intermitant power 1988 150hp black max

[Elkhunter173]

I have read many posts and manuels and tried most of them. I bought the boat and motor 3 years ago and have had the same problem all along. At times the motor will take off with full power 4900 rpm ( I know it should be 5000-5500 rpm) I live and use the boat at 4600 feet. At other times it will only get up to 4300 rpms. If I run it for about 5 minutes it will suddenly take off and run good. I have changes all the fuel lines, fuel plug, rebuilt the carbs and rechecked the floats two more times, rebuilt the fuel pump, checked the vent on the gas tank and ran off my smaller portable tank.

Today I checked the jets on the carbs and noticed the manuel called for .080 main jets, .048 idle jets and .078 vent jets. The manuel said the main jets and the idle jets should be .002 smaller and the vent jet should be .002 bigger. My carbs had .074 in the main jets and .048 in idle jet, .078 in the vent jet. I guess I should have a .082 main, .050 idle, .076 vent. Is this correct and would this cause an intermittant surge in power due to loading up. My carb is a WH40
The spark plugs dont get really carboned up but I replace them every year. I guess the last options will be to check reverse polarity on the coils and any ohm resistance. I do not have the equipment to check the CD ( power box) and the nearest place that does is 165 miles away.

If it looks like I am on the right track I understand no replies. If you have any suggestions or have come across the same problems please let me know.

Compression on all cylinders is 102-104.

 

[CharlieB]

Re: intermitant power 1988 150hp black max

Other than making sure that the carbs are spotlessly clean, I wouldn't change any jets just yet.

Also, recheck your math, your switched a few #'s around in your posting.

The fact that the plugs are not carboned and that the motor does run correctly sometimes, leads me to want to test the ignition system to be sure that the stator is consistently producing adequate voltage from the high speed windings for the engine to perform. Low or intermittent readings on a DVA Meter will show if this is a problem.

Prop size is also a consideration, overloading the motor, even that small amount, will shorten the engine life by excessive pressure on rod and crank bearings.

 

[j_martin]

Re: intermitant power 1988 150hp black max

A list of comments, not in any organized order:

Look at the plugs. If they're not carboning up and burning brown, you're probably pretty close on mixture.

Sombody will tell you your compression is low. Considering altitude, it looks good to me.

Running at high altitude lowers the output of the engine, just by the fact that it can't get as much air as at sea level. It gets critical that you not overprop it. That engine would be happier at 6000 rpm than at 5000.

Does it run better right after you change plugs? If so, and your plugs read rich, then carburetion could be the problem.

There may be an idle stabilizer, or an advance module, or both, or a combination idle/advance module on it. Disconnecting the module might require retiming to compensate. I say all this because it would be worth the while to investigate the possibility of a malfunction in one of the modules. They are so important to the function of the engine that Merc doesn't stock any of them. Many folks just remove all of them, and retime idle to what the engine wants, and WOT to 23* BTDC. If yer overpropped, don't run it there, either get the right prop, or back off on WOT timing. Correct prop would be preferred.

You could be dropping a cylinder. Look at the comment on carburetion. It wouldn't be a stator or trigger, could be a coil or switchbox. You'd have to try to discern which cylinder is failing at speed.

If you're experiencing a fast fall-off from WOT, you could be looking at a fuel delivery problem. If it seems to run strong at the lower rpms, and doesn't first go higher, fuel delivery is probably OK.

Octane, anything over 85, would not be the problem.

The backup I've been waiting for is done. Gotta get back to work

hope it helps
John

 

[Elkhunter173]

Re: intermitant power 1988 150hp black max

My boat is an 18 1/2 foot crestliner. The prop I am using is a stainless steel Steletto vented 3 blade. The prop is a M-1419. What other prop would you go down to.

I just changed the spark plugs and it didn't make any difference. If I just start the motor when it is cold and just let it warm up for a minute it will run as good as it has at 4900 rpm. It seems that if I idle it or troll it for a couple minutes it will only get up to 4300 rpms and 35 mph and then I have to run it for a few minutes and then it takes off back to 4900 rpms and at 50 mph. Once it takes off to 4900 rpms it will stay running good all day WOT even at a troll for a minute unless I troll, idle, or shut it off for a few minutes.

How can I check the coil, stator, trigger, and the Cd without all the neat equipment. I only have a ohm/voltmeter.

Thanks for all your help! As you know it can be frustrating when its not working the way it should.

 

[j_martin]

Re: intermitant power 1988 150hp black max

Check your plugs when it's acting up. You're probably just fouling one out. Probably carb adjustment, partially sunk float, float setting or such.

If you're runing the VRO (auto oiler) and it's smoky at idle, it could be misadjusted. Look in the book and adjust the link arm 1/2 turn at a time toward lean if that's suspected. They're a little touchy in that adjustment.

Go down 1 or 2 inches in pitch on the prop. Get a GPS reading on the top end, and run the numbers through a prop calculator, just to see if your tachometer is reasonable. Unless you're real heavy or waterlogged, that motor should pull a 21 or 22 at sea level, 19 at altitude sounds good. I might be all wet instead of your boat, though.

DING DING DING
I just ran the numbers you gave. Theoretical speed is 47 (at 0% prop slip)
This does not compute. This does not compute.
GPS and tach check time.

hope it helps
John

 

[Elkhunter173]

Re: intermitant power 1988 150hp black max

Today I did a polarity check on the coil with a pencil. The #2 sparked right away ( I know because I didnt have enough insulation between me and the spark plug wire). I tested it again and there was no spark and many more times and still no spark. The other coils had good spark and polarity. How lucky could I with this ongoing intermitant problem. I had checked spark with a timing light on the water when it wasnt running right and there was spark but probably not enough. The motor is 20 years old and no payments so I ordered 6 coils and wires, and two switch boxes. Old electronics go bad. If this dont work I guess I will go back to the drawing board.

I would just like to thank everyone whom responded to my thread. You guys really do make this web site very helpful to us struggling do it your selfers.

Thanks again
Ron

 

[MERCrazy]

Re: intermitant power 1988 150hp black max

before you go nuts lookin for this ... try taking the covers off at night and look for a lightshow ...if its real dark you may see a coil discharging to ground
...
otherwise try using a coil that works in place of the suspected coil..

real important tho .. never let a coil fire without a spark gap tester or the plug grounded... good luck !