2007 40hp Merc carb and compression questions

[renelson]

I just bought a 2007 mercury 40hp 4 stroke. Rebuilt the 3 carbs and it starts and runs good but it’s a little smoky. I left the mixture needle screws where they were at. Can someone tell me what the proper setting is for this screw on each carb? How many turns from tight? I used to my old single carb motor, not 3 lol. Also, the compression on each cylinder is 100, 110, and 115. Is this acceptable?

Thanks!

 

[airshot]

In my opinion the 100 seems a bit low, have you looked into why it is low ? Scored cylinder, stuck rings, easily fixable ???

 

[renelson]

Thanks. I hadn’t looked into it. I can rebuild a carb but going into the cylinder head is above my pay grade. I thought that within 15% from the high to the low was acceptable.

 

[racerone]

Most 4 strokes have 150+ PSI as far as I know.

 

[renelson]

Well that’s not good…

 

[renelson]

Would someone mind checking a service manual for that carbureted motor? I would sure appreciate it.

 

[airshot]

Just looked up compression numbers for a 2007 four stroke outboard....Over 150 psi on each cylinder according to a video. I would double check your compression gage or try another one.

 

[renelson]

I hooked my comp tester gauge to my air compressor at 150psi and it reads about 135psi so it may be off just a bit, but not a lot. Using that as a correction factor, I’m coming up with 115 125 and 130 ish.

 

[airshot]

I hooked my comp tester gauge to my air compressor at 150psi and it reads about 135psi so it may be off just a bit, but not a lot. Using that as a correction factor, I’m coming up with 115 125 and 130 ish.

Still a long way under the 150 psi. minimum. Good compression is more critical for a four stroke than a two stroke..

 

[boscoe99]

What is the engine serial number?

How was the test performed? Motor should have been hot. Throttle wide open. Known good gauge. Battery full charged. Motor cranked until the pressure stopped rising.

 

[renelson]

Serial number is 1C050911. I heated it up first, removed all plugs, WOT, then pulled 8 or 9 times until it quit going up (pull start). I just bought the gauges at Oreillys but they are not “top shelf”.

 

[boscoe99]

Serial number is 1C050911. I heated it up first, removed all plugs, WOT, then pulled 8 or 9 times until it quit going up (pull start). I just bought the gauges at Oreillys but they are not “top shelf”.

Looks to be a Yamaha power plant.

Yamaha data for a 2007 model year F40 indicates the minimum compression pressure as being 87 psi.

Yamaha data shows the idle mixture screw setting to be 2 and 1/2 turns out from being just slightly seated.

 

[renelson]

Looks to be a Yamaha power plant.

Yamaha data for a 2007 model year F40 indicates the minimum compression pressure as being 87 psi.

Yamaha data shows the idle mixture screw setting to be 2 and 1/2 turns out from being just slightly seated.

Wow, so this 4-stroke mercury was manufactured by Yamaha? I had no idea they did that.

Are you 100% sure on that compression? Because the compression went from way too low to way too high. Or maybe more likely my new gauges are incorrect. I noticed the other compression setting on your spec sheetwas 120, which would be similar to my numbers. What was that number for?

Edit: thank you for the information

 

[boscoe99]

Yamaha made a number of motors for the Mariner brand and the Mercury brand for many years. All the way up to the Mercury 225 HP four stroke model. Known by many as a Mercaha or Yamamerc.

I can only report what Yamaha information says. Following is from the Yamaha service manual.

 

[boscoe99]

Your model motor has an automatic decompression device so that it can be more easily pull started. This throws off the compression test numbers.

 

[renelson]

Thanks a bunch. I wonder if my motor is the equivalent to the 40BTR (120psi), rather than the 40BMH? Did your research of my serial number line it up with the BMH?

 

[boscoe99]

The BTR has an electric starter motor. The BNH has pull start.

You mentioned pulling the rope when doing your compression test so I assumed (yes, I know) that you don't have electric start.

Are you wanting your model to have low compression?

 

[renelson]

Your assumption is correct It just seems strange that I am reading much closer to the 120 mark than the 87 mark, and I’m just trying to figure out why that would be. Doesn’t that seem strange that I would be much higher than the rated compression?

 

[racerone]

It clearly states " minimum " compression is 87 PSI

 

[renelson]

It clearly states " minimum " compression is 87 PSI

Good point. Thanks to everyone for their help.