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Carb question(just humor me....)

johnatthebox

johnatthebox

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Vallejo, Ca.
Silly question? Maybe, but Im askin anyway....
Okay, lets say someone has a carb that is a little too big for his(or her) motor-
It runs good with it, but it just doesnt need that much cfm for daily driving-
Would smaller jets on both sides make it act like a smaller cfm carb?
 
No. But keeping your fat foot out of it would

What size carb? The factory Qjet is a 750, which is too big for a stock 350
 
Wouldn’t be worried. The jetting is controlling the air fuel ratio. Get that, the accelerator cams and squirters working properly and it’ll be just fine

Early 454s came factory with 850 Qjets
 
It probably is. Check all your timing and ignition settings and tune the bog out.

Learn to read spark plugs and keep a good ear. You can hear what the engine is doing
 
Vacuum gauge and an wideband air fuel.

Learning how to read plugs will kind of substitute the AF gauge but the vacuum gauge is very handy
 
Thats how I tune it now(vacuum guage). The plugs look good- a little bit of dark but clean electrode and its not fouled
 
I dont trust my timing tab. It was all bent up when I got the motor. I set the time where I get the highest solid vacuum reading. That works , right? Seems to......
 
It’s not necessarily wrong

Use the vacuum gage to get a reading on the differential between idle vacuum and vacuum at about 2000. That will help select the right power valve.

The other trick is partially shrouding the primary side with one hand and running the butterfly with the other. Shrouding mimics a richer condition, so if it seems to be bogged, it’s probably still rich. If when you shroud it, it actually picks up rpm, it’s probably too lean.

Play with it let us know
 
Bent77 said:
It’s not necessarily wrong

Use the vacuum gage to get a reading on the differential between idle vacuum and vacuum at about 2000. That will help select the right power valve.

The other trick is partially shrouding the primary side with one hand and running the butterfly with the other. Shrouding mimics a richer condition, so if it seems to be bogged, it’s probably still rich. If when you shroud it, it actually picks up rpm, it’s probably too lean.

Play with it let us know
Click to expand...
Thanks for the info! I will.....
Just to clarify: I should be adjusting the jet size, or the air bleed screws here?
 
Keep in mind when the primaries open, three circuits can activate at once: the jetting, accelerator cam, and the power valve. You may need to make adjustments to each of the three to get it clean. Go one at a time and be patient
 
Bent77 said:
Keep in mind when the primaries open, three circuits can activate at once: the jetting, accelerator cam, and the power valve. You may need to make adjustments to each of the three to get it clean. Go one at a time and be patient
Click to expand...
Got it. Which first though?
 
Without being there hard to say. But the cam is easy to move by just pulling the screw and moving it. Go one way, try it. If it’s worse, go the other. You may need a different color cam, but not usually.

If you pull the bowl(s) off, Vaseline the gaskets so you can pull the bowls without them sticking. I usually go up/down 2 number sizes at a time
 
My older brother tried running an 1150 CFM Holley Dominator on his 327 in a '68 Impala that had over 10:1 compression and a hotter than stock cam and it actually ran very well,it didn't run rich or foul the plugs or soot them up at all..it was more an experiment to get by with it until he got a suitable smaller carb for it,but he was surprised it ran good and didn't seem to be over carbed ,without doing anything as far as swapping jets or any other parts around on it..

A Q-jet is kind of "self liminting" as far as using a larger CFM rated one on a smaller V8,the secondary air valve only opens as far as engine demand makes it open,so the metering rods and secondary barrels wont open 100% if the engine doesn't need or have enough capacity to need all the fuel & air it can deliver..it was not uncommon to see the 800+ cfm ones used on 305's and 307 olds engines..
 
UPDATE;
Okay, Ive been tryin some stuff, like chanching the jets (went down to 68 and up to 72 stayed with 71) and have it to where it idles ok, starts ok, pulls ok BUT- if I mash on the pedal hard like Im drag racing, its like everything stops for 1 second and then kicks in. I checked the front squirter and its squirtin ok. I advanced my timing a bit cause it seemed to like it better.
 
That is all on the accelerator circuit. Cam/accelerator pump/squirter

Sometimes to can put your hand over the primaries and work it and get an idea what’s happening. Or change the squirter. Sounds like a bog, too much gas too fast
 
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