I just swapped a fresh 462 BBB into my '85 chevy pickup. I'm not having very good luck getting it to idle well, and pickup past 2000-2500 rpms is lousy. The carb is from a '76 electra which is also where I got the motor from. The intake I am using is also from the '76 electra, and is equipped with EGR. I have the EGR valve mounted, but I do not have a vacuum source hooked up to it. Right now, the only lines I have hooked up to the carb are the PCV valve, and the two that are attached to those vacuum pulloff/vacuum break/whatever 2 units that are on there. The rest of the vacuum ports on the carb are blocked off. The thing runs like it has a massive vacuum leak, but I don't know why it would. I can't find any leaks. I'm using brand new gaskets between the carb, 1" spacer (necessary for TV linkage to clear), and the intake. This is the 5th time I've installed an intake on a buick motor, and I've never had one leak before, but I know there's a first time for everything...... Any help would be appreciated. Thanks!!!
The 76 carbs like to leak around the secondary throttle bores at the base of the carb. A lot of the aftermarket gaskets are not wide enough at this point and an air gap can form. Give it a listen with a stethescope (old piece of tubing) and see if there's a whistling from the base of the carb. A shot of Carb Medic squirted around the base will seal the hole for a few seconds and allow you to spot it as well...
Clint - Thanks for the reply. I checked the carb gasket, and it's actually wider than the old stock gasket, although not as thick. I put the old gasket on anyways and it seemed to run the same as before. Right now, my setup is like this (from top to bottom) - Stock '76 Air cleaner '76 Carb Gasket 1" Plastic Spacer from Jegs Gasket thin steel plate (I assume is there to protect the carb base from direct flow of EGR gasses) '76 Intake The 1" spacer is necessary so that the TV cable linkage will clear. I noticed, however, that if I hold the choke plate partially closed (with engine at full operating temp.) , the idle smooths out very nicely. I am thinking that maybe I need to adjust the idle mixture. I looked at the Doug Roe book and I noticed it says that the q-jet is very sensitive to cam and head changes, especially when the strength of the vacuum signal changes. I'm running '72 Heads and my cam is a straightline perf. 220/235, .473/.476 lift, 112 LSA. It is a fast ramp profile. Other details - motor is 9.0:1 comp., uses '72 exh. manifolds into '76 y-pipe. My idle setup is not correct right now, I am sure, becuase I used the idle stop screw to increase the idle speed. I guess I'll need to pull the carb off and make sure that the primary throttle blades are properly aligned with the idle transfer slot before I continue tuning.... I'm using the '76 Manifold which is equipped with EGR. I purchased a new EGR valve from NAPA, and according to the chart that came with it, used the #27 orifice washer. I do not have a vacuum source hooked up to the EGR. Are aftermarket EGR valves known for leaking at all? I'm thinking of using a block off plate to try and rule that out too. Thanks!
Hmmm, once you've eliminated vaccuum leaks then you'll want to address the mixture. I'd run a vac source to the EGR just to be safe while looking for leaks. You can at least see if it's moving when you hit the throttle. Closing the choke improved idle (less air, richer mixture) so you need to try and back out the idle screws a bit and see if that helps. Otherwise, FWIW I think you're ont he right track with the throttle blade alignment. Before getting too deep into the carb though... is the distributor all set up and OK? Hope that helps Scott...
Scott, Just curious, Are you running a 455 carb on the car? The biggest difference between small and big block carbs are in the idle passages. A 455 will never idle correctly with a small block carb on it. Check the carb # and make sure it came off a big block.
Scott, Have you checked the #? Are you sure? I had a 71 350 carb on my engine when I first bought my car. It wouldn't idle right, especially with the cam. Wouldn't be the first time someone put the wrong carb on an engine. What is the carb #, we'll check it.
I'll check the # out tonite. Thanks! :TU: I tried backing the idle mixture screws out a bit. So far I have turned them out 1.5 turns, and there is no difference at all in how the engine idles. I'm not sure how far I can turn them out, it feels like they would fall out if I go much further. The carb responds well right off idle with no hesitation. There is a flat spot after that, but if I give it more gas it picks up a bit. If I push it enough to get into the secondaries, the acceleration is very underwhelming. It doesn't exibit that classic qjet "bog", it's just that it doesn't make much power. I'd say it is a fair amount less than my stock '65 electra w/ 401 (low 16s in the 1/4). This truck *should* be brutal off the line, especially with 3.73 gears, and the 3.06 1st gear of the 700-r4. I was hoping that this combination would get my truck into the 14s, with the power it's making now, not a chance. I tried doing a little WOT testing last night. I held the trans in L1, and floored it from about a 5MPH roll. I don't have a tach, but I'm guessing somewhere between 3000-4000, the motor started to cut out, sputter, etc. I'm wondering now if that old HEI unit isn't up to the task. I put a new cap and rotor on it, but the rest of the internals of the unit are the same. I am tempted to put an old points dist. on to see how it runs with that. The HEI dist. has very little mechanical advance, about 12 degrees. I have the timing set at 18 inital, 30 total. The vacuum advance unit on this dist does not work. Thanks for your help so far, guys!
What you may be experiencing here is the result of the power piston sticking. I did not read anywhere in this post that the carb was rebuilt. If the power piston is sticking, the result will be that the engine runs fine on the fixed passages, such as idle and off-idle, but when the engine begins to breathe beyond the se passages, the lack of power piston movement may lean it out. Try this...looking down the bowl vent with a flashlight, you should see the power piston under there. With the engine not running, try moving this piston with a long slender screw driver, GENTLY. If it moves, this is not the problem. My other concern involves the EGR. If this is an old manifold, it is possible for the exhaust to have eaten up the passages, allowing EGR to dilute the mixture, without the valve's control. It is also possible that a problem with the idle feed passages are blocked or restricted. Determine whether any of the cylinders are not running at idle, by pulling off the plug wires at the cap. If some have no response to the lack of spark, make a chart for me. Tell me either which are running, or which are not. We'll go from there. One other point: check that all of the vacuum passages are properly covered, including the one to the vacuum modulator on the trans. A leak here will cause cyls 5,8 not to fire due to vacuum leaks. Ray
Ray - Thanks for the input. I will check the power piston as well tonite. The carb has not been rebuilt. I just cleaned up the outside with carb cleaner and installed it. One thing I was concerned about, is when I took the fuel filter housing off to inspect the filter, I noticed that someone in the past, for some reason put RTV on the threads of the filter housing. There was also RTV on the threads where the fuel line goes into the carb. I don't know why this was done, becuase I cleaned the RTV off the threads and everything sealed up just fine. I'm thinking there is a possibility though that a little piece of thet RTV could have made its way into a passage in the carb and is causing problems. The manifold is old, it's a stock '76 manifold, but when I replaced the EGR valve, that area looked really good. The passages didn't look deteriorated, and the mounting flange is in good shape. The motor/car only had 68,000 miles on when I took it apart, so it's relatively low-milage. Thanks
Carburetor Problems Larry, what is the difference between a small block and a big block carb? I'm running one (350) on my 455 now and it runs great!!! The only change was I rejetted it to what the 455 spects would have had. I have two big block carbs and they both run rough. AL.
All - I do have another q-jet sitting around, it is from a '72 Electra with a 455. I had previously confirmed that it was a big block carb by the model # on it. Would it be worth trying the rods/jets/hanger from this carb in the '76 carb? I could put the '72 on and run it with the truck in place, but I wouldn't be able to drive it becuase there is no provision for the TV cable on the '72 carb, and the TV linkage on the '76 carb would not be able to be removed easily, to install it on the '72. BTW, the 76 carb did not originally have TV linkage, the throttle/TV arm from the '85 chevy 305 carb was taken off and put on this one.
If you have another carb there, try running it as is. This might eliminate the carb as the issue. It is possible for the other carb to have similar issues to yours presently, but not likely. The '72 will be metered differently as well as the '76 was metered lean to accommodate the cat. The '76 was cammed differently as well, allowing different metering from the '72. My suggestion: try it as is. You might happen on the problem. BTW, the main difference between the big and small block carbs is the primary metering and venturi size. Ray
Larry - The carb I am using is model # 17056240 Right next to that # is another set of characters : 1636 AD7 Thanks
Scott, That carb number is an emissions era cab, and it's probably jetted lean from the factory. There was also a provision to limit the travel of the power piston in that carb. I don't know if it is a small block carb or not. AL, If you have a Buick Chassis manual, take a look at the description for the idle system in the Q-jet section. It states that the carburetors on the 455's had what they called a "fixed idle air bypass" Basically, it was extra passages that allowed additional air to bypass the throttle blades at idle. This allowed the throttle blades to be nearly closed at idle, avoiding the old nozzle drip, and positioning the throttle blades clear of the transfer slot. Small block carbs do not have this. A 455 carb will work really well on a hot cammed 350. It is possible to run a 350 carb on a 455 if it is jetted correctly, but it will not idle correctly. I had to set my idle out of gear at 1200, so it would idle at 750 in gear, when I had the 350 carb on my engine. I also had a slight hesitation off idle. THe right carb cured all of that. Still, if you are satisfied with the way your car runs with the 350 carbs, leave em on. Mine idles much nicer now. I have a John Osborne 800 CFM, 7042240, and I love it.
you can use the baseplate off of the later carb on your 72 cant you? i used my 67 q jet baseplate on my 73 q-jet becausae it has a linkage for the switch pitch. i would def get the doug roe book and rebuild the carb. does it sound like a vac leak with the aircleaner off? mine does the same thing, but i hear that these cars always "suck" at idle without the aircleaner acting as a silencer. btw i also have a 76 455, in my 67 gs. im gonna rebuild it, AS SOON AS IT STARTS RUNNING BAD!!!! its all stock 76 455, with the exception of 67 exh manifolds, ignitionman dist (that helped idle tremendously), a 73 800 q-jet rebuilt by me. with 2.93's, a worn posi, and 195 series tires it ran 14.30's last summer. some better meat and some tuning and itll be in the 13s easy, not bad for 205 hp.
"Q-Jets" Larry, wow!!! I didn't know you had that "Q-Jet" knowledge!!! I think the base has been switched out because I think the side linkage on the 455 is different too. 455s use a flat intake and the 350s are higher . The 350 linkage would hit the intake at half throttle. AL
AL, Maybe, but the 350 carb I had on my engine fit and worked just fine. No linkage problems whatsoever. The only way I caught it was when I checked the number and it came back to a 1971 350 Buick.
Well I switched over to a '72 455 non-stg q-jet to see how it would run. As soon as I bolted it on, it ran much better than the '76. Idle was much smoother, throttle tip in was great, and the carb just generally ran real nice. I had some problems after a bit of driving where it seemed like the idle and primary circuits were starved for fuel, and it ran very poorly. I sprayed a bunch of carb cleaner in the air tube that's on top between the primaries, and then it seemed to straighten up. I drove it a bit and the truck seems to have much more power than it did with the '76 carb, however, I think this carb may be a tad on the lean side yet for this combination too. It looks like I may need to try tuning this one a bit as well.
Because this is a TV cable controlled trans, there is no vacuum modulator. From all of the symptoms you mention and the idle smoothing out with partially closing the choke flap, there IS a vacuum leak somewhere. When you said "BTW, the 76 carb did not originally have TV linkage, the throttle/TV arm from the '85 chevy 305 carb was taken off and put on this one." I take it you did NOT mean the throttle shaft, is that right? If you actually did swap the throttle shaft, that is the first place I would begin looking. Since you now have a carb that works, except for the TV cable issue, do you want to stay with that carb and just add the pieces necessary to TV cable function? Sounds like your best bet. If you want to go back to the other carb, buy a few cans of carb cleaner, WD40 or some other combustible spray and start spraying at the bottom and go all the way around the carb, then move your way up. You mentioned that the only two vacuum lines you have connected are the PCV and the choke pull off. Do you have any kind of vacuum advance connected? That may be aggravating the whole issue if not. You will need to find and fix the vacuum leaks first to be able to nail down the right timing curve for your particular engine.
