Firebird1984

I am going to try to rebuild my CC carb for my 1984 Firebird with the 305 V8. It's the stock Rochester Qjet. Has anyone tried to rebuild this electronic feed-back type of carb? Any insights or warnings that I should be aware of? How successful were you at rebuilding it?

I've rebuilt a number of carbs over the years, but not a computer controlled one. Just wondering what the rest of you have experienced in rebuilding one of these. Thanks for any insights! Brad

Damon

I've done quite a few of them.

BEFORE you remove the carb and start messing with it you need to make sure the 2 main "computer" components inside them are working correctly. The mixture control solenoid and the throttle position sensor.

The m/c solenoid is pretty easy. Turn the key to the "run" position but don't start the motor. You should hear a ticking noise coming from the carb. This is the m/c solenoid switching from rich to lean and bouncing the primary rods up and down. If it's not making noise the m/c solenoid is shot, or isn't plugged in (big 2-wire 90* plug on the top of the carb). Without this working it'll never run right. Check the plug to make sure it's in good shape- no damaged wiring or plug. If it's still not working you'll want to add the m/c solenoid to your shopping list.

The tps sensor you'll need a scan tool to diagnose. Again, key-on, engine off look at the TPS sensor readings. They should be around .5V at dead-idle and over 4V at WOT (open the throttle manually all the way while looking at the readings). The idle reading will usually be about right but if the sensor is dead you'll typically not get anywhere close to 4V at WOT. If yours is shot, put it on the shopping list along with the rebuild kit.

I won't go into great detail about the carb rebuild, since you should already be generally familiar with rebuilding them before you tackle one of these carbs. I'll hit a few of the critical areas:

Tools- you can't adjust anything other than the lean-stop without the proper tool designed to fit the adjusting heads of the various stops and adjustements. Don't know the part number for it but others can get that for you and tell you where to buy it. In general, if the adjustement block-off plate for that particular unit is still in place, don't mess with the adjustement. Factory setings are probably closer than anything you can do yourself.

Lean stop adjustement- set it 4 turns out from all the way down for starters. Do this before touching any other adjustements.

Rich stop adjustement- if the adjusting plug has not been removed, don't touch it, jsut run it like it is. If the block off plug has been removed you want to adjust this until the total travel of the m/c solenoid is 1/8" from full lean (against the lean stop, rods all the way down) to full rich (against the rich stop, rods all the way up). Measure total travel by removing the Idle Air Bypass valve and sticking a screwdriver down there- press it down until it bottoms, make a mark on the shaft of the screwdriver. Let up up and make another mark on the screwdriver. Measure the distance between the marks. Shoot for 1/8" total travel. Assuming you set the lean stop first, make all adjustements to the rick stop until total travel is 1/8". Always make sure the airhorn gasket is in place when making these adjustements as it adds about 1/32" to the height.

TPS adjustement- If the block-off plug is still in place, don't mess with it. If it's been removed you want to do the adjustement on the car, engine running, and hooked up to the scan tool. You need to make sure the idle speed and such is reasonably close before you set the final adjustment on this. It's a simple up and down kind of adjustement that affects both idle and WOT readings in the same direction so you'll probably have to adjust it for the best "compromise" between the two readings. If in doubt, always set the idle reading the closest and let the WOT reading fall where it may.

TPS "intermediate rod". SUPER CRITICAL little piece. It's a dowel about 1/2" long that goes from the underside of the accelerator pump arm to the plunger on the tps deeper inside the carb. Don't lose it. Don't forget to put it back in. Your tps will never work right without it. You want to install it from OUTSIDE the carb after you have finished the rest of the re-assembly. You need to remove the accelerator pump arm and then slide the dowel into it's receiver hole in the top of the carb and then put the accelerator pump arm back on. Don't try to do this by installing it inside the air horn and then installing the air horn on thecarb- you'll almost certainly wedge it at a funny angle (or it will jsut fall out) and your tps won't work correctly.

Idle air bypass valve. You'll need to access this even for a basic rebuild. It sits under a block-off plate on the top of the carb airhorn inside the "tower", held in place by 2 rivets. Drill out the rivets, remove the plate and you will see a large brass screw head under there. That's the iab and it adjusts your idle mixture, in conjunction with the regular mixture screws in the base of the carb. Remove, clean, apply a little WD-40 to the o-rings and re-install. You want to bottom it out lightly and then back it out 4 turns for a good starting adjustement point.

Idle mixture screws. Again, if they've still got the block-off plugs over them- leave them alone. If they have already been messed with start them both at 5 turns out from lightly seated for a good starting point.

Good luck. You'll need it. These carbs are SUPER easy to screw up unless you really know what you're doing and have an eye for detail.

Firebird1984

Damon-
Many thanks for the huge amount of info! That was great and I shall print this out for future reference on these carbs. It sounds a bit overwhelming but after opening the carb up late last night, I guess I may just have to switch to a different carb. It seems that someone else has been into this thing before and wow...what a mess!

The car currently has a non-CC carb on it as the previous owner removed the stock CC carb and said that he had problems with it.
The car runs fairly well, but without the CC carb, I think the poor computer and computer controlled spark advance just don't know what to do with it not in the system. I think it would run better with the CC carb redone and back in the mix.

Well, to make a long story short, in trying to carefully remove the long screw that threads down the middle of the fuel bowl, right next to the float pivot (can't think of the exact term for this, sorry, I don't have the carb diagram in front of me) it appears that moron-boy who owned this thing tried to rebuild it and hopelessly stripped the threads out of it BIG TIME! The carb body is thus TOAST. I found several other things he messed with that appear to be beyond repair, so I won't go into it here. Needless to say the previous owner didn't do me any favors when he gave me the stock carb he had already screwed up.

Ok, question time here. Maybe you can answer this. I am not sure what the state regs are on all this electronic controlled stuff. I know that they don't test any of the cars in our county. I would really like to get rid of the computer stuff and go to a non-CC QJet (I have one already on the car that the previous owner installed before I bought it), and a regular distributor setup as it's just easier to deal with. I would keep the cat converter and AIR pump as I think they do alot for emissions. It's just all the elaborate electronics I'd like to get rid of. Any idea if replacing the stock carb and distributor would result in a good driving and dependable car, or produce more problems that I want to deal with?
Thanks for your help and any additional insights.
Brad

PS- I guess I could just buy a rebuilt QJet from Jet performance, but it would be more expensive and I've had no dealings with them.

BowtieLOVER

Hi, just chiming in for a sec.

I picked up the MCS at O'Reillly's for about 34 bucks, then, following a tip from a fellow on these boards, I went to the NAPA website and ordered the following tools:

700-2003 CCC carb sockets
700-1134 CCC carb guage kit

The tools in these sets will allow you to set your MCS adjustments when you are reassembling the carb.
Not sure if there are other sources for the tool sets, but just in case, the carb sockets are THEXTON #380, and the guage kit is THEXTON #370.

Each tool set comes with a small instruction sheet.
The two tool sets plus shipping came to $41.07, though you should be able to cut a few bucks off that by just going to your local NAPA dealer and having them order the tool sets directly.

Hope this helps.

Damon

Brad- many people have done as you are considering- ditch the cc-carb and cc-distributor. Non-emissions Qjet and HEI distributor are drop-ins and will run well (often better than stock) if tuned reasonably well. Just make sure you do them BOTH- changing only one will drive the computer bonkers and result in worse performance. Your only loss will be the computer controlled lock-up torque converter. It won't lock up any more. And you'll have to yank the check engine light bulb since it will be on constantly once you ditch these things.

Emissions- ditching anything computer controlled is technically illegal everywhere. Big deal. If it passes perfume out the tailpipe who cares? Only California and other **** states will fail you for a visual check (if they even do one in your state) for switching to non-computer controlled stuff. Basically, most mechanics won't notice you did the change since they won't bother to take the air cleaner off and see if it's been monkeyed with. If you have problems ask some friends- usually there's a shop in town that will "play ball" with performance-oiriented customers and cut you some slack on the visual test.

Tailpipe emissions? Fahgedaboudit! If it's got a functional cat in the exhaust then that covers up for a multitude of sins. If it's tuned even reasonably well it should pass no problem.

Firebird1984

Thanks for the input guys, I appreciate it. Damon- I have all winter to work on this thing, so I guess I have some time to think about it. I agree that the non CC carb with the stock CC distributor makes for a lousy combo as that's what I have now. Actually I'm surprised that it runs so well with this Frankenstein mix of parts! I may knuckle under and get a CC QJet from Jet Performance if I decide my conscience can't stand it, but I prefer to go non CC as I understand it easily and I've been working with that kind of setup for many years (I'm 47 years old and remember how to replace points in the old mechanical distributors... of all things!).

Is there any way to preserve the lock up torque converter on this thing? If it doesn't lock up, doesn't that affect mileage adversely? I would think that getting into overdrive may be impossible without it. Any thoughts? Thanks a bunch, you da man! Brad

Damon

Yeah, somebody makes a lockup converter switch so you can continue to use it even without the ECM. Uses a vacuum switch and, I think, a switch that screws into a fitting on the trans so it only locks up in top gear under part-throttle conditions. Maybe check with B&M and other tranny-oriented companies.

Or do what I usually do- big tranny cooler and don't drop it in OD unless you're on the highway.

I know what you mean about old technology being more comfortable. You've got a few years on me, but not many. Try to find anyone under ~30 years old who knows what a dwell meter is used for!

Firebird1984

Thanks Damon! As usual you come through with the info. Yeah, I hear ya on the dwell meter! I remember one younger fella wondering what that "little silver window thingy" was on the side of a small block chevy distributor cap! Lots of good (and a few not so good) memories on the older technology. I could fix anything except the AC, trans and rearend on the older vehicles. Now you have to have $30,000 in specialized equipment just to find the problem! Hmmm... checklist for older vehicle
1. Got spark?
2. Got fuel?
3. Got air?
It should run!
Ok, it's the same checklist for the newer cars, but there's a lot more to go wrong in that mix now-a-days. I'm wondering if I should just trade in all my newer vehicles and find some late 60's to 1980 car that lacks computer control to restore and drive. At least I could easily fix it! Again, many, many thanks! Brad

Lancelaut

I believe it was first used to set timing but also used in setting the mixture on Computer controlled carburetors, but man I just turned 30 lol good call

five7kid

iTrader: (14)

Nope.

It was used to set the "dwell" of the points, the number of degrees the points were closed. Indirectly, it is the setting of the points gap.

For computer carbs, it is used to adjust the idle air bleed for a certain mixture control solenoid "dwell", 30 degrees on the 6 cylinder setting, which translates to a 50% duty cycle.

(And, you were only 23 when Damon posted that.)