Idle mixture screw for 1999 Mercury 150 xr6

[JOE54]

OLD one remember the early WMV carbs from MERC had pumps too , sho did

[Stubborn Hook]

As posted above, the factory service manual for the XR6 says approximately 1-1/2 turns out, +/- 1/8 turn. However, it mentions that the idle screws were adjusted at the factory and shouldn't need adjustment. I'm guessing Mercury balances the idle screws with some specialized equipment and voodoo magic, so you don't want to lose the original factory settings.

I rebuilt the carbs on my XR6 this summer and with the all of the idle screws turned out at 1-1/2 turns exactly, the engine idled high, around 1,000-1,100 RPM, causing the gears to slam when shifting. Luckily I took pictures of the original screw settings so I set them back to the original settings and it brought the idle RPM back down to where it idles just enough to keep from dying now and the shifting is very gentle. Adjusted back to the factory settings, the bottom two carbs on mine are more like 1-3/8 turns out the top carb screws are around 1-1/2 turns out.

What makes the idle adjustment screw so wicked to get right if you're trying to tune by ear is that when you adjust one, it alters flow volume to the rest of the carbs, since the carbs are interconnected by the fuel line. So you need a carb balancer to do it, but no way to connect a carb balancer to the Mercurys as far as I know.

[OldTimer57]

Don't follow the last paragraph. The fuel lines have nothing to do with balancing, and don't carry fuel from one carb to the next. All three are fed in parallel from the fuel pump. Difficulty in adjusting by ear is that each carb only feeds two cylinders, leaving the other 4 unchanged. Still the wrong way to adjust the idle speed. There is an idle stop for that. If you remove carbs, new gaskets can be thicker altering the linkage measurements. The idle screw (just one, on the throttle linkage) is your friend. Adjusting the screws on the carb can cause a lean condition when the throttle is opened, which produces a "bog".

As far as pumps go, my WMVs did not have any. Motor was a '92 (first year of XR6 I think) They had an enrichment circuit that dumped raw fuel in the top carb and trickled it down to the other two via a special purpose rubber hose. And that was only activated when the ignition key was pushed in while cranking. Mine had no plungers that pumped fuel when the throttle was opened. I only needed to use that when motor was cold, as in first start of the day. Rarely after that unless it sat for several hours in cold weather. Then it would need it again. Can't comment on other model years as those are the only ones I ever had scattered on my workbench. Main reason I ever took em apart was to check needle valve seats, replace a leaking float, and change the jet size when I ramped up compression significantly.

Edit: I just took a look at one I had apparently stuck up on a shelf. I replaced the stock carbs with 175 carbs soon after buying it. When I looked at the parts catalog first thing I noticed was block, heads, etc were identical between the XR6 and the 175. Carbs were different. When I got the 175 carbs they had a much larger air passage then the XR6 carbs. Everyone uses a "ring" inside to produce a low-pressure effect for fuel flow. The XR6 had a ridiculous ring. Starving motor for air. I suppose that (and the ridiculous exhaust tuner plus the infamous idle spark advance module) were the hoops mercury had to jump through to tone down that motor to "approximately 150 hp." That is in quotes for a reason. :)