MCM 3.0 181 dripping/flooding after shutoff, brass float level settings

[MidwestFloater]

Hi all, I'm having issues with my MCM 3.0 181. I recently rebuilt the carb on this unit using a kit from Mike's carb parts. While the system is generally performing much better than prior to rebuild, I'm not having significant flooding issues when I shut the engine off. After running and getting the engine warm, it continues to drip gas into the carb after shutting the engine off. It's a pretty significant flow and resulted in large amounts of gas collecting on top of the throttle body. From the sounds of it, this can be caused by a few things. Here's what I have on my troubleshooting list.
- Needle / seat issue
- Float level / drop settings
- Fuel system pressure

As I rebuild the carb recently, I'm guessing it's something associated with that. I took the carb off/apart today and didn't see any debris or obstruction in the needle/seat area.

I then went on to check the float level/drop settings and ran into an issue. From my manual, I'm getting a float level of 9/16" and float drop setting of 1-3/32". However, I believe the instructions with my float indicated measure these settings to the toe of the float. On the Mike's carb parts site, it indicates measuring to the bottom of the float for float drop.

Float drop settings: https://www.marineengine.com/parts/technical_information/sierra_18-7097.pdf
Mike's Carb settings: https://www.carburetor-parts.com/Float-Adjustment_ep_615.html

However, 1-3/32" to the bottom of the float would seem to have the float nearly at the same position as the float level settings (see pic). You can't really tell from the pic, but the ruler is on the gasket and the float is hanging. I had initially measured the distance to the toe (upper right portion in pic).

So a few questions
- Where am I supposed to measure to on the brass float? The toe or the bottom?
- If it's the bottom, does it seem like I have something wrong in my setup (other than the setting obviously)?
- What other things might be causing the flow after shutoff?

Let me know if other info would be helpful. Thanks for the help!

 

[MidwestFloater]

For reference, here's a thread on a similar problem another user had. https://forums.iboats.com/forum/eng...ives/55312-mercarb-problem-starting-when-warm

Another question: Would float drop or float drop setting more likely be the culprit if it is in fact due to the float?

Thanks!

 

[MidwestFloater]

Bump!

 

[havoc_squad]

My initial assessment:

Float drop very likely is not your issue, the drop regulates how quickly and and how far should the needle should allow more gas in.

In short, it sets/regulates the max flow rate demanded by the carb so it doesn't overflow/flood in all situations where the engine/carb is requiring more fuel.

The needle, seat, and float height/level are the two biggest factors that are caused in the carb causing it to flood. Other less likely but possible scenarios are leaving out/incorrect installation of certain gaskets or check balls/springs.

If the entire carb rebuild attempt was to solve a flooding issue, you need to check the fuel pressure to the carb after you re-assemble it correctly.

Questions that need to be answered:

Has the brass float been checked for leaks? I recommend replacing that with the OEM polymer ones as the brass ones will develop holes/leaks over time. If it's new, well keep it until it fails, then get the polymer one.

Which Mercarb carb kit did you get, did you get the one with the two piece solid needle, or the one with the spring loaded needle?

There are two different float height settings, they depend on which type of needle and seat you installed.

If there is a spring that is supposed to go with the needle, then you must use the OEM service manual specifications for the spring needle float height setting. Otherwise it is solid needle and you need to use the float height specified for that instead.


My guesstimate diagnosis:

Sounds like you used the wrong float height settings.

9/16 height sounds like the spring needle setting and I'm guessing you have a solid needle (not spring needle).

You probably accidentally applied the float height of the spring needle, which is actually a larger gap/height and would flood the engine.

Post your results of your findings when you can.

 

[MidwestFloater]

Thanks much Havoc.

The original reason for the carb rebuild was due to poor idling, dead zone during acceleration, and engine rpm cycling at lower RPMs. For the most part these issues have been resolved with the rebuild.

For the float setting, the kit from Mike's carb comes with both needles. I also purchased a new float (brass) because it was stated the polymer ones adsorbed gas over time. At this stage I don't think it has any holes in it. In the install I used the spring needle, which is why the 9/16" setting. That said, in the Mike's Carb video he says to "set it to be level" rather than doing a specific measurement.

I have the solid needle and could go do the 3/8" setting with that but it seems like the spring needle is the preferred one.

From a functionality standpoint, wouldn't a lower float level setting actually increase the gas to the system and flooding? From my understanding, a lower setting means the float has to be higher in the bowl for the needle to close.

Thanks!

 

[Rick Stephens]

From a functionality standpoint, wouldn't a lower float level setting actually increase the gas to the system and flooding? From my understanding, a lower setting means the float has to be higher in the bowl for the needle to close.

Correctomundo.

So... start by looking at fuel pressures. Then move back to carb rebuild job. Most common job needing doing again, and sometimes, again and again. A crack, a speck of corrosion in the throttle pump check ball. Anything that lets unmetered fuel by.

I don't think it matters a bit which needle you use.

Rick

 

[Bt Doctur]

You can also test a needle and seat by turning it upside down and blow into the inlet fitting .

 

[havoc_squad]

Correctomundo.

So... start by looking at fuel pressures. Then move back to carb rebuild job. Most common job needing doing again, and sometimes, again and again. A crack, a speck of corrosion in the throttle pump check ball. Anything that lets unmetered fuel by.

I don't think it matters a bit which needle you use.

Rick

Some days I think I hang upside down like a bat mentally looking at things from the upside down view.

Having the items in hand helps quickly fix that issue most of the time.

 

[MidwestFloater]

Haha, thanks guys.

For fuel pressure checking, does anyone know offhand the pressure spec or best way to measure it?

And things have improved some but I'm not sure it's fixed. I cleaned the needle area and put the old float back into it. This one had a greater float level setting than spec and I wasn't having the flooding issue prior to the rebuild so I figure maybe the prior owner set it there intentionally. It's been for the most part better about stopping gas flow after turning it off but we've had one brief incident of it not wanting to shut down for a few seconds after turning the key off.

Thanks for the help all!

 

[havoc_squad]

The fuel pressure test fitting part is OEM# 91-806901.

You'll want to get a combination low fuel pressure and vacuum gauge if you don't already have one.

If you have the solid line only, you will probably want to pick up some flexible USCG A1 rated pre-made connector to either temporarily replace or re-route the line to include the pressure fitting and connect to the carb.

I almost thought I was going to break my metal fuel line putting tge tester direct between the fuel connector and the carb.

A flexible line would have made it a cakewalk to diagnose.

If anyone has a better and cheaper approach involving connections, I'm all ears.