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1990 Century Resorter, 350 Chev MercCruiser - Engine has no power after ~30 minutes running

90Resorter

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I have a 1990 Century Resorter. It is an inboard (direct drive), 350 Chev MercCruiser, carburetor, set up. Last summer it developed an issue where after about 30 minutes on the lake, it will idle, but has no power when I try to accelerate. It restarts right away, but won't do much more than idle. This leads me to a fuel starvation theory, but what is puzzling me is that it runs fine for 20-30 minutes and after it gets warm it won't run properly. I replaced the mechanical water pump just 3 summers ago for another issue. I replaced the coil, distributor and points just 2 summers ago. Next, I plan to check the fuel tank venting by loosening the gas tank cap after the symptoms start. I'm also going to see if any of the fuel lines seem plugged due to higher solvent gas...even though I only run non-oxy, premium gas in it. Wondering if anyone has had similar issues and found the cure.
 
could be crud in the tank building up on the screen at the bottom of the pickup tube......could also be a venting issue with the fuel tank.....a vacuum gauge plumbed into the suction hose at the fuel pump will help in troubleshooting a fuel supply issue.

Whats not clear is the difference between "...runs fine for 20-30 minutes..." and "...after it gets warm it won't run properly."???

Did you mean mechanical fuel pump (vs water pump)??
 
Hi makomark. Thanks for the reply. I'm sorry, I meant mechanical fuel pump. I associate running for 20-30 minutes as "after it's warmed up". There is no difference. I think the most important factor is the time. The engine is approximately the same temp after 10 minutes as it is at 30 minutes. Do you happen to know the approximate vacuum gauge spec the fuel pump should be running at during normal operation?
 
No problem....you just need to remember those of us reading ONLY have what you write to go on.

Time may be a factor but its likely to be only one and may not be the critical one....it could be volume of fuel moved by the pump, boat motion, etc....bottom line is the more detail you provide, the better the forum help can be.

As far as vacuum readings, it will vary based on the installation and the condition of the pump....I strive to keep the vacuum to a minimum. I would say 4" Hg is the max vacuum (minimum pressure) a mechanical pump should have to work with.....with 2" Hg for electric pumps....and less is always better for 'normal' conditions.

If you run it and the vacuum starts to increase and then the engine loses power, shut the engine off and watch the gauge.....and record vacuum vs time after shutoff.....and note the water conditions.....I've seen the crud build up take over an hour on smooth lakes and less than 5 minutes on a rough day in a small boat....
 
Thanks again makomark. I did not have a chance to get a gauge yet to check the fuel pressure. I was able to find the proper fuel line pressure in my manual, 3.7 PSI.

I did however spend a little time looking at it last night and found that the choke thermostat (spring mechanism) was loose in it's carriage. I took that apart, reset it for the proper position and then crimped the forks tighter so it would not be loose in the carriage. I ran the engine up to operating temp (on the trailer using an inboard engine flusher) and for the time I ran it (20 minutes), it ran fine. I can't determine though if this was the root cause of the issue. I can't reason why that might have caused the carburetor to choke off after it was warm, thus causing it to have no power. To me, that would have caused hard starting and flooding. What are your thoughts on this?

I also purchased a miniature cable inspection camera that I can use to look down the gas filler opening to see if I can see any crud in the tank.
 
I was suggesting checking the suction side of the pump....it wouldn't hurt to verify the output pressure but with a combination gauge, you can only do one at a time.

On the choke, the housing (stove or chimney) just helps to concentrate the heat around the spring so I doubt its a contributor. As far as 20 minutes on the hose, thats a good sign but doesn't hep with you issue....big differences i see are no motion and much lower demand of fuel. as long as the choke was open, it shouldn't factor in....again, it wouldn't hurt to add that to your checklist.
 
Suction side of the pump - got it. That side of the fuel pump is rubber hose and fairly easy to plumb in a gauge. The delivery side of the fuel pump is tubing, probably aluminum, so that's would be more complex to measure the delivery side.

I agree with your comments, not sure that the thermostat choke spring is the issue. I plan to get it on the water this weekend just to see if there's any difference with the choke thermostat fixed. As you stated, no motion and much lower demand for fuel are key elements. I'll let you know.

Thanks again for your insight!
 
Hello Makomark. Sorry that it has been several weeks since I posted. I have been consumed with work commitments. I wanted to let you know that at the end of the day, I tore down my Rochester carburetor and rebuilt it with a new kit. That ended up being the fix. While I don't exactly know what component was failing in the carb (float?, accelerator pump?, vacuum leak?), it has worked fine since I rebuilt it. My guess is that the accelerator pump was not working correctly, or the float was stuck/not floating properly. The old float was made of a foam type material which can get heavy/soaked after time. I replaced it with a brass float.

Thank you for your help during my troubleshooting!
 
You are welcome. Thank you for the informative site and active members!

One more note...I've never rebuilt a carburetor before, so I was a bit tentative about taking it on myself. But, I found several Youtube videos to reference and help me through the process. It was a great learning experience! I encourage others to DIY also if you have a similar situation.
 
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