2 barrell Rochester question

[jfrey8899]

Recently rebuilt carb on a 1973 sea swirl with a 140 mercruiser sterndrive stuck to it. That went very well but have a lingering question. Not to sure about the two spring loaded needle screws at bottom of carb. Looks like since there was one on each barrell that each screw would control the fuel mixture for that barrell or for those two cylinders. That's my thought. Someone told me that the forward screw might only control air and the back for fuel. Anyone know. And would you happen to know what the factory specs are (I.e. Number of turns from seat). Thanks

 

[Gary H NC]

Re: 2 barrell Rochester question

Those should just be idle mixture screws.If i remember right start at 2 1/2 turns out.

 

[EddiePetty]

Re: 2 barrell Rochester question

Recently rebuilt carb on a 1973 sea swirl with a 140 mercruiser sterndrive stuck to it. That went very well but have a lingering question. Not to sure about the two spring loaded needle screws at bottom of carb. Looks like since there was one on each barrell that each screw would control the fuel mixture for that barrell or for those two cylinders. That's my thought. Someone told me that the forward screw might only control air and the back for fuel. Anyone know. And would you happen to know what the factory specs are (I.e. Number of turns from seat). Thanks

Whew!! You're listening to somebody with some pretty weird notions!!

Each screw controls the fuel to its repsective barrel in the idle circuit only. Screw in to lean the mixture, out to enrichen.

 

[jfrey8899]

Re: 2 barrell Rochester question

Thanks guys. I thought so

 

[Alpheus]

Re: 2 barrell Rochester question

The mixture screws are for idle only. Here is how I adjust them.

If you are planning on doing hole shots or pulling people and want to get up on plane as fast as you can. follow this procedure.

1 Get engine up to normal operating temp making sure that the choke is open full.

2 Turn you air mixture screws all the way in till they just bottom out and turn them out again 2 turns.

3 adjust your idle to around 600 to 700 rpm, or as low and stable as you can get it.

4 You can do this by ear, vacuum gauge or with a tach.
Turn your air mixture screws out until you start to here or see (tach or vacuum gauge) the engine rev just a little and just when it starts to bog down a little turn in until max RPM is achieved. Do this with both needles one at a time.

If you are looking for better fuel economy and are just spending your days cruising around and not at WOT all the time follow the first 3 steps accept this time turn your needles in till the engine revs a bit and the starts to bog and turn out till you are at maximum RPM.

5 After your needles are set return the idle of your boat to around 700-750 RPM

The reasoning on this is just that the mixture curve is a little small but you can choose which end you want to run at. For hole shots you want to be at the rich end just for that little bit of extra fuel before the carb switches to the primary circuit. And for cruising you can run a little on the lean side as not to waste the extra fuel dumping at idle it also dosen't carbon up the plugs as fast.
__________________

 

[Fishermark]

Re: 2 barrell Rochester question

I always thought the initial setting was one and a half turns out. Then you set the idle to the correct specs with the engine warmed up. Then turn the screws in one at a time till the engine starts to slow down. Then turn back out about a quarter turn. They should both be pretty close to the same. If they aren't, then you have some issues in the idle circuit.

EDIT - I was posting the same time as Alpheus.

 

[Fishermark]

Re: 2 barrell Rochester question

It's interesting how vague the manual is concerning this adjustment. Here's what it says in the Mercruiser manual for a Mercarb adjustment: (I know the Mercarb only has one screw, while the Rochester has two, but I would think the principle would be the same for both).

I love #6 - "Adjust idle mixture screw."

Yeah, that helps a lot.

Final Adjustments
IMPORTANT: Boat MUST BE in the water and engine
at normal operating temperature to accurately
check and adjust idle speed and mixture.
Carburetor should be set so that engine idles as smoothly
as possible, with boat in the water, engine normal operating
temperature and drive unit in forward gear. To adjust idle
speed and mixture, proceed as follows:
IMPORTANT: DO NOT attempt to compensate for
other engine problems ( incorrect ignition timing,
faulty ignition components, low compression,
vacuum leaks, etc.) with carburetor adjustments.
This will only cover the problem, which must be
corrected if engine is to achieve maximum fuel
economy and performance.
1. Connect a shop tachometer to engine.
2. Start engine and run at 1500 RPM until engine
reaches normal operating temperature.
! WARNING
DO NOT leave helm unattended while performing
idle speed and mixture adjustments, following.
BE CAREFUL NOT TO ACCIDENTALLY
ACCELERATE ENGINE WHILE PERFORMING
ADJUSTMENTS.
3. With boat in open water, place remote control in
forward gear, idle position.
ADJUSTING IDLE MIXTURE SCREW
To adjust the idle screw correctly, the throttle plates
must be just about closed. To make sure this is done
do the following:
4. Disconnect the throttle cable
5. Reset idle speed (RPM) screw so engine idles at
550-600 RPM in neutral gear
6. Adjust idle mixture screw
7. Reset idle speed screw until engine idles at
550-600 RPM in forward gear.
8. Adjust and connect throttle cable

 

[Gary H NC]

Re: 2 barrell Rochester question

Just follow what Alpheus said....dead on the money!

 

[jfrey8899]

Re: 2 barrell Rochester question

Sweet. Excellent info. Did alpheus mean to do all this tied to a dock. Probably huh. Will read it again. Thanks everybody.