Mercruiser 3.0 idle rpm increase when hot

[Niels45]

Hello,

my 1980 mercruiser 140hp full rebuild, has the folowing problem:
After half an hour boating, when engine is hot, the rpm is increasing from 650 to 1000 rpm.
The engine runs fine, carb is rebuild, ignition has al new parts, well adjusted, ect.

Could it be something like false air or something? Leak manifold?
When cold and the first half our, this issue isn’t there.

Who can tell me what can be the problem.

 

[alldodge]

My guess is there is next to no pressure from the throttle cable holding the carb closed. The carb does not have a return spring (so to speak) like a auto carb does.

Remove the throttle cable and adjust the barrel nut so it just slides on the carb and the bracket. Once there, remove again and back the barrel nut one full turn more and reinstall. The one more turn will keep some pressure on the carb linkage

 

[havoc_squad]

My guess is there is next to no pressure from the throttle cable holding the carb closed. The carb does not have a return spring (so to speak) like a auto carb does.

Remove the throttle cable and adjust the barrel nut so it just slides on the carb and the bracket. Once there, remove again and back the barrel nut one full turn more and reinstall. The one more turn will keep some pressure on the carb linkage

I would say this problem serves as a reminder to check one's work completely before a lake test.

This slack in throttle would have been found if tested on the muffs when engine fully warmed up to operating temperature.

When returning the throttle from high idle speed on the controls back to minimum, nearly every time this problem will show up.

It's a common pitful we've all fallen into, at least a few times.

In neutral idle RPM in or out of water should be around 700 to 750 RPM when engine is fully warmed up.

After fixing the throttle cable pre-load, make sure to do the OEM service manual procedure for idle air mixture adjustment.

The initial/default mixture adjustment in service manual that you tweak is to get it to idle at all.

Assuming you have ignition timing at service manual specification and good compression:

On the water, in gear, at 650 rpm is when you do final adjustment for that mixture per service manual.

If higher RPM, need to get the idle speed screw down to 650 in gear.

 

[Niels45]

Thanks for answering. I did what All dodge said. It’s a bit better now, but another problem is that the motor often dies after running forma while, when i come in the harber, motor goes to idle speed, 800 rpm, and when i shift to neutral it somethimes dies. When i test it in the dock, it works fine, but after an hour boating the problem is there. The kill shitch is checked, and works ok. It looks like that the engine drops a little to much rpm to shift. Could something be wrong with the outboard unit? Or lower shift cabel? The shiftcable goes smooth in and out when i loosen it and test it by hand.

 

[alldodge]

It can be many things, low compression, incorrect carb mixture, shift interrupter adjustment, engine timing, spark plugs, wires and/or cap. If the motor is not running good at idle it can die

Idle should be 650-700 rpm in Forward gear. This can only be adjusted in the water, either tied up at the dock or idling along.

Plugs MR43T, RV8C, BR6FS and gap .035
Timing 6 BTDC
Point .022
Dwell 28-34 degrees

If all the above is good, then check shift interrupter adjustment

 

[Niels45]

It can be many things, low compression, incorrect carb mixture, shift interrupter adjustment, engine timing, spark plugs, wires and/or cap. If the motor is not running good at idle it can die

Idle should be 650-700 rpm in Forward gear. This can only be adjusted in the water, either tied up at the dock or idling along.

Plugs MR43T, RV8C, BR6FS and gap .035
Timing 6 BTDC
Point .022
Dwell 28-34 degrees

If all the above is good, then check shift interrupter adjustment

I have MR43LTS plugs. They are a bit longer. My boat man used these spark plugs. Is that a problem? Everything is set like you said, only the dwell I can’t check. I don’t have a tool to check this.

Yesterday motor starts arts fine and runs great, but stil drops to much rpm when shifting from gear to neutral when engine is hot.

 

[Bondo]

I have MR43LTS plugs. They are a bit longer. My boat man used these spark plugs. Is that a problem?

Ayuh,..... Those are for Vortec heads,..... Yer older motor should have MR43T plugs, the shorter plugs,.....

 

[Niels45]

Just come back from an hour boating. After an hour, motor still bogging at the and. Runs bad and cann’t Keep it running in the harbour. Could this be the wrong plugs? Or engine temp to hot? Water temp is fine.

 

[alldodge]

Just come back from an hour boating. After an hour, motor still bogging at the and. Runs bad and cann’t Keep it running in the harbour. Could this be the wrong plugs? Or engine temp to hot? Water temp is fine.

Don't see it being temp of the motor. The wrong plugs can cause issues and also as mentioned before

It can be many things, low compression, incorrect carb mixture, shift interrupter adjustment, engine timing, spark plugs, wires and/or cap. If the motor is not running good at idle it can die

Check adjustment on shift interrupter


Do a compression test
Check timing 6 BTDC

 

[Niels45]

It can be many things, low compression, incorrect carb mixture, shift interrupter adjustment, engine timing, spark plugs, wires and/or cap. If the motor is not running good at idle it can die

Idle should be 650-700 rpm in Forward gear. This can only be adjusted in the water, either tied up at the dock or idling along.

Plugs MR43T, RV8C, BR6FS and gap .035
Timing 6 BTDC
Point .022
Dwell 28-34 degrees

If all the above is good, then check shift interrupter adjustment

I have som Ngk bpr6fs on stock. Are these also correct for a 1980 3.0 140hp?

 

[alldodge]

I have som Ngk bpr6fs on stock. Are these also correct for a 1980 3.0 140hp?

No, that's a cross to MR43LTS

 

[Bondo]

Man, I hate it when ya got 2 threads goin' on damn near the same subject,.....

I'd merge 'em, but the results will be a unreadable mess,......

As I've said, repeatably, in both of yer threads now,....

You've got the WRONG Spark plugs,....

 

[Niels45]

Problem fixed!
changed spark plugs, new points, wires, rotor ect. Set timing, dwell and rebuilt the carb. Now it runs fine! Thanks for al your support!