1989 5.7L 4brl won't idle below 900 - 1000 rpms

[mr300z87]

Yesterday I helped another friend prep his boat, my old Invader. After gear oil change, flat tire repair and a failed attempt to replace temp over heat sensor her 5.7L Rochester 4brl bravo 1 fired right up. This is a reman 5.7 with all the original bravo bits and pieces from the original engine including the carb and TB 4 ignition. When I owned the boat it always ran great even after over heat and water in the oil.

The issue we have is it won't run below between 900 and 1000 RPMs. We checked timing and have it at 8 before TDC as specified in the manual. Launch is not scheduled but I would to trouble after launch. The carb is original to the boat 1989 and to my knowledge has never been apart. Tuning carbs has never been one of my skills, I am a if it ain't broke don't fix it when when it comes to them. The rebuild on my last boat ended in a tow in and a new carb from Edlebrock.

Any advice or things to check, before or after launch? I was thinking vacuum leak somewhere. We had the carb and intake off a couple times for another issues. Other than the high idle that the Bravo drive doesn't like, it runs great. Last year same idle condition she jumped on plane no bogging or other drivability issue. Any advice is appreciated.

Mike

 

[Scott06]

First disconnect the throttle linkage and make sure throttle is coming back to seat on idle speed screw. adjust idle speed screw down See if idle lowers, may have to adjust throttle cable to get it all the way down.
usually if there is a vacuum leak idle quality will be terrible.
also check that choke is coming off and not hanging up linkage some where due to gum or corrosion.

if it ran well with high idle would think carb is workable as is just need to set idle speed and maybe mixture

 

[jimmbo]

Someone might have fiddled with the Fast Idle Screw, or the Fast Idle Cam isn't dropping down.

 

[dubs283]

Someone might have fiddled with the Fast Idle Screw, or the Fast Idle Cam isn't dropping down.

Marine carburetors don't have a fast idle cam/setting. That is for automotive only

OPs issue at first sounds like a vacuum leak however their mention of being apprehensive about properly adjusting the carburetor leads me to agree w/scott and perform some preliminary adjustments before investigating a vacuum leak or other issue

 

[mr300z87]

I may have forgot to mention if I adjust the the idle screw to lower it to normal 700ish rpms the engine stalls and will not fire or idle again without added throttle. Once I re adjust the idle screw back up the 900 + RPMs she will run fine all. The idle screw has to be turned to the point of slightly cracking the throttle plates, making the mixture screws do nothing. I screw them all in and no change. To be honest forgot to look at the choke . It is an old school divorced stove type pull off.

After launch and at the slip I will go through all adjustments list per the manual. I know it will run the way it is and I would hate to get it to ramp and have an start issue.

Now if the weather would just cooperate we can get this thing wet.

 

[ScottinAZ]

Sounds to me as the idle circuit is getting gummed up and not passing fuel. Having to crack the throttle plates gets it slightly into the main jets and is a bandaid for a dirty carb in need of a Berrymans bath and a rebuild

 

[Scott Danforth]

I may have forgot to mention if I adjust the the idle screw to lower it to normal 700ish rpms the engine stalls and will not fire or idle again without added throttle.

your idle circuit is plugged

The carb is original to the boat 1989 and to my knowledge has never been apart.

time to pull the carb apart. soak in a tub of berrymans, run a piece of mono thru each passage to ensure they are clean, re-assemble and set idle mix and speed

 

[jimmbo]

Marine carburetors don't have a fast idle cam/setting. That is for automotive only

If it has an Automatic Choke, it will have it.