Bad Fuel Pump??

77SceptreOB · #1 11-01-2009, 11:59 AM

I'm in the process of restoring an older carbed outboard. It's a 1989 Yamaha V-6 Excel (225hp). Recently I had the 3 carbs rebuilt. Run very nice now.

EXCEPT, at about 4300 rpms and up, the engine seems to surge up and down about 200 rpms. It is almost like one of the three carbs is getting fuel starved. Bring the rpms back to 3500 and it's rock solid. Then take it up to 4600 rpms and the problem starts again.

But, if you hammer it, from say 1000 rpms it rockets up onto a plane (I run a 15" pitch alum prop) and runs solidly up to 5000 rpms or so. No problems and GREAT throttle response, no hesitation at all, just exactly what you would want. But after about 5 to 10 seconds at 5000 rpms the slight to moderate surging starts again.

Could this be that I'm not getting enough fuel to the engine / carbs. IE old weak fuel pump??

A couple of clarifications:

Carbs rebuilt by Yamaha Dealer last week.

Fresh, non-ethynol, gas run out of a new 6 gallon tank.

fuel line is 3/8"

Thoughts??

76Red18 · #2 11-02-2009, 05:37 PM

I'm having that same problem with my 150 johnson. I've been through my entire fuel system and found nothing. Went through my wire harness this weekend and found a bad ( high ohm reading) wire on a temperature switch. I'm hopeing this was causing my s.l.o.w. (safety feature to limit rpms) feature to momentarily kick in. I'm in the process of putting it back together.

77SceptreOB · #3 11-02-2009, 06:21 PM

76RED,

Let me know if that is the problem. You are right, maybe I should be looking at the ignition side of things rather than fuel side. Hmmmmmm.

Bushwacker · #4 11-03-2009, 12:22 AM

Quote:

. . .But after about 5 to 10 seconds at 5000 rpms the slight to moderate surging starts again.

Could this be that I'm not getting enough fuel to the engine / carbs. IE old weak fuel pump??

What you've described is how long it takes for fuel in one or more float bowls to get low enough to reduce fuel flow thru the main jet, so one or more carbs is starting to lean out. I'd bet on the top one. Be careful if you're running premix, because you're starving that cylinder for oil as well as gas, so it's a quick way to burn a piston or score a cylinder. Don't run it long in the surge condition!

You can tell which carb is running lean. Take off motor cover and pull the air baffle/inlet cover off the carbs. Have someone else run the boat, and shine a flashlight down the carb throats with the engine running wide open. Fuel will be pouring out of the venturi on the good carbs and it'll be obviously less on the one being starved for fuel.

Could be a weak fuel pump OR some restriction between pump and carb. Did the dealer replace all the individual hoses between the pump and carbs? Old fuel lines can build up a film of crud on the walls if you let fuel sit in them long enough. This stuff will eventually liberate and can PARTIALLY plug the carb inlet needle and seat; it flows ok at lower speed but won't allow enough fuel to pass at higher speed. Ethanol fuel will make it happen quicker. I had this happen once on my old 115.

Also make sure the fittings on your portable tank/fuel line are compatible with a big V-6. I'm not familiar with Yamaha's but OMC used to use 5/16" fuel lines on V-4's and 3/8" on the V-6's. Not sure if the connectors were any different. My old dealer told me of a problem they found on a boat one time that had been repowered from a V-4 to V-6 . . . the inside passage in the 90 degree fitting in the fuel tank wasn't drilled out to correct diameter! It was OK for the V-4 but caused a blown powerhead on the V-6!

eggsuckindog · #5 11-03-2009, 01:44 AM

Be somewhat careful running too much WOT without the air box - tends to lean everything out. I don't think its the fuel pump personally, might be one of the carbs as Denny mentioned. This might be a something to try if you do pull the air box - put a finger over a small part of each carb - just a little - that would tend to richen that carb up a bit and see what happens. I would lean towards some coil breakdown or something ignition related though.

Fr. Frank · #6 11-03-2009, 08:12 AM

I had an '88 Johnson 225 Looper that did just what you describe. After replacing the VRO pump, rebuilding all the carburetors twice, and replacing all the fuel lines, we discovered I had a bad stator. I replaced the stator and it ran great afterwards. The problem I had was uneven voltage and current to the powerpack. But I spent over $900 on the fuel system, before a friend came over and we tested the electical system. Fortunately, I kept all my old parts.

77SceptreOB · #7 11-03-2009, 10:53 AM

Thanks for the advise guys!! Good stuff. I think it could be fuel related or ignition related, or maybe a combination of both.

So, to eliminate some of the fuel concerns, I'm going to replace both fuel pumps and the fuel filter. They aren't expensive ($50/ea for fuel pumps & $10 for the filter). They probably need replacing anyway, as they are 20 years old. Then we will see where I am, hopefully fix the problem.
But if not, at least a few items have been eliminated. Then it will be on to ignition issues.

76Red18 · #8 11-03-2009, 06:05 PM

Denny I'm with ya on the lean carb. Fuel lines and filter were the first check and fuel dumping from the mains was the second; all o.k.. I'm hoping resistance in temp sensor plug is sending a false signal to my power pack. It's the sensor in the starboard head, not just a regular sending unit for a temp. gauge.

NoBones · #9 11-03-2009, 06:57 PM

Several other things to check.
If you have a filter type water seperating fuel
filter, check for any signs of corrosion on
the the aluminum frame. They do get pin holes
and suck air. Also do not use a cheap primer ball
they tend to leak as well. Make sure fuel vent
has no restictions. Last but not least update
your fuel pump. The new ones have gaskets that
stand up to todays fuel.

See ya, Ken