4.3 thunderbolt IV idle adjustment

[1216bandit]

Engine serial 0F075410. Every once in a while the engine will diesel a bit when shut off and from the reading I have been doing it seems like it could be something as simple as too high an idle speed. I'm just a little confused about the proper way to check and make any adjustment. Some of what I've read says neutral on the trailer is fine, others say check or adjust in the water at idle in gear, some also mention disconnecting the throttle at the carb while at idle in gear. Can I do this while the boat is on the trailer?? I will be using a friends timing light with tach to check rpm's.
Thanx! Dan

 

[DFW Miles]

Re: 4.3 thunderbolt IV idle adjustment

Just went through this myself with 4.3 Mercruiser. IMHO... You can make adjustments to the idle while the boat is on the trailer and get your idle close to where it needs to be. The final adjustment needs to be with the boat in the water though. The reason is that the water provides backpressure to the exhaust that it does not have while on the trailer. The exhaust gasses need to be pushed down through the water before they exit through the outdrive. If you set your idle speed about 100 RPM high while on the trailer it should drop to approximately the correct RPM when in the water. Make sure that when you make the adjustments that the engine is completely warm, adjust the idle adjustment screw first (two screws if you have a 4 barrel carb) then the idle speed screw (located on the cam on the side of the carb). Also be sure to check your timing while you have the timing light. improper timing could cause your engine to diesel also. As far as disconnecting your the throttle cable while making final adjustments in the water, I do not think this is necessary. The reason for this is to be 100% sure that the cable is not opening the butterflies at all while at idle. You should be able to tell if it is bottomed out by looking at the linkage while in neutral. Hope this helps some.

 

[NHGuy]

Re: 4.3 thunderbolt IV idle adjustment

If you have a timing light set the timing too. Dieseling can be from a few things. Set the idle mixture, which is done in gear warmed up, and set the idle in gear too. You can just set the idle a few hundred rpms high in neutral, it drops when the prop is engaged.
Until you get rid of the dieseling you can run the engine above idle for a short cooling "shot" before you bring down the throttle to idle for shutdown. When mine was dieseling I would run it for maybe 15 or 45 seconds at 1200 rpms, that lowers my temp gauge to 160 or less, then I could get a no diesel shut down. Or if I had just been running along and the motor wasn't too warm I could just shut it off with no issue.
Mine was dieseling if I went to shut it off with temp gauge readings over 180.

 

[180shabah]

Re: 4.3 thunderbolt IV idle adjustment

top causes for engine run on - carbon buildup in combustion chamber and not allowing proper cool down time.

Timing, mixture, and idle speed adjustment (if they are not out of whack to start with) are just band-aiding the problem.

 

[Don S]

Re: 4.3 thunderbolt IV idle adjustment

top causes for engine run on - carbon buildup in combustion chamber and not allowing proper cool down time.

While that is a reason, it's not the most common. Hi idle speed, and shutting off the engine immediately after coming off plane are the most common.
Adjusting Idleto be 750 max in neutral, and allowing the engine to idle for a couple of minutes are two very easy fixes.

 

[180shabah]

Re: 4.3 thunderbolt IV idle adjustment

top causes for engine run on - carbon buildup in combustion chamber and not allowing proper cool down time.

Timing, mixture, and idle speed adjustment (if they are not out of whack to start with) are just band-aiding the problem.

While that is a reason, it's not the most common. Hi idle speed, and shutting off the engine immediately after coming off plane are the most common.
Adjusting Idleto be 750 max in neutral, and allowing the engine to idle for a couple of minutes are two very easy fixes.

I think we are agreeing here???

 

[Don S]

Re: 4.3 thunderbolt IV idle adjustment

I meant to copy only the carbon build up part of it.

 

[1216bandit]

Re: 4.3 thunderbolt IV idle adjustment

Thanx for the tips...I brought the idle down a bit today and that seems to have taken care of it!