Bad Fuel Pump??
 

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??

Re: Bad Fuel Pump??
 

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.

Re: Bad Fuel Pump??
 

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.

Re: Bad 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! :eek:

Re: Bad fuel pump??
 

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.

Re: Bad Fuel Pump??
 

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.

Re: Bad Fuel Pump??
 

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.

Re: Bad fuel pump??
 

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.

Re: Bad fuel pump??
 

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

Re: Bad fuel pump??
 

Not using a "seperator" now. I will when I hook up my new 100 gallon tank.


New ball from West Marine, I plan to replace that one with a "yamaha" one before the next test/outing.




Working off a 6 gallon portable, plastic tank w/ manual vent. So the problem is not there.





That's in the works.

Re: Bad fuel pump??
 

the most important adjustment in any carb job(rebuild) is the float height adj. i would almost bet the floats are set too low and the pumps cant constently supply enough fuel to run wide open. therefor the engine will run WOT until the fuel level is lowered, then it slows to an rpm that the pump can keep up with. all of this can be caused by the float too low and not opening the needle early enough and the pump is playing catch-up. what a coincidence the carbs. were rebuilt and this problem shows up. just my .02

Re: Bad fuel pump??
 

Interesting....

Maybe they heard the change rattling around in my pocket and figured they (The Dealer) haven't roughed me up enough!!
LOL! There coming back for more. Hard to win when you play against the Stealership.

Re: Bad Fuel Pump??
 

Bringing this to the top. Found my problem. Key switch was cracked. Tested fine once. Found the problem when it was wet. Got a little shock when shutting it off. Replaced it and have been out 3 times with no problems.

Re: Bad Fuel Pump??
 

76red18, im sorry i didnt see your problem. i had a 1985 evinrude 150 before my merc. the keyswitch caused it to have no spark. replaced it and it was fine after that. i couldnt beleive the key switch would do that.

Re: Bad Fuel Pump??
 

Variable mid to high rpm miss. Don't know if 77SceptreOB has fixed his problem or not but it something he can check

Re: Bad Fuel Pump??
 

Maybe thats it, I'll check into this.

Re: Bad Fuel Pump??
 

I would start with an original shop manual, multimeter, and a compression guage. It could be a lot of things really. Many a high speed problem can be attributed to an electrical problem like a timer base, rect., power pack, stator, temp... etc. I have seen coil packs do something very similar at higher rpms also, from old wires arcing or the pack being weak, or corrosion. If it is ultimately a fuel problem then I would take it back to the person who did the carbs. The float height is a straight forward thing. All that said, it's pretty easy to pick up a piece of crud in the needle and seat or a jet. If it goes on for a while it can score the cylinder a little and it will read a good comp.# but at higher rpm under load it can wipe the cylinder wall clean when that piston temp comes up. Especially the top two cylinders.

It's a tedious job of process of elimination for a broad problem like you have but it'll be like a checkup for your motor.
Good luck

Re: Bad Fuel Pump??
 

I've replaced or rebuilt just about every fuel related component. So I doubt that is the problem.

Compression was checked and was good. 130 psi max, 122 min. almost equal side vs. side at the same cylinder elevation. the lower 2 were 122 (left) and 124 (right). middle 128/126 and top 129/130.

Electrical, now that a different story. I believe that is where the problem lies. This was a saltwater motor and no parts, except sparkplugs, have been replaced.

Any ideas on how and where to start troubleshooting the electrical system. Preferably with instruments and cheaper parts first. LOL.

Re: Bad Fuel Pump??
 

The oem shop manual for that motor. Ebay. At this point if you have a local marina guy who dealt those motors he'd probably let you borrow the manual for a good while. The "assumed knowledge" terminology sometimes is a pain with oem manuals....get yourself a haynes/chilton also and you will be set to tackle almost any problem that motor can give you. Get yourself some beer and a friend for the electrical stuff or get yourself test harness. Bring your patience in abundance. Don't skip anything if anything do it thrice and log every number. The fact you belive it might be a corrosion problem means maybe clean everything up and lubriplate, grease. Check for pins that are broken off inside the boots. They have been my nemesis numerous times their crimped shape holds them in place when they snap right behind the crimp. Talk about intermittent.

Re: Bad Fuel Pump??
 

If you can't figure it out, go over to the Iboats forum and post. There is a master Yamaha mechanic that hangs out there and he is very good at trying to help out with problems. He lives is Kittyhawk N.C. and goes by Rodbolt.

Re: Bad fuel pump??
 

Coincidence? Check the floats.
Bones is right on the bulb. I had an emergency buy on a Sat. that fell apart in my hand when replacing. Pulled 4-5" of vacuum @ idle.
Cheers,
GFS

Re: Bad fuel pump??
 

Not sure I follow you guys on the above.

"if the engine consumes 10 gal/hour or 21oz/min and the bowls are full because the float levels are correct". And the fuel pump supplies more than 21oz/min all is well. If the fuel pump provides less than 21 oz /min then the bowl fuel level will drop until the a bowl or bowls dry up and the engine surges due to the engine starving for gas. Then I have to back off the throttle to 3500 rpm at which the Fuel pump can fill the bowls faster than the engine can dry then.

If the above makes sense, then it wouldn't matter what the float level was set at. The fuel supply simply can't keep up with the consumption.....right???

I replaced both fuel pumps BTW.

Am I missing something here?? Please explain.

Thanks,

Jim

Re: Bad fuel pump??
 

There are potentially 2 separate issues here, both related to fuel starvation, but for several different reasons.

1. If the floats are set too low, the motor will tend to run lean (not good if you run premix!), even if fuel pump is good. If they rebuilt the carbs, hopefully they would have also replaced all the internal fuel lines too. Otherwise some crap from old lines could break loose and plug up the inlet needle and seat or jets on the newly rebuilt carbs!

2. If fuel pump is weak and can't match engine demand, then yes, motor would start to lean out/surge as soon as fuel level dropped enough. Low float level would just let this happen sooner.

3. A fuel restriction is between pump and carb, like a partially plugged needle valve, might act like a low float or weak pump, but fuel primer bulb would be hard. A fuel restriction upstream of primer bulb would result in collapsed bulb.

Don't know if this helps, but a fuel starvation or blockage problem shouldn't be that hard to find. Pumping the bulb when engine acts up might give you a clue as to what's going on.

Re: Bad fuel pump??
 

No, the internal lines were NOT replaced.


That make sense.

Thanks, I'll try that!

Re: Bad fuel pump??
 

If there was a fuel restriction, say a plugged filter or line, would the motor surge or would it just loose rpm at a constant level? For instance, "The motor used to hit 6000 now it only reaches 5500"

Re: Bad fuel pump??
 

I think it depends on amount of restriction. If filter is seriously clogged, yes, that'll reduce max rpm. If just slightly plugged, it could reduce fuel pressure just enough to reduce flow to the top carb, making it act like float was too low.

On my old 115, I had a slightly plugged carb one time; don't remember if it was inlet needle and seat or a main jet. It didn't show up until after about 10 seconds at WOT, when it began to surge, but it would still hit max rpm between surges.