70hp evinrude won't idle when hot

[Jimex500]

So I have a 1985 evinrude 70hp on my aquasport 170. It starts and idles fine when cold. After I have taken a quick ride and idle down, the motor will run and then start to sputter out. It almost seems like it is starving for fuel. But after it shuts down I can immediately start it again and it will idle and sputter out in another 10 seconds. No clue what I should check myself before I send it to a mechanic and spend the big bucks. The fuel/water separator was replaced at the beginning of the season.

[fitz73222]

Does the engine run fine above idle without any problems? Will it cruise normally without sputtering or hesitating?

[Jimex500]

Yeah runs and cruises great above idle.....it only happens when I fully power down to enter 8 knot area around the marina....it makes docking very difficult

[fitz73222]

Will it idle in neutral but then dies when you put it in gear? What is your current idle RPM's while in gear? Does pushing in on the choke keep the engine running when it starts to die? I'm thinking you just need to bump up the idle speed.

[Jimex500]

It acts the same in neutral when warm. I've already bumped up the idle to try to solve it but it didn't work. I haven't tryed the choke or checking the fuel bulb when this happens. Those are my next steps. When I bought it over the winter, it had brand new linkage installed. I'm not sure how new the fuel line is but I can't see any cracks or leaks. No gas smell anywhere. Also it had a new gas tank, so I didn't know how big it was when I first fueled it up. Overfilled it a little and some gas came out of the gas vent so I'm guessing that's clear also.

[fitz73222]

There are many things this can be. The fact that you mentioned the linkage has been replaced got my eyebrows up. This engine may be simply suffering from incorrect idle synchronization with respect to throttle butterfly position, idle pick up timing and idle stop position. Your engine was the first year for the lost foam cast block and VRO which was a huge improvement over the old style sand cast block with the VRO exception. I only mention this because I have a factory service manual for a 1979 70 hp (old style block) that we could assume the 1985 base idle synchronization is the same but not sure. I could scan the idle set up pages and email them to you if you want to give this a try. You're going to need a timing light and a reliable tachometer to do this correctly. There are other mechanical conditions in this engine design that caused idle problems and is something called lybrith (sp) seals that were machined into the block that basically maintained crankcase pressure and vacuum balance in the crankcase to maintain even fuel distribution between cylinder pulses. These actually supported the crankshaft and were subject to wear on high hour engines and once worn, the block was scrap but we won't go there. If you don't want to do the link and sync yourself your going to need to find an older OMC mechanic 50 something (LOL) who group working on these engines. Let me know.

[fitz73222]

"Grew up working on these engines" damn spell check!

[Capt. Bob]

"Grew up working on these engines" damn spell check!

The man that invents the "what I really meant to type" checker is the guy who will get wealthy beyond his dreams.

[Jimex500]

Update: so trying to figure this out, I installed an external gas tank on the engine and it idled perfectly for over 15 minutes on the trailer. An in water test is next. Has anyone had a problem where their vent line is clogged or kinked?

[fitz73222]

Yes, water test with remote tank and report the results.