300HP Evinrude rpms drop after warm

wizzo · #1 June 5th, 2003, 12:34 PM

I have a 91 V8 evinrude. Its been to 5 outboard repair shops. It seems all the have is parts changers.All the electroics have been changed.

The engine runs great out the water.When I run it in the lake it reaches 5500 rpms and runs for about 30sec then it drops down to 4000 rpms and it surges.I bypassed the vro pumps and install a electric fuel pump that pumps 5 lbs psi through 10ft of 3/8 hose.Still runs the same.Could it be one of the carbs?Or do I have enough fuel pressure?

Bear · #2 June 5th, 2003, 02:56 PM

I'm not sure but I'm thinking lack of fuel too.

10 feet? Maybe the fuel hose collapses, or a bad primer bulb, the pick up filter in the gas tank or the anti-siphon valve.

Is it possible to try another gas tank?

JB · #3 June 5th, 2003, 02:56 PM

Hi, Mark.

You seem to be dropping one or more cylinders when warm.

Possible electronic/ignition problem? Start her up with the bonnet off and go through the whole ignition system, heating components one at a time with a hair dryer. If you can make the idle rpm drop you have found your heat failure.

Next, inspect the inside of the bonnet for any signs of parts pressing against it when it is on. If you find any, inspect that part for possibly shorting out to the block when pressed by the bonnet.

Possible fuel problem? Run her on the lake with the bonnet off. If the rpm drops (it may not because of the cooling air on the electronics) pump your primer bulb. No improvement? squirt some premix into each carb throat.

If pumping the primer bulb makes her pick up you have a restriction in the fuel supply or a weak fuel pump.

If premix into a particular carb throat makes her pick up, overhaul that carb with attention to the fuel inlet and float needle. It is probably running out of gas after a few seconds.

Hope this helps. Let us know how you make out.

Good luck.

wizzo · #4 June 8th, 2003, 11:53 AM

I took the wot needle out of all the carbs and found 1 stopped up.I took it out and ran it in the lake and it still does not reach max rpms.We had the baffle off the carbs and when you put your hand over the carbs to choke them the rpms go up.I tried a bigger pump still the same.Im going to take all the carbs off and go through them.I will let you all know how it turs out.

Thanks for the responses.

HAVING A BIG NICE BOAT THAT DOESNT RUN IS LIKE HAVING A BIG $@&^* THAT DOESNT GET HARD.

wizzo · #5 June 15th, 2003, 04:04 PM

I took my 25' Bertram out in the lake today after I replaced all the hoses to the carbs.The hoses to the carbs was 5/32.I change them to 3/16.I made up my own hose mainfold with all bigger hoses going to all 8 carbs.It ran 5600 rpms at 44 mph.Its running great !!! What a fathers day surprise.After 2 years off trying to get someone to fix my boat.Its fixed.

Thanks for the help

Hope this helps someone else too.

Bear · #6 June 15th, 2003, 04:12 PM