How to check a Merc Weber 4bbl Carb Secondaries?

[colsoncj]

I've searched, but not quite found my answer....

I dont have the boat here, but to save a trip to the boat and back, 1st can someone tell me on the weber's (edelbrocks) if the secondaries are mechanical or vacuum? (94 454 Merc)

2nd: Short of pulling the spark arresters off, and gunning it in the water, is there a good method to check that the secondaries are opening?

I'm assuming in a boat if they're mechanical they open around a specified RPM or throttle position (easy to check without gunning it in the water and staring at it).

If its vacuum, should the open around a set RPM, or is it really all just load dependent generating to proper vacuum to open?

 

[Black as]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

What is the actual problem ? eg: flat spot, hesitation or are you just interested in how your 4 barrel works.

 

[Bondo]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

Ayuh,... Vacuum...

 

[John_S]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

The secondaries are mechanical.

The air-door is vacuum.

It is considered a vacuum secondary carb.

Confusing, huh?

With engine off and at wot position, you can remove flame arrestor, push the air-door open with finger, and see if butterflies are full open. The secondaries start to open at about 1/2 the primaries. Unless you have been messing with the linkage, or had carb apart, this is usually not an issue.

As far as the air-door openning and actually getting fuel flowing through the secondarys, you have to watch them under load, out on the water. I doubt that that 750cfm carbs air-door will open all the way. It is just a counter-weighed door. When you have the throttle at full open, just check to see if the door moves smoothly, and not catching.

I suspect you are chasing a loss of power problem.

 

[colsoncj]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

Actually I'm not chasing any problem yet. no issues on the water at all. I'm just trying to verify everything is working as it should and trying to get a ballpark RPM that the secondaries open, so I know where to cruise for best fuel economy.

 

[achris]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

Actually I'm not chasing any problem yet. no issues on the water at all. I'm just trying to verify everything is working as it should and trying to get a ballpark RPM that the secondaries open, so I know where to cruise for best fuel economy.

Why not do as I did? Invest in a fuel flow meter. It gives me instantaneous fuel flow. I watch the GPS speed and do a simple math in my head and come up with litres/nautical mile. I also recorded the different flows and speeds and came up with this graph....

Fuel flow meter... Easy to install. (Can be set to litres per hour or gallons per hour)

And the fuel usage graph in Excel.... Once I put that together I know when I'm between 3000 and 4000 I'm in the right range. At 3400 I'm at the best...

What was really interesting was that I did this graph for my 17" prop. I put the 19" on one day and compared them. For the same speed, the fuel consumption was the same, even though the engine speed was less. Sort of proves a point, yes?

Chris........

 

[180shabah]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

Sort of proves a point, yes?

Agreed.

power = speed, doesn't matter at what RPM/throttle setting it takes to get there, 100hp = 100hp


OP - don't worry about when the secondaries open, it is irrelevent, unlike a true mechanical carb, it will only draw what it needs. If it is still setup as it came from the factory and in good condition, those webers are damn near perfect for your engines.

On a boat the sixe of yours, a flow scan will probably pay for itself in a single season.

 

[achris]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

... Sort of proves a point, yes?......

Agreed.

power = speed, doesn't matter at what RPM/throttle setting it takes to get there, 100hp = 100hp

I was thinking more along the line of using a slightly smaller prop to 'under-load' the engine. Thus decreasing stress on the engine, and improving driveability... without increasing fuel consumption..

And, as you point out, doesn't matter how you do it, a speed needs power, and power means petrol....

And on the point of 'pay for itself', mine certainly does. Just last Sunday I was towing another boat in. At 6.4 knots I was burning 13.5l/hr. I felt comfortable so push the throttle up a little. Got to 7.1 knots, but noticed the fuel burn went to 22l/hr. I figured an extra 0.7 knots wasn't worth 8.5 l/hr! So we dropped back to 6.4knots for the rest of the trip.

Even when I'm just running around normally, I start looking for the 'sweet spot'. I'm guessing I've saved quite a bit in fuel just doing that.

Cheers,

Chris...........

 

[colsoncj]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

Well, it sounds like a flow meter may be the way to go. Locations that sell them and any installation advice? Obviously I would need to hook mine up at or very near to the tank, but wouldnt it also see return overflow as well?

 

[180shabah]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

There is no return.

 

[bomar76]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

wouldnt it also see return overflow as well?

which makes me suspect you are seeing fuel in the clear tubing that runs from carb to fuel pump...that is NOT a return. It means you needed a new fuel pump yesterday 'cause the diaphram is ruptured.

 

[colsoncj]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

Nope, not seeing any fuel on that line the last time I checked... I'm just not near the boat right now, so I couldnt check if there was a return system on it.

After looking into the gauges, it looks like I'll need a system capable of handling two sensors, one for each engine, and the sensors should be located between the water seperators and the carbs...

Any recommendations on setups or places to purchase?

 

[achris]

Re: How to check a Merc Weber 4bbl Carb Secondaries?

Nope, not seeing any fuel on that line the last time I checked... I'm just not near the boat right now, so I couldnt check if there was a return system on it.

After looking into the gauges, it looks like I'll need a system capable of handling two sensors, one for each engine, and the sensors should be located between the water seperators and the carbs...

Any recommendations on setups or places to purchase?

True the sensor needs to be AFTER a filter, but trying to cut into the fuel line between the water separator and the fuel pump is not an option. I put a small filter in the line from the tank, then the sensor, just before the water sep filter....

I plan to relocate the water sep to the engine room bulkhead and put the sensor after that....

Chris......