Mercruiser 165hp I6 issue

[lamphega]

Tonight my 1975 Mercruiser was acting a bit strange. Started fine, temp good, ran fine at slow speeds but more than 1/2 throttle it would rev up and down. Sounded just like I was feathering the throttle back and forth. Slow down and it would settle into a good speed and tach would hold. But throttle up and tach and engine would start climbing and falling again. Good back to dock fine. As temp was good do I look at fuel delivery? I have changed fuel filters and have seen rusty looking sediments from time to time and I am wondering about water in gas but would that cause the problem to increase when going faster? Maybe fuel pump? I keep Aristocraft Nineteen at marina. Any ideas would be appreciated, thanks.

 

[ziggy]

Re: Mercruiser 165hp I6 issue

hi lamphega, hope yer having a great summer.
as to your issue. imho, i'd do a compression test first. just making sure ya got nothing wrong with the engine mechanically. then i'd do a minor tune up on it. ya know, clean or replace the plugs, maybe replace the points and condenser since they're cheap, check the plug wires, set dwell then timing to proper specs. ya already did the fuel filters. that's good. i will say from personal experience that i've had poor performance from dirty fuel filters. rust on the filter is signs that the fuel tank may be rusty. mine was when i got my boat. i was close to having to remove and clean my fuel tank. but i opted to replace fuel filters real often instead. just to see if the rust would clear up. in my case it did after several fuel filter changes. as for the fuel pump. check the service manual and find the specs on pressure and output and test it to see if it's ok. on mine. i had to do a diaphragm. when i opened up my fuel pump, it had much rust inside it. just replacing the diaphragm was enough to disrupt the rust enough that much of it was caught the screen at the carb inlet. this got cleaned several times too.
i've not had water in the fuel, yet. but to test i think ya just take some fuel out of the system and put it in a jar and let sit. if there's water in it, it'll separate. fuel on top, water on the bottom.
lastly, if all that produces no cure. i'd rebuild the carb to merc. specs w/ a merc rebuild kit.
good luck, hope ya find yer problem.

side note. interesting. i didn't put the links in for the word fuel tank. iboats did. not sure i think that's a great idea modifying my post with advertising. granted i understand that's what supports this forum. i just don't like it that it's modified w/o my permission or knowledge. +, no way for me to delete the link..... just saying.

 

[lamphega]

Re: Mercruiser 165hp I6 issue

Thanks. Points, plugs etc were changed beginning of summer. Carb was professionally rebuilt last year. I am changing filters again, 2nd time this summer. I think the old fuel tank is rusty on the inside, like I said I have seen specks of rust and sediment before in filters. Have not thought of changing diaphragm but makes sense. Is that a difficult job? Have not tried taking pump out of engine yet.

 

[fishrdan]

Re: Mercruiser 165hp I6 issue

Add a water separating fuel filter, it will filter out all the rusty junk before it hits the fuel pump and carb, areas you don't want rust getting to. The bigger filter element will trap a lot more junk than the pump mounted filter and it will also remove water, kind of a win-win thing.

My old 73 Boat had a rusty tank and I had to change filters often, it was a 140HP instead of a 165, though the pump/filter was the same between the two. At one point there was so many rust particles built up in the fuel pump that it wouldn't work right, clogged up, low pump output.