850 merc Idle issue

[hereto]

Hi all,
Hope someone out there can shed some light on my idle issue, as I have spent the last year trying to find the problem with no success. I won?t go into what all I have done to try and find what is causing the problem but it has been considerable.
Here is the problem. After engine is warmed up, Idle is at 700 to 800 RPMs. Once I accelerate and bring it back to idle speed my RPMs will be at 1100 to 1200 RPMs in neutral, then I will wait for about 30 seconds while the engine slowly drops back to 800 RPMs where it belongs. If while the engine is at 1200 RPMs and I shut it off with the key switch and immediately restart it, it will be idling at 800 RPMs. I just don?t get it, and just can?t find what is going on. Other than the idle problem the thing runs great!!
The engine in question is a 1976 Merc 850 85hp. Compression 131-131-131-136. Thanks in advance for any help you can give in finding the problem.

Hereto

 

[FLATHEAD MAC]

Re: 850 merc Idle issue

:facepalmOES the sound of engine change with rpm?? sound at 800 , @ 1200 , then back to 800 ?? ans what does it sound like when at 1200 and cut back to idle, does the sound change with throttle position or does it take those 30 sec to come back down. if sound changes with throttle with no hessitation , it would seem there are issues with the tach.

 

[Faztbullet]

Re: 850 merc Idle issue

This is normal for a engine of that age as the crankcase is "drying" from high speed run. I was told in service school that this is caused by the fuel/oil held in the labyrinth seals in reed blocks burning off and possible bleed restrictors missing. Turning it off the engine looses the charge as it is held in place by centrifugal force and into # 2 and #4 cylinder. When you restart it it should be be a tad lower on the rpm for a couple of seconds.

 

[hereto]

Re: 850 merc Idle issue

Thanks for the reply Faztbullet, so maybe old age is catching up to both my motor and myself. Just to clarifly, after running a short or long distance, then throttle down and shift to neutral, it takes about 30 secs for motor to idle down from 1200 RPMs to 800 RPMs, or I can turn motor off with key switch while it is at 1200 RPMs and immediately restart it and it will be at 800 RPMs ( not for a short time ) but will STAY at 800 RPMs.
When you refer to missing bleed restrictors, I assume you are talking about the small bleeder
line with one way valves at both the top and bottom of block. I recently pulled both valves, cleaned and tested them and replaced the hose. Could the valves have been in wrong?
I replaced them as follows.
Bottom valve is one way OUT of block
Top valve is one way INTO block.

Thanks again
hereto

 

[Faztbullet]

Re: 850 merc Idle issue

The bleed restrictors are on the intake side of engine and they restrict puddled fuel in low spot of intake port from flooding cylinder.

 

[hereto]

Re: 850 merc Idle issue

Well it apears those restrictors could very well be my problem. I found a thread on installing them and will be giving it a shot. I have not been able to come up with a part number, was hoping someone might have this info. Thanks in advance.

hereto

 

[Faztbullet]

Re: 850 merc Idle issue

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