I recently did some work on a friend's 1978 Camaro Type LT, 350 with automatic. One of the things he wanted done was a tune-up. Compression check was even, around 125 - 135 psi per cylinder. I did replace the distributor cap, rotor, plugs, and wires. I also rebuilt the 4 bbl Quadrajet carburetor. I use an acid dip to soak the parts in and installed a new overhaul kit. The car does not have headers, but does have a dual exhaust that the previous owner installed. I also replaced a couple vacuum hoses that were leaking or brittle. I did a vacuum test and the reading was within spec.s and stable. The odometer reads just over 41,000 miles which the owner believes is actual (I don't think so).

This car has always been difficult to start. Even after I finished with it, it still takes two or three attempts of cranking the engine to get it to start. Once it does start, it idles correctly, and runs great. I spent quite some time adjusting the air/fuel mixture, fast idle, and normal idle settings to get it to idle and run correctly, and to try to eliminate the hard start symptom. It has a heated choke rather than electrical. I removed the choke tube and housing and checked to make sure it wasn't full of carbon. I also observed the choke when idling and it does function properly. It will come off fast idle by itself just like it should. I changed the accelerator pump setting from the middle hole in the pump arm to the lower hole to give the pump a longer stroke and add more fuel, which helped a little. I have rebuilt Q-jet carbs before, and if they are working correctly, it should only take a couple pumps of the accelerator pedal and the vehicle should start. Not the case with this car.

There were several things wrong with the Q-jet when I first got the car - the choke housing was broken, apparently from too many attempts at adjustment; the linkage connecting the top-rear (secondary) throttle butterflies was missing. There is also a small amount of play in the lower throttle plates, more so on the rear (secondary) throttle plates. I also smothered the carb by covering it with a towel to see if there was a major vacuum leak around the base of the carb, and the engine died immediately. If I spray carb cleaner around the lower secondary throttle plate shaft, the idle will go up indicating there might be a small vacuum leak here, but the secondaries should not have anything to do with starting or ilde. I do not know whether the heated crossover passage in the intake manifold functions, and the warm air intake to the air cleaner is non functional. We live in the southwest where it seldom gets below freezing, and I worked on the car during the summer and fall so it was quite warm outside. I also made sure the fuel bowl wasn't emptying itself overnight by removing the fuel inlet line and filter, and watching for drips. I also placed a paper towel under the inlet and watched it over a period of several hours, looking for wet spots. I did install a new filter of course.

Okay, that's what I have done, and I know there is no simple answer, but now my friend is thinking of spending more money on an Edelbrock or similar carburetor but I don't want him throwing his money away. You will notice that I did NOT replace the ignition coil, but as I said the car runs great after it starts so I don't think it is an ignition issue. It just seems that it is starving for fuel when cranking. If this carb has 141,000 as I suspect it does, I would strongly recommend a new unit, especially noticing the play in the throttle shafts, which I know is a bad sign.

What does the group think? If I do recommend a replacement carb what would you suggest, and what is the original CFM of the Q-jet?

 

Have you held the choke open and pumped the accelerator by hand to see if gas is actually squirting in the carb?
It could still be electrical as well,if the starter is getting weak it may be pulling too much juice from the coil,tell tale would be if it fires just as you turn the key from start to run disengaging the starter motor.
Are the battery cables in good shape and clean?
Are you sure the timing advance is working and it's not timed off a little?
Does the car have a cat that may be getting plugged up?
Had the car been sitting for quite a while? Maybe the fuel filter is getting clogged already from crap in the tank?

 

Did you set the choke angle?
There is a lot of things that can cause a hard to start condition. Pretty much anything that will cause a slightly lean condition will cause hard to start. Any fuel port partially blocked Something not letting fuel to flow. More common issues are where the throttle plate shaft goes thru the base plates, EGR valve stuck partially open. Distributer/module not functioning.

After you have done everything you can think of retorque the intake and the carb.

If you have never had the car running right that opens up a whole new bag of worms. Is it jetted right? Is it setup for your engine? The reason there are so many model numbers on a GM carb is they are designed per application. This means very little adjustment. That is the opposite of an after market Carb that can be adjusted in just about any way.

Lastly there is Carbon. Valve and intake can get carboned up. It was common back in the day to pour water down the carb to fix stalling and rough idle. This was mostly on cars that where babied. After school I did not believe that was real. Once you see it the results are hard to argue with. You have to be carful to not hydro lock the motor, pour slowly. This allows the water to vaporize under vacuum. You are steam cleaning the inside of the motor.

 

Thanks for the suggestions,
I did verify that there is fuel going into both primary venturis. It seemed weak to me so I changed the acc pump adjustment, and after the adjustment I could see increased amount of fuel going in. The car cranks over fine, timing is ok, but you have to pump the accelerator, crank the engine for a few seconds, then try again, sometimes three times, etc for it to finally fire. The owner says it has been that way since he bought it years ago. It acts like a lean condition, which I hoped the carb overhaul would help, and it did, somewhat but it's still not right. It does not have a cat, it has been modified to dual exhaust with aftermarket mufflers, the fuel filter was changed when I did the tune up.

 

Thanks for the reply. I'm thinking the same, that it is a lean condition. I did check all the measurements as outlined in the carb overhaul kit, although I do not have a choke angle gauge. I set the choke (heat riser type) so that the butterfly closes when you depress the throttle - aka setting the choke - and it starts to open once the engine starts. Since I don't have the angle gauge, is there a choke opening measurement that be taken between the butterfly valve and air horn, or should it be closed tight? As I said, I have rebuilt Q-jets in years past, and if they are working properly, one pump sets the choke, then one or two max to prime the venturis, and the car should start right up. I used an acid bath to soak and clean the carb and parts, but there is the possibility that there could be a clogged fuel port. I am suspicious of the looseness of the throttle shaft, as this would cause a lean condition. The owner says the car has always been hard to start. I also saw a lot of things that had been modified, larger aluminum radiator, aftermarket dual exhaust, cruise control removed, etc. and the previous owner's work wasn't the best, I found missing fender bolts, a mickey mouse rigged hood latch release (which I fixed) and other issues such as the damaged choke housing and cover, and missing linkage to the secondary throttle butterflies. It is entirely possible that they may have even swapped the carb. I am thinking a stock 350 should run about a 600 CFM carb? Another responder said that aftermarket have much more room for adjustment than the stock Q-jet which are designed specifically for the engine they are to go on, so maybe that is the way to go?

 

When you think about it the aftermarket needs to have more adjustment since the manufacture has no idea what the carb is being mounted to. Heck you may see a 600 CFM on a Buick V6 or a truck Small block 400 that never goes over 4000 RPM.

There is idle adjustments speed and air control on the quadrajet. In my opinion your best bet is to set the air bleed screws to the factory setting and fix the real issue instead of trying to hide the real problem with adjustment. that does not work if the carb is not from that motor configuration.
There are experts out there in quadrajets that make all kinds of adjustment by bending rods and levers and even using drills. To me it was always "if the have the original carb just fix what is wrong. If you have modified the engine or plan on modifying it just get a Holley. There are a lot more choices in aftermarket carbs these day. The old school carb are no where near as touchy as newer fuel injection is tuning wise. Your car would run find on the original setup with headers.

When you set your chock angle you are setting the pull off that opens as soon as it starts. I believe in 78 these where Vacuum, I have not messed with a quadrajet since I left the GM dealer in 1990. I have a choke angle gage so I never had to be creative but there must be some way to use your phone to preset and angle and compare that to your existing setup. That would make sure your where in the ball park.

We used to repair the base plates/butterflies at the dealer with a bushing kit and a drill press. I am not sure those kits are still around. Making sure the drill is square going in was considered critical.

Do not completely rule out spark, a lazy pickup or a weak spark module can fake you out, With the car cold get a buddy to spray some carb cleaner or starting fluid directly into the carb when you first try to start it. If it starts instantly it is a fuel issue and there is still a delay then you have some type of spark issue.

How many times are you pumping the gas? this is not EFI you are required to pump the gas. Check you owner manual for details. My old rule of thumb was if it was more then three then something is wrong, 2 I consider normal. If it will start on one pump you are a lucky guy. You have a very tight motor and the carb is right as can be.