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CARBURETOR PROBLEMS PCM 351

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64stang View Drop Down
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    Posted: August-24-2012 at 11:44am
I decided to start a new post because there were so much non related issues in my other posts.

My 1990 351 PCM started idling very poor at first, then had an intermittent loss of power on acceleration under load.

First I replaced the plugs, wires, coil and points. Set to .18 gap and 10 Deg at BTDC. With no change. I took it to a Ski Nautique dealer and they said the Distributor was worn out so I replaced it with a new Prestolite point mechanical advance, just like the original, no change, I replaced the power valve with a 6.5 and it fixed the idle issue but no change in the loss of power under load. Now very very hard to start. I installed an electric fuel pump 5-9 psi rated. No change. I rebuild the carb, paying attention to completely cleaning the primary metering block and secondary vacuum port. Replaced all gaskets and now I have an idle issue (could be one float is set to high dripping gas into the carb, because now when I turn in the idle adjustment screws the motor continues running, it did not when I rebuilt the front and replaced the PV. For some reason the rear 2 bbls are not opening properly, one symptom seems to be it gets worse as the motor gets warmer. All new carb gaskets and no signs of any vacuum leaks. Still very hard to start? Above 3K I can hear the motor running just OK, then it will surge forward and motor sounds great for a 1/2 second then back to dogging and back and forth. The carburetor looked very clean it worked fine before all this started, so I did not replace or adjust the floats. I am guessing gas is leaking into the carb from the rear, because it idled fine after I replaced the PV. I sprayed carb cleaner through the vacuum port for the rear 2 bbls, do I need to clean it better? The only other thing I have not done is checked the valve on the top of the gas tank. I would think the 4 bbls should open even if it were starving for gas. Still very hard to start? Appears to come out of the hole OK, but starts cutting out and surging forward above 3K rpm.

Thanks again for all the help.

Mark
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GottaSki Quote  Post ReplyReply Direct Link To This Post Posted: August-24-2012 at 1:02pm
One issue sounds like fuel delivery issue. Get at that antisiphon valve.
Adress fuel filtration too.
Also be wary of the electric fuel pump pressure, can overcome the needles.
Your idle mix screws have to be effective else it will always be compromised performance. It will be hard to start. It will stumble off idle.
So either there is a vacumm leak, or the idle fuel orifice is still, or re clogged.
Don't concern about the secondaries opening till you address the above, else confound the issues.
"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."

River Rat to Mole
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64stang View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 64stang Quote  Post ReplyReply Direct Link To This Post Posted: August-24-2012 at 1:59pm
Thanks, filter is the original canister type, cleaned with new element. Have a new pump on order, should be here soon, I was getting too much gas in the oil, so I figured it was the pump, but no gas in the overflow tube, so I bypassed it with an electric temporarily, new one in next weak. Now I am thinking the gas in the oil was coming from the bad power valve and flooded idling. Will check the anitsiphon valve today, and I am betting you are right about the electronic pump overpowering the needles.

On the starting issue, it cranks for ever with no significant gas smell. Even if I pump it 3 or 4 times. Plenty of gas is getting pumped into the carb. Will check a plug today. One symptom is it attempts to start just when I turn the key off, no matter how long I crank, it will not fire a single time, then at the moment of "key off" it will fire a couple times. Will do that 2 or 3 times then start up.

Mark
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GottaSki View Drop Down
Grand Poobah
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GottaSki Quote  Post ReplyReply Direct Link To This Post Posted: August-24-2012 at 2:07pm
Ok, you either have a key switch issue inhibiting the ignition while cranking. Or your dash is gettign pulled down too much while cranking inhibiting spark.
That little wire is trying to power the coil, dash, electric choke and alternator enrgize. Its undersized, and if compromised at all, weird things happen.


Tell me about your battery. Its not a deep cycle, is it? Their Voltage while loaded drops abruptly due to their internal resitance, and can similary inhibit ignition, and well as poor cranking speed.
"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."

River Rat to Mole
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64stang View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 64stang Quote  Post ReplyReply Direct Link To This Post Posted: August-24-2012 at 4:13pm
Interesting that you shuold mention the ignition, I wired the electric fuel pump to the ignition according to the instructions. I will change that. Boat as a newer 750 cranking amp automotive battery in good shape. I think I am going to assume that the gas in my oil came from the failed power valve running gas into the carb, and because the new pump will not get here until middle/late next week I am going back to the original pump today and check the anti siphon valve this afternoon.

I really appreciate all your help

Mark
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64stang View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 64stang Quote  Post ReplyReply Direct Link To This Post Posted: August-26-2012 at 6:42pm
Things are looking good, thanks to all the help and guidance.

I removed the electronic fuel pump from the ignition switch and went back to the mechanical. Using the mechanical fuel pump fixed the carburetor idle issue. Antisiphon valve is perfect.

I hot-wired the coil directly to the battery and the boat ran perfectly all the way through the power curve. Went back to the ignition supplied coil power and experienced loss of power above 3500 rpm. Jumped the ballast resistor terminals and it ran strong above 3500 but coil got hot like it did with the hot wire from the battery.

I tested the power into the passenger side of the ballast resistor purple w/red stripe and it was 12+ volts (I have a crude analogue tester) and the other side (driver side) solid purple was 10+ volts at idle. At the same time it showed 12 volts at the coil?? The ballast resistor said 700 on one side, only mark. Where do I get the correct replacement? I have a coil that said it would run with or without one, but it is way too hot without one, I think.

Sure was good to se my boat run right, of course everything has been replaced or rebuilt in the process of locating the bad resistor.

Mark
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64stang View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 64stang Quote  Post ReplyReply Direct Link To This Post Posted: November-27-2012 at 11:14am
The final update for my 1990 351 PCM motor. It turned out to be the coil. The local Orielly auto parts store gave me the wrong coil! I replaced the wrong one with a new epoxy filled 12 volt ballast resistor coil and that was the problem. It started with a ruptured power valve which kept fouling out the spark plugs and was getting gas in the oil. Not knowing how easy it was to rebuild the carb, I went after the symptoms not the problem. The new coil was clearly marked, but the one I got before had no markings on it and only some stocking number, so I had no way of knowing it was actually a 6 volt coil until I tested it.
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