yeahbuddy

Looking to replace my current 7028250 and I got my hands on a pretty nice condition 7028251 Quadrajet that needs a rebuild. I know it was made to go on the 400/455 so my question is, what would I be required to do in order to get it to 350 specs? I heard that the jets are larger for the 251 vs 250? Thanks

bccan

http://www.442.com/oldsfaq/ofcrb.htm#Carbs

According to carb calibration table, primaries are the same w/ both carbs 71/49. Different secondary rods - AT vs AU.

yeahbuddy

Would I run into performance issues I this carb on a 350 without changing the secondary rods? Other than poorer gas mileage.

Rickman48

A larger cam, stall convertor, and steeper gears would take advantage of the richer mixture @ WOT!
If you're not going to be 'pounding' on it, you should be fine!

Make sure you check the primary throttle shaft - wear gives you a vacuum leak!

yeahbuddy

I use it as my daily driver. I like to punch it here and there but who doesn't. Consider myself light-footed about 80% of the time. I just dont want to run into anything that could be avoided while rebuilding.

Rickman48

Rich is better than lean - you're good to go!

bccan

Rich is safer but response generally mushy if the secondaries are too rich - a long drawn out drone "wooooooooooOOOOOOOOOOOOO," but not a lot of response. If secondaries are closer to correct/lean, response will be quicker & much more noticeable "wooOOOOAAAAAHHHH!" It will certainly work w/ what you have but you might want to hunt down some "AT" or "CL" sized rods & swap them in. http://www.oldspower.com/vb/showthread.php?t=32057 will provide a table of secondary rods, find AT & CL on second page. Power tip size is .667" (vs "AU" rod's .527"), larger is leaner & you want a long or medium tip length. AT or CL is the best choice as there is a "hole" in increments w/ the long or medium tip right in the range that would probably work best for you. I have hit that same hole on a carb I'm dialing in for my kid's red car in sig.
Very easy to do (1 min job), if switch is done, you may find a slightly tighter air door adjustment will be needed to prevent a bog. That adjustment is also very simple & quick (1 min job) If you are unfamiliar, search around & procedures should be outlined.

While editing I just saw Bill's comment about Pwr Piston spring - good point! Do you have the ability/willingness to do some disassembly? You need to pop the top off carb to make this change & you would need a lighter spring & maybe a gasket. You may have a pretty rich idle if this detail is not attended to.

BILL DEMMER

what may bite you is the power piston spring, the 251 spring is much stiffer and will likely over-richen your primary mixture. the 250 uses a 7036019 spring which has a cut-in point roughly half of the 7037734 used in the 251.

everybody always overlooks this critical detail.


bill

yeahbuddy

Great info thank you very much! Just what I was looking for. I plan on buying a rebuild kit, CL secondary rods, accellorator pump and float from quadrajetparts.com. They dont carry specific power piston springs. Any idea where I would be able to find one? Is it something I am going to be removing while rebuilding anyway? Thanks again

bccan

The power piston springs typically come in a 4 pack of different tensions which correspond to your engine's vacuum demand/production(chicken/egg). You can test them & find one that is lighter than what comes out of there. Check your choke pull off (little vacuum can near choke linkage) & make sure it is good before ordering parts. This would be apart when you go through it, under the primary power piston that has the primary rods hanging off it.

Bill - Do you have any vacuum specs that correspond to the OE spring part #'s?

BILL DEMMER

if you want a rochester 7036019, i can supply it.
iirc, the cut-in point is 8" HG. the 7037734 cuts-in about 14" HG.


bill

yeahbuddy

Thank you everyone who posted.

I had one more question. I purchased this carb from someone who has no knowledge of its history. Its overall condition based on looks alone is cleaner/better shape than the QJet that is currently running on my car. If I were to want to test it before rebuilding, should I just blow compressed air through the whole thing? Or possible spray everything with carb cleaner? Thanks.