1994 Mercruiser 3.0 high idle off plane

[markhodges78]

I have a 1994 3.0 mercruiser when coming off plane the boat will idle at about 1000 rpms I rebuilt the carburetor thinking that would fix the problem and it didn’t. I have checked to make sure the the choke is fully opened and that the throttle stop screw is fully seated and it is what I have found is that if I give it a quick blip of the throttle it will settle right down and idle normal I have sprayed all around the engine no vacuum leaks I just can’t figure this out. Any help will be appreciated.

Thanks Mark

 

[kenny nunez]

Idle issue
Disconnect the the throttle linkage at the carburetor bellcrank. Check if you can wiggle the bellcrank, if it is loose then the “preening” of the throttle shaft to the bellcrank needs to be tightened, try using a center punch and a small hammer to expand the “preening”. Also it is possible to remove the throttle shaft and get the shaft brazed to the bellcrank if you know a good welder.

 

[markhodges78]

I’ll check that tomorrow

Thanks Mark

 

[Bondo]

I’ll check that tomorrow

Thanks Mark

Ayuh,...... While yer at it, make sure the throttle cable is properly adjusted,......

We don't use throttle return springs on boat motors,.....
We rely on the throttle cable, pushin' the linkage against the idle adjustment screw,....
If there's no tension against the idle adjustment screw, idle is all over the place,......

Generally speakin', throttle cables can be lengthened in 1/2 turn, or full turn increments,.....

 

[markhodges78]

Yes there is preload on the cable and when the boat is idling high if I go back to engine and push the throttle tightly closed the rpm doesn’t change the only way to get it to idle down is to blip the throttle open and back closed then it settles right down until I go across the water then back off throttle

 

[wahlejim]

I would go back to the carb and check your float and needles to make sure everything is seating properly. It could not be shutting fuel off completely and that little blip could be enough to jog it into place

 

[markhodges78]

Any idea where I can get a manual for adjusting idle and idle mixer on this engine

 

[kenny nunez]

The idle mixture screw/s should be 1.5 from lightly seated or 1/4 either way.
Something else to check is the centrifugal advance linkage in the distributor.
Remove the breaker point plate and lightly oil the pins that mount the counter weights. The pins should not be loose either. When you rotate the rotor shaft it should spring back freely.

 

[markhodges78]

The idle mixture screw/s should be 1.5 from lightly seated or 1/4 either way.
Something else to check is the centrifugal advance linkage in the distributor.
Remove the breaker point plate and lightly oil the pins that mount the counter weights. The pins should not be loose either. When you rotate the rotor shaft it should spring back freely.

I have the EST ignition so no points just put new carb on and it runs great but idle is so so i adjust it to 650 and then will go up to 1000 for a minute or so and back down to 650 so i just wanted some documentation on proper adjustment procedure

 

[Rick Stephens]

The online service manual can be found at

Boatinfo - Mercruiser Service Manual nr. 13 - 1990-1997 Engines GM 4 Cylinders

boatinfo.no

 

[markhodges78]

The online service manual can be found at

Boatinfo - Mercruiser Service Manual nr. 13 - 1990-1997 Engines GM 4 Cylinders

boatinfo.no

Rick I see that you repowered from a 3.0 to a 4.3 how much weight did the repower add to the boat (or how much lower does your boat set in the water now ) lol

 

[Baylinerchuck]

Ayuh,...... While yer at it, make sure the throttle cable is properly adjusted,......

We don't use throttle return springs on boat motors,.....
We rely on the throttle cable, pushin' the linkage against the idle adjustment screw,....
If there's no tension against the idle adjustment screw, idle is all over the place,......

Generally speakin', throttle cables can be lengthened in 1/2 turn, or full turn increments,.....

Ok, I know this is an old post, so don’t smack me.....curiosity has got the better of me.

Why wouldn’t we want a return spring on the carb to help the linkage seat against the idle screw. Just curious if this is just preference, or an absolute no no. The Quadrajet on my 4.3L has Such a spring. This is in addition to the Rochester return spring on Carb the linkage itself. Perhaps added by the POs mechanic?

Again, curious.

 

[Grub54891]

Someone must have put that spring on there. It may not bother the throttle, but it's never on there from factory.

 

[Scott06]

Ok, I know this is an old post, so don’t smack me.....curiosity has got the better of me.

Why wouldn’t we want a return spring on the carb to help the linkage seat against the idle screw. Just curious if this is just preference, or an absolute no no. The Quadrajet on my 4.3L has Such a spring. This is in addition to the Rochester return spring on Carb the linkage itself. Perhaps added by the POs mechanic?

Again, curious.

The Webber 4 bbl used a return spring as well.

24-897454 - Spring​

I bought one and installed when I upgraded to a Edelbrock 4 bbl on my 5.0, used that and the throttle bracket off the Webber equipped 5.7 and 4.3 s

 

[markhodges78]

Ok so I installed a new carburetor and adjusted according to the manual and for the life of me I cant tell that the idle mix screw makes much difference i can get the idle down to 650 in drive take it out of drive the idle goes up to 900 -950 for 30 secounds are so then it comes down to 650 idles ruff put back in gear and idles down to 550-600 and its ruff i take idle screw almost all way out and it will not change but if i turn it in past 1 1/2 it will change and run ruffer i did spray wd 40 every where and no change in idle

any ideas

 

[Rick Stephens]

Might be an intake leak.

 

[Scott06]

What gasket was used between the intake and the carb. I ran into a sI I,at issue on my 1990 3.0. It had a single hole in the intake but yet had a two hole gasket below the carb. When doing a routine carb rebuild , the kit came with two gaskets a two hole and one with an oval hole that more matched the intake, so I used the oval hole gasket.
my idle mix screw basically did nothing until I replaced it with a two hole gasket ... didn’t make any sense to me but after replacing gasket was able to get the mercarb tuned in perfectly.

 

[markhodges78]

Ok update on my high idle off plane .... The problem is fixed.. After seeing water in my oil I found that I had a bad exhaust elbo so I replaced the elbo and the manifold..... now when I come off plane it settles right down to 650 .... the mixture was set with a vacuum gauge and I was pulling about 15inches of vacuum now It pulls around 20inchs of vacuum and it will idle down to 480rpm in gear if I wanted it to and not cut off.... my guess is running across the water I would pull water in causing a lean condition when coming back to idle which is what made the engine reve up to 1000 rpm for about 30 seconds are so and idle like poo