Low rpm's

[CaptainKen1]

I have a 24' Searay 240 srv with a 351 omc motor and an 800 stringer drive. My problem is this: I can only get 3600 rpms at WOT. I am down to a 151/2x13 pitch prop. Carb was rebuilt, new points, plugs, etc. I do live at nearly 6650 ft. elevation. Mechanic recently suggested installing 21:18 gears...Any suggestions ?

 

[Mikedg]

Re: Low rpm's

Ken:<br /><br />I wouldn't change gears. It is more than likely the motor. As part of your tune-up you should pay special attention to the ignition advance. If it is hanging up the engine will not develop full power.

 

[CaptainKen1]

Re: Low rpm's

The advance on the distributor seemed to be working...motor runs smooth; still need to adjust electric choke on holley carb-seems to run a little rich. We ran the motor wot for a while and then shut down very quickly. Checked color of plug tips-#1 was biege like I think it should be. Thanks for your reply !!!

 

[hoot]

Re: Low rpm's

had a similar problem this summer. wot rpms were down. replaced ignition coil. problem solved.

 

[Bondo]

Re: Low rpm's

Has this boat Ever Ran Better, At this Ele.???<br /><br />

nearly 6650 ft. elevation

If Not, I'd say a Gear Change is your Only Option.......

 

[CaptainKen1]

Re: Low rpm's

Bondo, I don't know if this boat ever ran better at this elevation because I just bought it last May. This is a 1978 Searay and it's really heavy. It didn't run any different at Powell at elevation 3613 than at McPhee elevation 3795. Thanks for your reply...I think the motor is original equipment; the OMC serial # on the outdrive ( upper-gearcase ) is 983489. From reading service manuals, this looks to be a 1984 model gearcase. I'm not even sure what gears it has in it, but the boat was in California for the last few years. The original owner was from here...P.S. Coil looked good

 

[Walt T]

Re: Low rpm's

First things first, stop messing with props. With the 13 pitch prop you should be overreving that engine. Next, find another mechanic, the one you are using is trying to cover up symptoms without finding the cause.<br /><br />Verify good compression, good tune up, and clear exhaust. Verify you are getting full throttle by looking at the throttle valves down in the carburetor. I suggest you do this with the engine off. Verify the tach is correct. Make sure the boat hasnt soaked up a bunch of water. Then start leaving the fat fishing buddies at home.

 

[CaptainKen1]

Re: Low rpm's

Thanks for your reply WD. I like the part about leaving my " fat fishing buddies at home ". I accuse my boss of being overweight in the brain all the time !! I wonder if this engine has a weak valve spring or lifter..I'm just talking off the top-it does'nt use any oil or sound bad. It does'nt run hot. I do have a valve gasket leak on the port side, but it's not that bad. Former owner even left fel-pros gaskets to replace. The motor will rev up to 6500 rpm's in neutral ( and maybe more ). That's what frustrating about this situation-the dang engine purrs like a kitten at the dock - I've had friends with Merc outdrives walk up and say " Why don't you start her up and let it warm up ? " I say it already is running...

 

[Walt T]

Re: Low rpm's

Weak lifter or springs are most likely not the culprit. As I said verify the basics first, then you can start worrying about other things.

 

[CaptainKen1]

Re: Low rpm's

Again DW thanks for replying...the service manual for this outdrive does suggest using 21:17 gears instead of 21:16 at high altitude. Mechanic in Page said that probably would'nt be enough. Only thing I can say is I haven't paid any mechanic anything yet, but I have spent a few $ on 2 different prop pitches..I was wondering if I could do the work myself-I'm not a great mechanic, but I have a few friends (lean) that can fix practically anything.

 

[Bondo]

Re: Low rpm's

CaptainKen1,<br />You've got to figure out Where the Problem Exists...<br />You're Already running the Smallest Prop I've ever heard of... So you're at the End of that road now.....<br />A Full + Complete Engine Survey would be My next move.....<br />A Compression,+ Leak-down tests... Valve Lash... Timing... Carb... Igntion... Exhaust Flow...etc...<br />This will tell you if the Engine is Up to Snuff......<br />Next, Is this a Dock Boat ?? How Dirty is the Bottom,+ Outdrive ???? Even the Slightest Flim of $hit will Drag a boat Way Down......<br /> IF Everything Else Checks Out.... Then go to the gears.... I Don't Do OMCs, so I can't help you pick the right number of teeth..... <br />I'm Thinking a Gear Ratio of about 1.84/1 Should get you into More Normal prop pitches.....<br />You Might have to go to a 1.98/1......<br /><br />BTW, A Rebuilt Carb is Not the Same as a Well Tuned Carb.... The Plug Test is a Good Start,(ya gotta check More than 1, All is Better) But you state, the Carb's Still Not Right.....<br />Your Valve Cover Gasket Issue,+ the Question about the Lifters will be Answered when you Run the Valves....<br />Oh ya, FreeWheeling an engine much beyond 2500/3000RPMs Shows/Proves NOTHING......<br />All It'll get you is a 600lb Block of Scrap Iron......<br />DeiselWalt is giving you All the Right Places to Look....<br />Good Luck...

 

[Bhamil]

Re: Low rpm's

Hey, all you guy's are giving sound advice. The really good advice is an old adage; the simplest answer is usually the right one. That being said, I'll bet your lack of WOT is a tuning problem, not prop, not gear, none of those complicated ones. Give the boat a thorough tune up, set the timing properly, make sure you have good movement (no binding) on all throttle and choke components and linkage, etc. Finally make sure you recommend the Adkins diet to your fishing buddies (how's that for PC, DW?).

 

[Walt T]

Re: Low rpm's

You can probably do it yourself. Check all the basic tune up items, points then gap then timing. Verify full throttle. Check exhaust shutters, Check compression. Always check basics first. If you dont find it then, try to get more specific about the problem, ie; Does it backfire, does it run smoothly, any noises, hesitation, rough running, etc.<br />No one can say its the carb or ignition yet we simply dont have enough information.