but under what load? I don't believe the transition from low to high speed stator windings occurs at exactly 3000rpm and ONLY at 3000rpm.

That motor has 2 high speed and 2 low speed windings, one "set" for each switchbox. If one high speed winding goes ka-putt the motor will run at rpm on 3 cylinders and most likely idle great on all 6 cyl's. You may or may not see 3000rpm because of the load on the engine.

Quantum, it sounds like a thermal problem when an ignition part gets hot and malfunctions, then cools off enough to "make" a connection. Looks like you have about 20 minutes to diagnose and find the malfunctioning part. Also, make sure you disconnect the 2 yellow wires off the voltage rectifier and tape them off when it does this again. Mine went from 2000rpm straight to 6000 when I did that test. Just had to replace the rectifier....cheap and easy!

I will take a guess and your 17" prop will work great for about 20 minutes, but the rpm's will be where they need to be for that engine.

Let us know what you find

 

[roncoop75]

Re: propeller is too big? must read!!!!

Sure. I'll take a shot.

Your theory sounded good, but according to my manual high speed coil doesn't kick in until 3000RPM.

DRAT! Well, I learned something anyway. Thx!

 

[Chris1956]

Re: propeller is too big? must read!!!!

Mike, when you jumped from the 17" pitch prop to the 23" pitch prop, you should have lost 1200 RPM at WOT, or maybe more, due to loss of teh engine's powercurve. I am surprized the that TOP didn't self-destruct. I recommend a compression check and a trial run with the 17" pitch prop to see if you can get 5200-5500 RPM.

 

[arsenalpsu]

Re: propeller is too big? must read!!!!

i agree 100% with the last post, if she only got 4800 WOT you lugged her a lot. A S.S. prop is not the problem, you should have got a 17p SS prop, the "upgrade" as you call it would have then been fine. When you lug an engine like that it puts added stress on the pistons and will heat things up, causing detonation and carbon build up. These engines like to spin, prop WOT for 5500 and don't run WOT all day long unless your 100% sure your fuel system is good. With the amout of fuel these old girls use at WOT, they can empty the carbs in 30 seconds if you have any air leaks or your fuel pump is not up to par. If your pistons are still good you are one very lucky guy.

 

[j_martin]

Re: propeller is too big? must read!!!!

Agreed. The weight of the prop is of no consequence, except for shifting gears. When it's running, it's a balanced rotating mass, so the weight has no effect on power consumption.

The pitch change from 17 to 23 has me totally baffled. Why would you expect the engine to pull that without lugging?

I run a similar bass boat, (18' tracker) with an XR4 putting out about 165, into a 24 pitch wheel. Mine's set up with a jack plate and setback so I'm piercing the surface with the prop. That's 50% more ponies on an equivalent boat, with a high performance setup.


Now, if you put a 17p stainless prop on it, you would experience a modest improvement in performance because the stainless prop has thinner cross sections and thus is more efficient. If you're light enough and get your setup just right, you might be able to run a 19, but that's a big if.

hope it helps
John

 

[quantumman29]

Re: propeller is too big? must read!!!!

I only ran the boat about 30 minutes with the 23 inch prop on it,I really havent used the boat much since I bought it,im still tweaking with it,
all cylinders are over 125lbs at this time,
the engine has strong spark on all plugs,
my next move before I use the boat/engine is to replace the high speed stator....

is it wise for me to try the boat/engine with the original aluminum prop before replaceing the high speed stator?

I was getting 5500 rpm with the aluminum prop the first day I ever test ran the boat after I bought it,since then I switched to the ss prop and had these problems,but again,I never forced the engine to run,when it would begin to bogg down id shut it off before it stalled and always used the trolling motor to get back in to the dock........82lb thrust motorguide great white.

im pretty sure its the high speed stator though,
i'll follow up on it.

mike

 

[BoatNoobie]

Re: propeller is too big? must read!!!!

I only ran the boat about 30 minutes with the 23 inch prop on it,I really havent used the boat much since I bought it,im still tweaking with it,
all cylinders are over 125lbs at this time,
the engine has strong spark on all plugs,
my next move before I use the boat/engine is to replace the high speed stator....

is it wise for me to try the boat/engine with the original aluminum prop before replaceing the high speed stator?

I was getting 5500 rpm with the aluminum prop the first day I ever test ran the boat after I bought it,since then I switched to the ss prop and had these problems,but again,I never forced the engine to run,when it would begin to bogg down id shut it off before it stalled and always used the trolling motor to get back in to the dock........82lb thrust motorguide great white.

im pretty sure its the high speed stator though,
i'll follow up on it.

mike

But you were 100% sure yesterday it was the prop.

 

[quantumman29]

Re: propeller is too big? must read!!!!

hee hee,
either way,the engine is still attached to a large hole which sits in the water in which we all throw a lot of money into !!!!!!!

mike

 

[Chris1956]

Re: propeller is too big? must read!!!!

Mike, At what speed does the high speed winding of the stator take over?

If you are getting 4000 RPM or more, it is not your high speed winding. If you couldn't get more than 2500, you may have a stator issue.

 

[CharlieB]

Re: propeller is too big? must read!!!!

I'm curious about the fuel tank venting, is the primer bulb flattened or very soft when the 20 minutes are up?

A restricted tank vent could allow the motor to run a while before tank vacuum seriously restricted fuel flow, waiting an hour till the tank vented would restore lost fuel flow and power again.

The larger prop and additional load could be more fuel demand than the restricted tank vent can flow. The lower pitch prop would not create the same load and fuel demand.

 

[arsenalpsu]

Re: propeller is too big? must read!!!!

I'm curious about the fuel tank venting, is the primer bulb flattened or very soft when the 20 minutes are up?

A restricted tank vent could allow the motor to run a while before tank vacuum seriously restricted fuel flow, waiting an hour till the tank vented would restore lost fuel flow and power again.

The larger prop and additional load could be more fuel demand than the restricted tank vent can flow. The lower pitch prop would not create the same load and fuel demand.

I think you're a bit confused on one area, the larger prop would cause the engine have a lower RPM at WOT, the slower the RPM the less fuel us consumed meaning a higher pitch prop producing a lower RPM at WOT would use less fuel......less "sucking and blowing" going on at lower RPM

 

[CharlieB]

Re: propeller is too big? must read!!!!

This is a common misconception, that fuel flow is directly related to RPM.

Fuel flow is directly related to LOAD.

Even with a severely restricted fuel system a motor will rev like crazy in Neutral with NO load, yet once in gear in the water and LOAD, it won't pull any more RPM than the fuel flow will allow.

More load requires more fuel. It least until you get to the limits of the carbs airflow.

He 'could' make a test run, WOT with the large prop, kill the engine at WOT,(watch out for the following wave), pull the plugs and see if any or all are white from running lean. But it may be safer to install a pressure gauge between the fuel pump and carbs to see if pressure begins to fall off at WOT.

I it quite possible that his fuel system has a problem and should be checked out, running lean with the larger prop could toast pistons.

 

[arsenalpsu]

Re: propeller is too big? must read!!!!

That's true but i wasn't talking about a no load condition. I was talking WOT with a 23p at 4000RPM and his 19p at 5500 RPM, In both conditions of WOT the carbs are open fully open allowing max. flow, with an extra 1500 rpm those cylinders are putting a higher demand on the carbs.

anyone else want to chime in??