1993 Volvo Penta 500B runs great for about 30 minutes then .... Please Help !!

M2bsing

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Boat runs great for 30 minutes then i feel it's running on 80% power. Funny thing is that when it's running poorly, I can push trim up or down and the motor gains RPMs. If I turn it off and let it sit for only a minute or 2,it runs well again for a short while. I'm confused as to whether it is a fuel or electrical issue. I run the blowers at all times. Started at the end of last year and has been bugging me since.
To date I have replaced plugs ,wires, rotor and distributor cap. Replaced coil, trim and starter relays as well as looked for any unplugged wires.
I have replaced fuel filter/ water seperator, fuel line from tank as well as the anti siphon valve. I even wrapped the metal fuel lines with a foam tubing to help keep fuel from vaporizing. I even blew out the vent line from tank ( overflow line )

Please help
 

M2bsing

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Thanks for the link!! Pressure gauge is next . I kind of ruled out fuel pump after the latest test today . I ran the boat for 35 minutes until I felt the hesitation starting at 3000 RPM. Turned the boat off for about 5 minutes and opened the engine compartment seat ( motor is under rear seat on my deck boat.) boat ran terrific !! Ran it,stopped it ,ran it , stopped it without any performance issues. Shut the seat cover and within 10 minutes it started acting up again. Maybe an air issue? I run my blower all of the time.
 

alldodge

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Maybe an air issue?

Could very well be. Blowers should not be needed while on plane and normal boating but does not hurt anything to keep them going. How clear are the vents on the boat?
 

M2bsing

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I will check them tomorrow, the Chaparral deck boats have large exterior vents but I think that they feed into a 3"line. Do you think that it would hurt to run boat without the large black motor cover and only the spark deflector for awhile to see if it gets better air to the carb?
 

alldodge

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The plastic trim cover has no purpose except for looks and it has your motor serial number info on it. Removing it will also have no effect on how much air it gets. Do make sure the flame arrestor is clean under the cover
 

M2bsing

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No charging or starting issues. Boat will always start and run, bur when it's acting up , runs at about 80%. I just went and checked the vent hoses and all clear. Blower is pushing nice air out of the vents exterior. I just cleaned the air box on top of the carb. ( didn't appear too bad ) and going to run it this afternoon without the plastic engine cove to see if it helps? Will let you know tomorrow. Thanks for the suggestions !!
 

M2bsing

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Well I cleaned the flame arrestor but it looked pretty good. No obvious clogs or debris. I think that I'm going to have to put a fuel pressure gauge on it when I can . Maybe a week or two before that happens. I will post the readings when I have therm. Going to be away from the lake and this frustration for form 10 days. Ha
Thanks again all for the posts, it keeps me from going crazy !!
 

M2bsing

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Well, I'm back at. I skipped the fuel pressure gauge step and replaced the fuel pump ( had fun with that chore ! ) I figured it's a 24 year old boat and the pump would need to be replaced sometime and it was only a $110.00 part and I could do it myself. Well after a 30 min. ride then went thru a long no-wake zone. Went full throttle after no wake and it could barely plane off again. I'm beginning to believe that it is a vapor lock problem. I always run blower. I just installed another blower that pulls outside air into doghouse and pointed the 3" hose at the base of the carb. Haven't had a chance to try it out yet,but I will post after this weekend.

Thoughts?
 

alldodge

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Still don't see it being vapor lock, your either getting to little or to much gas. Starting to think to much as in the carb is flooding. If you were running lean there would be lack of power while on plane and maybe some backfiring.
 

M2bsing

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Still have same issues !! Just don't understand why the engine gains RPMs when I either tilt up or down when it's acting up !
 

M2bsing

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Is it possible the trim motor gets hot and is a problem? Now I'm back to thinking electrical ?
 

M2bsing

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Well I took her out yesterday. I put the trim in a fairly neutral position and turned it off. I then unplugged the power to the trim pump to see if the motor was somehow getting hot and causing a short. Ran it for 35 minutes in and out of no wake zones and then it wouldn't run right at full throttle. Started missed and hesitating and could only get to 2900 RPM's . I stopped and plugged the trim power back up , know now that this wasn't the problem. I decided to run it with the engine compartment open, and within 5 minutes, it ran fine. Ran it hard with no problems with the engine compartment open, shut the cover and if I idled through a no wake zone, power was then an issue , especially at full throttle, hesitates and couldn't get to 3000 RPM.
 

alldodge

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Runs fine with engine hatch open and when closed power is reduced, yet leaving the blowers on does not help. Do you have an exhaust leak, and do your blowers push air into the engine compartment?
 

M2bsing

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No leak that I know of or can hear. The main blower that came on the boat pushes air out of the compartment. I added a small blower to pull air in and directed it toward the base of the carb. I think my next step will be to do a through cleaning of the spark arrestor and shoot some Gumout in the carb. Have you ever used Seafoam for carb cleaning?

It just acts like it needs more air or fuel with the hesitation. The gaining of RPM's has me stumped when lowering or raising the trim when it's acting up.I will post later this weekend if carb cleaning does any good.

Thanks
 

alldodge

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Seafoam is snake oil, its made of Naphtha, machine oil and isopropyl alcohol. There is no way I would put that kind of curd in my tank.

Cleaning the arrestor is a good thing, more so if its clogged. Could just remove it and take it for a run to see if things go away. The blowers should push air into the engine room
 

M2bsing

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Hmmm The original blower has always pushed air out. I may try to reverse main blower ? Thanks
 

M2bsing

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Well cleaning the flame arrestor and shooting a bunch of Gumout down the carb didn't do much good!!
 
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