Mercruiser unsteady under load.

[bobecke]

I have a 1990 5.7 mercruiser ski inboard. It runs OK with no load up to 3000 RPMs. But when I put the throttle down it usually won't take it. It will occasionally run up to about 4200 RPMs when colder but is inconsistant. Trying to pull out a skier just results in sputtering. I've been through the roch carb (clean as a whistle),<br />replaced coil, plugs, points, cap, rotor,wires are 15K ohms at longest. Repaced just about everything but the resistor ballast which I can't find. Help. Any ideas out there??<br /><br />Thx.<br />Bob

 

[achris]

Re: Mercruiser unsteady under load.

Have you done a compression test?<br /><br />The ballast resistor is just one of the wire in the harness that is resistive.<br /><br />Chris............

 

[Don S]

Re: Mercruiser unsteady under load.

You shouldn't have points or a resistor wire/ballest resistor in that engine. It should be a Thunderbolt IV electronic ignition.<br />What do you actually have there?

 

[bobecke]

Re: Mercruiser unsteady under load.

Don<br /><br />It's a mallory ignition --points codensor etc...<br />1990. Points at .016" Dwell as per spec's. Timing is at 8 degrees advanced as per spec's. The engine runs strong when it is not sputtering and it only has 379 hrs. so I'm not even considering that it might be a compression problem.

 

[Don S]

Re: Mercruiser unsteady under load.

Well, like I said before, you don't have the OEM setup for your distributor, so don't expect to find a resistor wire in the harness. Merc. never put one there on that model, and the didn't use Mallory distributors in 90 either. So the question is, why was it changed? Possibly an attempt to fix a sputtering problem?? <br />Boats are not like cars, and if you don't have a marine distributor, you probably have the wrong advance curve for one thing, along with a potential explosion/fire hazard. That is why the distributor needs to be marine certified.<br />You could also be getting water into the exhaust due to leaky manifolds, risers, or gaskets that could cause problems. Automotive doesn't have water close to the engine like that..... so consider doing the compression test. Hours don't mean anything.

 

[flashback]

Re: Mercruiser unsteady under load.

mayt want to check the fuel system, filter, pump, etc.. if all else is in good tune, then you may not be getting enough gas.... goood luck...

 

[muc]

Re: Mercruiser unsteady under load.

Bob,Your engine could very well have a Mallory dist. in it, they were very common in the ski engines around 1990. Take a look at micro fiche card #50 it shows Mallory, Prestolite and Mercury Thunderbolt used in that serial number range. If you want to test for low volts to the coil, just hook up a jumper wire from the positive battery post to the + side of the coil. But that problem almost always shows up first as a hard start symptom. Don S is right about doing a compression test; hours mean nothing on a 1990 engine. I have had to replace more than a few tulip valves on engines with less than 200 hours. Might want to check total advance on your timing (those centrifugal advance dist. can rust up pretty easy). Tough to give you much help without better info on your part (serial number, how long you have had this problem, what rpm’s engine gets at W.O.T. cold and normal temp.) Good Luck, Muc

 

[bobecke]

Re: Mercruiser unsteady under load.

A couple of other hints as to what is happening are; 1/ when testing the engine on the water at full throttle (it's only coming up to 3900-4000 RPM's), my buddy advanced the timing to about 11-12 degrees advanced and it came right up to 4500 RPM's. However I don't want to run it that advanced. 2/ Sometimes when it's relatively cold, it will come out of the hole just fine and run right up to 4400-4500 just like it should. However there still is a hesitancy at the end of the throttle. If I have a compression or valve or fuel problem, how could it do that???

 

[waterone1@aol.com]

Re: Mercruiser unsteady under load.

As MUC suggested, you should check your advance curve with an advance timing light. This sure sounds like a timing issue at this point.

 

[Wellcraft 89]

Re: Mercruiser unsteady under load.

Just because the carb is clean doesn't mean you don't have a fuel delivery problem!<br />float level or fuel pump maybe<br />Also is max advance happening with the dizzy? <br />Rich

 

[waterone1@aol.com]

Re: Mercruiser unsteady under load.

Bobecke, I think the key here is that your friend manually advanced the timing and the boat "came right up to 4500 rpms". Low compression or fuel issues will not be solved by "playing with" the timing. Check that advance curve.