jets for 85hp merc

i have looked all over the webb and cannot find jets for my 850 merc, the ones im looking for i belive are the fuel bowl vent jets, i have ran 72,76,78.5, 78.5 a little lean on bottom 72 way to rich on bottom 76 still a little rich,would like to go to a 77 or 77.5?any help on where to find these would be very welcomed

Re: jets for 85hp merc

The fuel bowl jets are for the backdrag carbs. If they make the motor run too lean, drill them out or disconnect the system, as merc discontinued these in later years.

Re: jets for 85hp merc

If i drilled them out would this make the motor run that much leaner? or disconnect the system how would i go about doing that?
im not sure what carbs they are i think it is a 1976 or77,but i have had no luck with the serial number that is on the motor as to finding the year.
I have tryed several different size jets,but can not seem to find 77 77.5 for the bowl jets.
i have 76 in it now with 90 mains seems to run ok until idleing down loads just a bit.i am actually wanting to lean it just a bit on these jets,not sure if i am useing propper terminoligy for them?

Re: jets for 85hp merc

The backdrag jets are located in the carb bowl covers. Is that the jets you refer to? If so, at midrange and higher RPMs, the carb throats pull air thru the small rubber tube that connects to the carb bowl. The carb pulls more air than the backdrag jet can supply, causing a partial vacuum in the carb bowl and leaning the mixture a bit. Backdrag jets have no effect at idle or during initial acceleration.

You do not want to lean the mixture by reducing the backdrag jets size. You can disconnect and plug the rubber tube to eliminate the backdrag effect. What exactly is the issue?

Re: jets for 85hp merc


This is the carbs that i have on my motor,#2 on the graph is the jet im talking about,is this the same as you are talking about? I have not looked at the back of the carb to see if there is a tube comeing out or not. my motor originally had a size 72 jet on the number two on this graph with 90 main jets.
with that configuration the boat ran great mid to high,but on the lower end off of idle circuit it would load up, So comeing off plane, lets say to come into a no wake area or slowing down cause there are fish on the finder it would load up REAL BAD.
So i talked to the guy who built the motor for me and he gave me some size 78.5 jets to replace the 72 jets. I should mention at this point the setting on the carbs was two and one quarter turn out! rich
He gave me 78.5 told me to try them this would allow me to lean the carbs to eliminate the rich issue, suggested one and three quarter turns out,so this is what i did,78.5 jet on the number two jet on this graph 90 main one and three quarter turns out,went to the lake ran great at mid and high but this time ran a little lean on the transition from comeing off plane and was lean at idle so when i would try to take off it was not getting enuff fuel.
So i then went back to the guy that built the motor and he gave me 76 jets.The carbs are now set at one and three quarter turns out and have 76jets and 90main jets, runs good mid and high idles well and is just a hair rich comeing off of plane and lower and still wants to load up a tad. i want to lean this circuit just a hair by going to a 77 or 77.5 jet. i have run at various altitudes rangeing from sea level to 1700 ft above sea level with the 76 and 90 main and it does the same thing at both maybe a little more at sea level.
If i could lean this a tad it would not have to be messed with from one place to another?

Re: jets for 85hp merc

The vent and main jets you started with are the ones Merc recommends for your motor from sea level up to 2500' elevation. I would stick with those. Normally you would not change vent jets to control mixture just off idle.

I suspect you have other carb issues, incorrect float adjustment, bad inlet needles or incorrect mixture adjustment.

A good way to set idle mixture is to adjust for max idle speed. The carbs will then be pretty well synched. Now try to accelerate. If she bogs (likely), open mix screw on top carb 1/8 turn and try to accelerate. Repeat with bottom carb, then repeat, testing after each adjustment. You will get her dialed in...

Re: jets for 85hp merc

I have tried to do as you stated with the 72 and 90 jets,if i leand the motor it would not accelerate,then tried to richen and it would want to flood out or choke up real bad.
i assume this points more towards incorrect float adjustment or bad inlet needles?
If this is the case i will take motor back to guy who built the motor beings it is still under warrentie. plus he needs to do 20 hour tune up anyhow.
how hard is it to adjust floats and replace needles?
Kinda new to boat motors but have built many small block chevys and done lots of work on harley and dirt bikes.i would almost rather do the work myself rather than have mechanic do it!
things just never come out good when i let other people do the work!!!!
thanks for all your help so far i appreciate it!!!

Re: jets for 85hp merc

If your mechanic is handing out vent jets to fix a carb problem at sea level, you may be better off not going back to him.

Get a Merc service manual and check or replace the inlet needles and seats. Set the floats to factory spec, and replace all gaskets. Now adjust the idle mixture screws and see if you can get them right. They usually need to be a tad rich, from the best idle quality to accelerate well.

Re: jets for 85hp merc

Sounds like the way to go to me! how do i find the year of my motor so i can get parts?
Ive tried putting in the serial number many times and comes up with nothing?
was easy with my old evenrude!

Re: jets for 85hp merc

Look on the maxrules.com website Merc Model guide. That will give year by HP and serial number.

Re: jets for 85hp merc

I was looking ont that sight today and found the parts i need,will let you know how turns out!
thanks for your help!!!!!!!!!!!!!

Re: jets for 85hp merc

Hey chris thanks again got it all tore down and cleand up,put new floats needles and seats,runs great agian thanks for your help

Re: jets for 85hp merc

My pleasure. Glad it worked out!