rochester q-jet information

[cerryblazer]

Hi guys!
a question please...
can i bend some linkage of my rochester quadrajet to early open the secondary opening?!
time ago it has been modified, i think bending linkage and now it opens just a few of millimiters and only when i push down accelerator to the floor.
i'd like to have at least an opening of 45° not lus 5°!!

it's a 305 engine and carb is a Rochester Q-Jet M4MC or M4ME, they are very similar but my carb has rear vacuum break connected to secondary opening so i don't know if it's possible bending linkage.
thank you very much guys!
Fabio

 

[Hogback Fabrication]

The secondaries won't really open with the truck sitting there, once you really put load on it (when driving the truck for instance) they will open.

None of that linkage is meant to be bent.

 

[Greg Ducato]

Where the secondary top butterfly shaft extends through the carb top on the passenger side there is a small screw/spring assembly that adjusts the tension on the secondary butterflys relative to vacuum. If the bottom butterflys are not opening then you may have an interference of the gasket on the base plate or a choke linkage that is preventing the secondaries from opening.

 

[Cornfield creations]

Yeah check the butterfly shaft on the bottom side. There should be a stop on the base plate. If that opens up all the way then it should be good. The upper butterflies operate off of engine vacuum. Those will only open during on a high vacuum. Check the choke pull-off to see if it is operating correctly. If it is bad they won't open or they will only open partially.

If you replace the choke pull-off, you need to adjust it so it takes 3-5 seconds for it to fully open. If it opens too fast you will have the infamous quadrabog. It needs to open slowly to build power.

 

[cerryblazer]

ok!! thanks guys!
finally i found the little screw (really small and hidden!) and i adjusted it just 1/4 turn on. i think it's enough.
in the beggining i didn't see changes, but with truck running, sound of secondary opening is unmistakable!
i don't know how many degrees is opened, because with truck sitting there, as you told me, butterfly opens always for a little bit, but certainly faster.

if i can, i have another question about carb...

for idle adjustment, haynes manual says:
idle speed for 305 engine:
high 1600 and low 600 RPM

600 is final idle, right!? when i turn idle speed, after adjusting mixture screws at the maximum engine speed?
and 1600!? fast idle adjustment on the left!?

thanks again
Fabio

 

[Greg Ducato]

Fabio, I would think that idle speed is the hot idle rpm in gear. I usually use a vacuum gauge when adjusting idle mixture, shooting for highest and most steady vacuum signal, others may have an easier/better way. The fast idle adjustment is on the choke side of the carb.

 

[cerryblazer]

hi Greg!
so tell me if it's correct...

- transmission in D with parking brake
- i turn back mixture screws until maximum engine speed is reached
- then i will turn idle speed (on the left) until initial speed, 600 rpm in my case correct?
at this point manual says:
- "..now turn each idle mixture screws, until idle speed is as shown for lean drop in the spec...."

so...if initial idle speed is 600 rpm, fast idle speed is 1600, how many rpm has idle speed for lean drop?!?

certainly i must use a tachometer to adjusting idle speed and as you told me, i must use vacuum gauge to check the right balance between air/fuel, if vacuum gauge needle rises, means that there is more air than fuel, right Greg?!
thanks Greg!

Fabio

 

[Greg Ducato]

Fabio, it sounds like you have it figured out. The engine will go to fast idle when you depress the throttle before starting the engine when it is cold. Once the engine is started, depending on ambient temperature, you would "kick" the throtle to idle down, The choke ramp has a couple of steps so it may not kick down to hot idle for a few minutes depending on choke setting. A change of seasons means a change in choke settings but don't wear yourself out on exact rpm settings, you will know by sound and feel when it seems right as much as the tach will tell you, your engine may need a few more rpm depending on state of tune and modifications.

 

[cerryblazer]

Hey guys!
maybe 305 engine is a junkyard engine as many people said me , but now i think it's running tremendly good!
i spent 2 hours to setting in the better way my q-jet, in the end this is the setting:

- mixture screws : 2 3/4 turns from zero point.
- idle speed : 600/700 rpm in D position.
- 1/4 turn for secondary opening adjustment screw
- just a problem with fast idle screw... i set the screw with cold engine because it works with cold engine, but i know maybe it's better with warm engine.

now when i start engine, it runs around 1500 rpm as spec., but if i push down accelerator, it's not smooth. i have to wait just 1 minute or push down again.

however it's fun to see how many new Fiat 500 "supercars" i can burn all together!
thanks!
Fabio