Rochester Quadrajet tuning...

[undrh2o]

good morning all!

I just replaced the motor in my houseboat and in the process am rebuilding the carb. It has two adjustment screws in the front of the base plate but I can't locate any info as to the correct settings and what they adjust.

Can anyone help?

its a Rochester Quadrajet model # 17059286, 1980 OMC 260hp/350

Thanks!

 

[jeromeb]

Re: Rochester Quadrajet tuning...

You are more than likely referring to the idle mixture screws. Turning them in, (clockwise), leans the mixture. Out, or counterclockwise richens the mixture. With the engine up to operating temps, turn the screws in until the engine starts to stumble. Then out until you get a smooth idle. Do this for both screws. The name of the game is have a smooth idle with screws in as much as possible. Too rich a mixture wastes fuel and leads to "dieseling effect" when you turn the ignition off.

 

[cr2k]

Re: Rochester Quadrajet tuning...

Don't screw them in hard or you will ruin the needle part of the screw. Just lightly until they bottom out. Start at about 1 1/2 turns out and go from there you will need to have the motor warmed up for final adjustment. Set Idle around 650 or lower if it will and set again.

 

[Silvertip]

Re: Rochester Quadrajet tuning...

Before you even fire the engine, make sure the choke is adjusted properly.

 

[undrh2o]

Re: Rochester Quadrajet tuning...

ok, I have rebulit this carb twice and can not get the unit to idle. If I lock the choke pullout bracket down so the butterfly is only open about 1/4" the motor starts fine, idles and has great throttle response! I have started with the air/fuel mixture screws at 1 1/2 turns out and slowly adjusted both with no effect...

could it be the carb is sucking air elsewhere?

at the moment it is starting/running GREAT but I know this is not the correct way to set it up with the choke locked...

 

[oldjoe789]

Re: Rochester Quadrajet tuning...

Forcing the choke to 1/4" closed is making the fuel mixture much richer, if it feels 'good' like this with the engine warmed up, I would guess that there is a sizable vac. leak, but then again the idle RPM should be too high with vac leak. Check with carb spray to ID leak. If idle RPM is OK, verify that you have the correct gaskets for your carb, the wrong or low quality parts could result in large vac or internal leak. Verify that the idle passages are not restricked by a wrong gasket.

If your not sure that you have the correct gaskets, go to http://www.quadrajetparts.com, you can search for your rebuild kit based on yoour carb # and verify your gaskets with the detailed pictures of the gaskets in the kits.

 

[undrh2o]

Re: Rochester Quadrajet tuning...

kit came from NAPA and looks identical to the one from quadrajetparts...I tried the carb cleaner to locate a leak and came up empty. I am completely baffled by this as the boat fires right up and runs smoothly.

 

[oldjoe789]

Re: Rochester Quadrajet tuning...

When you say it runs smoothly, is that just at idle or also at cruise RPM?

If its running fine under load, I would think that there may be a mismatch of castings. Maybe wrong airhorn or base. Try to visually see what the carb is doing (fuel flow) at idle and under load. Something is definelty out of wack.

 

[undrh2o]

Re: Rochester Quadrajet tuning...

it runs smoothly both at idle and under load...I have run the boat a total of 4 hours the last couple days and it fires right up, idles and doesn't miss a beat. It just doesn't make sense to me that I would need to lock the butterfly down like it is.

I have sprayed carb cleaner around the base & gaskets and the motor never stalls/revs so I can't explain it...

I think I am gonna leave as is and enjoy...oh wait, I already have!