Rough Idle after warmup

**tmbiv56** · 11-05-2017, 09:40 AM

Hi all,
I have 1989 Mercury 150 xr4 on a 1990 Skeeter 175 DX.
The motor hasn't been run in about 3 years and I decided to spend some time on this boat to get it running.
I'm a shade tree mechanic at best. So, I try stuff to save money, but I am not trained by any means.
Before cranking it, I took the spark plugs out and sprayed a bunch of WD40 in the cylinders.
I spun the starter and it seemed to spin easily.
Then I bought carb kits and an impeller and soaked and cleaned the carbs and reassembled.
I replaced the spark plugs with new ones.
I replaced the impeller which was still in good shape.
Really, the motor is in very good condition.
The compression is kind of low. All cylinders around 92 +- 2psi.
I have a Clymer Mercury repair manual and followed the directions for doing the link and sync.
In general, I am pretty happy with the way the motor runs.
I have fresh gas in the tank. Someone had removed the oil injection a long time before I got it. I mix the gas 50:1.

The motor starts well, has good power. I got the boat up to 57 mph yesterday.
However, when I come back to the ramp and have to idle for about 5 minutes going through a no-wake zone, after a few minutes at idle, the motor gets rougher and rougher and eventually dies.

On going out, the motor idles fine.

Can anyone point me in the right direction? The motor temperature seems okay.
This is not a big deal for me as I am pretty happy otherwise, but if I can fix it, then I would like to do that.
Thanks for any help.

**FORBESAUTO** · 11-05-2017, 09:57 AM

Sounds like it's loading up a little. If it jumps up on plane satisfactory, I would try raising in gear idle a tad by raising the idle timing slightly. Make sure all throttle plates are fully closed at idle also. The higher timing will clean up idle. If it stumbles getting up try and lean out idle fuel just a tad. It's easier to try raising idle timing a couple degrees and see what happens though, if doesn't help lean out idle some. If it has the thermal valve on starboard head for carb bowl vents also make sure it's opening when warmed up

**flabum1017** · 11-05-2017, 10:02 AM

Carb float levels being too high can cause that too...........

**tmbiv56** · 11-05-2017, 10:21 AM

Thanks for the responses.
Forbes: When I did the link and sync, I checked the in-gear timing (with muffs) and it is set at 8 deg ATDC. The book says between 3 and 12 (I think).
So, this puts it in the middle. I agree that this might be a resolution and I will give that a shot.

Flabum: The boat ran this way a few years ago when I ran it a lot. I always suspsected a float issue even though the guy I bought it from said the shop mechs went through the carbs. When I took the carb bowls off, I checked the floats by holding the carbs upside down. I found that only one float was inconsistent. The bottom carb had one float hanging down about 1/8" lower than all the others.
I didn't really measure, but when I assembled the carbs, I eye-balled them to hang down about 1/16 - 1/8 below the top of the bowl. At least they are all consistent now.

**FORBESAUTO** · 11-05-2017, 10:37 AM

I'd bring that dude to about 4 ATDC and see what ya got. It's best to set it on the water in gear to what makes it happy. Each motor/ carb combo, and prop setup makes a difference what is needed. Get it to operating temp and set timing to where idles smooth and steady in gear. It's possible it could not have factory carbs installed that have different amount/size holes in butterflies that could cause for a different amount of idle timing than spec. calls for.

**XstreamVking** · 11-05-2017, 10:46 AM

I am guessing electrical parts breakdown. (switchbox?) Check the spark to all cyls b4 you do anything else.

**tmbiv56** · 11-05-2017, 10:48 AM

Thanks. My carbs are WH. 1374-5427-C is stamped on each one. They also have 41-11458, 41-21458, 41-31458 stamped on them.
Sounds like a logical conclusion.
I may not get back to the lake for a while, but I'll bring my timing light with me next time and try a bit of timing adjustment.

**tmbiv56** · 11-05-2017, 08:23 PM

Thanks Xtreme. When I was adjusting the timing before, I checked spark on all wires and it was good.

**Bruce2** · 11-05-2017, 09:00 PM

Is the idle stabilizer still hooked up?? They cause problems..

**tmbiv56** · 11-05-2017, 09:09 PM

Yes, the idle stabilizer is still there and hooked up. I read some comments about that, but I never tried to run without it.

**Bruce2** · 11-05-2017, 10:51 PM

Originally Posted by tmbiv56

Yes, the idle stabilizer is still there and hooked up. I read some comments about that, but I never tried to run without it.

Most everyone takes those off and throws in trash can...they can screw with the idle...take it off and do a link and sync ...

**j_martin** · 11-05-2017, 11:22 PM

I have an XR4 on a fiberglass Tracker. I notice that if I trim it all the way in at warm idle, it'll load up. I think it's a combination of carbs not level and high water (back pressure) in the exhaust. Solution is to trim it out for closed throttle operation.

When I first got mine, it had problems. I shot the bugs out and set it up like I wanted. The Idle stabilizer/high speed module is chit canned. Carbs set up on the money, and Carson fiberglass reeds. I've messed with timing, including external modifiers using the bias circuit. 25° with modules removed and high test gas, and it runs great, probably about 175 HP.

If you get 92 psi compression, either you live way high on a mountain, the throttles are shut tight when testing, or your gauge is shot.
\

**tmbiv56** · 11-06-2017, 08:33 AM

haha... You are probably right about that pressure gauge. I bought this boat in 2012 and there is a receipt from a mechanic shop which stated the compression was around 140 on each cylinder. The motor seems to run just about the same as it did when I bought it. I'll do a check on that gauge.

**tlwjkw** · 11-06-2017, 09:25 AM

forget tha book number for idle timin'... put it in tha water, warm, in gear, trimed level and set timin' to whatever makes you and tha motor happy............

**Dave Strong** · 11-06-2017, 11:06 AM

Originally Posted by tlwjkw

forget tha book number for idle timin'... put it in tha water, warm, in gear, trimed level and set timin' to whatever makes you and tha motor happy............

The way to do it but make sure all the carb throttle plates are fully closed, if they are cracked open can create a rich idle. Make sure the timing box was disconnected when timing adjustments were made.

Dave