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Carb/Distributor Suggestions 85 K5 305

H

Hobbes9112

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Hello all-

It has been quite a while since I have posted in the forums. I have an 85 GMC Jimmy with the 305 and the California emmissions package. As soon as gas prices went up, I stopped driving it. My daily driver, a 1996 S-10 is starting to give me trouble. So far this year I have replaced the water pump, fuel pump, ac compressor and now the ABS unit has malfunctioned. The truck does horribly in the rain and snow. I am thinking of selling it and keeping the Jimmy, but if I do, I will have to do something about the gas mileage. On the highway the Jimmy is only getting about 13-14 mpg. This seems way too low for a 305. I am wondering if this could be due to a malfunction in the computer controls for the carb.

I am thinking of gutting the current system and replacing the carb and distributor. The factory quadrajet that is on the engine now is about 800cfm and I was thinking about something more like a 500. Any thoughts about which carb and distributor to buy? What kind of gas mileage increase should I expect? I am working my way through college and I don't have a lot of cash, so I really need to get rid of one of these vehicles. Thanks!
 
If it's a stock motor I'm for keeping things factory.

The qjet is rated for up to 750cfm, but the way it is designed it works just fine on a 305.

I would stick with the HEI, again it is more that sufficient for a stock motor.

I would say rebuild the qjet and do a tune-up. Plugs, wires, cap and rotor.

13-14mpg doesn't seem that bad to me, but I've never had a K5 with overdrive. I'm happy that I'm getting 11mpg.
 
mrk5 said:
If it's a stock motor I'm for keeping things factory.

The qjet is rated for up to 750cfm, but the way it is designed it works just fine on a 305.

I would stick with the HEI, again it is more that sufficient for a stock motor.

I would say rebuild the qjet and do a tune-up. Plugs, wires, cap and rotor.

13-14mpg doesn't seem that bad to me, but I've never had a K5 with overdrive. I'm happy that I'm getting 11mpg.
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Thanks for the suggestions. I am not concerned about keeping things stock on this truck, at least with the things I can bolt on and take off with no major modifications. I am driving over 300 miles a week back and forth to school so the gas mileage is a much bigger deal for me. I will just put the factory parts in the attic for the next owner. Getting 13-14 (on the highway) in 2 wheel drive is bad enough, I can't imagine how much worse it will be in 4 wheel drive.

I read somewhere that this carb is a 795cfm... that could be wrong. But even a 750 seems like way to big of a carb for a 305. I understand that it was designed to work that way, but I am thinking that the computer controls are busted. The previous owner messed with the carb, drilled out the rivets on the choke and busted the adjustment screws loose. If the computer controls are busted, the computer should default to full rich. I think that is the problem.

Thanks again for the tips, I might go grab a distributor cap and wires. I am pretty sure the plugs and wires are new. The engine only has a few thousand miles on it and for some reason the previous owner decided to throw the old carb and distributor back on the engine without making sure they were working properly.

Any other suggestions for carbs or distributors so I can ditch this computer controlled system and increase my gas mileage?
 
If you keep out of the throttle , its only running on the very small primaries . The secondaries only open as needed , if the engine only flows so much , thats how much the carb flows . Thats why the Q-jet works on small motors :D
 
That probably is an 800CFM carb. 305 trucks got 800CFM carbs from what I've seen.

Does your check engine light work? With the CCC system, you should probably see the bulb come on when you turn the key to run, then off once it starts unless the ECM sees problems.

They are pretty robust systems, but the TPS is a KNOWN problem with these carbs as they age. I would definitely test it first thing.

It also might be as simple as a bad bulb and a dead O2 sensor if it is old. If you feel inclined to try a new one, get a Delco.
 
roadnotca said:
Man, the Camaro is sharp; use it as equity and get a loan, not.:bow:
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:D Thanks!!! If I could keep one of my trucks running for more than a few months, I might be able to save enough money for my new engine.
 
dyeager535 said:
That probably is an 800CFM carb. 305 trucks got 800CFM carbs from what I've seen.

Does your check engine light work? With the CCC system, you should probably see the bulb come on when you turn the key to run, then off once it starts unless the ECM sees problems.

They are pretty robust systems, but the TPS is a KNOWN problem with these carbs as they age. I would definitely test it first thing.

It also might be as simple as a bad bulb and a dead O2 sensor if it is old. If you feel inclined to try a new one, get a Delco.
Click to expand...

The check engine light works as far as I know. I have no idea what sensors have and have not been replaced. The engine and tranny have been rebuilt recently, but the previous owner definitely was not a detail man. When I took everything apart to replace the heater core, the computer was sitting loose under the dash with all of the plastic mounting tabs busted off. I have no idea if it even works anymore. I know next to nothing about these computer controlled systems. Would the bad O2 sensor cause it to default to full rich? Is there a simple way to test the TPS?
 
Other than physical construction, I don't believe the TPS sensor on the CCC carbs is any different in operation than other TPS', which means any TPS test should work.

You might head over to oldsgmail.com or the monte carlo (spots.ca is the email list) website and see if there are any tech articles on testing CCC TPS.

If no luck finding it, let me know, I'll put you in touch with a guy that knows all about those carbs.

If the ECM fails, the system DOES go full rich on the primaries. My Olds 307 with bad ECM was getting 10MPG no matter how I drove it. (bought it that way)

First things first, you should have a check engine light when you turn the key to run. If you don't, that MAY indicate an ECM failure. If you've got a check engine light in that condition, you should hear the MCS clicking in the carb if you pop the hood and listen. I don't recall if that is on a timer and shuts off after a few seconds, or keeps clicking as long as the key is in run.
 
dyeager535 said:
First things first, you should have a check engine light when you turn the key to run. If you don't, that MAY indicate an ECM failure. If you've got a check engine light in that condition, you should hear the MCS clicking in the carb if you pop the hood and listen. I don't recall if that is on a timer and shuts off after a few seconds, or keeps clicking as long as the key is in run.
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Went and picked up the Jimmy today. It has been sitting in the driveway at my Grandma's place since gas prices went up. The check engine light does not come on when the key is in the "run" position. The only lights that come on are "Choke" and "Brake". What should I check next?

If the ECM really is bad, would I be better off to convert the system to a non computer controlled carb and eliminate the electronic spark control distributor?
 
Is there even a check engine light place on the dash? I'm thinking that may be tied into the choke light? If there is a check engine light, either the bulb is bad, or there is something wrong with the ECM or wiring to the bulb.

It's ok to replace the carb, but once you figure out the problem, an easy fix might simply be to replace the ECM. Car ones are probably quite similar if not exact (minus the calibration data) but that would require a bit of research to determine.
 
The 85 Factory Service Manual says there is a check engine light. I ordered an ECM from O'Reilly. It was only $45, so i will plug it in and see what happens. If that doesn't fix it, I will check the bulb and wiring too. Considering the shape of the current ECM, I am really hoping that is the problem.

I will let you know what I find out tomorrow. If you guys think of anything else in the meantime, let me know. I will be sure to check the boards before I head out to work on this thing tomorrow.
 
Be careful throwing parts at this. The system isn't that bad, but when it fails, it isn't really worth spending much money on.
 
dyeager535 said:
Be careful throwing parts at this. The system isn't that bad, but when it fails, it isn't really worth spending much money on.
Click to expand...

I agree with that. You can get comparable performance with an aftermarket carb and distributor for a lower price. The parts on this system can add up to a big chunk of change if you just start throwing factory replacement parts at it.
 
dyeager535 said:
Be careful throwing parts at this. The system isn't that bad, but when it fails, it isn't really worth spending much money on.
Click to expand...

I agree 100%. The plan at this point is to check the ECM and the check engine light wiring. If that isn't the problem, I am probably going to replace the carb and distributor. I just thought I would try the ECM since it was under $50 (assuming they ordered the right part).

The problem I have is that I don't really know if I am going to keep the Jimmy. It is all going to come down to gas mileage and I won't know much until I replace the carb and distributor unless I run into someone here with a similar set up and get some numbers from them. I don't want to spend the money to replace them if I am not going to keep the truck. I will let you guys know what happens after I plug in the ECM.

Does anyone have any idea what kind of mileage I would get on the highway with the 305, in 2wd, overdrive, with a different carb and distributor?
 
Whats your gear ratio? Probably won't make much difference, but here's what mine used to be:

'83 (305/700R4/208/3.08/31" tires) 17MPG best on the freeway, 10-13 city.
 
dyeager535 said:
Whats your gear ratio? Probably won't make much difference, but here's what mine used to be:

'83 (305/700R4/208/3.08/31" tires) 17MPG best on the freeway, 10-13 city.
Click to expand...


I have a 3.73 rear and I will be running lock outs on the front. Your city mileage is about what I am getting on the highway right now. Which carb and distributor were you using?

By the way, the store ordered me an ignition module instead of the ECM so I won't get to do my little experiment until Monday or Tuesday. I am going to check the wiring on the engine light tomorrow. I will keep you guys posted.
 
Time for a really dumb question...

I started working on the truck today (this time with the right parts. I can't find the ECM. I found the electronic spark control behind the glovebox, but I have no idea where to look for the ECM. I have a factory service manual and I haven't had any luck. Page 6E8-10 shows the ECM on the passenger side. All I have found is the ESC. Pg. 6E8-77 shows removal instructions and diagnostics for the ECM, but it doesn't tell me where it is.

I am starting to think I am blind or worse yet, this thing is hiding behind all the duct work (some of the diagrams look like the ECM is near the center of the dash). Would someone please tell me where this thing is???

Thanks!
 
Only place I know for the ECM is above the glove box on the underside of the metal dash piece. If it's not there, look over near the gas pedal. Thats also where the cruise control module is mounted, but there aren't many other places to hide the ECM.

Not sure why there would even be an ESC module with an ECM though...
 
Carb and distributor were both stock for 1983, 800CFM Q-jet atop the 305, and the distributor was "rebuilt" with one of the recurve kits from Moroso. The weights were binding, but even after fixing that, the mileage didn't change.
 
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