Carb selection

[greggome]

Any opinions on a replacement carb for a 68' 327/350 stock spreadbore manifold? I have a 650 cfm holley double pumper on now. Its worked fine for years but its been giving me some issues. I am considering replacing it with either a new Holly double pumper spreadbore or a rebuilt quadrajet.

 

[Hib Halverson]

On a stock 327/350, I'd use either a Rochester Quadrajet or a vacuum secondary 650 Holley.

 

[tshera388]

I find that the 600 CFM Edlebrock works very well in this application. You may need to change metering rods and/or jets to tune it to the engine.

 

[kpic]

greggome:
As you had a 650 CFM double pumper Holley for years; I'd stay with what you have.

Holleys can be rebuilt also; so why pay for a rebuilt Rochester?

 

[minifridge1138]

greggome:
As you had a 650 CFM double pumper Holley for years; I'd stay with what you have.

Holleys can be rebuilt also; so why pay for a rebuilt Rochester?

+1 rebuild before you replace

 

[kpic]

+1 rebuild before you replace

Exactly and it's a lot less expensive than replacing..

AFA mechanical or vacuum secondaries, personally or back in the day of cheap leaded gas, I preferred the mechanical with dual shooters or accelerator pumps.. That being said, vacuum secondaries get better MPG and as they were vacuum controlled the same flow adapted well to a variety of engine sizes and requirements. The chief advantage of vacuum controlled secondaries is it's almost impossible to over-carburete an engine. Even with the softest diaphragm spring.. IMO, the British SU's variable slide operated by the engine vacuum and was one of the best.

Mechanical secondaries are a bit more complex to setup and double pumpers (2 accelerator pumps) were the usual. Typically, the secondaries would hit at ~40% of the primaries opening.. Without the dual pumpers, typically the engine would bog at low RPM.

To be honest, Hotrod.com says it better than I can..
"A mechanical-secondary carb’s enrichment characteristics are regulated by the accelerator-pump capacity, the accelerator-pump cam configuration, and (most important) the accelerator-pump nozzle-size (aka the pump “shooter” or “squirter”). All can be tailored to fine-tune the fuel curve, developing smooth linearity with no bog or hesitation in a performance motor. However, dialing in a double-pumper requires more effort and know-how than the more forgiving controlled secondary design. In summary, mechanical-secondary carbs are much more sensitive to engine size, drivetrain gearing, vehicle weight, and rpm range compared with the controlled secondary configuration, which is why the double-pumpers are offered in many more cfm ratings than the vacuum-secondary carbs."

In all honesty, with the cost of gasoline today, vacuum secondaries are probably the better choice. Unless, it isn't one's daily driver and all one wants is speed...

 

[GTR1999]

I haven't bought a new Holley in 10 years, the last being a 670 street avenger. I had to tune it out of the box but it works pretty good. I have heard recently they are another brand outsourced and the QC isn't the same. I can't back that up but I wouldn't be surprised. I ran a Q-Jet for years on my 69 & 72 and they worked better then the Holley's.

 

[kpic]

Where I used to work, a co-worker former employer was Demon. According to him, basically Demon cloned the Holley and fixed its well known short comings.
However, as I have no personal experience with the Demon carburetors; I don't know if they are worth pursuing or not. Perhaps, someone who does will weigh in....