New stator?

[mirage500]

my '78 150 V6 stumbles when full power is used from idle and does not like full throttle at high rpm picks up if throttle is backed of a little, only spins to 5,300 with a 17"prop. Mechanic (merc dealer who I trust) thinks its the stator, carbs have been rebuilt and new cdi unit fitted.
any other ideas?

 

[jerryjerry05]

Re: New stator?

Compression test then spark check.They putting Ethanol in the fuel there?Rebuild the fuel pump.Then check for any air leaks in the fuel line.Inline connectors etc.
Check the leads for broken,burnt wires.
With the 17p prop did it ever rev past 5300?Or is it a new prop? J

 

[mirage500]

Re: New stator?

Compression test then spark check.They putting Ethanol in the fuel there?Rebuild the fuel pump.Then check for any air leaks in the fuel line.Inline connectors etc.
Check the leads for broken,burnt wires.
With the 17p prop did it ever rev past 5300?Or is it a new prop? J

compression was 135 - 142 all cyl's, no Ethanol running premium unleaded 95
tried 3 prop's now never goes over 5,300rpm
thanks

 

[CharlieB]

Re: New stator?

Still can't tell if it is a fuel restriction or weakened ignition cuasing the RPM limitation.

Need to test two things;

Fuel pressure while under load and at speed to see if the pressure falls off at WOT vs when you slightly pull back the throttle.

Also, rig extended leads to DVA stator voltage, looking to see if any differences at WOT vs when slightly throttled back.

One of these should show the problem and give you a direction to proceed with further testing.

 

[mirage500]

Re: New stator?

he checked fuel system said its all good so fitting a new stator next

 

[CharlieB]

Re: New stator?

Can't see giving up and just throwing parts at it hoping that a new stator will cure it.

GOT TO find the problem.

Possibly the top bearing just begining to show wear/loosen, allowing the warmed/expanded stator to brush against the inside of the flywheel magnets.

Remove the flywheel and CAREFULLY inspect the outside of the stator and the inside of the magnets, looking for any brightness/contact marks/scratching, proving contact, or not.

 

[j_martin]

Re: New stator?

I bought a boat with an XR4 150 that did that. "Certified" mechanic had been throwing parts at it for months.

Long story short, I shot the bug out, mostly by noting fuel pressure under different circumstances. There were about 8 bugs in it, half of them installed by the "certified" mechanic, such as misadjusted carbs, insufficient (atwood) fuel bayonet connector, etc.

Charlie's trying to guide you on a systematic diagnosis of the problem. If you follow his advice, you'll get the answer as quickly as possible, and also probably save a lot of money.

my 02
John

 

[mirage500]

Re: New stator?

I bought a boat with an XR4 150 that did that. "Certified" mechanic had been throwing parts at it for months.

Long story short, I shot the bug out, mostly by noting fuel pressure under different circumstances. There were about 8 bugs in it, half of them installed by the "certified" mechanic, such as misadjusted carbs, insufficient (atwood) fuel bayonet connector, etc.

Charlie's trying to guide you on a systematic diagnosis of the problem. If you follow his advice, you'll get the answer as quickly as possible, and also probably save a lot of money.

my 02
John

not sure what you mean by "insufficient (atwood) fuel bayonet connector" tried a new stator same problem so old one went back on which looks like it has been replaced recently anyway. He said fuel pressure is fine but maybe cylinders are out of round even though the compression is good. He says theres nothing left and its maybe just old and tired.

 

[j_martin]

Re: New stator?

not sure what you mean by "insufficient (atwood) fuel bayonet connector" tried a new stator same problem so old one went back on which looks like it has been replaced recently anyway. He said fuel pressure is fine but maybe cylinders are out of round even though the compression is good. He says theres nothing left and its maybe just old and tired.

It's pretty simple. An atwood fuel bayonet is made of plastic and has a smaller bore than the original Mercury metal one. It works fine on a 4, but cannot flow enough fuel for a V6. If you have aftermarket plastic fuel fittings on your engine, change them. They are part, if not all of the problem. That comment is based on experience and gauge reading.

If your mechanic actually said an out of round cylinder is the problem, fire him. He's most of the trouble.

You cannot possibly know that the fuel pressure is fine unless you've rigged a fuel pressure gauge and observed it under operating conditions. (instructions in post #4) I'm pretty certain you haven't done that.

In short, you have no idea what's wrong, and you haven't given us enough information to even hazard a guess.

 

[CharlieB]

Re: New stator?

With compression at 135 that motor ain't nowhere near tired!

Did you carefully inspect the stator and inside the flywheel for witness marks that would prove ANY contact between the two once at temp and RPM?

A motor of that age could only need a new top bearing, but without proof I wouldn't take anything off other than the flywheel for inspection.

Pull a couple of drops of oil from the lower unit to inspect for metal, worn bearings/gears can also create drag.

 

[1979checkmate]

Re: New stator?

time to find a different mechanic!! I find that the mechanics who operate a little ma and pops shop to be the most competent. They do it because it is what they love doing, and most absolutly HATE being defeated by a motor.. and wouldnt say that a cylinder is out of round. Chances are your mechanic knows that that is a load of BS, he is just bored with the motor and doesn't like the challenge of it. I do all of my outboard work myself. They really arent that complicated, sure i was confused as all get out at first with just a huge 6 cylinder chainsaw engine sitting in front of me, but i got a service manual and read up. Not hard stuff.

 

[j_martin]

Re: New stator?

This might be a good place to explain the theory of binary reduction in trouble shooting.
Simply explained, you devise tests or parts swaps that divide the possible faults into 2 nearly equal groups. For instance if there's 30 possible faults, only 5 tests should be required to isolate the fault. The more possibilities there are, the more this system saves in time and parts.

You modify this procedure somewhat based one experience and cost of the step. For instance if there's a miss, the first thing you do is throw 15 bucks of spark plugs in, because it's not expensive, and experience says that is often the fault.

In this case, first step is to look at the LU lube (drops of it) to see if there's a major problem there. That divides the problem into LU or powerhead.

The next step has already been given. You rig a fuel pressure test and check it at speed and under load. If that's good, it basically eliminates the fuel system. (another half of the possibilities)

and so forth.

 

[mirage500]

Re: New stator?

ok thanks for all the info. when I get it back I will check what fuel bayonet it has and put a fuel pressure gauge inline and check it out. What would you want to see, 5 psi?

 

[j_martin]

Re: New stator?

ok thanks for all the info. when I get it back I will check what fuel bayonet it has and put a fuel pressure gauge inline and check it out. What would you want to see, 5 psi?

Anything over 2 will keep it supplied. Over about 7 will overpower the needle valves and flood it. Usually if you have a fuel delivery problem it'll drop to 0 at WOT under load.

hope it helps
John