how fast and what rpm ?

[farmboss]

new to boating and have a few questions.... bought a 1984 cheetah 166. its got a 140 merc... runs pretty good... its got a 14x17p prop on it.... rpms top out at 3800 at 20 mph.... is this right ? thought it would be a little faster ..... no real other issues.... am i exspecting to much from the little thing or not ? what could i do to make her strecth her legs alittle more ?

thanks for any of the help . like i said im new to the hole boating thing. just trying to get into a family friendly sport...... these jeeps are killing me!

 

[Bob_VT]

Re: how fast and what rpm ?

welcome to iboats.

Break out the shop manual and start troubleshooting. You should be running between 5500-5800 rpm at wot. Your motor might have...... a compression problem, fuel problem, carb problem or electrical problem. You need to start by checking spark on all cylinders and compression too.

Get right on it before you burn a piston and trash the motor.

 

[steelespike]

Re: how fast and what rpm ?

I'll second that,^ I would say your at about 1/2 power Check the condition of the plugs and do a compression test and check the spark.If you have carb trouble and are running lean you could burn a piston.

 

[SparkieBoat]

Re: how fast and what rpm ?

I 3rd that...read my sig below and follow links for helpful guides.

 

[farmboss]

Re: how fast and what rpm ?

are you sure the motors suppose to turn out over 5000 rpm ? i read the book that it closer to 4200......?
complete rebuild of the carbs is done .. much better.... mallory solid stat ignition.....fresh plugs gapped....good spark.... havent done a compression test yet...

got an off the wall one for ya... is there a chance i wasnt trimming it up high enough ?

i dont believe i was ... im gonna check the speed tomorrow....

i figured 30-35 would be about all i get out of her ? but your saying half power ? i guess i might have more trouble then i thought ....

wish me luck . and thanks again for the help...

 

[fishndirk]

Re: how fast and what rpm ?

Before you go breaking out the crazy tools, answer some basic questions. How long is that boat, approximate weight, type of hull and a typical load that you carry on the boat. Assuming that the motor runs good you may have the wrong prop on it. Does it take forever to get out of the hole? Details?

 

[farmboss]

Re: how fast and what rpm ?

140 horse merc 16'6" cheetah....fiberglass v-hull....2 people , 35 pounds of gear... seems to get out pretty quick... doesnt nose up too bad at all....it has a 14.5 " 17 pitch prop...

anything else

 

[fishndirk]

Re: how fast and what rpm ?

Id say the problem is definately in the motor not the prop. With a 16ft boat and a light load, 140hp at 17 picth should definately get you faster than 20mph. Should be more like 35-40mph. If it gets out of the hole quick and then stops at 3800rpm you may have blockage in your high speed jet system in the carb. Im assuming that it is an older 2 stroke motor which probly has a fixed high speed jet valve in the carb. These can get clogged with gunk over time. How many times have you had it out? Did it always do this? Bob_VT is right, it should be running atleast 5500rpm.

 

[farmboss]

Re: how fast and what rpm ?

no its a inline 3.0 merc inboard...4 stroke ...im gonna try to get a gps to check the speed ... im guessing the speedo my be off a bit...ive only had her out once... gonna take it out in the morning with the gps.... and gonna try to trim her up alittle more.....

 

[fishndirk]

Re: how fast and what rpm ?

Trim probly wont make the difference of 15mph unless you were way off but im assuming you werent, im sure you played with it a bit. Would'nt bother with the gps. If you know its only running 3800 at WOT than thats a problem in itself. Your Cheetah looks like a heavy boat but if it was too heavy for the prop pitch it would take forever to get out of the hole and if it did, would achieve WOT rpm. If it was too little picth it would get out of the hole very quick then level out. Iether way, you should still get more than 3800rpm out of it. Engine rpm is a balance between boat weight, motor hp and prop size. There are other factors but those are the big 3. Even my 20hp 4 stroke runs at 5500rpm at WOT.

 

[farmboss]

Re: how fast and what rpm ?

ok got some more info..... running 3800 rpm 25 miles per hour (gps)...boat empty, gas and people...just my 140lbs ***.. still not sure.... so got her home and thought what the hell.. just for the heck of it i pull the distributor.... the weights on the mechanical advance were stuck .... i mean solid... no freakin way they were moving..... freed those up . lubed the shaft and weights... gonna take her out in the after work tomorrow.... got a got feeling.....

so what ya think.....

still gonna replace the prop... might go up a pitch.... 19...

 

[fishndirk]

Re: how fast and what rpm ?

You mean the linkages on the spark advance were stuck and not allowing it to compensate? That would explain a lot. The exact position of the spark advance can have a drastic affect on the engines performance. There is a way to do it with basic tools but it involves a lot of playing around with the linkages with the motor cover off and having someone adjust thread position while someone else plays with the throttle. Not the most prefered method. Done it before on a buddy's 1982 50hp Johnson. Pain in the a**. Timming is something I wont play with and would definately have a service center with proper tools deal with it.

 

[Bob_VT]

Re: how fast and what rpm ?

ok got some more info..... running 3800 rpm 25 miles per hour (gps)...boat empty, gas and people...just my 140lbs ***.. still not sure.... so got her home and thought what the hell.. just for the heck of it i pull the distributor.... the weights on the mechanical advance were stuck .... i mean solid... no freakin way they were moving..... freed those up . lubed the shaft and weights... gonna take her out in the after work tomorrow.... got a got feeling.....

so what ya think.....

still gonna replace the prop... might go up a pitch.... 19...

Well.....now that it is understood that it is an I/O ............. 3800 is still not proper WOT and moving up in pitch will reduce your rpms.

 

[SparkieBoat]

Re: how fast and what rpm ?

the fact that it is an I/O was an important one. 4500 RPMs is usually about right for most I/O motors. yours could be different, you need to find out what the recommended range is and target for the upper end of the range. make sure the motor is running right first.

 

[farmboss]

Re: how fast and what rpm ?

we have success !!!!!!! its amazing when your mechanic advance is frooze up and doesnt move .. she just wont get over 3800.... fixxed that and wow she really moves..... cranked up to 5000 rpm at 35 mph..... gonna try and get a 19p-21p prop , and make a nice little cruiser..... its got a little lifter noise but we can fix that...... need to do a little adjustment on the timing.... i think its a little too far advanced ,, she starts alittle hard but runs great........ id like to get her to run 4400-4500 wot...

thanks for all the help....

 

[fishndirk]

Re: how fast and what rpm ?

Awesome deal good job. Be carefull when moving to a steeper pitch prop however, you may find that your enjoyable fast acceleration turns into a real engine lag. Especially if you plan on increasing the amount of weight you carry. Steeper picth does equal more speed, but only on lighter loads. I wouldnt expect much more from a boat in the class that you have. Its a cruiser, not a newer bass boat.

 

[farmboss]

Re: how fast and what rpm ?

so what ya think 19p or 21p ?

 

[farmboss]

Re: how fast and what rpm ?

its a 17p now..which is what it came with factory ...

 

[SparkieBoat]

Re: how fast and what rpm ?

each pitch up will reduce your RPM about 200, so a 19 should get you to around 4600, which would be about right, especially loaded, probably only hit 4400-4500.......................by the way..JEEPs sux bad, the absolutey worst possible vehicle I have ever seen...poor power, cant pull anything, cant haul a load, no room, horrid MPG, ride is terrible, hard to cool with AC because its not air tight, noisy, and just an overall useless piece of crap. I can not for the life of me understand why people buy them.

 

[farmboss]

Re: how fast and what rpm ?

i own 3 of them... and love them.... but it killing me to run them through the woods..... breakin stuff hurts....