150 GT running out of options..

I have a '91 150 GT that swallows gas at an alarming rate and breaks up badly anywhere below 4500 RPMs. At full throttle it tears up to 5600 RPM like no engine I've seen. Compression is 95 to 100 on all cyl. amd I've decarbed with deep creep. At idle, if I put a spark tester on the plug wires, often one cyl. or another won't be firing. At around 3500 rpm it will clear and bounce up as if a cyl hadn't been firing or a plug were fouled. I checked resistance across all the coils and came up with 250 to 265 ohms on all. At this point I changed the powerpack, plug wires and plugs. Still lousy. So I tore down the carbs expecting a hung needle valve or a plugged port. They were spotless and dead on. I changed out all the fuel lines and both the little filter under the cowling and the water separator. It acts like a cold engine that never warms up, so I replaced the thermostats and bypass valves. No good. It still seems to cut a cyl. out whenever it wants. The stator puts out a very steady 14 volts and it's the heavy 35 amp unit. If it were the stator or a charge or switch coil, why does it run hard at full throttle? If one of the coils it bad, why doesn't it cut out when warm at hi speed? If the timing is off, why so much top end?

Re: 150 GT running out of options..

You say it really drinks the gas? Check the fuel system recirculation valves and any inline valves that your motor might have. You manual should show the locations of the valves and testing procedures.

Re: 150 GT running out of options..

I think the next step is to confirm whether it's an ignition problem, or something that mimics an ignition problem. You can tell alot with an inductive pick-up timing light. Run the motor with the light and try each plug wire. If the spark sparks, the light will flash. If there's a miss you'll not only hear it, you'll see it.<br /><br />The stator has separate coils for the ignition and electrical system. You tested the electrical system. I suspect the ignition portion of the stator has a problem, but I certainly can't guarantee that. That's where the timing light/misfire trick can help. Now it should be that if one cylinder fires, the rest should fire. But the stator could be weak and most of the coils fire, but maybe a weak coil isn't able to translate the voltage into spark. The coils are close to the same, but not exactly the same. There are tiny variances between like components that render one as the strongest and one as the weakest. The rest fall inbetween. Resistance might only vary by a fraction of an ohm. But if you were to use a digital ohmmeter, you would probably get slightly different readings on all 6. That could be enough. Replacing that coil won't help because the next weakest one will be the next problem. I hope this helps.

Re: 150 GT running out of options..

Let's start at the beginning of the ignition system. The ignition sensor's have to trigger that pak, and coils. What is the out-put on the sensor's?

Re: 150 GT running out of options..

Walker, I did suspect the priming solenoid might be leaking by or stuck and it isn't. I pinched of the hoses and the smoke continues. It appears to be a selective thing with the ignition. I have a spark checker but it's tough to see it flash in the daylight. As I understand it, there's a separate sensor coil for each cyl. The cyl. that cuts out at any given time isn't a constant. It could be any one of them at anytime and there's no way to tell at part throttle without falling in the drink at 25 knots. As I understand the charge coils, there is one per bank but I've seen them cut out on either bank.

Re: 150 GT running out of options..

It's a little darker out now and I can see that the stbd. bank top cyl. is dead right now. Last time it was port bank middle...

Re: 150 GT running out of options..

Each time I think about this problem I tend to dismiss coils of any kind simply because any problem with a coil would be amplified with respect to RPMs. The time frame between firing is much shorter allowing for less time to reach saturation. My rhetorical is always the same.. if it's a coil of any kind, why so strong at full throttle when it's hot?...

Re: 150 GT running out of options..

Those coil's will only fire, if the sensor coil trigger's it to fire. Think of the whole system as a chain. What goes in, must come out. It's not the charge coils, or it would shut down one whole bank. If a sensor coil fails, it will not trip the SCR in the pak. If the pak, or I should say SCR in the pak fails, it will not fire the coil. You have to start at the beginning.

Re: 150 GT running out of options..

As I understand it, there's one sensor coil per cyl. but the cyl. that fails is never the same one. Why do they all fire strong at full throttle

Re: 150 GT running out of options..

The sensor's, being nothing but a coil of wire can fail at a slow rate. Generally starting with a break down of the insulation of the wire ,leading to an internal short. The sensor's on your engine are contained in what is called a "potted component". They will fire at a given voltage, depending on the speed of the flywheel with it's magnetic field. The more the coil, or the wire's in the coil breaks down increasing the resistance, the more speed it will take up to total failure. It takes a peak reading volt meter to make a running check on this system. The bad part is, they can be exspenive for just occasional home use.

Re: 150 GT running out of options..

Normally I would consider changing parts to solve a problem as a novice action however, if the cost is minimal for a stator/charge coil assembly and the sensor coil ring, considering this is a single engine application, I think it may be the way to go. This would now be a new ignition system on a powerhead with good even compression. It appears the flywheel removal and parts change is straight forward. What will these parts cost and is there anything else I'd want to accomplish while I'm there?

Re: 150 GT running out of options..

Let me elaborate a little on the timing light test. Wrap the trigger with electrical tape to hold it on and anchor the light someplace where you can see the flashing at a glance, it doesn't have to be in your hand or flashing at the flywheel. Run the boat until it starts acting up. Connect the light to one plug wire and go for a run. Do this to all 6 in successive runs.

Re: 150 GT running out of options..

Well, it turned out to be the timer base had 2 bad coils. You guys were right. Resistance readings were good and now I'm firing all six cyl. and my smoking has cleared considerably at idle. However, midrange is still rough and smokey. I don't have a timing light so I don't know where that may be but the idle is smooth and top end is 5500rpm. The pieces removed are bolted or keyed as you guys know and though I should recheck timing, I believe it to be fine. What would cause lousy running and smoke from 1400 rpm to 3800 rpm? New timer base, new powerpack, new plugs and wires, new thermo and bypass valves, carbs rebuilt.

Re: 150 GT running out of options..

What spark plugs are you using?

Re: 150 GT running out of options..

I had the original QL77JC4 and switched to NGK BZ7HS-10. Mistake?

Re: 150 GT running out of options..

If the carbs have the stock jetting put Champions back in as soon as you can.

Re: 150 GT running out of options..

I'll try it

Re: 150 GT running out of options..

still breaking up in the midrange...