1993 70 TLRR power loss issue

[grouper sandwich]

I've been stumping my toe on this problem for a couple of years now and am at my wits end. First I'll describe the problem, steps I've taken and what my certified Yamaha master mechanic suggests. I'm hoping, praying, someone can at least give me some ideas. I've posted on this engine before and some have given me good advice. Unfortunately, the issue still exists..

Problem:
RPM loss when running over 4500 RPM for more than a mile or two. RPM drops to the mid-3000's and slowly loses from there until until either A) the engine sits off for an extended period of time, or B) the hood is removed for about 3-5 minutes and the engine is idled. After either one of those scenarios, it rockets off again. If the RPM's are held to 4500 or under, it will generally run all day unless the boat is loaded heavy or the water and air temps high. When I back it down at around 2000-2500 RPM (on the way down) the RPM's surge briefly. If I push the throttle all the way home, as in WOT, it begins dieing. In the winter and spring when the water is cool (about 80* and under) and the air temps are in the low-mid 80's or lower, the problem doesn't seem to exist. I can get 5800-5900 RPM under the right sea conditions WHEN THE PROBLEM DOES NOT EXIST. I'm running a Powertech SCD3-15 prop (if that helps). However, the engine itself does not seem to be overheating. No, I haven't taken it's temperature, but when the issue happens I can place my soft office hands on the top of the block and have a conversation while holding it there.

What I've done:
Cleaned the carbs, and cleaned the carbs, and cleaned the carbs. I'm convinced I have the cleanest carbs in all the boating world. Installed carb kits. Replaced the fuel pump, fuel lines (both on the engine and too the tank). Replaced the on the engine fuel filter. Installed a Yamaha Mini-10 filter. Replaced the fuel tank and used ONLY E-0 fuel. Replaced the fuel pump. Replaced the exhaust cover gaskets (water passages look clean underneath). Replaced the thermostat and gaskets. Tested all of the electronics, both on my own and by the certified Yamaha master mechanic. Installed a full water pump kit, including replacing the lower housing. Have done multiple compression checks (115, 115, 110). Changed plugs and inspected old ones. They look oily.

What my mechanic says:
It's old, can't be touched because bolts would break. Let me show you our new line of 4-stroke engines. My response? Find new mechanic who doesn't also sell new engines.

If there is anything in what I have written that sets off any alarms to ANYONE, please, for all that is holy in the ocean, let me know.

[99yam40]

seems like you have never tested the fuel pressure/ vacuum to see if there was a problem with supply.
Never tested alarms to see if they function properly and sound buzzer.
Never tested temp switches to see if they function in spec
never tested voltages to cdi and out of it to see if that was in spec before and during the problem to see if that changed due to heating.
Never checked timing to see if that was changing while problem showed up

[grouper sandwich]

seems like you have never tested the fuel pressure/ vacuum to see if there was a problem with supply.

Didn't think of that. I just assumed it was done when the pump was changed by my Yamaha master mechanic (I love saying that, BTW). How is it done and with what tools?

Never tested alarms to see if they function properly and sound buzzer.

Assumption on your part. Did it... tested good.

Never tested temp switches to see if they function in spec

Another assumption. Did it...tested good.

never tested voltages to cdi and out of it to see if that was in spec before and during the problem to see if that changed due to heating.

Tested before with DVA, but not during.

Never checked timing to see if that was changing while problem showed up

Never thought about that, but will certainly try it.

[rodbolt17]

its a dirt simple little motor.
however it does require certain things to run.
air fuel and spark all at the correct time and ratio.

I understand old saltwater motors.
I am also fairly good with a drill and heli-coils.

your describing a fuel issue.

[grouper sandwich]

its a dirt simple little motor.
however it does require certain things to run.
air fuel and spark all at the correct time and ratio.

I understand old saltwater motors.
I am also fairly good with a drill and heli-coils.

your describing a fuel issue.

Thanks. I'm not a mechanical moron, but only by a narrow margin. If the fuel pump, fuel lines, fuel bulb, fuel tank, fuel filters have all been replaced and the carbs cleaned to the point being brand new, where else could a fuel issue be? What would I look for and how would I test? And why would the problem only occur when the atmospheric conditions are hot and not when it's cold? How would removing the hood for 5 minutes at idle correct it?

[99yam40]

If you think it is heat related but does not set of alarms and they are functioning properly, test while problem is occurring to see if you are loosing spark or timing is changing.

Have you run without cowling on to see if it still has the problem?
Might have exhaust gases building up under hood and starving motor of O2

[grouper sandwich]

Yes, have run it without the cowl and it runs great. I've changed the exhaust cover gaskets and the problem still exist. My mechanic told me to look for any sign of exhaust around the base of the engine at first start up. I've done that several times and can't see any. I guess it could be undetectable to the naked eye? Also, if it were exhaust I would think taking the cowl off for 30 seconds to a minute and then putting it back on would correct the issue, but it doesn't. It seems to take about 5 minutes for SOMETHING to cool down. Could some electronic device be heating up and effect the timing? What about that timing arm on the CDI? Does that go bad and cause intermittent power and/or timing issues?

[99yam40]

I had a problem with the CDI on my C40 the idle timing was too far retarded when wiper was set according to book.
being electronic there are things that can go bad

all you can do is test to see what is going wrong when it is happening.
Guessing and throwing parts at it gets very expensive.
Does you motor have a high and low speed charge coil for feeding CDI?

[grouper sandwich]

Does you motor have a high and low speed charge coil for feeding CDI?

No, just a single charge coil, from what I can tell. I only see one and the manual only refers to one. I'll drag it home next weekend and do some more testing, I guess. And you're right, throwing parts at it is expensive, but at some point that may be my last option.

One question. You mentioned exhaust gasses building up under the hood. I replaced the exhaust cover gaskets (looked fine, but I wanted to get a peak at the water passages). If there is not a leak there, is the only other place the intermediate housing gasket? Is there anywhere else exhaust can come from?

[99yam40]

I guess corrosion could eat through anywhere

[grouper sandwich]

I guess corrosion could eat through anywhere

So supposing exhaust is the issue and it's NOT the intermediate housing gasket and corrosion could eat through anywhere, what is the remedy? Is it no powerhead time? Are there other gaskets that contain exhaust that I'm missing?

[grouper sandwich]

Updated with videos:

I was able to get to the coast this past weekend and run the boat. I didn't have any of my test equipment but did shoot a couple of videos. If anyone cares to watch them and sees ANYTHING they think I could use, please do. I appreciate all of the help this place provides to us novice wannabe boat mechanics.

Cowl on, power loss occurring after running at 4700 RPM for about a mile:

http://www.youtube.com/watch?v=uZtP1...G6DTXh8D6uflgg

Cowl off immediately after initial power loss then trying to plane up again:

1993 Yamaha 70 HP power loss cowl off - YouTube

Cowl off after idling at about 1200 RPM for 10 minutes, running at 4700 RPM:

1993 Yamaha 70 HP cowl off underway - YouTube


Thanks in advance!

[99yam40]

If the compression is good, and the spark and timing are good while problem happens then it is a fuel to air ratio issue.
But you have to test things to see what is happening.
Why did you leave the test equipment at home and miss a good chance at finding out what was happening with spark and timing?

[mackwrench]

While I cannot explain the "runs great wtih cowling off" It surely sounds fuel related......I'm also in Tallahassee (go noles).... I'm not a "master marine yamaha tech" like some here, but I'm local and perhaps take a look at with you....PM sent

[grouper sandwich]

If the compression is good, and the spark and timing are good while problem happens then it is a fuel to air ratio issue.
But you have to test things to see what is happening.
Why did you leave the test equipment at home and miss a good chance at finding out what was happening with spark and timing?

It was an impromptu trip with the wife and daughter and honestly, I just didn't think about it. I'll be back down again in two weeks (going fishing on a friends boat this weekend) and I'll test away then. I think you've given me some good ideas of, if nothing else, ways to eliminate things, Eventually there will only be one left standing, hopefully.