3.7l bogs down after running under load for 30 minutes

[captquint]

Hello to all,
I have been up and down this forum and want to thank you all for he great information. And here is my 1st post.

I purchased a 1989 Thompson 20' Carrera with a 3.7L. The previous owner replaced the bellows, shift / throttle cables, regular impeller changes, regular oil changes, etc. then fell on hard times. The boat sat for 2 years.
She starts easy and idles smoothly and gets on a plane quickly,but when running at mid-cruising speeds, bogs down after about a half hour. re-starts , but low power. This has happened twice.
Here is what I have done so far... the usual...changed the filter at the carb, cleaned the blue stone filter in the fuel pump, changed the points, plugs, cap, rotor, wires. Set dwell , gap and timing.

Compression ~150 in 1,2,3 and 140 in 4. I have a cheap compression meter and could not get it to seat completely in #4, so that could account for the lower reading. I know the elbow gasket can cause problems with #4, so I changed the gasket and flushed and replaced the coolant. While the cooling system was drained I checked the heat exchanger (4") and thermostat....everything is good there.

She fires right up on the muffs and runs smoothly... I am hoping to take her to the Hudson River for a test run tomorrow if the conditions allow....We got a double dose of mother nature, hit be Irene and TS Lee. The water is quite turbid, so I may hold off.

I just bought a full set of replacement gauges, so that is my next move. I am not sure I trust the ones currently installed. But the tach seems to agree with my volt, tach, dwell meter. The fuel gauge and sender tests OK.

She had a 19" prop, which I thought was too tall for this boat so I replaced it with a 17". I will check for WOT RPM's once I solve the engine bogging problem.

So here are a few questions....

I see conflicting information about timing some say 4 deg and some say 8deg. Is there a consensus? Is 4 deg more conservative with respect to longevity vs. performance?

Assuming I run the correct prop and can achieve ~4500 RPM WOT... Is there any danger in running at considerably lower RPM's...on a plane, but in the low 3000's?

Regarding octane.... is any point in running high octane fuel?

Thanks again.... Lee

 

[SSTKO81]

Re: 3.7l bogs down after running under load for 30 minutes

in regards to octane it's just kinda dependant on what your boat likes. I've got an '81 140 and she'll ping on 87, but runs smooth on 91. If you're forced to buy 10% ethanol like we are here I'd recommend the higher octane, but for the simple fact that its a better burn. But again. It's what your boat likes...
As far as the RPMs, find out what operating RPMs are for your motor and make sure that reguardless of pitch or prop height, you dont exceed or fall below those recommendations. As long as you do that I can't think of any problems to be had by running in the low 3Ks.

 

[captquint]

Re: 3.7l bogs down after running under load for 30 minutes

Thanks for the info... Yes... there is no avoiding E10. I just finished installing a water separator / filter and cleaning / changing all the other fuel filters. The marine mechanic in my area, that takes care of all the law enforcement boats, had suggested setting the timing at 6deg. instead of 8. I'll give that a whirl.

I hope to take her to the river tomorrow for a test spin.

 

[Rocky_Road]

Re: 3.7l bogs down after running under load for 30 minutes

Since your problem starts after about 30 minutes of running, you may want to make sure that your tank vent is not blocked. If it is restricted, then the longer that you run the more vacuum is created within the tank, until there is enough vacuum to restrict your fuel flow. An easy way to eliminate this as your problem is to 'crack' the gas fill cap to allow air to replace the used fuel. If this makes you 'good to go', then all you have to do is clear your vent line.

Happy boating!

 

[captquint]

Re: 3.7l bogs down after running under load for 30 minutes

Makes sense. I have a large inspection hatch over my tank. I think I'll check the vent line prior to shoving off.
Thanks for the good info.

Lee

 

[mpdive]

Re: 3.7l bogs down after running under load for 30 minutes

Only run the octane fuel your manual tells you to. Running high octane in an engine not designed for it is a waste of money. High octane is designed for alternate compression engines and your engines timing and controls may not be set for it.

 

[captquint]

Re: 3.7l bogs down after running under load for 30 minutes

Rocky Road... you hit it on the head! Something is not correct with the vent tube. At the filling station I pressed the rubber "skirt" from the gas pump nozzle to seal around the fuel inlet mounted high on the stern of my boat. I had less than 1/4 tank and slowly added fuel. I expected to feel a little air come from the vent tube, instead fuel spilled out the vent and filler inlet. This suggests the vent is low in the tank.. This can be correct.
I slowly added a few gallons (not sealing off the filler) and took her out for a water trial. I left the gas cap "cracked open" The boat ran for a few hours with no problems.

I am going to clean the carb this week and look into the tank vent..

BTW.. I was running 87 and no pinging... plenty of power... cruised nice a little below 3K RPM. I brought her to WOT, but could not stay there for more than a few seconds as the Hudson is still recovering from TS Irene and Lee...lots of stuff in the water!

 

[Rocky_Road]

Re: 3.7l bogs down after running under load for 30 minutes

Rocky Road... you hit it on the head! Something is not correct with the vent tube. At the filling station I pressed the rubber "skirt" from the gas pump nozzle to seal around the fuel inlet mounted high on the stern of my boat. I had less than 1/4 tank and slowly added fuel. I expected to feel a little air come from the vent tube, instead fuel spilled out the vent and filler inlet. This suggests the vent is low in the tank.. This can be correct.
I slowly added a few gallons (not sealing off the filler) and took her out for a water trial. I left the gas cap "cracked open" The boat ran for a few hours with no problems.

I am going to clean the carb this week and look into the tank vent..

BTW.. I was running 87 and no pinging... plenty of power... cruised nice a little below 3K RPM. I brought her to WOT, but could not stay there for more than a few seconds as the Hudson is still recovering from TS Irene and Lee...lots of stuff in the water!

That's great! Nothing to stop you now....

I use mid-grade (89) in my boat because I only have ethanol spiked fuel available. Gasohol loses octane fairly quickly, and the small amount of money involved is worth it to me to keep my engine from pinging. I also use Marine StaBil at every fillup, since I cannot be sure just when my schedule will allow me to hit the water next.

Happy boating!