99 force built by merc idle issues

[milehighboater]

I have a 1998 force built by merc that is having idleing issues, I did a post about a month or two ago "99 force bogging at idle" and thought the problem was fixed... its not. During the previous process I did the fuel pump, plugs, gear oil, impeller, and decarbed, took it to the lake and I putted around between 900 and 2000 RPM for about an hour with no problem. I went to a high mountain lake and it was terrible again (~8500 feet). I am normally at about 6000 feet. When I came back down it is still having an issue but not as bad, I can idle around for about 10 minuites. My question is how high in the RPM range do the idle screws effect the mixture in the motor? the problem goes away above 2000 RPM. Because the variation in altitude I am thinking my problem is still with the idle mixture but I'm not sure. Also when it was running right it was about 30 degrees colder than it was last weekend. This is starting to become a problem seeing I really like to troll for lake trout. Oh the enrichment on the key will sometimes help bring the motor back but only for about 10 seconds or so. The motor will start right back up when I give it a bit of throttle and will idle out of gear forever. Any thoughts?

 

[roscoe]

Re: 99 force built by merc idle issues

So, this is your 75 hp motor?
A 1998 or a 1999?

What size is your main fuel jet?

These are the jet sizes for a 1998 75hp

If operating between 6000 and 10,000' elevation, you need a .082 main jet.

(090) (0 - 1500 FEET)

(088) (1500 - 3000 FEET)

(086) (3000 - 4500 FEET)

(084) (4500 - 6000 FEET)

(082) (6000 - 10000 FEET)

It also sounds like Your air/fuel mixture may be out of adjustment.
Needs to be adjusted precisely to not lean out the mixture too much. Then adjust idle to 800 in gear.

Something as simple as switching to a full synthetic tcw3 oil, mixed precisely, can also make a difference.

 

[milehighboater]

Re: 99 force built by merc idle issues

Thanks for the numbers, I will pull a carb and find the jet size. The previous owner said they had the jets redone but did not tell me to what size. It should be stamped on the top of the main jet in the fuel bowl and all three are the same size, right? As far as the oil I did switch from a valvoline tcw3 which was I think semi synthetic to a penzoil full synthetic but didn't think it would make too much of a difference. It has been acting up with any oil I have been using. As far as the gas/oil mixture I am preatty precise. I will normally mix a full 20 gallon mixture with a measuring funnel with the standard 50:1 in 91 octane fuel. I will go ahead and check the jet size this evening and post back. Any other ideas would be helpful, many of the "duh" fixes have been tried. The fuel fittings and lines are new including the bulb plus everything else already posted. This problem is getting kinda tiresome. I asked a few of the motor shops in denver about the problem and none of them have the capibility to test the motor in a tank or lake and it runs well on muffs so I'm kinda at a stand still with them.

 

[roscoe]

Re: 99 force built by merc idle issues

We really need to know if it is a 1998 or a 1999 model.

The jet sizes are different since the 99 model has 3 carbs, and the earlier models had only 1 carb.

I guess by the comment in your last post, you have 3 carbs, so it must be a 1999 model.

The jet sizes for the 99 model are:

(.056) (0 - 2500 Feet)

(.054) (2500 - 5000 Feet)

(.052) (5000 - 7500 Feet)

(.050) (7500 - 10000 Feet)

Yes I believe they should all be the same.

 

[milehighboater]

Re: 99 force built by merc idle issues

ok it looks like I have .052 jets installed, I guess this leaves me with messing around with the idle mixture screws? If anyone would like some light reading here is the previous post on this subject http://forums.iboats.com/showthread.php?t=381501&highlight=1999+75hp+idle