Carb Guys...Need Help...Please!

[cheyennesuper]

In my '73 blazer on my 350 lies a pos holly.I think its shot because no matter how much i turn the mixture screws any which way it does nothing.I was trying to adjust it by the book too.!Its so rich my eyes really burn.Well my buddy just gave me an old Rochester Q-jet but hes not sure what it was taken from.I ran the id# 17080272 and it sais that the carb is a 1980 49state Q-jet made for pontiac division...and these guys say its for a pontiac with a 301 140hp
http://www.carburetion.com/Number/oemnumber.asp .I'm pretty sure its 800cfm because of the prominent bumps in the venturies as shown here
http://dyeager535.topcities.com/qjetidentify.html .I was told that 800cfm is way too much for my engine(350).Is this true?My truck has headers+dual flowmaster exhuast and NO catalytic converters so it all gets out pretty good.Does anyone know if this will work on my 350 in my 79 blazer?

 

[Cricket]

Make sure you don't have an intake manifold leak. If your eyes are burning that badly when you are under the hood and the carb is not adjusting at all then the intake leak is a real possibility.

 

[cheyennesuper]

I thought of that and sprayed around the manifold but did not notice any changes.I'd rather just put on a Q-jet anyway but the one i have is 800cfm and i think its too much...

 

[pauly383]

Do you think your Holley has blown a power valve ? Sell it and run the Q-Jet . I don't think its too big for youre engine , you will only use what you can . My 305 has a 750 Q-jet , pass emissions , get decent mileage for an old brick .

You just need to get an adapter fitting to point the fuel inlet sideways like a Chevy style Q-jet

 

[dyeager535]

I wouldn't worry too much about it being an 800, although as you are thinking, it's not likely necessary on your engine.

You may want to pay attention to see if the truck surges once you put that Q-jet on it. That 301 I'm sure had EGR, and my experience on carbs that EGR has been removed, is that they will surge while cruising on the street. Richening the primaries up a bit has always solved that issue for me.

 

[Hogback Fabrication]

Don't be fooled by the 800 CFM rating (though, I would have thought it would be a 750). The reason an 800 CFM Holley would run poorly on your engine is due to it having the same size primaries and secondaries. The Q-jet has much smaller primaries with huge secondaries which make it much better for streetability, mileage, not to mention at angles.

 

[cheyennesuper]

So 800cfm is fine then?Is there even such thing as too much cfm?

 

[original balzer]

exactly if it was on a 301 why would it be too big for a 350???? lol but yea the small primaries will work and personaly i love q-jets you just have to learn how to rebuild and ajust them correctly

 

[cheyennesuper]

Ok...i just don't know much about carbs....and the dxxxhead at autozone told me that 750cfm or higher would drown out my engine....whatever that means..But I believe you guys know more then some pencil-pushin-so-called-parts-expert like Autozone employees.....I'm gonna rebuild this 800cfm Q-jet and try it out.Thanks guys.

 

[dyeager535]

All of the Olds 307's in the 80's that I have seen are 800's as well, so it's not unusual that a 301 was using one.

All of the 80's 305 truck Q-jets I've seen are also 800's.

IIRC from a Car Craft article a few months back, the larger the carb, the more power, generally. It seems kind of contrary to popular belief, but I remember David Vizard found the same thing.

 

[diesel4me]

They used the 800 CFM Q-Jets on most of the 305 chevy 4 barrel motors factory--as already stated the Q-Jet carb is a spread bore(small primary barrels,large secondaries)and is set up so the secondary barrels only open as far as the motor can suck them--the more cubic inches,the more the motor can "inhale" and pull the air valve open and open the metering rods for the secondary barrels-..I have no idea why GM thought such a big carb was needed though--I've run carbs as small as 390 CFM 4 barrels,in many cases the smaller carbs performed better and got better mileage (on the street,not talking dragstrip performance),and I think the motor will last laonger if its not constanly "gas washed" by an oversized carb or too rich a jetting..the Q-Jet is more forgiving due to the way the secondaries work...

As for what CFM your motor needs,there is a formula to figure it out you can find at edelbrock.com --I didnt have it handy or I'd post it for you--but 600 CFM is plenty if your talking a holley or edelbrock carb--anything bigger would probably be a waste,and it will run more economically with a smaller carb,if however you have a hot cam,headers,hi-rise intake and other "goodies" the larger carbs may be better suited for that application----

I had an oldsmobile 455 V8 (1968) that had only a 2 barrell carb(!) and it would smoke the tires like nothing,and still got 16mpg on the highway!--my 307 with a dinky "economaster" 2bbl carb cant even do that well!--And my brother tried an 1150 dominator holley carb on his 1962 vette 327 powered 68 Impala when he rebuilt it for a freind just to see if the floats were set correctly,etc,before he returned it to him--we thought it would be way too rich--but we drove it 65 miles to my parents house on about the same amount of gas as his holley 600 that was on it before--I guess the the bigger carbs let more air in as well as fuel--it went like a raped ape too--he hated to take it off !!..But bigger aint always better on carbs--the 800 CFM Q-Jet will be fine for your application if GM felt it was ok for a 301......