RPM's dropping on starboard motor when trying to get on plan

djohnson52

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I have a 2001 Grady white Islander with twin 150 hpdi. When taking off the starboard motor will easily reach 3500 rpm and then drop to 2500 rpm for a second and then come right back to 3500. The motor will do this maybe 4-6 times and then runs just fine. This only occurs on heavy acceleration, not if I ease the rpms up slowly. I changed the low pressure pumps about 2 seasons ago and the motors have run great since until a few weeks ago when this started. I feel this is some type of fuel starvation possibly related to the high pressure pump seeing that this only occurs while trying to get on plane.

I have been doing some research and reading about micro filters and also VST filters being clogged. Any advice or input would be much appreciated. Thank you.
 

DaveHNL

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Re: RPM's dropping on starboard motor when trying to get on

I'm experiencing the same symptoms, all the research I've done points to bad fuel, clogged VST filters/injectors and possibly high pressure fuel pump.

I've pumped out all my old gas, changed fuel lines/fuel water separator filter and will be replacing VST filters and injectors on Friday - I also asked the mechanic to bring a HP fuel pump just in case.
 

Hammertime

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Re: RPM's dropping on starboard motor when trying to get on

djohnson52 said:
I have a 2001 Grady white Islander with twin 150 hpdi. When taking off the starboard motor will easily reach 3500 rpm and then drop to 2500 rpm for a second and then come right back to 3500. The motor will do this maybe 4-6 times and then runs just fine. This only occurs on heavy acceleration, not if I ease the rpms up slowly. I changed the low pressure pumps about 2 seasons ago and the motors have run great since until a few weeks ago when this started. I feel this is some type of fuel starvation possibly related to the high pressure pump seeing that this only occurs while trying to get on plane.

I have been doing some research and reading about micro filters and also VST filters being clogged. Any advice or input would be much appreciated. Thank you.

Definately sounds like fuel. Checking the VST filters is easy enough... Also put a fuel pressure gauge on your HP pump and check for pressure. I believe 37 pounds is where it should be at....I know you changed your LP pumps, check them again or just change them outright. They should be changed every few years anyways because Ethanol eats away at the diaphragm. Also check for any pin holes in your fuel lines... My buddy just had this same exact problem on his 200 HPDI. Idle perfect, runs fine at low RPM, as soon as he would gun it the motor would bog down. Air was getting into the fuel line starving the HP pump of gas.. and he changed basically everything you can think of on the fuel side of the motor and all it was just a simple, small pin hole on his fuel line, just outside of water seperator under the cover.
 

djohnson52

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Re: RPM's dropping on starboard motor when trying to get on

Thanks for the input.

I am not completely ruling out bad fuel, but I would find it odd for bad gas to only effect one motor and not the other. I changed the water separators last week and there was not a drop of water in them. That being said, when I disconnected the fuel lines the last few times I tucked them up against the inside of the bilge and between some other lines and connections which may have been able to pierce the fuel line. I will start with the easiest things first and work my way down from there. Will check the fuel lines and HP pump output, but my gut tells me it is a clogged VST.

Thanks again for the advice. I will post my findings when I get this resolved. The boat still runs great besides the few instances when it lags on acceleration. After a few seconds it corrects itself and runs perfect for the duration. Not a dire situation, but I definitely need to get this resolved soon so it doesn't cause other issues.
 

teaklejr

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Re: RPM's dropping on starboard motor when trying to get on

If I was betting on it I would say VST filter. Had a similar problem on port motor a few years ago. Twin 2007 200HPDI motors and when coming off a wave the port motor would bog down, would be fine if not trying to keep the rpm's up. changed out the VST filter and problem was solved. I now keep a spare filter and o-ring on board just in case.
 

djohnson52

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Re: RPM's dropping on starboard motor when trying to get on

Teaklejr - you would be in the money! I finally got the time to address this issue. VST filter was completely clogged/blocked. I could not see through the strainer at all. Swapped it out and took for a spin today and it ran great with no dropping RPMs at all - nice and smooth.

Thanks everyone for the info and advice!
 

teaklejr

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Re: RPM's dropping on starboard motor when trying to get on

Awesome glad you got it fixed.