Vacuum Gauge Connection

[Tom Kazakoff]

I have a refurbished Mercruiser 357 Alpha 4v. I want to tune the carbs using a vacuum gauge. There is no place on the base of the carburetor that I can find to connect the gauge. Do I need to take out one of the plugs in the manifold and connect the gauge there? I would think that if that is the case, I have to find/make an adaptor that fits into the plug hole and connects to the vacuum gauge.

 

[alldodge]

Why do you want to use a vacuum gauge?

 

[Tom Kazakoff]

I think it is more accurate then finding the "sweet spot" by ear.

 

[alldodge]

Sorry, but I must be missing something. Thought the 357 Alpha V4 is a EFI motor so there is no adjustment

 

[Scott Danforth]

no, the 357 is a reman'd 350 with between a .030 and .040 overbore (355 to 357 cubic inch) and a 4-barrel carb

 

[alldodge]

no, the 357 is a reman'd 350 with between a .030 and .040 overbore (355 to 357 cubic inch) and a 4-barrel carb

OK, how do they connect a gauge?

 

[Tom Kazakoff]

Sorry, but I must be missing something. Thought the 357 Alpha V4 is a EFI motor so there is no adjustment

There are two adjustments on the front of the carb, located near the base. Turning them definitely changes the idle.

 

[alldodge]

There are two adjustments on the front of the carb, located near the base. Turning them definitely changes the idle.

Thanks I understand
The big IF unless someone has an answer is to find a plug on the intake. If there is one on one of the intake runners then plug can be removed and fitting installed. If not, then its of no use.

Could use a Tach but agree not as good for idle as a vacuum gauge

 

[Scott Danforth]

If there is a port, it would be just aft of the carb on one of the runners. however I believe Mercruiser got rid of that port long ago. It is on the aftermarket intakes and the volvo intakes (edelbrock), however I believe not on the mercruiser intakes as they added threaded bosses for mounting other stuff

 

[achris]

Merc have never bothered with setting idle mixture by vacuum gauge. Even at Merc school, we're told to use an accurate tacho and your ear. And frankly after using both methods, ear is better. Takes less time to set up, and actually gets a better result. The only time a vacuum gauge is useful is multi-carb setups, like motorcycles (and those horrid V6 yamaha outboards).

Chris.....

 

[alldodge]

Kids

 

[Scott Danforth]

I personally use an un-lit propane torch to set idle mixture. if introducing a bit of propane raises idle, your too lean, if adding just a bit of propane lowers the idle just a bit, your perfect.

 

[Tom Kazakoff]

If there is a port, it would be just aft of the carb on one of the runners. however I believe Mercruiser got rid of that port long ago. It is on the aftermarket intakes and the volvo intakes (edelbrock), however I believe not on the mercruiser intakes as they added threaded bosses for mounting other stuff

There are threaded bosses. Could one of these be used for the vacuum gauge provided that the correct "adaptor" was screwed into the threaded boss?

 

[Scott Danforth]

There are threaded bosses. Could one of these be used for the vacuum gauge provided that the correct "adaptor" was screwed into the threaded boss?

no, none of them are intended to have a passage into the plenum

 

[TyeeMan]

Don't those carbs come with those idle jets "sealed" so you can't adjust them?

I have the same engine package, the previous owner had the engine package installed. I got the rig with about 20 hrs on the engine. Someone popped the caps off the idle and totally screwed up the jets to where it was running grossly rich.
I started from scratch, turned the jets in till they seated, then backed them out one or one and a 1/2 turns, fired up the engine in the lake, once it came up to operating temp I tuned them by ear. Works perfectly.

 

[Bondo]

There are threaded bosses. Could one of these be used for the vacuum gauge provided that the correct "adaptor" was screwed into the threaded boss?

Ayuh,..... I believe those go into the water jacket,.....