Losing RPM

[kdneuzil]

HI. I am looking for some help with a fuel issue. 240 Offshore Twin Yamaha 150's. I went out for a day trip on Erie and on the way in the SB motor started losing RPM. On the way in to port it would just seem to miss but after a couple of these events the RPM just went from 3200 to 1200. I would draw back on both throttles and throttle up after 15 seconds or so and it would preform at 3200 for there or four minutes. The next time out it got worse and I started to diagnosis it. I had someone press the primer bulb when it started to happen again. The motor would rev back up to 3200RPM but fade after 4-5 min. Press the bulb again and would get back up there. Well after two or three times the bulb collapsed. I decided to switch lines between the motors. Same issue when SB line was on the Port motor and the SB motor had the port line. The SB motor was ok. I looked at the gasoline's and primer bulb and decided I should replace the old black flaking lines and primer bulb. I did that and pulled the hatch to check out the anti-siphon fitting. Seemed okay. Under deck fuel lines look pretty good. Reassembled everything and decided to check fuel filter/water separator. Replaced with new. Test ran after new lines and same problem. It seemed to be not so bad. Well we went out fishing the next day and now both motors have this issue. My next step is to pull the fuel lines off of the tank and see if the probes on the tank are clean. I'll also check all 90 degree elbows. I'll pull the fuel gauge sending unit to look inside the tank. If there is a lot of gunk in the tank do I have someone clean it or purchase a new tank? Anyone have a ballpark cost on tank cleaning? Any Idea where to get a new tank comparable to my original 95 Gal tank? I have to be money wise. I know it may cost me to get it going. I just can't have it sitting around all summer....I would get bumped out. Thanks in advance for your help

Ken

 

[Bloodweiser]

Vst and filters good?

 

[kdneuzil]

I checked the Serria Water/Fuel filters which are located just prior to the primer bulb and they seem fine. There is one for each line. I am not a mechanic...what is the Vst? If it is the screen filter on the motor? If so we took it off and check the screen. Seemed fine. I can't get back down to the boat until Tuesday. My parents 67th anniversary weekend. I have a friend that is going to help me out on Tuesday. He is thinking the tank has sludge/water/scale deposits that may be blocking the tubes going into the tank.
K

 

[gradyguy04]

VST

 

[gradyguy04]

check youtube VST tank , filter/screen in tank on bottom gets clogged

 

[kdneuzil]

I have reviewed the fuel diagrams for my 1988 150's. they are two stoke motors. no fuel injection no VST. It has two fuel pumps, fuel bowls with screen filters and two in line Serria water seperator/fuel filters. A buddy is planning to help me Tuesday. Our plan on Tuesday is to take out the tank fuel tubes, examine, clean, visually check inside the tank as well as examine the bowl screens again and visually check the fuel for moisture/debris.

Headed out to see the folks. 90 and 88 years young, married 67 years!!! family social distance party planned...if it doesn't storm. They have been in lock down for three months now. Twice because of hospital visits they were 14 days quarantined. But they are safe and the entire complex is Covid free. The admin and workers have done a great job.

Thank you for your advice. I will report back Tuesday what we find with the fuel tubes, tank and screen filter.

 

[Sparkdog118]

I would replace the Sierra filters with oem racor filters for better protection. Dirt and water pass through Sierra filters easily. Then remove the screen inside the tank. This will let anything that is in the tank to get sucked up and caught in the racor. also Remove the check ball and spring of the anti siphon valve. To remove ball n.spring, remove the aluminum nipple and put it in a vise. Use a Hammer and a pin punch to knock it out the rear. This is not needed on an outboard. Designed for iOS. Overworks fuel pumps on 2 strokes and causes restriction problems all the time.

 

[seasick]

Sounds like bad fuel pump(s). The fact that squeezing the primer makes the motor rev up is a clue. These are carbed motors. Whatever you do, don't touch the individual fuel mix screws or idle speed screws on the carbs. You will need a multiport manometer to set them up and I am sure you don't have one. I have made the mistake of adjusting the screws and learned the hard way that you can'r sync the carbs without the special gauge.

 

[Sparkdog118]

Those carb idle jets are set by bottoming out the setscrew and backing it off 1 1/2 turn. Then turn in just a very very small smidge. That is the magic setting for the Yamaha carbs on all those 2 strokes. I’ve done literally over a thousand carb rebuilds in my time.

 

[seasick]

Maybe the mix adjustment but not the idle stop adjustment. You need the gauge to sync the carbs in my opinion, I used to sync with plastic tubing held up to my ears. My ears got older I suppose.
You did mention something that did cause my brain to remember that before you do anything with those adjustments, mark the current position of the screw and if you change settings keep really accurate notes on which screw, how many turns and in which direction

 

[kdneuzil]

Ongoing diagnosis. We have been motoring out on one motor at 8 MPH to do some trolling near shore and we have positions in our current derby at second and fifth place. However the high RPM issues remain. I have replaced all fuel lines. Primer bulb (all were 15-20 years old and I feel good having new lines and bulbs). replaced the water separator filters, replace back flow preventers, and tested motors with spare isolated tank. During the isolated tank test we did it straight to the motors and they ran great. Then put the filters in line...motors performed great. I am really puzzled. we inspected the tank for water and sludge and it looked very nice. Few specs of dirt on the bottom which after 32 years I would consider it a clean tank. No water observed during inspection. We tried to take dip tubes out but couldn't budge them (I plan to retry after my next test is finished) It almost looks like the dip tubes are welded in place but my manual says otherwise. Currently I have new fuel lines hooked directly to the motor and plan to test the motors this way. The only places I can see where the problem would be coming from is the dip tubes or the 90 degree fittings at the stern. I didn't think to inspect them when putting the new fuel lines on. I doubt that it is the fittings but I didn't rule it out. (Besides the fittings are in the battery compartment and comes through into the splash well in the worst position to inspect. I may have to position my phone and take pictures because I don't think I can get my eye down there). After I test the direct hook up tomorrow and if I find out it is the dip tubes, I will need to try to get them out. My plan is to spray them for a couple days with Blaster and then use a wrench and a pipe on the end of it for leverage. Short of getting them out if they run the same direct to the tank, I think I'm looking at a new tank or remove and install new tubes in this one (I like that option the best) . I don't look forward to that. If anyone has had experience with their tanks I would like to know if there is a screen filter on the end of the dip tubes. it would be nice to be able to turn the fitting at the top of the tank off and inspect the tubes. I have not looked into the company yet but it is a "FLORIDA MARINE TANK, INC" tank...the model number was covered with some type of epoxy or resin and I could only see a 93 9
but I hope to make a call on Monday. They are still in business! Thanks for all of your replies, Ken

 

[seasick]

Did you try running at speed with the gas cap off?

 

[Sparkdog118]

If you have antisiphon valves, get rid of the spring and ball out of the aluminum tank fitting on the pickup tube. I get rid of all of them since they are not needed on an outboard.

 

[Sparkdog118]

Anti siphon valves can be removed without removing the dip tube.

 

[leeccoll]

Hi kdnuezil,

Florida Marine Tanks are still in business.
I got a quote from them last summer when I replaced my 33 year old tank.
They are proud of their work, I saved nearly 50% by using Patriot Marine in New Jersey to fabricate my tank instead.
Grady White customer service will e mail you the diagrams for any aluminum tank fabricator to use. Just give them your year and model.
Good luck!

 

[Tarpongrady]

This is just an FYI..... I am not a mechanic (not even close) but I had a similar problem with my twin 350s about 2 years ago. I thought I read yours are two strokes but maybe the problems are the same.

My motors would do the exact same thing, start to lose RPMs and pumping the primer balls would prop them back up, but then they would lose RPMs again. If I idled the engines for a minute the boat would get back up on a plane but lose RPMs again. My engines are synced so once one engine starts to lose RPMs the other would follow suit. I have a great mechanic and he tried everything to fix it (replace fuel lines, replace primer bulbs, any and all filters etc) and all along he suspected it was something fairly simple. I don’t know if your engines have this but mine have a water sensor with a fuel filter in some sort of a housing. He bypassed the water sensor and the engines ran great. He ended up replacing the housing the water sensor and filter are located inside of and the engine has run great since. As best we could figure there was some sort of a hairline fracture in the housing that we could not see. This allowed air into the fuel supply. A distributor cap on an automobile will not run properly if it has any kind of fracture in it. Shortly after that my other engine started doing the same thing. Replaced the housing and everything is fixed.

 

[seasick]

In all likelihood the old fuel bowl was not seated correctly. That is fairly common on those Yamis with the water sensor. So as you run you start to suck a bit of air. As you speed up and increase the fuel consumption rate, the gas in the bowl can't keep up and you in effect 'run out of gas'.

If you had immediately shut down the engine when it died and removed the cowling, you would have seen that the clear fuel bowl was empty. Of course that doesn't tell you what the issue is but it does eliminate a lot of parts that got replaced.

 

[Sparkdog118]

Those filters have a problem with ethanol in that the Plastic threads like to melt into the housing and shred. I only have that problem with ones that have been exposed to ethanol. They can also warp with ethanol and cause the air leak.

 

[seasick]

Sounds like a good possibility since no cracks were seen.

 

[kdneuzil]

I will be working with a buddy today....again...we will check the fuel bowls and fuel supply.. Over the last few days I have replaced the back flow preventers with plain fittings. I have Filtered the entire tank with a water separator filter and particulate separator on a 8 gallon per minute pump. Circulated the gas for 15 minutes and then pumped gas into 5 gallon cans to empty tank. Gas has been going into my truck and it runs great. Filled with fresh gas and oil. Same issues continue. We hooked up motors to a remote 6 gallon tank direct to gas feed line and primer bulb. Motors ran fine. Returned lines to tank and the SB motor stalled out while motoring and the Port motor limped back into dock. The Port motor actually stalled backing into the dock. Today the boat is on a trailer headed to a small engine shop buddy of mine who says he will help look at the gas flow from the tank. I also will ask him to use his compressor to blow compressed air into the dip tubes to see if there is any junk in the dip tubes. I don't think it is the fuel bowls if the motors run fine on the 6 gallon portable fuel tank. But we will check them out. I am just about done with it. I have called just about every marine service company in a 75 mile range of Cleveland and nobody want to work on my problem or they tell me it is 4-5 weeks. As soon as they hear they are 88 Yamaha's they say they don't work on those motors. I have talked to some mechanics who work mobile calls and after we talk they offer advice but don't really want to work on it. I am just about ready to dry dock her in my back yard. Really frustraighted and at a loss of what to do. Today will be my last day working on it.