86 Mercrusier 170 not acting right

[seanbean]

Heres the short version.. took boat out and after about 1 to 1 1/2 hours starts missing/and wants to turn off when throttle is applied. When letting off the throttle it will continue to run but can still hear miss a little. Bring it home on trailer and runs fin after it cools off. This has happened twice and I added dry gas to 1/2 tank of fuel and topped off with 93 octane thinking there was water in the fuel. Took it out second time and the same thing happened. Oh and the first time it acted up I turned motor off to check all the fluids and when I restarted it kept turning off when I put it in gear. Let it rest 5 minutes and problem with turning off went away. That problem did not come back on second run but orginal problem did. Please help wants wrong with my boat?

 

[Reel Poor]

Re: 86 Mercrusier 170 not acting right

When was the last time it was tuned up, plugs, wires, cap, rotor, points, condensor, fuel filter(s), timming set (along with advance curve checked), carb adjusted ?

 

[seanbean]

Re: 86 Mercrusier 170 not acting right

Not really sure. I bought the boat last year and now am in the process of selling it. I have a buyer but told him that until I fixed it that I would not sell it to him. I'm trying to not guess at whats wrong with it and spend a bunch of money for nothing. I am going to clean the fuel filter tonight and check all the plugs. I checked the #4 plug before and it looked great but did not look at any others. I was just told by a boat mechanic to check the filter, then the carb, then the fuel pump. If that was not it then check all 4 plugs and if the #4 plug was cleaner than the others that the heat exchanger could be bad. Something about water getting in the cyclinder and cleaning the plug during detenation.

 

[Reel Poor]

Re: 86 Mercrusier 170 not acting right

Do a compression test, a cylinder leak test, and a fuel system test. If it passes these test it needs a tune up as mentioned above and possibly a carb overhaul and fresh fuel.

Test Outlined Here

Fuel System Test

 

[seanbean]

Re: 86 Mercrusier 170 not acting right

Cleaned the fuel filter last night, installed new spark plugs, inspected wires, cap and rotor. Everything looked good except for the plug I broke so I bought new ones. Points are 1 year old (forgot I put new ones in last season) Would them not being set right cause this? How about the coil? I don't know how old the coil and condensor are.

 

[Reel Poor]

Re: 86 Mercrusier 170 not acting right

Points are 1 year old (forgot I put new ones in last season) Would them not being set right cause this?

Yes...Set the dwell then set the timming....The condensor should have been changed with the points.

 

[seanbean]

Re: 86 Mercrusier 170 not acting right

*UPDATE* Took her out again last night, here's what happened. Ran boat @ 4000 rpms for about 45 mins and it ran great. Heres the bad stuff all of a sudden it acting like it was starving for fuel, I opened the fuel fill and no improvement, cut back the rpms and the problem went away, increased throttle and problem started coming back at 3800 and above. Started heading to shore went all the way down to neutral and went to WOT and she stumbled and fell on her face. Went back to neutral and slowly applied throttle and she ran great up until 3800 rpms and then started stumbling again. Had the mechanic out last night before the trip and set the dwell and checked the timing (timing was dead on and dwell was a little off) I asked him to do a compression check and he said it was not worth taking the gauge out of the case. (awesome compression) Any new ideas? I have just been told about a check valve in the tank could that be the problem? Fuel pump?

 

[Reel Poor]

Re: 86 Mercrusier 170 not acting right

The link to the fuel system test that I posted already will test everything from the fuel in the tank to the fuel pump. A fuel pressure guage installed between the pump and the carb will test the fuel pump pressure.

There may also be a fuel filter on the inlet side of the carb. Pic may differ slightly from your carb.

 

[seanbean]

Re: 86 Mercrusier 170 not acting right

Took my anti siphon valve out last night (which was good) but out of curosity wht is the pick up tube in the tank made out of? My tank is plastic and it doesn't appear that I can take the fitting out the the fuel pump is attached to. I don't have the stuff or knowledge to do a fuel test so I'm wondering if there is a hole in the pickup tube since it seems to be doing it at the same time and fuel level. Picking up a new fuel pump today since mine is 20 years old.

 

[Reel Poor]

Re: 86 Mercrusier 170 not acting right

Run the boat on an auxilary fuel tank (3 or 6 gallon portable outboard tank). Disconect the supply from the fuel pump and connect the hose from the auxilary tank to the fuel pump inlet and run the boat. This sould eliminate any problem from the pump to the carb. If it runs ok on the auxilary tank you have then isolated the problem to the tank.

 

[bruceb58]

Re: 86 Mercrusier 170 not acting right

You coiuld have many problems from junk in your carb, fuel pump problems or a fuel venting problem.

To diagnose a fuel vent, try running with your gas cap off and see if it does the same thing. If it doesn't check for restrictions in the fuel vent.

 

[Reel Poor]

Re: 86 Mercrusier 170 not acting right

To diagnose a fuel vent, try running with your gas cap off and see if it does the same thing. If it doesn't check for restrictions in the fuel vent.

He did that already.

acting like it was starving for fuel, I opened the fuel fill and no improvement,

 

[seanbean]

Re: 86 Mercrusier 170 not acting right

Problem Solved. Fuel pump was bad, and was sending fuel up the second line(?) which caused the safety feature to kill the boat. Thanks

 

[MrBill]

Re: 86 Mercrusier 170 not acting right

Thanks for letting us know what the final solution was...I wish everyone did.

 

[Don S]

Re: 86 Mercrusier 170 not acting right

and was sending fuel up the second line(?)

That second line is a clear hose to give fuel a place to go when the diaphram in the fuel pump leaks. On automotive pumps, they just had a hole in them and the fuel would drain out on the road. In boats it would fill up the bilge with gas and fumes and be an explosion and fire hazard.
So the fuel goes into the carb as a place to get rid of all the excess (leaking) gas. In the process, it makes the engine run excessivly rich and it doesn't run very good.
For anyone reading this thread. If you have gas in that clear line and the engine runs ok, you still have a bad fuel pump. Just not enough bad to cause your engine to run rough .......... YET!
And thanks for the update.