Partial Power... Part of the Time!

[PITBoat]

My question is, is there one or two standout things that might cause that?

I was thinking maybe water in the fuel, but now I'm wondering about something electrical. I don't want to just throw parts at it and hope I hit on the right thing.

I just replaced the anti-siphon valve and filter/water separator, and removed the screen from the pickup tube (I made another post about that stuff).

At the lake yesterday, takeoff power seemed reduced a little, and power was down a good part of the time, with little bursts or short sessions of more. It would do 3,500rpm pretty consistently, but usually at a throttle position that should have given more. The sessions of when the power felt right didn't last all that long.

Then at the end of our time out (maybe an hour's runtime), We had a nice good solid run (30sec or more, prob) up to the top speed.

I don't know what to think really. I would have expected water in the fuel to give bursts of bad running, but we've got lowish power with bursts of good.

If I knew a good mechanic, even a car one, I'd be ready to take it and say here, fix this. But I do most of my own stuff and unfortunately don't personally know one (they're out there, I'm sure...). You never know what you're gonna get when you take something somewhere - mediocre or expert.

I'll get some flak here, but I'll admit I haven't changed, but have checked the plugs (visible, gap), wires (resistance), and dist cap (visible). I could throw all that at it, but I'd be guessing at whether or not any of that is the problem.

The good part is the boat's getting us back and forth and not leaving us stranded anywhere. It's just a little frustrating to always have something not quite right with it...

 

[kenny nunez]

I had a similar problem with the Donzi. After chasing a non existent fuel problem and even with the Mercruiser Rinda scanning tool which would show no problem codes I replaced the plugs. I went to NAPA and asked for the most expensive plugs available. The boat had been in storage with just 30 running hours when I bought it. The whole time it turned out to be the plugs.

 

[Lou C]

Do you have a points distributor?
Last time points were done? Dwell checked? Total timing advance checked? Could the centrifugal advance be sticking? If you have the OE Prestolite all of these points should be checked. Always verify basics before looking for ghosts

 

[PITBoat]

Ok, plugs. I'm whining a little about not wanting to guess and replace good stuff, but it doesn't get much easier than changing those out. Especially if they're in stock locally.

Points and timing. I filed at the points before the outing yesterday, but didn't get too aggressive with it, so don't know how much good that did. I checked the dwell several times last year, and had replaced the points either last year or in '19. Boat's only been out 3x this season so far.

I looked at idle timing a couple of times last year, but not the advance thing. Can't remember how to do that. I added a couple of drops of oil to the felt while I had the cap off Fri. eve.

This thing manifested at the very beginning of last season I believe, but sorta straightened itself out for the most part if I'm remembering right, but it's here again this year.

Thanks for the replies.

 

[Redrig]

I got lazy one year and didn't file the points and gap , dwell , etc . It was a quick April trip and I figured " it was fine just back in September" the boat ran quite poorly and needless to say I havent done that again .

My money is on the same situation for you.

 

[Lou C]

You need an advance timing light to check total timing advance. The specs for the OMC Cobra 4.3 were base timing 6* BTDC+12* advance at 3200 rpm for a total of 18*.

 

[kenny nunez]

The main thing about points and condensers is only from some place like NAPA. All that foreign junk you should stay away from.
If the points have tiny white spots on them that is from moisture, you can file them but they will fail.

 

[Lou C]

Kenny what about OEM or Sierra?

 

[kenny nunez]

Of course OEM is very good, I like most of the Sierra products but I do not know where the ignition parts come from. If foreign parts are made to real factory specs using the same materials then yes I feel that would be good.
It is too bad that the Prestolite electronic conversion does not work with the OMC shift assist. I did not realize that until I read the small print in the Sierra catalog.

 

[Lou C]

However I replaced my original ESA with one from CDI electronics when the original failed about 7 years ago. These according to CDI can be used with points or an aftermarket electronic conversion like the Pertonix. So I could use that instead but heard mixed things about them too!

 

[kenny nunez]

I wish that OMC would have kept it simpler with the same shift dog from the beginning As Mercruiser. The very first 800s I installed were chewing the dogs up because they used the same 90* dog angles as the outboards. While that same arrangement worked on outboards it did not on stern drives because the engines idled smooth. OMC engineers mounted a Chevy flywheel on a 200 hp outboard which smoothed out the idle so much that it started jumping out of gear. That was how they found out the problem and started sending out the electronic shift kits.
And then they made the Cobras without the negative angle dogs and I had to replace those with the negative angle gearsets under warranty! Plus the electronic shift assist is a PITa also. I do not miss those days.

 

[Lou C]

I must have gotten lucky with this one because once it was set up right, it has worked fine for 15+ years and I have not even had to replace the shift cable that was on it when I bought it 19 years ago. Shifts with 2 fingers still.
PS what do you think of the Pertonix vs points? I have no problem putting in and adjusting points but if it runs better and smoother it might be worth it.
As far as the shift cable problems on the Cobras, I read that at first they thought it was the angle of the dogs, but when they looked further and after replacing gear sets it was actually defective shift cables, they were recalled and the original cables were black the recall ones were red. The cable had too much drag and it didn't engage all the way into gear and would jump out. That and the tendency of the pocket where the shifter bellcrank is in the pivot housing caused stiffness and difficult shifting. Once that was fixed on mine, no more problems.
What I like about the OMC system vs Merc is that the OMC esa will not stall the engine if its in good tune, it will lower the idle to about 450 rpm on mine and that allows it to shift out of gear into neutral. On the Merc Alpha if the cable gets stiff it will stall the engine.

 

[kenny nunez]

Things were pretty crazy when the first Cobras came out. I did a a few re powers which finally was able to straighten out with help from OMC. They were really good with warranty problems and never questioned any of my requests. Mercruiser was a different story sometimes.
Petronic ignition parts are good and that is what Chrysler passengers cars used in their distributors before they switched to electronic.

 

[PITBoat]

You need an advance timing light to check total timing advance. The specs for the OMC Cobra 4.3 were base timing 6* BTDC+12* advance at 3200 rpm for a total of 18*.

So Lou, are you talking run the engine in N at 3,200rpm?

I changed the plugs to ACDelco #10s. One of old NGKs had 3 hairline cracks in the external ceramic.

I checked the dwell angle tonight (running) and it was just under 37deg (spec 39 +/-2). I got in there and filed the points better before making an adjustment, and I either ruined my meter somehow or it just scrapped out on it's own, because it wouldn't come back on again. Batt and fuses were good. Man that was/is frustrating.

Base timing was at 8deg BTDC

I'm gonna try to take it out in the morning and see how it does. And order a new meter, sigh.

 

[Lou C]

It’s supposed to be 6* BTDC for base timing, plugs are AC MR 43T gapped to .035. Yes you run it at 3200 in Neutral to check total timing advance.

 

[PITBoat]

Ok, thanks. Those plugs are what the guy at Napa sold me. I did a quick cross reference search before I put them in, and I didn't see AC MR 43T, but I saw the Champion RV8-C that the book gives as an alternate, and the NGK BR6FS ones that came out of it.

I think it's back to normal now... There were no hiccups today. It ran smooth, planed easily, even without using full throttle, and hit mid-40s a couple of times. That was with at least a 700lb load, not counting fuel. Mighta coulda squeezed a little more speed out, but had to back off each time for one reason or another while I was still bumping the trim up.

I made an observation about the shifting that I want to ask about, but I'll make another post for that.

Thanks to all for the replies and helpful suggestions! I'll look closer at the timing and dialing the dwell angle in a little closer when I get my new meter. It almost looks like I could estimate the 18deg timing advance thing with the light I've got (but it's beyond the index, obviously), but prob oughta order one of those lights while I'm at it.