How to Adjust Air & Fuel Mixture on 2bb Rochester

[longstand]

hello again.

the mechanic is coming again to Tune the carburetor..

there is two idle needle adjustment on my carburetor for the Air and Fuel Mixture..

Question is. he will tune the carburetor on land. not on the water which will not put any load on the PROP..

Well ones the mechanic tune the carburetor and runs great on LAND. and if boat runs through the water slowly and sputtering which i assume its rich and if dies after after 10 or 20 feet its to lean..

to fix this problem do i have to adjust the two idle needle adjustment or i could just turn the screw which open the butterfly just a little?

i also assume If boat runs through the water slowly and sputtering because is to rich then i really have to turn the idle adjustment ?? or i could also fix this problem by adjusting screw which open the butterfly ???

 

[system-f]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

If it is like the carb in this post and has the two adjustment screws right beside each other horizontaly then those are only to adjust idle mixture.
It is my understanding that once off idle they do nothing, but I have misunderstood many things before.
http://forums.iboats.com/showthread.php?t=351361&highlight=rochester

 

[longstand]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

If it is like the carb in this post and has the two adjustment screws right beside each other horizontaly then those are only to adjust idle mixture.
It is my understanding that once off idle they do nothing, but I have misunderstood many things before.
http://forums.iboats.com/showthread.php?t=351361&highlight=rochester

yes i belive is the same carburetor that you have rebuilt. but my question is do i have to mess around with the Idle mixture adjustment after it has been set when i put the boat on the water??

reason is because.. ones there i load on the Prop. it will sometimes need to be adjusted to give more gas or to lean it out..

well again.. i ask do i have to mess with the idle adjustment for the Air and FUel Mixture to readjust the carb after it has been properly adjusted in LAND. or i just adjust the Screw which open the butterfly ??

 

[mla2ofus]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

If it's idling smooth, try speeding up the idle speed a little w/ the idle adj screw on the throttle. If it's idling rough or stumbles, try turning the idle mixture screws a quarter turn counter clockwise on EACH one. If this smooths the idle, it was too lean. As you turn the screws counter clock it enriches the idle mixture. If you go too far and enrich it too much the idle will sound labored and the exhaust will have a strong gas smell.Always turn them equal amounts. If your mechanic has them set evenly you don't want to deviate from that, always turn them each an equal amount each time.
HTH,
Mike

 

[longstand]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

If it's idling smooth, try speeding up the idle speed a little w/ the idle adj screw on the throttle. If it's idling rough or stumbles, try turning the idle mixture screws a quarter turn counter clockwise on EACH one. If this smooths the idle, it was too lean. As you turn the screws counter clock it enriches the idle mixture. If you go too far and enrich it too much the idle will sound labored and the exhaust will have a strong gas smell.Always turn them equal amounts. If your mechanic has them set evenly you don't want to deviate from that, always turn them each an equal amount each time.
HTH,
Mike

What about the Idle screw? doest the butterfly has to be fully close.. i just ask someone and told me that.. the butterfly has to be fully close and then adjust the air and Fuel Mixture.. ..

what is the first step before adjust the Air and FUEL mixture..

 

[Showtime22]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

My butterfly position is set by a screw off to the side of the carb. The screw setting is set to like 750 RPM (On a 350 Chev) or the lowest the engine will idle and still perform. The butterfly will end up where ever that setting puts it. The Idle screws on the front of my carb don't do a lot of anything, unless they are off. Aka Mega lean.

Also check your timing. I dialed my boat in by timing it on the water with a co pilot. You probably want to stay near the dock and make a mark where you started. Time it on land with a light. Make the mark...then dial the last few MPH in on the water.

 

[JustJason]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

How I do it...

remove IDLE MIXTURE screws and make sure they are good. Re-install IDLE MIXTURE screws until they "just lightly seat", back off 1 and 1/2 turns.

Start and warm engine

Verify base timing, quick check for advance

remove throttle cable

back out IDLE SPEED screw all the way

It should still idle low if everything else is correct and there are no other issues. If it doesn't idle, then you have....
1. bad fuel
2. defective carb
3. dirty carb
4. incorrect timing

back off IDLE MIXTURE screws 1/4 of a turn until the engine loads up and coughs and spits out the exhaust (RICH)

Turn in IDLE MIXTURE screws until the engine is starving for gas and almost wants to stall (LEAN)

For all intensive purposes, you set your IDLE MIXTURE screws in between the points where it is too rich, and where it is to olean. This will get you going. This can be done on the muffs or in the water.

Next set your IDLE SPEED. By the book this needs to be done with the boat in the water in forward gear
However... you can usually set it 100-150 RPMS above spec when the engine is on the muffs and when it actually goes in the water the RPMS will settle down and you'll be okay. But it's always best to double check in the water.

Thats all there is to it. No great secret. If the mixture screws aren't giving you a responce then you have something else wrong.

 

[Alpheus]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

There are 3 ways to tune up your idle air/fuel mixture. Tachometer,vacuum guage or by ear. I am so used to doing it I can listen to it and do it but I feel the most accurate is with a vacuum gauge.

1 warm up engine to normal operating temp.

2 Turn off engine and locate a vacuum port on your intake manifold and install vacuum gauge or watch the tachometer.

3 Turn both of your air/fuel mixture screws in all the way (lightly), back them off about 3 1/2 turns and start the engine.

4 This step is up to you, but I lower the idle screw as low as I can without the engine stalling. Turn you left screw in (clockwise) so that your vacuum (RPM's) gauge increase until you hit the highest peak that it will go until the vacuum (RPM's) starts to decrease. Then move to your right needle and do the same thing. You want the highest amount of vacuum (RPM's) you can obtain by turning the screws in.

5 Adjust your idle screw to the correct idle adjustment.

the idle are mixture screws are just for idle circuit and that's it. Your primary flow needles measure the air/fuel mixture after you hit the throttle...

 

[ENSIGN]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

Been reading your posts for quite a while,have you EVER taken some fuel out of the tank and ckecked it for water and debris??

 

[longstand]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

i never check the Fuel. but i had been putting Octane Booster and water Remover, and stabilizer.. i been putting a lot of chemical to renew the fuel.

as for dirt. i dont know how to check for that one.. but the engine starts. its just carburetor adjustment that i need to deal with...

 

[fishrdan]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

JustJason's procedure sounds pretty spot-on to me. I troll a lot, so I also fine tune it on the water (while in gear) so it idles perfectly. Pretty much play with the 2 idle mixture screws and idle speed screw until it purrs like a kitten and won't cough if I'm idling for a long stretch.

As someone else posted, if you're dealing with fuel (or ignition, or carb, or mechanical) problems, getting the idle set correctly will be difficult, if not impossible.

 

[Alpheus]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

If you don't know the condition of your fuel then you need to change it. All the carb adjustments in the world wont do a darn thing if you have bad fuel...

 

[intrepidvoyager]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

If you don't know the condition of your fuel then you need to change it. All the carb adjustments in the world wont do a darn thing if you have bad fuel...

That is the most succinct point in all this ......all the other posts about max rpm between rich and lean are for sure the ticket .... but someone mentioned ..."if you can turn the mixture screw and nothing happens to RPM's" .... that is BIG !! ...it means you got an idle circuit issue on that venturi .... what i often find is one screw will move the tach up and down and one wont .... no use going any further .....what you can do aand sometimes works is to get a can of carb cleaner with the little red tube .....pull the idle screw and blast the idle screw port with cleaner .... you want the end of the tube half way in the port with something like a piece of rubber with a hole for the tube and held in place with a long 8mm socket so the cleaner goes in and does not spray back at you .......and then blast with the tube all way in .....sometimes this will clean the circuit ...otherwise carb cleaning time again.

 

[longstand]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

If you don't know the condition of your fuel then you need to change it. All the carb adjustments in the world wont do a darn thing if you have bad fuel...

then i have A big Great IDEA..

Seens i have HALF FULL of Tanks. i should just put the boat in the water. and adjust the idle mixture so the engine could run good enought and have fun all day long until i have enougth Fuel to take me home.

and then Refill the Tank with Good Gas.. Sound like a GOod idea to me..

then with the new Gas. i will Adjust the Carburetor..

what you guys think..

also not to mention. its about 33 Gallon of Fuel.. thats alot of cash. i purchase that gas about 1 year ago. it cost about $100 dollars..

so not to waste any money. i will just take the boat out and run it around and waste the fuel.

Will it hurt anything by using this old fuel??? also i did add Chemical to remove water. and To boost Octane.. and also added the Marvelos OIL. which condition the Fuel and cLean the System

 

[Silvertip]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

One year old fuel (additives or not) it is still one year old fuel and no wonder it runs poorly. Adding fuel stabilizers and conditioners will not bring bad fuel back to life. You need to do one of two things. 1) drain the old fuel and use it in your lawn mowers, weed whackers, tractors or whatever. or 2) at least fill up with fresh fuel to mix with the old. Running around on old fuel is a good way to create other engine problems.

 

[Alpheus]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

Drain that fuel and start over. You don't know whats in it sediment,rust,water.ETC. I would not run that fuel through a carb that is having trouble to begin with.

PS.

Im not trying to sound cynical, but if you are worrying about coming up with some spare cash to pay for fuel. I think you may be in the wrong hobby. There are much more expensive things to worry about other than gas...

 

[intrepidvoyager]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

OK Longstand ......ya ...you could go for a run with the old gas ...but lets think about what might happen ........ the boat is not running ok right now so maybe you are already running on h2o contaminated fuel but thats ok cause your fuel water separator is soaking up a lot of the h2o ...... but oh oh ....you are now 5 miles from the dock and your fuel water separator is now saturated and you are dead in the water !!!!

This is only one issue you might be dealing with ..... like the previous posts are trying to tell you ......syphon the old gas out and use it elsewhere.

Once you ditch the gas ......

One other point not mentioned is the ball check valve on the top of your tank ... make sure it is working and not jamed with sediment. If the engine fires up right away after sitting for a few days its probably ok ....if you got to crank more than 5 secs it could be an issue.

 

[longstand]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

argg. another bad news... but yes. you guys have the experience.

hey. i like this hobby. saving little cash doest not mean iam in the wrong hobby.

Ok. i will remove the old gas and give it to my Wife Dad. he could use it on his Cars...

Arggg..

Got another idea.. how about this. one Tank is really empty. how about i put about 20 Gallon on that and some water separator to remove any water that is been inside the tank. just to be safe

also ball check valve on the top of your tank.. i dont really know where that is but i assume is where the Fuel line connects to the Water Separator/Fuel Valve?

if it is that . i have check.. i remove the hose. and Blow IN. and i Suck IN.. there seem to Flow without no problem

 

[intrepidvoyager]

Re: How to Adjust Air & Fuel Mixture on 2bb Rochester

Dont know the age of the boat ... how long its been sitting ...so i will give you a worst case senario ..... buddy had an older boat with 2 alu side tanks ...sitting for 2 years ..... lots of crap in the fuel filter and one stuck check valve ( antisyphon) with sediment in it ...usually an alu fitting where the fuel line goes into the tank... because sediment was an issue we pulled the fuel guage sender lid off one of the tanks .... lots of crud in the bottom of the tanks ... so we ended up pulling the tanks and cleaning them out ..... the problem with all this sediment is that at the dock its all sitting on the bottom of the tank....but when you go out it gets mixed with the gas and you got problems ........ at least if you have new fuel you are not going to have a water or octane problem but you might have sediment clogging your fuel system.

You got too many potential problems here ...... get rid of the old gas ...if you can have a look inside one of the tanks .....a bit of sediment is normal but if its bad get rid of it . ( we pulled a cup of crud out of each tank )

OK .....this was a worst case senario but you got to be prepared

Check the tanks ...if you can !!!

So gas it up with new fuel, change the FWS (fuel / water separater) and fuel filter(s) and go for a 1 hr shake down ..... then check the FWS ...dump it out over paper towel and see if there are any specs of crap ....if no ...then you can forget about tank sediment as a major issue.

As previous post have said .....you will be dealing with other issues as time goes by .....so best to get a manual so you can do pm stuff and fix problems yourself. I look at it this way ....if you can solve a problems yourself that would take a marine shop 5 hours ...... you have made a $1000 .....remember you are paying the shop in after tax dollars !!!!