57 FI question.

[Bruce B.]

My 4360 unit has been running fine for the last 2000 miles but I would like to lean it out a bit. The plugs are not fouling but I get a lot of black soot in the tailpipes.
I understand that by moving the lean stop forward one turn and also moving the rich stop forward one turn the resulting position of the ratio lever will lean out the mixture.
Is this worth messing with or should I leave well enough alone?
Thanks in advance.

[Roy B.]

Re: 57 FI question.

My 4360 unit has been running fine for the last 2000 miles but I would like to lean it out a bit. The plugs are not fouling but I get a lot of black soot in the tailpipes.
I understand that by moving the lean stop forward one turn and also moving the rich stop forward one turn the resulting position of the ratio lever will lean out the mixture.
Is this worth messing with or should I leave well enough alone?
Thanks in advance.

On my 57 FI the lean side ( looking at them "left side" ) shorten one turn "that's idel ", right side leave alone "power".

[Barry H.]

Re: 57 FI question.

Bruce, Roy is right, don't change the "rich" stop, just work with the "lean" stop. Only move 1 turn lean at a time, then test drive, check the "tip in " throttle responce. When you are at cruising speed on the level with light throttle pressure, try to just lightly accelerate with very little pressure on the pedal, if it accelerates smoothly, you are OK. If you get a stumble, you are too lean. Make sure you don't push the gas pedal hard enough to get into the "rich"stop, this defeats the test. You should be able to get leaner til you get the stumble at "tip in", then just turn back a l turn richer. Also watch your enrichment diaphragm rod for any interference. Go for a nice grey look in the tailpipes. Good luck, also keep records of the adjustments you make, (you can always change back). I used this method with my second 57 with a 4520 unit, & also my current 62 with a 7360 unit. If I remember correctly my first 57 had an early 4360 with twin spiders & 1 piece nozzles (no oriface plates) & it ran much better when it was rich. Barry Holmes #940

[Bruce B.]

Re: 57 FI question.

Barry & Roy,
I went through the adjustment procedure on the lean stop and after backing the lean stop out 4 turns I got a definate stumble. Then adjusted the stop in 1 turn and it seems to run fine.
I'll drive it again as soon as the rain stops and see how it runs.
Thanks.
Bruce B

[John D.]

Re: 57 FI question.

Hi Bruce, You really cranked the lean stop out a lot!!!! But whatever it takes.
I noticed my pal Barry forgot to tell you another adjustment. One that's overlooked a lot.
Try playing with the idle fuel screw on the air meter. Manuals say to turn it in all the way and out 1-1/2 turns. Guys think that's gospel but actually it's only good enough to start the car up. Playing with the idle fuel screw can also effect the rich and lean.
Another problem you have Bruce is that's it a 4360. LOL.
Now for a cheater. ssshhhh
If your dual spider '4360 once had a dual spider or has a dual spider right now the Branch T-the T fitting on the main diaphragm cover has a restrictor in it. Well in the old days when that unit came out it had a coasting shut off valve. Total flop it was and only a few guys in the country could figure out how to make those things work. Meanwhile when you had the coasting shut off valve all hooked up old RP had a huge restrictor hole for the branch T.
Now to the punch line Bruce. Use a smaller restrictor in the Branch T and that leans out the FI big time.
Bruce-check on the size of the Branch T. A number should be stamped on one side of it. Real deal POC '4360's (LOL) had a huge hole. Maybe .022 or .024 plus or minus. Wayyy to big. Try swapping out the restrictive T with a .016 (stamped 116) or even a .018 (stamped 18 or 118).
What this does is lessens the signal and gives you a leaner running FI. Yes I know this is not kosher but it works.
Story: '4520's were RP's and GM's first major attempt at 283-283. Now to get this they had a wild vacuum signal. Trick is to control the wild signal.
Gail Parsons was a pro at this sort of work. He taught me a lot about controlling the signal.
Now this is not for the beginner or back yard guy but Gail put a bleed in the airmeter's on most of the '57's. Just like the 4800R had. Also the '58 to 62 had a bleed that varied in size from .035 to .050. Bleeding off the vacuum signal told the fuel meter-I don't need all that gas. Result-leaner running FI.
Bruce I know you are a 62 fan. Well the trouble with a lot of the 62's is that the bleed signal in the air meter has too big of a hole. .050. Old Frank Sciabica taught me 30 plus years ago to use a .035 bleed. Works good. Ask Barry. JD

[Bruce B.]

Re: 57 FI question.

John,

Thanks for all the info.
I will look at the branch Tee and see what size the ID is.

At this point the old 4360 is runnung just great. It both cold and hot starts are fine and goes down the road nicely.

Bob Williams did the work on the FI and I have been told that he likess to set them up a little rich.

Since I drive the 57 I think I will try not to mess with it until the snow starts falling. I think It was Jim Lockwood or Tom Parsons who said if it runs good , Keep Your Hands Off It.

So for now it's drive it.

ps, I have a single spider.

Good hearing from you, I hope all is well and hopefully I'll see you at Carlisle.
Bruce B

[Jim L.]

Re: 57 FI question.

At this point the old 4360 is runnung just great. It both cold and hot starts are fine and goes down the road nicely.

Bob Williams did the work on the FI and I have been told that he likess to set them up a little rich.

Since I drive the 57 I think I will try not to mess with it until the snow starts falling. I think It was Jim Lockwood or Tom Parsons who said if it runs good , Keep Your Hands Off It.

So for now it's drive it.

Bruce,

Sorry to be tuning in late; I've been away.

I'd be pretty reluctant to lean out the economy stop setting without a compelling reason to do so and some way to measure the results, like monitoring the mixture with a wide band air/fuel ratio gauge. It's just too easy to set the air/fuel ratio excessively lean and not even know it.

As an example, one unit I had here recently came to me with the air/fuel ratio at 17:1! The owner had no idea since the cruising behavior of the car seemed OK to him.

For comparison, the original design goal for these old FI units was for an air/fuel ratio around 15:1; modern cars, as I understand it, dither around 14.7:1.

You'll notice that the tailpipes of modern cars are always black. So the color of the tail pipes isn't a good indicator of the mixture.

Even if you have access to equipment that will measure the air/fuel ratio, you really don't want to go much leaner than about 15:1 anyway. A lean mixture will increase the exhaust gas temperature which will increase the underhood temperature. What do high underhood temperatures cause? Yep, fuel percolation, the bane of Rochester FI units everywhere.

A mixture that is slightly rich, of course, has the opposite effect.... it will lower exhaust gas temperatures and underhood temperatures.

So, bottom line, it's just better to err on the side of too rich than on the side of too lean.

Jim