oldsconv

Any issues using a 7040250 Qjet (from 350)on my 70 SX with 455? I swapped from the original 2 barrel. It has run fine this way for several years. Just wondering if there any downsides using this lower CFM carb and presumably different metering rods. That is, will the car have less power vs the correct 455 QJet, will it run lean and do engine damage, etc? It seems to work fine, but not sure if car should have more power - hard to tell as I have 2.56 rear end, but car seems pretty fast as is.

oldcutlass

No it will not damage your 455. Look at it this way, your car was originally happy with a 2v, why wouldn't it be happy with the 4v?

joe_padavano

Don't confuse airflow with fuel metering calibration. The original 2bbl was factory calibrated to meter fuel for the needs of a 455. The '250 Qjet is factory calibrated for the fuel needs of a 350.

Ironically, there is another thread about a guy with a 70 Supreme with a 350 who is currently running a 7041251 carb off of a 455. You two should swap.

https://classicoldsmobile.com/forums...ml#post1041363

allyolds68

I run a 7042250 on my 70 455. It took some tweaking of the secondary rods and secondary air door tension to get it to run well at WOT. It was running very lean and pinging. It think the primary rods need to go thinner as well.

oldsconv

Now that I think about it, it does ping sometimes. I assumed my timing was off or not enough octane. Do you know what metering rod sizes you Used?

allyolds68

i think they were CV's

olds 307 and 403

Any leaner in the secondary rod department is just too lean from my testing, even on a 307. Nearly every Qjet, mid 70's to mid 80's I had came with the CV rods. As shown in the factroy manual, get the 455 primary spec rods or even one richer if running ethonal fuel.

allyolds68

i pulled the CV's from a 73 350 carb. The 72 350 carb had CG's on it which were way too lean.

I still get pinging at lower rpm's, especially when I shift from second to third and nail it. My initial timing is set at 11 and full in at 2200 rpm's is 25 so it shouldn't be a timing issue.

RetroRanger

A chart I printed out from idk where shows the au and cv secondary rods are very similar w the only difference being when the secondary air valve opening is less than 20 degrees, after that they appear to have the same spec. The au secondary rods were used on most of the 69-70 455 carbs, the 72 455s had ch rods which are generally leaner.

Many 69-72 455carbs used a 69 main jet w a 50b rod or a 70mj w a 49b rod.

I have similar conservative timing settings and sometimes ping when stomping on it at speed. My secondary rods are cg so I'm going to try the cvs, the primary side is currently a mismatched set up so I'm going to switch to the 69mj 50b rod and see how that goes.

allyolds68

FWIW AU's and CV's are almost identical. My primary jets are 70's. I can't remember what my primary metering rods are but I remember that I had to mic them because nothing was stamped on them.

RetroRanger

Yes that's what I meant lol but I was typing ch instead of cv

The difference between the au and cv is probably not even noticeable it's so minor.

The ch on the other hand is much leaner especially from 70 to 90 degree of secondary air valve opening.