Adjusting Idle Mixture....Rochester 4BBL

[Manipulator]

I can't seem to adjust my idle mixture. I can screw the idle screw (both) out without any change in RPM. I have a Rochester 4BBL and it was rebuilt last year. Idle is a little rough, has new plugs, wires, points, distrib/rotor and fuel filter. Runs fine off idle.

 

[bjcsc]

Re: Adjusting Idle Mixture....Rochester 4BBL

That's interesting. If you were running rich off idle, I would think your jets were too large. My guess is that either 1) you're running so rich at idle (secondaries misadjusted, other problem, etc. ) that the idle circuit is not even functioning as it is overridden by the problem or 2) The idle circuit has trash in it and is "preset" by the trash. If you screw the adjustments all the way to lean, and gently to the seats the engine should starve for fuel and stop very quickly. Yours keeps running the same? Is this a spreadbore?

 

[AdamB]

Re: Adjusting Idle Mixture....Rochester 4BBL

Is your idle adjusted right? If your throttle plates are too far open, they'll bypass the screws...

 

[Blk-n-Blu]

Re: Adjusting Idle Mixture....Rochester 4BBL

Heres a little item from the FAQ forum that mighy help,
http://forums.iboats.com/bbBoard.cgi?a=viewthread;fid=36;gtid=1105882
8)

 

[Manipulator]

Re: Adjusting Idle Mixture....Rochester 4BBL

If you screw the adjustments all the way to lean, and gently to the seats the engine should starve for fuel and stop very quickly. Yours keeps running the same? Is this a spreadbore?

Yes, this happens. If I screw them in all the way the motor wants to stall. I then can back out 1/4 turn and idle increases a little. Thing is, I can keep unscrewing them to they fall out without any change in engine RPM.

 

[jimmbo]

Re: Adjusting Idle Mixture....Rochester 4BBL

"Wants to stall". It should have died long before the screws are seated. sounds like your fuel level is too high. A misadjusted float, wrong float, wrong fuel inlet needle and seat, too high fuel pressure. Any of these can cause a high fuel level.

 

[bjcsc]

Re: Adjusting Idle Mixture....Rochester 4BBL

Thing is, I can keep unscrewing them to they fall out without any change in engine RPM.

This can be normal. First, as mentioned, make sure your idle is set to spec.. Next, check clearance on the throttle plate in your primary bores. You should have a spec. listed in your manual. Most are around .020 (20 thousandths). Check your secondary plate and linkage to make sure nothing is binding or sticking. The secondary linkage should be adjusted so that its plates close when you're off the throttle. Sometimes this linkage gets sloppy and (esp. after running hard and using the secondaries) it will not close them all the way. Secondary plates are adjusted to be completely closed, but not to the point that they will stick. These are adjusted by bending. Once all that is good and working properly (cycle your linkage several times and make sure it all stays where you put it) you can run the mixture screws lean and gently bottom them out. Back them out ~3 full 360* turns. Start it up. Adjust the idle to spec if necessary. Turn the mixture screws lean a quarter turn at a time each until you hear a drop in RPM. Then back them rich about 1/8th of a turn. You can do this a few times to make sure you get consistent results. If you want to be even more precise, you can also use a vacuum gauge or optical tach. In these cases you would lean out until you noticed a drop and then back up to the highest reading. It is always better to err on the rich side vs. the lean side.

 

[Manipulator]

Re: Adjusting Idle Mixture....Rochester 4BBL

Thanks for your help guys.