Mercury 90hp 2-stroke. Having a bit of difficulty getting mixture screws set.

[g_man500a]

Hey guys. I have a 2000 Mercury 90 ELPTO that I've been refreshing and when I recently went to set the mixture screws, I had a bit of trouble that I would greatly appreciate some input on.

The carbs are freshly rebuilt and I took the boat down to the ramp, submerged foot, started it up and let it warm up for 10 minutes or so. I then put it in forward and started attempting my adjustments. I had the screws on all three carbs set to 1-1/4 turn out from lightly seated at the onset. Heres my problem. On the top two carbs, I could turn the screws all the way in with no change in how the motor idled, the bottom carb would cause expected changes to idle quality as screw was adjusted. This just really seems weird to me that I can close the screws all the way like that yet it still run. All the idle mixture screws were replaced during rebuild.

Has anybody experienced something similar to this or have some insight on what might be the deal here?

Also, as a general question, when adjusting the carbs, do you adjust one carb down to where the engine sputters then 1/4 turn back out before moving to the next carb, or do you make small adjustments to each until you find that spot and then adjust them all together?

I probably have the screws turned out 2 full turns at this point, hoping that since the factory manual stated to use 1.25 turns out as a starting point that it would be safely rich enough to take out for a little while. It idled okay but there was a hesitation at about 2000rpm before it kicked over to the high speed jets, which sounds like a lean condition? It ran at WOT beautifully.

Any information that anybody can offer will be really appreciated. Thanks!

 

[GA_Boater]

I know exactly what you are seeing, On the two carbs with no affect, I would check the float settings.

I only have 2 carbs, but I start with 1 1/2 turns out and adjust one carb to the mid point between rich (out) stumble and lean (in) stumble, then do the same on the other carb. Then repeat several times to to fine tune. You would do all three carbs, of course.

 

[ondarvr]

If turning the screws doesn't make a difference then the carb is probably still dirty.

I do one at time, but you may need to go back do it a couple of times to get it just right.

 

[g_man500a]

Thanks for the replies fellas. When I rebuilt the carbs, I set float height with a depth gauge caliper to factory manual spec, so hopefully they're within adjustment. My method of cleaning them was a 30 minute soak in chem-dip, followed by about 20 minutes in an ultrasonic cleaner and then compressed air through all orifices, all of which seemed to be nice and clear. Something I was wondering though... on these carbs, there is the idle mixture adjustment screw and then to the left of that, there is a threaded hole with nothing in it that leads into the top part of the carb. Looking at the diagrams I don't see where it indicates that something should be in there but it seems weird that there's an open place there unless it's just a vent hole.

Do you ever have to run the adjustment screws further out from initial start-up setting or is it usually always just screwing in from that point?

Thanks again guys. I'd appreciate any further information.

 

[ondarvr]

You adjust the screws in or out as needed to dial it in for the best results.

 

[g_man500a]

Thanks again! I put it on muffs and messed around with it a bit today just to see and decided to see if I could get the engine to rich-stumble on all carbs and I was able to, so what I did was take it to where it was too rich on each and then backed it down 3/8 of a turn. I'll still need to get it in the water and readjust but I feel a bit better about it. Went ahead and compression tested the motor as well after it was warmed up since I hadn't done that yet and got a 113, 115, 115 so I'm happy about that. I noticed a bit of fuel leakage on the right side of the motor which I suspect is from where the hose from the enrichment pump t's into the carb hoses, but I'll sort that out the next time I have the cover off.

I appreciate it guys. If I end up taking the attenuator plate back off, I'll probably go ahead and pull the carbs and check the float levels and orifices again just in case. If anybody thinks of anything else. I'm all ears