Carb for 1985 305

[Hobbes9112]

I am getting ready to replace the carb on my 305. I only use my Jimmy for on-road use. What would you guys recommend? How many CFM for decent gas mileage? I read that the stock carb is over 750cfm, but I am thinking about something under 600? The current carb is computer controlled, but it isn't working. It runs super rich. The Jimmy is California equipt with the high altitude package. That doesn't do me much good here in Kansas. Suggestions?

 

[PhoenixZorn]

Already replied to this question in a hijacked thread... but here it is again...

How, praytell, is it computer controlled in a 85?? I may be electric choke, but it surely can't be computerized. Yeah, the Q-Jet you have on it right now has a 30psi pump and 750cfm air flow... the best bet is to drop a Holley 600 on it, you'll get more fuel and a bit less air... and it can be calbrated to run the way you want it...

By the way, how is it running rich with 750cfm air flow?? It should be running lean.

 

[dyeager535]

And my response from the other thread as well:

CA models for at least a few years have the CCC system.

Q-jets are 250CFM on the primary side for 750CFM variants, but I'd be real surprised if the one on his 305 isn't an 800. (300CFM primaries)

Cruising is the only place primary CFM really matters, at 300CFM on the primaries, that's the same as a 600CFM squarebore. No gain in economy or performance based on CFM alone.

GM apparently learned a long time back that more CFM's=more torque, because small CID V8's started getting 800's (305 Chev and 307 Olds) while the larger engines had 750's.

Could be a few things on the CCC system causing it to run rich, the failsafe mode for that carb is full rich on the primaries when the mixture control solenoid loses control for whatever reason.

 

[Hobbes9112]

It looks like the last owner took the caps off the mixture screws and busted them loose so he could adjust them. From what I have read, they shouldn't be accessible... I am wondering if that has anything to do with the computer going into failsafe. Whatever it is, it isn't working properly. Am I better off replacing the carb with an Edelbrock or Holly or should I try to fix the sensors and run with the computer controls? I don't have to meet strict emmisions standards here and the high altitude settings aren't doing me any good. I am most concerned about gas mileage and about 85% of my driving is highway. Thanks!

 

[dyeager535]

Did you figure out high altitude from the stamped in carb ID or?

Was unaware that you could have a CA and HA carb one and the same...figured one was Colorado, one was CA!

Been a long time since I had to mess with one of these. If you move the key to run, without starting it, does the carb click at all? I believe it's supposed to click, thats the MCS working. When my CCC system "failed" (first bad ECM I've ever had) the MCS didn't click. Replaced ECM and the clicking resumed, but I pulled the motor immediately afterwards, so no clue how it would have run. I DO know that I was getting 10MPG on the freeway in a 3500lb car with a 307 and OD, so you can see what kind of MPG you get with the primaries full rich.

Just give the wiring a once-over, see if anything in particular is messed up/disconnected. Check vacuum lines too, you have a MAP sensor.

If it's not something obvious, since the system has been tampered with, it's PROBABLY not worth spending time/money to fix. You'll need a non-CCC Q-jet, and a non-CCC distributor. But if you ever plan on moving, and taking the truck with you (or selling it) to an emissions controlled area, removing it is a bad idea.

Think of it as a TBI system without the injectors.

 

[Hobbes9112]

Did you figure out high altitude from the stamped in carb ID or?

Was unaware that you could have a CA and HA carb one and the same...figured one was Colorado, one was CA!

Been a long time since I had to mess with one of these. If you move the key to run, without starting it, does the carb click at all? I believe it's supposed to click, thats the MCS working. When my CCC system "failed" (first bad ECM I've ever had) the MCS didn't click. Replaced ECM and the clicking resumed, but I pulled the motor immediately afterwards, so no clue how it would have run. I DO know that I was getting 10MPG on the freeway in a 3500lb car with a 307 and OD, so you can see what kind of MPG you get with the primaries full rich.

Just give the wiring a once-over, see if anything in particular is messed up/disconnected. Check vacuum lines too, you have a MAP sensor.

If it's not something obvious, since the system has been tampered with, it's PROBABLY not worth spending time/money to fix. You'll need a non-CCC Q-jet, and a non-CCC distributor. But if you ever plan on moving, and taking the truck with you (or selling it) to an emissions controlled area, removing it is a bad idea.

Think of it as a TBI system without the injectors.

Thanks for all of this great information. I have zero experience with computer controlled carbs so I really appreciate it.

The carb doesn't click when you switch the key to run. I just bought the Jimmy with about 1000 miles on the engine and tranny rebuild so I plan on keeping it for a while. I am a student and I will be here in the midwest for school for at least another four years (knock on wood). I think I will be better off replacing it with non-ccc than to invest in replacing the sensors or ecm. Do you know what is involved in disabling the computer controls for the carb and distributor? I think I need the ecm for the other sensors and I read someplace that the lock out on the transmission isn't vacuum controlled on California models. As far as the high altitude goes... the sticker on the glove box and engine compartment list it. It is making diagnosing the problems interesting.

If I do switch to a regular carb and distributor, I will keep all the other equipment and let it go with the Jimmy when I sell it. That way it can be switched back later.

What size quadrajet would you recommend?

 

[dyeager535]

On lockup, no one has ever really spent the time to investigate if that is ECM controlled or not on the trucks. It is on CCC cars. Thats all it is for our years of 700R4, lockup. Everything else is still done "manually". Any non-CCC 700R4 truck will net you the parts to convert the transmission to non-ECM lockup. The lockup vacuum switch is on the drivers side of the engine compartment, near the brake booster IIRC. (thats if it's present)

Nothing short of the TCC (transmission locking torque converter) is going to be tied into the ECM, so that won't be an issue. Oil pressure and coolant temp gauges use their own sensors independent of the ECM sensors. Those are two that might give you pause. EGR, AIR, and EVAP might have switches for them, you may or may not run into that. Just spend time when pulling the system, and don't cut anything. It's not necessary and saves you from cutting wires that you need.

On the carb, if going Q-jet, either the 750 or 800 will work, running whatever you find won't hurt anything. Going to need some fine tuning to extract as much mileage as possible out of a carb you get from another application (or new) but it's not real difficult. Hard to get perfect, but acceptable is pretty easy.

 

[Hobbes9112]

If I buy a new or rebuilt carb, is there any advantage to buying a lower cfm carb? 750 or 795 cfm just seems like a whole lot of carb for such a small motor.

 

[PhoenixZorn]

From my research in the last few weeks... I've noticed that rebuilt Quadrajets cost as much as or more than a similarly rebuilt Holley... The Quadrajet is great for what it was made for, but I think you may be happier with a small CFM Holley like a 450, 550, or even 600... they are easy to tweak to get everything running right, and they look a hell of a lot nicer...

 

[dyeager535]

If buying new or rebuilt, yes, I'd agree aftermarket is fine, cost is probably comparable, if not less.

Not that I had ANY complaints about my Q-jet on the street (18MPG freeway with a 350 and no OD) but if not rebuilding one yourself, cost is crazy on them IMO.

 

[beater_k20]

having driven QJet, 2Jet, Holley, and Edelbrock/AFB carbed trucks, i would say hands down the QJet will win on all cases. the design of the fuel bowl allows them to operate on more severe angles than other carbs, the small secondaries offer greater throttle response at light throttle, the "toilet bowl" secondaries are massive, and may be considered overkill for a small engine, however what most people fail to realize is that a properly adjusted QJet will only allow as much air/fuel as the engine asks for. so while it may be rated for 795cfm, if the engine is only asking for 500cfm at WOT the secondaries will only open enough to satisfy the engines demands.