2000 Merc 135 carb help requested

[bpedrant]

Hello all, I have a 2000 Mercury v135, carb motor.

We have a very specific issue when going from idle to 1/2 or full throttle, it bogs down and stalls. It ONLY happens when giving it gas from an idle.
The boat cannot get up on plane when this happens, so the RPMs are still on the lower side.

I have been looking at the idle air jets in the card and was wondering if any of you folks have tuning experience on these old Mercs.

More info on this struggle. (My brother and I are pretty good on working/repairing these motors, but this one, we think, is a carb issue. That being said, we could be 100% wrong!
- The motor starts easy and idles fine. In gear or out.
- Giving it 1/2 to Full throttle in gear, under load, it will stall
- WOT (full throttle) works great (after its gets moving! LOL)
- After heating up, it *mostly* works fine, but still has some of the same issues
- It has been doing this for years, but seems to be getting worse or my tolerance is getting less. We use 93 octane gas, premium oil.
- 2 years ago, we completely rebuilt the carbs. They were spotless inside before and after rebuilding. This made no change.
- Plugs have been changed several times over the years. No change.
- We have triple-checked the timing (under load, on the water)
- All the linkages are correct, sync'd, etc.
- The ONLY way to get this thing moving is to have it in gear, hit the choke until it just starts to die out, then give it full throttle. It then takes off. As soon as it hits about 2500-3000 RPM, it launches as expected. [THIS is why we think carb issue.]

Thanks for all the advice I have received here!
-Brian

 

[JDusza]

Seems like a carb issue. But be sure fuel supply is good; primer and line, fuel pump, etc...
Running too lean: stall at throttle up, choke to accelerate.
Any idea what size your idle jet is? From a parts list there appears to be (4) sizes to choose from; 046, 056. 058, 060.
J

 

[Chris1956]

I presume your idle mixture is adjustable? Merc carbs usually do not have accelerator pumps, and as such the idle mixture need to be a bit richer.

So, richen it 1/8 turn on a single carb throat (top carb first) and see if she accelerates better. If so, repeat with the other throat, and retest. Then move to the mid carb, and finally the bottom carb. Test after each adjustment.

 

[bpedrant]

Thanks for the replies.

I went digging and documented all the idle air jets. They are what the book calls for (I don't have the list in front of me, but literally 4 or 5 different sized jets depending upon location). They are all clean.

I did what Chris suggested and adjusted the idle mixture on each throat. What seemed to work the best is about 1/4 turn on all of them.
When the engine is hot, I could not get it to bog down!
(Going more than 1/4 turn the idle started to suffer.)

The only issue we still have is when cold (and this might be defined as what the "cold" air valve in the head thinks is cold), it still has the stalling issue. I'll test that valve next, it might be defective. (According to the book, when the engine is under 100 degrees F, it blocks the idle air jets, thus making it richer.)

-Brian