1979 Fiberform 17' low profile w/ 1984 Mariner 90

[79Merc80]

I recently aquired this boat (Pictured). The owner told me that the fuel tank was rusty and that's why it wouldn'tgo faster than about 18mph. I installed a fuel filter and checked the tank and there was no rust to be found. I took it to a mechanic who said that the link and sync was dialed in perfectly but that both of the switch boxes was dropping a cylinder each. He also did a compression check and all 6 were between 135 and 142psi. I got 2 switch boxes from (Outboard Exchange (They said they tested them before shipping)). Before I changed the switch boxes it would surge from idle through about 2000 RPM (Not real sure the tach is correct). I took it out this evening along with the video camera (Forgot the camera's battery). The engine idles beatuifully and up to about 1500 RPM where it will surge (Not as bad as before) but that's pretty much as fast as it will go. It will SLOWLY speed up to about 2000 RPM (~ 20 MPH by GPS). What should I have checked? I have limited access to tools and no testing equiptment. Just wanted to know what to tell a different and closer mechanic.

Thanks for any information you can give.

Craig

 

[CharlieB]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

Two things,

First, both boxes dropping a cyl. You need to retest spark on all cyl's. Each module fires 3 cyl'''s and share the triggers. A bad wire or bad trigger can cause this. Always remember there is NO RETURN on electrical parts. You MUST PROVE any fault before ordering any parts.

Second, a surging idle may be caused by an idle stabilizer module, remove it, it is not necessary. A surging idle can also be caused by an intermittant trigger, poor trigger wire, connection, etc. Make sure all wires on the ignition modules are clean, bright, and tight.

DVA testing as directed in the CDIElectronic.com free Ignition Troubleshooting Guide is the most reliable method of determining an ignition problem. Follow their directions to the letter to prove any fault before ordering any NON-returnable parts.

 

[79Merc80]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

Where is the idle stabilizer and what does it look like?

The only reason I ordered the parts is this mechanic was going to order them at 220 a piece and I got them for 65 a piece. I still have the old switch boxes.

Thanks for the information.

 

[79Merc80]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

Update:

I took it to the mechanic and he tested all of the electronics and said they all were in good shape, great compression, so he is rebuilding the carbs. He said he is a "Master Mercury Mechanic"

Any thoughts??

 

[79Merc80]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

******************U P D A T E ***************

Had the carbs removed, cleaned, and re-installed, took it out and it hessitated a little then ran up to 30 MPH on the GPS. Tack said 3000 RPM's, but I doubt that is correct. I ran it at WOT for a bit, then slowed to an idle, turned around to go back and tell them that it does run above 15 MPH, Put the hammer down and it was back to 17 MPH. I took 2 video's, one when it went 30 and the other on the way back at 17. Let me know if you think the first one is actually reaching full RPM's, I think it's only getting to about 4200.

http://vimeo.com/13751498

http://vimeo.com/13751594

 

[CharlieB]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

Hesitation on acceleration is often a sign of a slightly lean idle mixture and/or slightly low carb float setting, either will not pass enough fuel thru the idle circuit necessary for crisp acceleration.

Limited top end could be a low voltage from the high speed windings of the stator

OR

a rectifier that is beginning to fail, which will also cause an inaccurate or dead tach. Test this by disconnecting the yellow wires (and tape them off) that connect the stator to the rectifier. Test WOT again on the water to see if it now gets up to speed. Again, no return on electrical parts, do not order either a rectifier or stator until PROVEN bad. You can Ohm's test the rectifier each yellow too the red should have continuity in the same direction and nothing when you reverse your meter leads.

 

[njrextreme]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

I have a 86 mariner 90 that was running simillar to yours in the second video. I replaced the stator,trigger,switch boxes and rectifire all in one shot. It probable wasnt the smart way to go but it was all new and made the motor run great. I would guess that your boat will probably do 40 mph tops w/ that motor. I have a 15 and a half foot ski and fish (750 pounds) and it would get it up to 45-50 mph on the spedo. I think mine has a torqe prop more for pulling a tube or ski. After I replaced everything, took it out twice and then it devolped a knock. Got a love the boating blues. Best of luck
Nick

 

[79Merc80]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

Thanks for the imput guys. He won't be able to look at it until Monday, but I will bring your suggestions to him.

Craig

 

[79Merc80]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

Still waiting for him to look at it.

can someone post a picture of the idle stabilizer?

Also, what is the WOT RPM range on the 90?

 

[79Merc80]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

*************U P D A T E**************

I talked with the mechanics other employee today and he said that the mechanic took the boat out and determined that #1 had an intermitin spark and #3 wasn't sparking at all. Going to talk with him tomorrow for more imformation

 

[79Merc80]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

*************U P D A T E *****************

It's fixed and runs like a champ!!! It turned out to be the stator. All of the voltages were off so he ordered and installed the stator and now it jumps out of the hole. Actually very impressive for a 90hp. Top speed on a choppy lake was 40 MPH, but I didn't play with the trim much. Idles at 2.5 MPH.

 

[crazyinkc]

Re: 1979 Fiberform 17' low profile w/ 1984 Mariner 90

Nice. A friend of mine has a 77 21ft Fiberform cuddy with a 175 OMC. I have a 90 on my 16ft Sea Sprite and it moves along the water just fine. I have yet to GPS it but I was side by side with a newer 18ft Bayliner with a 3.0l and keep up with my friends cuddy too. I love that little boat almost as much as my toon.