Engine loses power after 20 minutes

[SuppressedAR15]

I've spent countless hours online and in the engine bay, and simply cannot solve this mystery on my own apparently.

I have a 1980 Mercruiser 228 (Chev 305) in a 20' boat.
It starts easily and runs smooth... For 20 minutes... Then, it loses power and will only run at idle up to approximately 1600rpms. Any higher and it will bog down, backfire, and die. If I keep it at idle or up to 1600, then it sounds just fine.

If I place a bag of ice on top of the distributor cap and that general area for a few minutes, it will generally let me run it for another 10 minutes before losing power.

Initially I thought it was the ignition coil, so I replaced that. No luck.

I then relocated the ignition coil away from the engine (thinking it was overheating), with no improvement.

​​​​​​i replaced the condenser under the distributor cap, but it made no difference.

I am 99% sure it's a spark issue, as I can manually press the accelerator pump and see fuel spray, with no effect.

New plugs, wires appear good (about 3 years old).

The engine runs really cool in Lake Superior, and never goes higher than 175 degrees.

Flame arrester on or off makes no difference.

New battery as well.

Fuel aspects were cleared just for good measure, and replaced all fuel lines. Replaced fuel filter/fuel separator, disassembled and cleaned mechanical fuel pump (looks brand new). Cleaned out fuel filter at the carb too. Rebuilt upper half of carburator last week, cleaning out all Jets, readjusted fuel float to specification, replaced needle and seat and all gaskets. I messed with idle fuel/air ratio screws until they were a purring clean at idle of 800rpm.

What am I missing?!?! I feel like it must be something super simple that I just can't put my finger on.

So frustrating. Literally runs like a new boat, then I lose power after 20 minutes of running. Cool down, then runs again like new, until 10-20 more minutes go by under load, then same thing over and over....

Please help!

 

[tacx]

Check your fuel tank vent. If its plugged the engine will run fine until the tank can no longer suck in enough air, creating a vacuum, then starves the engine. Let it sit for awhile and the tank re gaines air.

 

[achris]

Next time it happens, loosen the fuel tank cap and listen. If you hear air rush in, you found your problem.

Chris.....

 

[Scott Danforth]

Its almost never the coil

Its your fuel system

 

[tacx]

As Chris said, if you hear that sucking sound, you have a plugged vent tube. Not sure how you are filling your tank, but if you are filling till the fuel gets to the fill cap, then you are also filling the vent tube and creating a plug.

 

[SuppressedAR15]

During my prior diagnosing, I had removed the fuel vent screen on the exterior of the boat and blew thru the vent tube several times. It seemed to be very free flowing, with no restriction at all. I will still try opening the gas filler cap if it happens today, but I don't feel that this will prove to be the issue.

I have never filled the tank to full yet, and generally just add 5-6 gal at a time, and keep the tank around 2/3 full.

I appreciate the ideas... any others? I really felt that it was not a fuel issue, but a spark one, since the fuel system seems to being doing well, and even when the problem is occurring , I can still manually press the fuel accelerator pump lever, and I can see additional fuel squirting, but with no benefit to the engine's running condition.

Thanks for the ideas guys.

 

[Scott Danforth]

you have spark because it runs 1600 RPM and below. its not a spark issue

it may be a spark control issue when was the last time you serviced the points and condensor?

what is your fuel pressure?

 

[SuppressedAR15]

Condenser was just replaced this summer. I have never messed with the points, as I am unfamiliar with that. I certainly can look into it, but is that something that would cause the issue only after 20 minutes of running? and then be fine after a short cool-down period?

I have not checked the fuel pressure yet, but I was thinking of picking up a guage of some sort today to look into that. Again, if it works perfect for 20 minutes, I suppose the only value in checking that would be to check it when the problem is occurring..? can a gauge be left in-line for diagnostic purposes while the boat runs around for 20+ minutes?

 

[nola mike]

I have never messed with the points, as I am unfamiliar with that.

This is the very first thing you should do if you suspect an ignition issue. That + fuel filters should probably be 1 and 2 for any engine running problem. Stickies have good info on how to adjust. They need to be cleaned as well (or replaced).

 

[SuppressedAR15]

Understood. I am looking into the points right now. Thanks.

 

[Jofayc]

I have this same problem with my 1984 Mercruiser 140. I have changed fuel pump, coil, new plugs, new battery, new points and condenser but engine continues to barf after 15 minutes and won’t restart until it cools down. I’ve seen this issue too many times on forums with no one really saying what the correct fix is. I actually never see anyone that says they fixed the issue, so I’m concerned. Would love to know before spending any more $!

 

[scoflaw]

Could be tired motors, how's that compression looing ?

 

[SuppressedAR15]

Ok, I set the points correctly, replaced the condenser again for good measure, and checked the fuel pressure (6.5lbs). I also opened up the fuel filler cap while the issue was occuring, with no effect. Still exact same issue.

runs great for around 20 minutes, then only let's me stay under 1500 rpms after that.

​​​​​​While it's at 1500 rpms, it sounds great and runs very smoothly, but no joy if I attempted to increase throttle. Engine temp at this time is around 165.

If I turn things off, have a refreshment and wet a line, I'm back to 100% in 20 minutes or so

Then, same cycle returns.

Very frustrated and completely at a loss

Engine was rebuilt about 3 years ago, with only 30-40 hours on it since. Motor sounds too good and has too much power to have a compression issue in my opinion.

Would Love other input.
​​​​​

 

[Maclin]

A couple of oddball things that I have seen in remote parts of the internet, both center around the doghouse, how big or open it is, and well vented it is or isn't. Even if valid, may or may not apply in your situation. So, ask yourself, if you just putted around at 2k rpm or so do you think it would go longer before any symptoms showed up.

First is just air availability to the engine. If it is pretty socked up in there then all the good air gets used up and then just whatever can be drawn in is available, and if tight enough I guess it can kill it or limit rpm's severely. Troubleshooting involved running with the blower on, assuming vents are plumbed, helped sometimes, also leaving the hatch propped open a bit.

Next is a slight exhaust leak into the engine compartment, before the mixing of water and exhaust, eventually making the air unusable. Same remedies, prop open the hatch or run the blower with good venting.

One person swore up and down it was because he had removed the directional vents and put them back on oriented the same direction in error, and they thought this cancelled each other out no fresh air was getting in.

 

[SuppressedAR15]

Thanks for the ideas Maclin. I keep the hatch wide open during the testing/diagnosing phase generally, so there is ample fresh air at that time. I have also opened up the entire cover, so basically the engine is sitting on the deck all by itself in the open air, and still no diference.

Some additional info , it seems like the puttering around at 1200 rpms also only works for about 10 minutes once the issue ofcirs, then after that I'm stuck at idle or shutting down for 20 minutes prior to self remedy.

Today interested the fuel pressure when the issue was occuring, and it still has full 6.5lbs of pressure coming out of the fuel/water separator (directly prior to the carb). So I feel that the fuel side of things is still on track.

Can a bad external resistor cause this possibly?

Any other ideas are welcome.

 

[dennis461]

If it is had to diagnose, then you gotta start throwing parts at it.
for example.
I keep a brand new distributor and spark plugs in my bag of tricks.

If not already done...
I would replace plug wires cap rotor ignition points capacitor ,.
Check timing and advance
Double check plug wire firing order.
Check all electrical connectors.
Rebuild the carburetor again.

 

[dennis461]

Most people start with an assumption which is never really validated, so they continue to have trouble.
for example.
I know it's not a spark issue or
I now its not a fuel issue.
or in my case.
I know I put the spark plug wires on the right cylinders ;-)

The first thing I would do is get a good timing light and keep it on the boat.
One person posted it will not restart until cooled down.
That's when you pull out the timing light and check for spark.

One person posted, "manually press the fuel accelerator pump lever, and I can see additional fuel squirting, but with no benefit to the engine's running condition."
This is not a valid test, the engine needs extra air to go along with the extra gas, so fuel squirt alone should not raise RPM's.

When it's running bad, check the timing, there may be some component affected by heat changing the timing.
Points not set right, loose screw on capacitor or points or coil.

A warm carb, if not in great shape will start to act differently.
Keep in mind that water temperature on the gauge is not the same as distributor or carburetor temperature.
The engine was rebuilt, this could be a problem if piston clearance was not done correctly.
Does the engine seem to be blowing a lot of oil through any PCV lines, possibly loading the intake manifold with oil.

 

[Kcommander]

Did you ever figure out what the problem was?

 

[achris]

Did you ever figure out what the problem was?

Please read the forum rules. If you have a problem, start a thread of your own.

Chris.............
iboats mod team.