1986 MCM 140 wont return to idle speed

[Amad2727]

New to the forum. We rebuilt the carb. New plugs new wires new cap rotor coil. I get the idle down to 700 out of water set the timing to 6 degrees btdc. In doing so the engine idles up to 1000. I throttle it up out of gear return back to neutral and engine stays idled up at 1800. I push in the leakage to return back to idle. What did I do wrong?

 

[alldodge]

Boat carbs don't have idle return springs,

Remove the linkage off the carb, then adjust the idle speed screw until correct rpm is reached. Then adjust barrel nut on throttle cable so it just slips back on carb and barrel nut slot. Remove the cable again and screw the barrel nut another turn so that there is a slight bit of pressure holding the carb at idle.

 

[Amad2727]

Thank you. What's the barrel nut? Also another problem just happend. Rpms and volts drop while using the trim at full throttle

 

[alldodge]

Unable to get pic right now, but the throttle cable attaches to the carb and some kind of bracket that holds the outer jacket of the cable. The thing that holds the outer jacket of the cable (barrel nut) either has a slot which it slides into and a cotter pin secures it, or a nut is removed off and the barrel nut slides off a stud.

The rpm and volt problem in most cases is a bad connection. The trim uses more current and if grounds or positive cable has corrosion then volts drop. When volts drop the rpm drops because there is a weaker spark. Clean bat cables and most important is the ground cable attached to the engine block. Get everything shinny clean

 

[Amad2727]

So I'm trying to adjust the barrel but all I feel is the cable twisting. Is this right?

 

[Amad2727]

Ground wires are tight and clean.

 

[alldodge]

So I'm trying to adjust the barrel but all I feel is the cable twisting. Is this right?

The barrel should turn, its probably corroded. Here is what I found in the manual. The second pic is the adjustment for WOT

 

[tank1949]

Thank you. What's the barrel nut? Also another problem just happend. Rpms and volts drop while using the trim at full throttle

If you linkage is old or worn (1986), you may need a small spring to apply a little pressure back towards idle. Normally, they are not needed. But, we all wear out.

 

[alldodge]

If you linkage is old or worn (1986), you may need a small spring to apply a little pressure back towards idle. Normally, they are not needed. But, we all wear out.

Agree WE wear out, but do not agree there is no way is a spring ever needed to return carb to idle on a boat. There should be no slop

 

[Amad2727]

So I was afraid to twist the plastic end. So I adjusted the brass barrel and tightened the anchor screw in place the idle screw seems to be returning to its natural position and idle is smooth. Now the only problem left is when I hit the trim rpms drop. Cleaned all the grounds from the battery to block with steel wool. Trim reservoir is low on oil. Could this cause the pump to over work and draw more voltage the usual?

 

[alldodge]

Trim fluid level has no interaction with voltage

Pull the 10 pin engine connector and check pins for corrosion

 

[tank1949]

Agree WE wear out, but do not agree there is no way is a spring ever needed to return carb to idle on a boat. There should be no slop

I totally agree, but I have fix one of mine by adding spring to apply pressure to reduce "slop." Most carbs have springs, but they weaken too.

 

[Amad2727]

Bought an ohms meters today where should I start?

 

[alldodge]

Sorry, kind of tired and old, but went back thru the thread a couple times, what problem are we working on?

 

[Amad2727]

Rpms dropping while using the trim.

 

[alldodge]

Ohm out the 10 pin connector. The pic below uses DDIS ignition but otherwise should be the same. The helm wiring is a bit different also but the wire colors are the same

 

[Amad2727]

Okay folks here's an update. After cleaning every ground and positive connection and find a couple broken ground terminals and positive terminals. Still had the trim and rpm issue. Put new points in reset the tab and trim issue vanished. Thank you everyone for your input it is much appreciated! Now back to fishing and tubing!