84oldsDelta88

Sorry if this post is a little long but i want to give as much information as possible, so that you guys can help diagnosis what wrong.

Still working on my quadrajet trying to get a good a/f ratio and good mileage.
(i'm sticking with the quadrajet, i'm not gonna give in and buy a holley or edelbrock, not to mention i'm flat broke at the moment and i dont have 500 bucks to spend on a new carb.)

Its a quadrajet M4MC carb on a 1970 olds 350 (originally a 2bbl. Motor is completely stock never opened up)
(the carb came off a 1978 olds 403 with 73/55 rods stock.)

The last time I posted about my carb problems, it had a hesitation off idle, that was fixed by replacing a bad spark plug.

But the car continued to run lean at idle and part throttle cruise. WOT was fine though. I tried many different rod and jet combos with little success.

I tried 71/55 rod/jet combo (which gave great gas mileage 16-20mpg, but was too lean on the a/f gauge) WOT seemed okay.
73/55 combo (gave worse gas mileage 12.7mpg good throttle response better WOT performance, but was still too lean on the gauge.)
73/51 combo (not much of a change still bad mpg, but was reading in ideal/slightly rich zone all the time on the gauge. sluggish performance.)

With all the difference rod/jet combos the idle mixture screws were always out 7-8 turns to achieve a 19 in. vacuum and a good idle.

I did lots of research for months. Finally i realized the idle channel restrictors (the little brass plugs in the body) were too small. I changed them out for slightly bigger ones from a chevy truck carb i had.

That seemed to make a big difference. The idle screws only had to be turned out 1.5 turns to achieve a 19 in vacuum with 71/55 rod/jets and the a/f gauge was in the ideal zone at idle.

But now at part throttle cruise its showing too lean. But if i push the pedal a little bit to speed up the gauge will jump back up to the ideal zone.

I'm starting to run out of ideas on what to do next. The only thing left i can think of is the power piston spring being to soft. Its the original for the carb.

I've also tried adjusting the APT screw which doesnt help part throttle cruise. Currently i think its at 2.5 to 3 turns out.

So guys based on the info. i provided what do i do next.

I'm hoping someone has a similar setup like mine and can tell me what rods/jets to use with this carb.

My goal is to get somewhat good gas mileage, good driveability, and hopefully not run so lean anymore.

Thanks for any and all help.

turbobill

I just rebuilt the original carb on a 35,000 mile '74 Omega with the 350 4bbl engine.




This is what I found in it for jets/metering rods:



Primary jet: 69

Primary metering rod: 53C

Secondary metering rod CG

Secondary hangar: N


I reused the original power piston spring and made no adjustment to the APT screw nor did I make any tension adjustments to the secondary air valve spring.

84oldsDelta88

Thanks for the details, but i think thats still too lean for my motor as my current 71 jets/55 rods appears to be richer then that when calculating the metering area.

I been doing more research, after reading through endless forums, and google searches I came up with this; A 1970 olds 350 had 70 jets and 52c rods stock. Which would have a metering area of 0.001724733.

All my rod/jet combos were always either too lean or too rich.
55 rods and 71 jets = 0.001583363
55 rods and 73 jets = 0.001809558
51 rods and 71 jets = 0.001916371
51 rods and 73 jets = 0.002142566

I spent an hour or so last night with a calculator trying to find a rod/jet combo as close to stock as possible. Finally I came up with this, a 72 jet with a 55 rod has a metering area of 0.001695671. Which is as close as i can get with this carb. Have to order new jets, will post back with my results when i get them. Probably wont have them until sometime after the new year.

Happy holidays to everyone on the forum.

-Joe

Octania

I'd like to know what size idle bleed restrictions you started and ended up with, and

What is this system you have that tells you the a/f ratio?

The QJet book I have shows a typical needle/ jet diff of about 0.030 or "30" in their part numbers:

49 rods in 79 jets for example

Whereas your combos range from a difference of 16 to 22
[13 on turbobill's numbers]

84oldsDelta88

I'm not sure what size the idle bleeds were to be honest. I used a carb. rod as a measuring tool to get a rough estimate of the difference. I think i used the 55 rod as a gauging tool, the old air bleeds you could fit the .026 power tip in. The bigger air bleeds i was about to fit the whole rod into, so i'd say the new air bleeds are probably .055-.060

The a/f ratio comes from a gauge and a o2 sensor, although the o2 is a stock narrowband it gives me a rough idea of how its running. t may not be as accurate as a wideband o2 but its close enough.

A .030 rod to jet difference seems way too rich. When i run the richest rod/jet i got, 51/73 the spark plugs are black, i have horrible acceleration, and i get horrible gas mileage under 10 on HIGHWAY. (and that was before i changed out the air bleeds, so if I used those now it would be a lot richer). Even 55/73 is too rich as my plugs were black with this combo.
So far with my 55/71 combo i get 20mpg highway, but i feel its just a tad to lean so i'm gonna try a 72 jet next, hopefully thats the 1.

The stock combo for my engine was a 52c rod and 70 jets. thats a difference of 18.