'87 OMC 5.7 stalls over 3000 rpm

[ed hobbs]

I have a 1987 OMC 5.7 with a Rochester 4bbl. I bought the boat as a project, and have only had it in the water 4 times. The first time it ran fine, considering it had been sitting for a couple of years while it had a new transom and stringer work done. I didn't run it very long, because I had not yet changed the oil. After changing the oil I took her out again. This time I burnt up an exhaust tube. While trying to restart, I got water in a cylinder and siezed the engine. I got the engine moving again with some WD-40 and replaced both exhaust manifolds and elbows. I also did a compression test (which was fine) and replaced the plugs. The third time out, it started and idled rough, but smoothed out as it normally does. After clearing the ramp area, I gave it some throttle and it sputtered a little and backfired a few times. After a few minutes of this, I was able to get up on plane and even once to wot! I figure I had a valve or two still sticking from the rust, and they freed up, because it ran good the rest of the day. This theory tested true when I went out the fourth time and the engine started easily and idled smoother than ever before. Now, up to this point, I had always used regular gas (86). This time I put in Plus (89) as the book recommends. After clearing the dock, I gave her some throttle and everything was great up to 3000 rpm. Then it sounded like it was stalling, but then it surged. It repeated this action several times until I backed off the throttle. Then it was fine, as long as I kept it under 3000 rpm. My first thought was carbeuration, so I pulled off the flame arrestor. It seemed to be getting plenty of fuel in the secondaries when my wife gave it the throttle, but it barely opened the butterflies. I didn't know how much to expect them to open, so I forced them open quickly. That didn't do anything, so we just spent the day swimming and tubing. We had a great time, but I want to go faster!<br /><br />I have talked to some motorheads where I work, and checked things like the centrifugal advance and spark plug wire connections. The guy I bought the boat from said it needed a tune-up, but I haven't gotten around to it yet. Would the difference in octane cause symptoms like I am having? Would a needed tune-up make it run bad suddenly? Would...? I appologize for the lengthy history, but I wanted to give all the information I have to this point.

 

[akriverrat]

Re: '87 OMC 5.7 stalls over 3000 rpm

sounds more like water in gas issue. if a tune up doesn't help try hooking a portable tank to fuel pump and see if that clears up your problem. but check filter in carb inlet first. if it has one.

 

[ed hobbs]

Re: '87 OMC 5.7 stalls over 3000 rpm

I pulled the fuel filter/water separator off today and poured it into a bowl. There was about a tablespoon of water in the bowl. That doesn't seem like enough to cause the problems I experienced, but I'm not sure. I put the same filter back on - that shouldn't be a problem, should it? It was new this year.<br /><br />I also pulled the carb filter. It looked like it had been in a while, so I will replace it. I then checked the exhaust tube to make sure one of my flappers hadn't broke off and gotten stuck. I got one side off easily enough, but wrestled with the other side until I myself was exhausted .<br /><br />Thanks for the response.

 

[akriverrat]

Re: '87 OMC 5.7 stalls over 3000 rpm

i am on second season with merc spin on filter and i dumped a lot of water out of it my first two trips this spring. i don't care what they say, i put a thin film of synthetic grease on my exhaust hose when installing it and never have to fight it. i am confused about the secondary butterflies just barely opening. are they vacumn secondaries like some holleys?

 

[ed hobbs]

Re: '87 OMC 5.7 stalls over 3000 rpm

NOW you tell me! The secondaries are vacuum, and I will be looking into that next.<br /><br />I replaced the ignition components and tuned it yesterday. It purred like a kitten at idle. I revved it up and seemingly could not get it past 3000 again. This is in neutral in my driveway, so I didn't want to try too hard while on the hose. Needless to say I was disappointed. I need to try it in the river, but it is a very large boat to tow around just to test.

 

[akriverrat]

Re: '87 OMC 5.7 stalls over 3000 rpm

i have never seen a vac secondary q jet. there is a lot i've never seen. when you manually opened secondary butterflies that was the round plates in the throttle body and not the rectangular plate at the top?

 

[ed hobbs]

Re: '87 OMC 5.7 stalls over 3000 rpm

I was refering to the rectangular valve at the top; perhaps I am using the incorrect terminology. I checked the valve in the throttle body today and it was opening with the throttle. It is mechanical, but the rectangular ones are vacuum.

 

[akriverrat]

Re: '87 OMC 5.7 stalls over 3000 rpm

my WAG is that the secondaries come in at about 3000 rpm and there is some thing wrong with them. at least a tear down and cleaning. they are opening but not providing the extra fuel? Q jet guys?

 

[magster65]

Re: '87 OMC 5.7 stalls over 3000 rpm

Your engine should rev up to max rpm on the primaries alone (no load) but don't do that. You'll screw up the water pump impeller. Do your testing in the water. <br />Q-jets are vacuum actuated secondaries and will only open as far as they need to.<br />The rectangular piece I think you're talking about is the choke plate, that should be all the way open when warmed up (this might be the problem).<br />The other two butterflies are pulled open when the secondaries open, they are not mechanically connected to the throttle linkage.<br />Put a timing light on the engine and if the base timing is good and the advance is working I'd then check your fuel pressure. If that's ok too put a kit in the carb. Have fun!

 

[ed hobbs]

Re: '87 OMC 5.7 stalls over 3000 rpm

Looking in the sevice manual, that is the direction I was heading in. I was hoping to stave off a carb rebuild untill the fall! Oh well - such is the life of a boat owner . At least I fixed my clothes dryer today!

 

[magster65]

Re: '87 OMC 5.7 stalls over 3000 rpm

The timer on my washing machine just packed it in. <br />Why do things break anyways?

 

[liquidasset]

Re: '87 OMC 5.7 stalls over 3000 rpm

I have the EXACT same engine and EXACT same problem. Luckily one of my family members is a GM mechanic. We traced the problem down to a microswitch on the throttle quadrant. If you look on the engine's left side(looking aft) there are two microswitches on the throttle linkage. There are teeth on a cam that engage a little leaf spring which ultimately trips the switch and advances the timing. My spring was bent slightly and wasnt tripping the switch. I had to bend the spring slightly to get the switch to trip. Send me your email address and Ill send you pics of the switches. Hope it helps.

 

[metwrench]

Re: '87 OMC 5.7 stalls over 3000 rpm

I have a 5.7 OMC also, First the rig should hit about 3,700 to 4000 rpm , always use 89> Octane as a Knock under acceleration could be fatal over time to the engine.<br /><br />Lots of things could cause your Condition, Including Size and Pitch of the prop. Hopefully it has never been changed to a Different one.<br /><br />I assume the Compression is all Good and even, The Tune-up of Plugs, Wires, cap and rotor and Filters, and The Points were Dwelled or gapped Correctly which is a BIG RPM reducer under load. Also Timing MUST be checked and set Correctly<br /><br />What I would check: Make sure the Vacuum Secondary on the carb is Opening and also the Metering rods in the Carb opening up. The rods are in the secondary barrel throat toward the primary barrel side. They Open Mechanically, But often they can have GUM and Varnish keeping the holes ( jets ) from allowing Fuel Flow. This would deffinetly cause lack of High RPM.<br /><br />In the Distributor, You should have advance weights. The easiest way to check them is Timing light at idle, Then rev the motor and see if the timing advances ( goes higher in numbers ). Another way of checking them is with the Distributor cap Off, Grab the rotor and see if you can twist it slightly. If you cant, the advance weights may be Stuck and need of oil. In fact in the tune-up you did you should have dropped a small amount of Oil ( 1-2 Drops from an oil can ) on the top of the distributor shaft ( Right Under the Rotor there is a cloth pad, drop the oil on that ) That helps Lubricate the weights below the point plate.<br /><br />try theses things, good luck !<br /><br />John<br />Tech

 

[liquidasset]

Re: '87 OMC 5.7 stalls over 3000 rpm

I put my boat in the water today after we adjusted that microswitch. No help there. Im still having the same problem. I manually opened the butterflies at the RPM where it started bogging down. Nothing changed. Im going to rebuild the carb this weekend and see if that helps. How do I check to make sure the metering rods arent gummed up?

 

[metwrench]

Re: '87 OMC 5.7 stalls over 3000 rpm

When you pull the top of the carb off they will be located in the secondary barrel area. 2 Long Rods that go down into 2 Small jets. Now its been awhile but I think you can unscrew the jets and actually take them out, Blow through them and run carb cleaner threw. KEPP IN MIND that the rods ARE adjustable, and when you set your float level and drop, I think there is a Guage for setting the rod heights.<br /><br />I would also advise that if you have never rebuilt a carb before, Have some manual to follow, they can be very complicated.<br /><br />Good luck

 

[liquidasset]

Re: '87 OMC 5.7 stalls over 3000 rpm

Luckily my father is a GM mechaninc. Hes going to supervise the rebuilding. I worked on aircraft for 6 years. I think cars and boats are more frustrating than them. I took the boat out yesterday to experiment with the carb. I took the spark arrestor off and watched for anything unusual under acceleration. I didnt see anything. I checked for vaccuum at the diaphragm and checked to see that it worked. Then I tried something a friend reccommended. I held the secondary butterflies closed when it started bogging and BOOM...she took off! We got all the way up to 4400RPM and 43 MPH. But if I let the butterflies go it would start bogging again. Strange. Any ideas there?

 

[metwrench]

Re: '87 OMC 5.7 stalls over 3000 rpm

Strange , No ....<br /><br />Im Guessing that the metering rods or main jets are Plugged Partially. By Holding the Secodary closed you are Forcing ( Vaccuum/sucking ) The Fuel into the barrels with less Air to Mix, getting a Richer Condition. Thats a Guess...<br /><br />I would also make sure I check the float Level..Im glad your dad is a GM tech, carbs can be complicated ! Good Luck

 

[liquidasset]

Re: '87 OMC 5.7 stalls over 3000 rpm

That was my first guess. I thought that by holding them shut I was creating a higher vacuum and drawing more fuel into the barrels. When I hit that RPM(around 3000) and the butterflies start opening, should I see fuel spraying into the secondary barrels? Because I never saw it.

 

[liquidasset]

Re: '87 OMC 5.7 stalls over 3000 rpm

We rebuilt the carb this afternoon. The float adjustment was pretty far off and the carb was dirty. Shes running like a champ now. 4400RPM at 43mph. I would check the float adjustment and vacuum pressure if youre having this problem.

 

[ed hobbs]

Re: '87 OMC 5.7 stalls over 3000 rpm

In the interest of those who might experience this problem in the future, here is what resolved the problem:<br /><br />I think replacing the filter in the carb and water separator fixed this problem, but changing the points created another problem. See ENGINE SHAKING.