90hp 2 stroke fuel pump(?) Issue. Won't run at WOT.

[S.B.Horace]

As a follow up to a thread from 2016 (See here)

I am documenting my almost identical query with the hope of assistance.

I have owned my circa 2000 mariner 90hp 3 cyl 2 stroke for about 3 years. It has has a intermittent yet increasingly frequent issue where it cuts out under throttle, then after a few seconds, cuts back in. The boat starts and idles perfect. This ia only an issue when you step on the gas.

I have rebuilt fuel pump and thoroughly cleaned the carbs after finding obvious fuel pipe debris in the carburetor bowl. I have also replaced all fuel lines (5/16), filter, primer bulb etc. This did appear to fix the issue and I had an autumn of trouble free boating.

Here comes the crux.

I failed to winterise my boat last autumn. I left it sitting for 6 months with E5 fuel in the system.

The issue appeared again with a vengeance. I can now barely get up on the plane without it bogging/cutting back.

This spring i pulled the carbs again (appeared clean), replaced questionable fuel lines and connections and inspected the fuel pump.

I did find a small piece (2-3mm) of fuel line in the fuel pump. I removed and it ran better but still not 100%.

I am now almost at the end of my wit with this engine which has otherwise been great.

I haven't managed to install a fuel pressure gauge and I have to trailer the boat quite a long way to test run.

My next plan is as follows:

Rule out air leak / blocked filter in tank
Fuel tank higher, fuel hose shorter.
Run aux tank above cowling straight to pump.

Before sea trials:
Drop carbs and fuel pump and inspect thoroughly. Soak carb in acetone.
Inspect floats, gaskets, needle valve and jets.

Pull fuel pump and thoroughly inspect.
Tear down clean and reassemble. Double check check valves.


i would love to fix this without throwing expensive parts at it as i feel i am close.

Pumping the bulb was definitely improving running. It has even compression across all three cylinders (105 - 109 psi).

Many thanks in advance from the bonnie highlands of Scotland

 

[Texasmark]

I didn't read your whole article. Your engine is 10 years old. Rubber degrades over time. Why don't you cut your fuel filter open and see what's inside. If it's full of little black specks (of rubber), you know what to do next. Read my answer to an article that I just responded to on the 125 HP Merc. post above about compressed air catching a problem!

 

[S.B.Horace]

Thanks for your reply. My engine is now coming on for 26 years old.

I have renewed just about every hose and filter associated with the fuel system. I will pull the carbs again and clean more thoroughly with compressed air to see if this helps.

 

[louc]

a couple of things..
might want to (carefully) pull the fuel pick up tube, some have screens on them that can get plugged up
if you see that, then I'd probably also pull out the fuel gauge sending unit and look down inside the tank to see if it's got a lot of debris in it.
also might want to temporarily use a clear hose in line with the fuel line from the tank to the filter to see if you get a steady stream of bubbles, which suggests an air leak
last thing are you sure that the tank vent is unobstructed?

there are other tests you can do, here is a link to them....

HOW TO: Testing your boats fuel system.

Note, this was originally posted courtesy of Don S. Minor edits and pictures added by CaptJason A lot of you folks out there seem to be tearing into carbs quite a bit. Before you choose to open up a carb, you really need to look at what's feeding the carb first, and that is the boats fuel...

forums.iboats.com

 

[racerone]

What other trouble shooting has been done?----Post actual compression values here.----Does spark jump a gap of 3/8" on all leads ?