Mercruiser carb rebuild but hard start and idle

[MilleniumFalcon]

I have a 1989 mercruiser 5.0 serial number B903262. After a season of losing power, hard starts, rough idle, engine stalls, and black smoke I decided to pull the carb and rebuild it. This is the first time I’ve done this and the kit seemed pretty simple. Now with the carb back on, I can’t get it to start without having the choke plate propped open and it will die if the plate closes. I’ve replaced spark plugs and wires, fuel lines, fuel pump. The carb was definitely a mess when I pulled it apart. The boat ran fine most of the season but I think I had a wire go bad to a spark plug and that’s when I decided to look at the carb. Adjusting the idle mixture screw does nothing but adjusting the fast idle so the throttle is open more seems to help it start and run.

 

[Bt Doctur]

If the fuel delivery is more than adequate a motor may not need full choke to start. A cold motor may need choke

 

[MilleniumFalcon]

So I understand, you’re saying a cold start might not need full choke? If that’s the case, does that mean I shouldn’t set the choke to close fully?

 

[Bt Doctur]

If you can have the choke open to start a cold engine the carb is delivering more fuel then needed for a cold engine.
You set the plate to close on a 1/8th inch drill bit on a COLD engine.If you have to hold the choke open to have the engine run, you have a carb problem

 

[MilleniumFalcon]

If the carb is delivering too much fuel, what can I do to resolve this?

 

[alldodge]

If the carb is delivering too much fuel, what can I do to resolve this?

Check you float level, its probably to high

 

[MilleniumFalcon]

The float level is set according to instructions but I have read that the original floats could go bad. I put this float in gas to make sure it floated and it did fine. Should I try a new float and reset it?

 

[Bt Doctur]

Who`s instructions

 

[Bt Doctur]

from manual 15

 

[MilleniumFalcon]

The set that came with the Sierra brand kit specify 9/16” level and 1-3/32” drop measuring from the toe.

 

[alldodge]

The set that came with the Sierra brand kit specify 9/16” level and 1-3/32” drop measuring from the toe.

Sierra that's kind of funny but your measurements are off when compared to manual 15
A whole lot of difference between 9/16 and 3/8

What kit number did you use and what type of needle seat?

 

[MilleniumFalcon]

Sierra 18-7098-1

 

[alldodge]

Well Sierra says that's the kit, but their dimensions are off.

and what type of needle seat?

spring loaded or solid?

 

[MilleniumFalcon]

I have the spring loaded or solid. The original was solid but I placed the spring loaded in per the recommendations. In the instructions posted here it recommends the solid 2 piece type. I can change that and readjust the float.

 

[alldodge]

Using the spring loaded, then the 9/16 may be correct, and without gasket.
The issue still seems that the float is either not sealing the fuel off, or an incorrect measurement

 

[MilleniumFalcon]

So the carb works by a vacuum sucking fuel down from the supply in the fuel bowl? If the needle isn’t cutting off the supply then the vacuum would continue to draw fuel also supplied by the pump and cause a flooding effect? The carb is needing to overcompensate by adding more air to counteract the amount of fuel? Does this sound like the cause for me having to hold the choke plate open?

 

[alldodge]

No
The motor is drawing fuel by vacuum across the venture's. The vent's are connected to the carb float bowl chamber with passages. If the fuel in the float bowl is higher then spec, there will be more fuel sucked in then needed.

If bowl is less then spec, there is to little fuel. I'm seeing to much fuel so the bowl level is to high

 

[dlogvine]

Sorry to bother you, but did you say you used the old float from 1989 carb, when you rebuild the rest? If you are using the old float this could be the problem.

 

[dlogvine]

The float might look ok but if it is old it might lose its buoyancy and it gets more fuel to stop its flow into a fuel bowl.

 

[MilleniumFalcon]

I've ordered the new float and will be installing in a few days when it comes in. I'll update after this. I really appreciate all the advice and I'm hoping for a good laugh if after all this, skipping one simple and cheap step has stopped my boat. I'm confident after replacing and adjusting the float properly, this thing is gonna be back at it! Thanks!