Well, I thought my carb problems were over, but here I go again. After rebuilding my carb, with much advice from you people here, all seemed to be well. I have done all the regular tune up stuff, and tuned the carb which seems to work well most of the time. However, there are problems which seem to be getting worse, and my mileage dropped by 30%.

Symptoms:
1. Mileage dropped by 30%
2. When the car is cold, it runs so rich that it misses and chugs for about 1 minute. Once it gets the slightest bit warm, everything smoothes out, and it runs normally.
3. When I start the car after it has sat 15 to 45 minutes since the car was driven, and reached operating tempurature, it runs really rich just as it does when it is cold. It also smoothes out after about 1 minute.
4. No check engine lights come on.

State of the carb:
1. Rebuilt with used, supposedly working, MC and TPS sensors. The F-body shop said they worked when they came off the last car.
2. Mixture screws set about 1 1/2 turns out from total lean. Seems to idle with the right mixture when engine is at normal operation tempuratures.
3. The spring tension on the secondary air valve is about 5/8 of a turn tighter than 0 tension.
4. MC selenoid cycles with the key on.

From what I have learned by reading the forums, I have some guesses as to what the possible problems(s) might be:
1. The TPS is broken or out of adjustment. I am going to put in a new one since they cost very little. Do I still need to adjust it?
2. The MC is not working properly even though it is cycling with the key on. Is it possible to be broken even if it is cycling? If so, I will put one of those in also while I have the carb open.
3. The MC selenoid plunger is not adjusted properly. I am not sure how to do this. I was looking at a parts diagram of a Q-Jet, and it has a 'stop rich limit' which prevents the plunger from going too high. My carb did not have one. Should it?
4. I have not adjusted the Air Bleed Valve from the setting it was at from the rebuild. How do I know what to adjust it to?

Any comments, ideas, or suggestions will be much appreciatted.
Thanks so much,
Draug.

[This message has been edited by Draug (edited July 26, 2001).]

[This message has been edited by Draug (edited July 26, 2001).]

 

Your cold-start situation is a malfunctioning choke. It's too tight and/or the vacuum break isn't opening the blade enough. Very common.

It sounds to me on the whole like your float level is too high. Your hot start problem is caused by fuel boiling in the fuel bowl and spilling into the intake. Again, very common.

5/8 turn on the air valve is excellent for a 305. Any more and it will run too rich, any less and it will have Quadra-Bog.

Mixture screws do almost nothing at all in these carbs. I don't even know why they're there. The threads are REAL fine, like 60 threads per inch or so. IIRC the factory setting is in the 5 to 7 turns out range. Set them to 7 and forget them.

There are 2 stops to the MCS travel: a lean stop and a rich stop (duh). The lean stop is the thing that holds the MCS into the carb, it's a silver-colored screw with a head that looks like a screwdriver. For a good starting point, set it to about 5 turns backed out from all the way in. The rich stop is a big flat thing in the air horn. It has a plug covering its adjustment end, which is identical / similar to the lean stop. You can unscrew it from underneath with needlenose pliers or something like that if you have the air horn off; first screw it all the way in, counting the turns, then take it out and knock the plug out, then put it back where it was.

First, lower you float level about 1/8 - 3/16" from wherever it is.

You can tune these carbs most easily by setting the rich stop to where the car has good power and throttle response in 4th gear at like 35 mph, with light to medium throttle. If you (from outside the carb) turn it clockwise until the car sort of hesitates and bucks a little under those conditions, then back it out until it clears up, it will be about right. Then adjust the lean stop clockwise until the car starts to surge at a 35 mph high-gear cruise, then back it back out until it clears up. These adjustments will interact. Then if you have a dwell meter, adjust the idle air bleed screw to get about a 50% duty cycle (30° on the 6-cylinder scale of a dwell meter) at idle. This will also interact with the other 2 adjustments. To really get it to run its best, you may have to go through several iterations of all the adjustments.

Set your vacuum break diaphragm linkage to open the choke blade about ¼". Your diaphragm might even be defective, they often are. If you can push in the linkage and the diaphragm pops back out with a finger over the vacuum hole, you need a new one.

Your MCS is not defective if you hear it ticking. Your TPS might be but it will not cause the problems you describe. The computer simply doesn't have enough "authority" to change the mixture by enough to make that huge of a difference.

------------------
"So many Mustangs, so little time..."
ICON Motorsports

 

RB83L69, Thanks for your informative response.

It is my understanding that if the float was too high, I could make it spill gas by stopping hard, accelerating hard, and surging the car, and as a result smell fuel, but I cannot. This is how it was when my float was bad in the past.

Ok, so I wont replace my sensors until I set the lean stop and the rich stop.

Just to make sure I am on the same page, I included a link to the picture and article that I am getting my names from.
http://www.monte-list.nu/tech/qjetparts.shtml

My first problem would be that I just screwed in the Lean Stop (#226) until in was tight. I did not know any better, I thought it was just a screw to hold the selenoid in.

My second problem is that the carb I bought rebuilt did not have 'Stop Rich Limit' (#227). I guess I will have to take that out of another carb I have lying around.

Thanks so much,
Draug

[This message has been edited by Draug (edited July 26, 2001).]

 

Your carb has part # 227, I feel fairly sure, unless you have mechanical failure (the kind where the mechanic failed to do his job right). I have never seen a CC Q-Jet without it. It's kind of like a screw with a completely flat head about the size of a dime, with no phillips or hex or anything on it, just a round flat head with a couple of barely noticeable flats around the edge for a spanner wrench, that only protrudes from the casting by about 1/8" or so when properly adjusted. I believe if you take the carb apart you'll find it there, waiting patiently to be discovered.

If you have a carb without that, then you do indeed have a seriously fornicated carb. No way it will run right Hopefully the provision for it at least is there. If you are missing that, then the carb may run real rich, because the rods will be able to come almost completely up out of the jets, maybe all the way out, on the up-stroke of the solenoid plunger.

Having the solenoid screw run all the way down will tend to make the car run lean, because the large part of the rods will be in the jets more of the time. Of course, running too lean will make a car use more gas, even though that sounds counter-intuitive. But think of it this way: if you introduce 10% too little fuel into the air stream so that the air-fuel mixture is 10% leaner than it should be, but you have to use 20% more air-fuel mixture to compensate for the inefficient combustion, you have a net increase in fuel consumption of about 10%. Emissions might be lower, but performance and economy will suffer.

{edit}OBTW — I just went back and studied it, I have no clue what part #227 in that callout is. #72 is the rich stop.
------------------
"So many Mustangs, so little time..."
ICON Motorsports

[This message has been edited by RB83L69 (edited July 26, 2001).]

 

Hey,

Part 227 is not present in my Camaro carb. Part 72 is. However, part 227 is present in a CC Q-Jet that I took of an mid 80's Oldsmobile at the auto wrecker.

I am working on the carb right now. I will keep you posted on how it goes. There must be others going through the same learning curve as me, so hopefully someone else will find these posts usefull at some time

Draug