How do carbureted engines run at idle?

[The motor guy]

The carbureted gm350 in my boat always seems to idle higher after it's good and hot and nicely warmed up. The difference is about 100rpm and it's kind of annoying honesty. Also if you let it idle for 30 seconds or so it will generally slowly lower back down from say 800rpm to 700 where it should be. Then when you put it in gear it drops to 600rpm. It has an original 715cfm Holley 4 barrel made for Volvo penta. Someone told me it's probably my shift cable that might have some slop in it and that's what caused it. Then the engine vacuum pulls the idle back down on its own. It's frustrating because on a boat you need the engine to idle right so you can dock the boat. Is this just the nature of a carbed engine?

 

[Fawteen]

If it's set up correctly it will idle consistently.

 

[mk378]

If it's a single handle control you could try tightening up the throttle cable to be sure it stays against the idle stop during low-speed forward / reverse maneuvers. With dual handle control of course the throttle is independent of the shifter. Also you could try turning down the idle some, a V8 should be able to stay smooth below 600 rpm.

 

[demarpaint]

This should help. Holley Carb Adjustment A lot of people don't do the adjustments on a fully warmed up engine, in gear. Make sure the engine is timed properly too.

 

[mk378]

If there is a vacuum advance system make sure it is working. On cars the vacuum advance really helps consistent idle.

 

[Lolvoguy]

Originally Posted by demarpaint
A lot of people don't do the adjustments on a fully warmed up engine, in gear. Make sure the engine is timed properly too.

Originally Posted by mk378
If there is a vacuum advance system make sure it is working. On cars the vacuum advance really helps consistent idle.

 

[Kestas]

Some carburators have an antistall dashpot to prevent the throttle from slamming closed. Maybe it doesn't let it down quick enough.

 

[JeffKeryk]

Start with timing; it has to be right or carb adjustments cannot be made.
Get the idle mixture right; I like to use the "highest vacuum" method.

That will get you to a good starting place.

 

[slo town]

Unlike today's modern throttle by wire engines, your mechanical linkage operated carburetor doesn't have an ECU controlled idle speed. Idle speed will ALWAYS drop when you put it in gear.

Scott

 

[The motor guy]

Originally Posted by JeffKeryk
Start with timing; it has to be right or carb adjustments cannot be made.
Get the idle mixture right; I like to use the "highest vacuum" method.

That will get you to a good starting place.


For whatever reason there aren't vacuum ports.on this carb. Just a blind hole where it's supposed to be.

 

[mk378]

There may be a place to hook a vacuum gauge into the manifold. You can also go by highest rpm but you will need a service tach that can measure low rpms accurately, not the one on the dash.

@SLO_Town makes an excellent point that the rpm on this setup is always going to drop when putting in gear. There is nothing to compensate for the additional load on the engine. This is one of the things you get when paying big money for a new boat.

 

[atikovi]

Originally Posted by motor_oil_madman
The carbureted gm350 in my boat always seems to idle higher after it's good and hot and nicely warmed up.


Hotter oil=less internal friction so idle speed increases.

 

[NYEngineer]

On a Holley you have to make sure the power valves aren't blown out. Idle mixture set to highest vacuum and the throttle cable and stop are dialed. Timing too. Everyone here is dead on.

We used to set carb'd boats to idle consistently at 550-600 rpm.

 

[CT8]

to add to the posted diagsostic posting see if the throttle shaft is worn.

 

[Traction]

I have a 1950 1.5 ton Chevy fire truck 216 inline 6 with only 2500 miles, with the original carburetor, and no kidding, it starts/runs/idles like it is fuel injected.

 

[spasm3]

Originally Posted by CT8
to add to the posted diagsostic posting see if the throttle shaft is worn.


+1 used to be a problem on quadrajets.

 

[caprice_2nv]

Originally Posted by spasm3
Originally Posted by CT8
to add to the posted diagsostic posting see if the throttle shaft is worn.


+1 used to be a problem on quadrajets.


I've heard of this happening but it hasn't happened to any of my quadrajets the past 17 years on me. All of my cars had/have a quadrajet except the 89 Caprice I had was TBI. Of course now that I've said this I'll end up with this problem tomorrow.

 

[Bluestream]

Originally Posted by CT8
to add to the posted diagsostic posting see if the throttle shaft is worn.

Possible but not likely in a marine engine. They dont have a gas pedal to move like in a car, and generally run low hours. 1500 hours is a well used engine, so throttle shaft dont wear out

 

[Dinoburner]

So many possibilities, a vacuum leak, heat expansion in the linkages, dry mechanical advance weights. Please let us know what you find.

 

[The motor guy]

In have a feeling the power valve are blown because someone adjusted the idle mixture screws and one was 1/2 a turn out and the other a 3/4 turn out.