Where do I find fuel/air mixture carb adjustment? 1986 Bayliner Ciera 2755 w/Volvo-gctid398278

[Guest]

The engine is the 5.7L Volvo Pentadrive (original engine). How do I adjust the fuel/air mixture on this engine?

TIA!

Dave

[2850Bounty]

davesisk wrote:
The engine is the 5.7L Volvo Pentadrive (original engine). How do I adjust the fuel/air mixture on this engine?

Dave, without getting into the carburetor, the only f/a changes that we can make to this carburetor, will be the low speed fuel metering.

These will be the two air blend screws at the front of the primary throttle plates.

Screws IN = less air/more fuel

Screws OUT = more air/less fuel

Shoot for the highest vacuum reading while you continue to keep idle speed within specs.

BTW, these have zero to do with any fuel metering once out of the low speed metering circuitry.

In order to change anything on the high speed side, we need to get into the carburetor and/or secondary metering rods.

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davesisk wrote:
The engine is the 5.7L Volvo Pentadrive (original engine). How do I adjust the fuel/air mixture on this engine?

TIA!

Dave

What problem are you having that necessitates adjustment?

5.7L Volvos have some unique, uhm, operating characteristics as they age.

[Guest]

2850Bounty wrote:
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In order to change anything on the high speed side, we need to get into the carburetor and/or secondary metering rods.

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I have been curious, is the new fuel with so much ethanol effecting the "jetting" of carb style boats? I would think with boats being under such a hard load in its power range (on plane) and already close to detonation it may cause problems. I am asking you because I know you are invloved with these type boats more than most people I know.

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I am curious what the problem is. I think I might have an inkling on what he is complaining about.

5.7L Volvos have a dead spot when accelerating to plane. Every single stock 5.7 VP I have ever piloted has had it to some degree, including my old boat.

[Guest]

Actually, the problem I'm trying to resolve is that it stumbles and cuts off at low idle rpm's...AND it is occasionally very stubborn to re-crank after it's cut off. It's already had a carb rebuild, and that seems to have fixed about 80% of the issues...there's still that lingering 20% that it stumbles and cuts off below about 900-1000 rpms and can be really hard to restart (like 1 out of every 10 times, it just won't start...but it seems that if I wait a while, it will then start). On another thread, a couple of folks repeatedly said "sounds like it's running to rich at low rpms", so that's what I want to troubleshoot/adjust. It runs pretty well at higher rpm's, and gets up on a plane reasonably quick for it's weight and age, I think.

Thanks!

Dave

[Guest]

OK, here's a pic of the carb with the cover off. The outdrive is toward the top of the photo. Where are the two screws I'm looking for? (The one's to adjust the low-rpm fuel/air metering.)

[img]/media/kunena/attachments/vb/705640=29879-BaylinerCiera2755_1986_carb.jpg[/img]TIA!Dave

[2850Bounty]

This should be very similar to your Rochester Quadrajet carburetor.

Attached files [img]/media/kunena/attachments/vb/705646=29880-Volvo Penta Q-jet low speed air screws.jpg[/img]

[2850Bounty]

You mentioned that the engine does not want to idle well. Have you look into your low speed air jet orifices?

These need to be clean and free from any restrictions, or the low speed air adjustments will not help any.

Using eye protection, carb cleaner and the carb cleaner red straw, give these a blast to clear them.

Attached files [img]/media/kunena/attachments/vb/705694=29882-Rochester Q jet low speed air ports.jpg[/img]

[Guest]

2850Bounty wrote:


Screws IN = less air/more fuel

Screws OUT = more air/less fuel

Shoot for the highest vacuum reading while you continue to keep idle speed within specs.

.

I'm no mechanic...how do I measure vacuum reading? What kind of guage do I need for this, and where do I hook it up?

On my motorcycle, I adjusted the carb fuel/air mix by turning the mix screw until I got the highest idle speed (then adjusting the idle speed down). Can I just do something similar here with the boat engine without a vacuum guage and get it mostly right? The boat engine won't idle at the spec'd 600 rpms (stumbles and shuts off), so that's what I'm trying to fix. I'm also hoping this might help with getting the starting a little more reliable too.

I will take a look at those low speed air jet orifices. They *should be* clear since the carb was rebuilt recently. However, I shouldn't be having this idle issue either, so I'll check them. Thank you very much for all this info!

Thanks!

Dave

[2850Bounty]

davesisk wrote:
I'm no mechanic...how do I measure vacuum reading? What kind of guage do I need for this, and where do I hook it up?

On my motorcycle, I adjusted the carb fuel/air mix by turning the mix screw until I got the highest idle speed (then adjusting the idle speed down). Can I just do something similar here with the boat engine without a vacuum guage and get it mostly right? The boat engine won't idle at the spec'd 600 rpms (stumbles and shuts off), so that's what I'm trying to fix.

You would use a standard vacuum gauge typically used for this.... most auto parts store will have these.

Not all of the Marine carburetors will offer a port for this. You may need to open a port in the intake manifold.

This carburetor from MM does offer the port where you see the large rubber cap.

This port should be a direct manifold pressure (vacuum) port.



You can also do what you suggested by adjusting these for the smoothest and best idle speed.

Usually this involves a combined adjustment..... air screws and idle stop adjustment.

[Guest]

Ah, OK, cool. Next time I have the boat at my house, I'll look for that vacuum port.

I tried adjusting this without a vacuum gauge...I got it to idle a *little* smoother, but still can't get it to idle smoothly below about 900-1000 rpms without cutting off. I've already had this carb rebuilt...would I get better results if I just bought a new carb and replaced the old one?

I'll pick up a vacuum guage, and try it again next time I have the boat at my house.

Thanks!

Dave

[2850Bounty]

Dave, there are no new Rochester Quadrajet carburetors. All today are re-manufactured.

In post #9, I pointed out the small orifices for the air inlets to the metering circuits. I just looked again, and I believe that I pointed to the high speed air orifices. Look to front of this area, and you'll see two additional orifices.

Try spraying these out, and then do your adjustment again.