Omc 120 3000 Rpm At Full Load?

Discussion in 'Sterndrives' started by H180DSC, Sep 22, 2009.

  1. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    There's no debate over this, I'm familiar with the engine, having owned several This is what they pull.

    I do think there may be a misunderstanding about the distributor. I don't see a good reason to convert this engine to coil packs and a crank trigger. It's much more likely the distributor is still in place and the points, breaker plate and condenser have been replaced (a good option) with an electronic ignition upgrade.

    So the question goes back to H180DSC, "where do the spark plug wires come from?" If the distributor, it's an electronic ignition upgrade, if on the other hand they come from multiple coils, then a whole different ball of wax.

    I still doubt it's an ignition issue, but think it's fuel delivery. There's very little that can go wrong with either of the ignition types mentioned.

    Honestly a mechanic can sort out the effected system in 5 minutes with some simple tests. Once you've located the system (fuel, ignition, controls, etc.) you can then focus on fault finding that system.

    [​IMG]

    It'll look like this.
     
  2. H180DSC
    Joined: Apr 2007
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    Location: Hillsboro, Oregon

    H180DSC Junior Member

    OK fellas, Here is an update, but first let me summarize everything up to this point.
    Boat: 16.5 foot Seaswirl open bow. "whale tail" and trim tabs.
    Engine: OMC 120, which is a GM 153 I believe.
    Outdrive: OMC 400 stringer.
    Carb: Rochester 2 jet.
    Ignition: Distributer with points removed, and pertronix ignitor (electronic pickup) installed. Standard coil. Tach signal is being pulled off the coil.

    Issue: won't rev past 3000 rpm under load, and boat only runs 23 mph (verified on gps)

    What I have done:
    Checked compression: 126 psi on all 4 cylinders.
    Basic tune up; cap, rotor, plugs, wires. effect: none.
    Changed props from 15x15 to 15x13. effect: none
    Pulled plugs after full throttle run. insulators bright white idicating lean mixture.

    Removed carb. Disassembled and cleaned. Assembled using carb kit. Removed the bowl and filter from the fuel pump, and it was clean. There is a steel hardline from the fuel pump to the carb, and it is clear. For the rest of my tests, I ran a new hose from the inlet of the fuel pump to a 5 gallon can of fresh fuel to eliminate fuel supply issues.
    Checked timing and set to 6 degrees btdc at idle (550 rpm), and am getting 34 degrees total advance at 2000 rpm and above. Compared tach readings on dash to tach readings on snap-on digital timing light, and they are the same.
    Put the boat on the water today and did another trial run. I picked up 400 rpm. Now am running at 3400 rpm with the 15x13 prop. Pulled the spark plugs. They have picked up a little color (faint tan) Maybe I need bigger main jets or????????????
     
  3. Frosty

    Frosty Previous Member

    Ok before we can move on can you confirm full throttle on the carb linkage.

    You have picked up 400 from somewhere may be by disturbing throttle linkage.

    After that we need to make sure the carbs are right for the motor. Then jetting
     
  4. H180DSC
    Joined: Apr 2007
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    Location: Hillsboro, Oregon

    H180DSC Junior Member

    Frosty, I have confirmed full throttle with the linkage. Its funny you say that though. I work on CAT equipment, and don't know how many times a "low power" complaint is just worn out governor linkage that can no longer pull the injection pump to full fuel.

    As I said earlier I am not a "gas engine guy", but I am a fast learner.
    I just had a thought, that will probably bring some chuckles from you "gas engine guys", so here we go. A diesel engine's speed is controlled soley by the amount of fuel that is delivered to the cylinder while the air intake remains wide open, or even pressurized in a turbocharged application, thus a diesel engine will never run lean. But take away the inlet air flow, and it will run "rich", which is seen as black smoke. I have been going after what seems to me a fuel problem on this damn gas engine, but, wait, could it be much more simple? A gas engine rpm is controlled by inlet airflow right? So if the inlet air is restricted, its not going to run rich right? I want to make sure that I am understanding this properly. If you limit the intake airflow on a carbureted engine, it will limit the gas due to the venturis? ****after reading Frosty's reply below, I see the error in my thinking. I forgot that the throttle valves are below the venturis. The effect would actually be like running with the choke on...way rich)****

    After hearing what you guys have said, and looking at what I've got, I am starting to think it is that damn spark arrester. If it was running lean, it would start to run crappy, but it doesn't. It acts the same as if you were to just give it half throttle, which could be achieved by a restricted spark arrestor right? It looks clean, but is it? I think I better look into this. What do you guys think? Is it safe to do a test run with spark arrester removed?
     
  5. Frosty

    Frosty Previous Member

    If the air intake is restricted the vacuum will pull fuel from the carb and it will be way rich.

    can you get some one to drive the boat as you lean over the back and tinker with it at 3400.

    Listen carefully to the motor, is it on 4 cylinders, yes take the filter/spark arrester of, look down the carbs and see if BOTH barrels are open and feeding fuel in,( be careful it could spit back and take your eyebrows off) just slightly cover the carb intake with your hand , this will richen up the flow considerably.

    Does it reach 3400 way before full throttle or does it need full to reach 3400. If you back off slightly will RPM drop?
     
  6. H180DSC
    Joined: Apr 2007
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    Location: Hillsboro, Oregon

    H180DSC Junior Member

    Frosty:
    when running wide open at 3400 rpm, i can back off to about 2/3 throttle before the rpm's start to come down. If I move the throttle back and forth between 2/3 and full, the rpm doesn't change, but I can hear a slight change in pitch at the carbureter.
     
  7. Frosty

    Frosty Previous Member

    That sounds like the engine is loaded, do you trim the leg up a bit once its planing.
     
  8. H180DSC
    Joined: Apr 2007
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    Location: Hillsboro, Oregon

    H180DSC Junior Member

    Frosty;

    Can't trim it. OMC 400 stringer. I can change the trim angle by adjusting the front engine mount. I am wondering if maybe the float foam is waterlogged. Maybe this engine is at max output, but the boat is too heavy.
     
  9. Frosty

    Frosty Previous Member

    Could be, the throttle response sounds about right for that conclusion. If it wont trim it will tilt right? alter the stops some how, even if it means temporarily gluing some bits of plywood to the stops.

    If it then wont get on the plane you've gone too much.

    If its breaking water up forward by the bow then it needs some trim adjustment.

    Take it to a wieghing station and get it weighed.

    Have you any pics of the leg and of the boat on plane.
    Not neccessary,--but if you had!.
     
  10. jonr
    Joined: Sep 2008
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    jonr Senior Member

    You could borrow a smaller prop - just to see if it will spin full rpm at some reasonable load.
     
  11. fasteddy106
    Joined: Apr 2009
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    Location: connecticut

    fasteddy106 Junior Member

    You won't get 5,000 rpms under load, that's why they call it load, nor do you want to. But 3,000 or 3,400 is a bit low. I had a similiar issue when I repowered my 23ft Winner. When I set installed the 4.3 I built for it I found that the timing adjustment I used to get the motor running in the yard would only give me about 3,000 rpm and like you I played with props and all sorts of stuff. See I work with car engines so I went by experience with those. I then talked with a old timer at the marina and he told me to set the timing under a load while on a run. So I got a buddy and we went up the Niantic river to the flat water and did just that. I put the boat up to 3,000 rpm and while my buddy took the helm I advanced the timing by turning the distributor until I got the max rpm, and then backed off 50rpm and picked up 700rpm over what I started with.That setting with the other tweaks give me a max of 4,000 rpm, enough to get me places in a reasonable time at 3,500 rpm cruising with a little reserve for trouble. Once you have the max rpm, mark the distributor and the intake with a scribe.

    Good luck!
     
  12. tuantom
    Joined: Jan 2005
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    tuantom Senior Member

    Actually you do. If you can't wind it up to the rpms it was designed for at wot, you probably have it under too much load - or something's amiss. I think those 4.3s call for 4500 or so at wot.
     
  13. fasteddy106
    Joined: Apr 2009
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    fasteddy106 Junior Member

    I should have explained that better. I tried a number of props while setting the timing. I achieved the maximum speed vs rpm with a 14x18 prop at approx 4,000 +- rpm depending on wind and waves. I was able to achieve more rpm's but not speed with 14x14 and 14x 16 props and the 14x20 prop dropped me down to bogging like a tug boat. I saw little value to subjecting my engine to 5,000 rpm if all I accomplished was rpm with no increase in speed. With the 14x18 prop the boat gets up on top quickly and hits 25mph without having to listen to the engine scream plus it gives me great manuverabilty and steering maintenence at low rpm. It did take me several days worth of trial runs to come up with the optimum for my boat.
     
  14. stanl
    Joined: Aug 2010
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    Location: portland,ore

    stanl Junior Member

    brand spanky new to "boat forum",was looking for the "factory recommended "WOT RPM for my 1978 seaswirl when i came upon your "3000"max rpm problem. everything was going along fine untill you posted pictures of said engine,my boat is a FOUR cylinder "120" yours is a SIX,so i looked up in my CLYMER stern drive manual and see that the last STRAIGHT SIX was used in 1975,are you sure you are useing the correct tune up specs? i"m not a genious at this stuff but i thought i would at least throw it out there . fuel pumps can loose "pump pressure" ,maybe?,,, anyway ,does anybody know the WOT rpm from the factory? thanks. sl
     

  15. stanl
    Joined: Aug 2010
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    Location: portland,ore

    stanl Junior Member

    stanl

    hello powerabout,you didnt answer the question i snuck into that reply of mine,if you dont know theFACTORY "wot" give me a ball bark if you can,i absolutely have no idea,,,,thanks, stanl after reading what i just wrote seems as if i left out some pretty important info. 1978 seaswirl 120 four cylinder,,,what would be a good "sustainable" RPM, RUNNING A 13.75X 15 PROP,I run it now at about 2800-3000 kinda slow on water speed i think,but seems like over 3000 is windin her up pretty good.
     
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