Need help with Yamaha 150 hp 150TXR 6G4-1015800-... 2 stroker, carbed, 26L V6

[luckydude]

Hi, I'm back, looking for more help. The short summary of the problem is the engine won't idle. If I throttle it up to about 1100 RPM it will sit there forever.
If I throttle it down to idle, once it is warm, it appears to be set right, it idles at 800 RPM but after maybe 10 seconds it will slow, slow, slow, quit.
If I throttle just a tiny bit above 1100 RPM it will go up a little and then it zooms up to 3000 or 3500.

Things I've done.
New impeller (I was in there)
New fuel filter
New spark plugs
Threw away 70 gallons of gas (most of it clear but the last 1/2 gallon or gallon was yellow and shellacy, I couldn't suck that out so I mixed it with 30 gallons of brand new premium, pulled the input hose off the fuel filter, pumped the bulb until the gas ran clear, but the hose back, ran the engine for 20 minutes to get an old gas out of the carb bowls, no joy).
New oil pump and got the oil pump working
New computer (didn't need that as it turned out)
Cleaned the fuel filter after it was getting clean gas

What I'm really looking for is a mechanic that knows these engines. I'm a semi decent home owner mechanic, I've fixed a lot of stuff, my impacts go up to 3/4", but my area is mostly stuff like tractors, excavators, skid steers. I'm way more diesel than gas. I have dealt with engines like this, I had a Honda 550 4 (or was it 500 4, I dunno) that had 4 carbs and I had to balance them, but that was decades and decades ago.

I don't just want a marine mechanic, I specifically want one who has fixed these larger Yamaha 2 strokers with carbs. Ideally one local to the Santa Cruz area.

Because I have to decide if I put expensive mechanic money into this engine or if I man up and repower to the tune of $13K for just the engine.

I'm kinda wondering about a vacuum leak. The engine runs just fine at speed, when I bought it we were in a harbor in Eureka, flat water, that engine pushed my 215 CC at 45MPH. So it's not junk (though Barry and I agree it's sort of a moot point, it's never gonna be that flat out on the ocean, if you can do 25, that's about as fast as you want to go).

Help me out with a pointer to a mechanic or a pointer to a forum that deals with yamaha engines pretty please!

 

[Dave LeGear]

1st thing I would check is the mounting bolts to the carbs, possible you are sucking some air under them... This can happen if the engine / intake manifold got real hot for whatever reason OR the last person did not snug them down to the correct torque or (worse yet) over torqued them and smashed the base gaskets. Check the intake manifold mountings at the same time if you can reach them? Again, don't over torque them... Just make sure they are sung and as Dad would say "just wrist grunt" if you do not have a torque wrench

I would also shoot some throttle body cleaner into them while running. One of the idle air bleeds / circuits might have some crud build up them. And the reason I say Throttle Body cleaner is it not as harsh as old fashioned Carb cleaner which "can" strip some of the intended factory coatings off of them if not careful...

Lastly, you can always spray some cleaner "around" some of those components while running, and see if the base engine RPM's increases any? This would show you a general location of where she is sucking air "past" the carbs which would then need to be addressed in some manner.

Hope this helps and let us know how you make out!
Dave

 

[luckydude]

OK, progress! I pulled the carbs off and cleaned all the jets, put in new gaskets, and checked the float valve height like the service manual to,d me to do. Guess what? 5 out of 6 were about 3mm too low which means the system was starved for gas. Fixed that and did the checks in the book, timing was good, etc. The control cable was way out of wack, it wouldn't let the control arm go back to zero. Fixed that.

So now it starts but it won't idle below 1200. Start adjusting all that, finally got that right, it's idling right around 700RPM if I trust my gauge (borrowing a more accurate one soon).

Starts _way_ easier, is responsive to the throttle, almost there. The one remaining problem I have with the engine is it's like one cylinder is pooping out an extra big wad of exhaust. I need to go time it to see what the frequency is but if you get a same, same, same, same, same, big poop of exhaust, does that say one cylinder is messed up?

I ran a compression test and they were all 130-140 so I don't think that's it.

 

[Harper]

With compression good on all 6, the reeds are probably all good, and you have carbs, so can't be an injector. Guess the only thing left is ignition on that cylinder. Spark plug (I've seen 'em bad right out of the box)? Wire? I don't suppose you've been able to figure out which cylinder it is. I'm not at all familar with the engine in question. Does it have a regular high voltage plug wire? Since it's carbed, I'm just guessing it's not a coil-on-plug setup.

 

[Dave LeGear]

Sounds like you are making great progress Larry! And I have to agree with Mike, sounds Ignition related at this point...

What year is that engine so we can find a blow up of the ignition system on that engine

 

[luckydude]

Sounds like you are making great progress Larry! And I have to agree with Mike, sounds Ignition related at this point...

What year is that engine so we can find a blow up of the ignition system on that engine

2006. And you guys were right, I had one spark plug that wasn't firing, replaced that, and it's better. I'm buttoning it up and gonna take it for a spin!

 

[Dave LeGear]

Sounds great! and check the plug wires also... Break out your multi-meter, and anything over 10 Ohms resistance on any wire, I just say replace it if you can?

Hope this helps?
Dave

 

[Offshoreman]

+1 on the plug wires. With its age, I would replace them.

I had a past experience with individual coil failing causing a similar problem. It would fire, but not consistently.

I have a fleet of Ford's for work and once in a while an individual coil will fail causing all sort of issues. Simple fix. Get them tested if you have any doubts.

 

[luckydude]

Sounds great! and check the plug wires also... Break out your multi-meter, and anything over 10 Ohms resistance on any wire, I just say replace it if you can?

Hope this helps?
Dave

So how much resistance should they have? I'm getting 8-9 Ohms. 9.15 in one case. What would new ones be?

 

[luckydude]

Also, you can hear/see the engine here:



If you listen/watch closely, it runs for a bit, pops, runs for a bit, pops, repeat. That looks like a misfire, right?

 

[luckydude]

So how much resistance should they have? I'm getting 8-9 Ohms. 9.15 in one case. What would new ones be?

OK, read the shop manual, it said take the caps off of the spark plug end, measure just the cap, that's supposed to be 4-6K Ohms, they were all 4.3-4.8K ohms. Wires unscrewed on both ends, all wires were 0.0 ohms.

I've got a buddy who is a real mechanic and he thinks the coils might be wavering under load. I need to get one of those inline spark checkers and see what it looks like.

 

[Dave LeGear]

I would replace both the wires and the coils with that amount of age. Normally though anything over 10 Ohms are not worth keeping and most likely breaking down internally.

Hope this helps?
Dave

 

[luckydude]

Dave, the wires are fine, they have 0.0 Ohms. The caps are designed to have 4000-6000 Ohms and they were all within spec.

Some progress, I got a little spark tester and tried it. At first it seemed like the flash was red, not blue but that was my old eyes. I went and got a magnifying glass and I have 5 solid blue, one that is either nothing or once in a while it manages to do something. So I have two new coils on order and we'll see if that is THE issue or if there are more. Boats, sigh.

Thanks for the help so far, I'll report back when I get the parts, should be midweek.

 

[Dave LeGear]

And don't be shocked if you find a new yet defective Spark Plug also... Been there done that (and NGK's which are usually perfect out of the box) thus my old saying... "I will take Known Good over Brand New any day"

 

[luckydude]

Coils came, doesn't fix it. Engine still fades to a halt.

 

[Offshoreman]

I'm just not a 2 stroke guy.

An often overlooked item is/are the grounds throughout the engine systems. If there is a bad ground, plenty of resistance can interfere with electronics like coils, and electricals like fuel pumps.

Might also be time to start with the fuel vent. Make sure it's clear. Clean or replace the fuel pickup tube. Check fuel lines and filters.

If it were a 4 stroke, I'd clean the VST and replace its internal filter. Clean injectors and check fuel pressure.

But it's a mysterious 2 stroke...

My dollar bet is fuel supply issue or a ground wire is corroded.

Hope this helps you.

 

[luckydude]

At this point, I hate 2 strokes and am inclined to repower the boat with a new Yamaha 150HP 4 stroke.

 

[Offshoreman]

Well, I'm not trying to sell you one of those, I'm not in that line of work. However, you hang a new four-stroke on the back of that boat and you will be very, very happy.

 

[Dave LeGear]

Amen on the grounds and I would just clean and secure them again.

Also....
Just how old is the fuel in this tank / lines? This almost strikes me a fuel issue or carb float that is sticking...

And if this is true (old fuel) it will cause havoc with even a new engine

Hope this helps?
Dave

 

[luckydude]

Amen on the grounds and I would just clean and secure them again.

Also....
Just how old is the fuel in this tank / lines? This almost strikes me a fuel issue or carb float that is sticking...

And if this is true (old fuel) it will cause havoc with even a new engine

Hope this helps?
Dave

I drained the gas tank, it's all new fuel. I also cleaned the carbs.