73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Performance => Topic started by: LTZ C20 on February 01, 2012, 06:13:43 pm
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I have 79' Chevy Cheyenne C20, SB350. Over the past two months I have rebuilt the entire top end, rebuilt the carburetor, rebuilt the distributor, put on a new intake, completely reset the timing, firing order and idle and mixture. Now the truck is running great while your actually rolling. When it is cold it runs great until about 15 minutes later, usually my drive to school from home. At that point it idles rough but doesn't die. If I give it a rev, sometimes it idles normal again or continues to lope. And finally now that I have done all of this, when its idling normally, it has this whistling and I can't figure out why or where it's coming from., it doesn't do it while accelerating or driving at a constant speed. I have eliminated everything I can think of. I think it might be a vacuum leak and that's whats causing the rough idle when it warms up. Anything you can offer or suggest is appreciated. I have reached my mental know how, not very good with troubleshooting. Thanks ya'll.
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Can you hear the whistling more in the cab or in the engine compartment?
If it's in the cab and it is/was originally air conditioned, sometimes the vacuum diaphragms or vacuum portion of the control panel switch can develop a small leak that will whistle.
If it is coming more from under the hood, then you have to verify all the vacuum hoses are connected and/or unused vacuum ports on the carburetor are capped and that the vacuum hoses don't have any cracks in them.
To eliminate the vacuum hoses as a culprit, verify that the following hoses are firmly connected: vacuum hose to the distributor, vacuum hose to the PCV valve, vacuum hose on carburetor for the choke pull (if automatic choke), vacuum hose to the vacuum reservoir on passenger side firewall (A/C truck), vacuum hose to the power brake booster and vacuum hose to the transmission (if automatic transmission).
Also check any vacuum fittings that screw into the intake and back of the carburetor. You can use teflon tape on the threaded vacuum fittings to ensure they make a good, tight seal.
If the vacuum hoses aren't the problem, then you might try the old trick of spraying carb cleaner or WD-40 where the carburetor mounts to the intake. While the engine is running and warm, spray the carb cleaner around the base of the carburetor. If the engine speed changes, then you found a vacuum leak. The same thing can be done where the intake manifold meets the head.
There are other things to check, but first I'd ensure your vacuum hoses are all properly connected. Hope this helps.
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Thanks, I have done all of that including the carb cleaner spray trick. I can't find it anywhere. It's under the hood and its loud.
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This is not what you are going to want to hear, but the things you described do sound like a vacuum leak. Now you said you tried the carb cleaner spray trick, did you check it everywhere there is a hose and around the engine you worked on and around the carb ?
Next is process of elimination, start with one hose at a time and check it twice, both ends (you might have loosened the other end of a hose while working on engine and not realized it). But I would suggest you start with the hoses and parts (intake and carb specifically) that you actually moved/messed with while rebuilding the top end of your engine.
As far as intake and carb. make sure the gaskets are there (maybe a piece is missing and you didn't realize it), and you did make sure the intake was flat against the heads when you installed it ?
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Well I will redo a spray check on everything and see what happens, yes the intake was flat and everything was working. It just started after I rebuilt the distributor. I'm sure my gaskets are ok, but I will check again. Thanks.
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If it started after you rebuilt the distributor, have you tried working back from there?
I'm just throwing out some more ideas to consider: Remove the hose to the distributor from the carburetor and cap the port on the carb to see if the whistling stops. If so, then you either need a new vacuum line or a new vacuum advance unit.
Same thing with the remainder of the vacuum lines, remove and cap the ports at the intake or carburetor...
If you remove all the vacuum lines, the vacuum ports are capped off and it still whistles, then you have a vacuum leak at the carb or at the intake to head gasket location.
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So, here's a little trick that might help you locate the source of the whistle (and many other troublesome noises that are difficult to pinpoint). Get a 3' length of 1-1/2" ID rubber radiator type hose. While the "whistle" is "whistling," with one end of the hose to your ear, move the other end around the engine compartment until you recognize the "tune." The hose filters out much of the background noise and amplifies the sound entering the open end. I've pinpointed many an impossible noise this way, even when stethoscopes failed.
Curious. When you O/H the distributor, did you remove the drive gear from the shaft and lube the bushings? What carb do you have?
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At Jason, I have actually put a new vacuum advance and new hose on the distributor. I believe all vacuum lines are in good condition, they all flex well and aren't crispy. At BD, holy crap that's a great idea! I am so going to do that as soon as I have the chance! Thanks a bunch! Almost forgot, I did pull the drive gear yes, and I have a Edelbrock Performer 600 50state electric choke.
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The distributor gear has a 1/8" dimple on one side adjacent to the roll pin hole. The gear should be installed so the dimple is pointing in the same direction as the rotor is pointing. If the gear is installed 180 out, the rotor will be pointing between cap terminals when the spark fires instead of aligned with a cap terminal. This can cause frustrating misfire, stumble, etc, and may not display consistent symptoms. > It's one more thing to consider if everything else fails to resolve your drivability complaint.
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Thanks, I installed the gear correctly and I knew about the dimple. I tried the radiator hose trick, just with a straight shop vac tube cuse I didn't have radiator hosing. That led me to the EGR which I had suspected but wasn't sure about. Now I believe I need to replace those EGR gaskets and that should eliminate the problem. I believe the whistling is causing the rough idle after warming up. Aside from the whistle, the truck runs great!
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Great! Let us know how it turns out. :D
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Will do.
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Ok guys. So I replaced the gaskets that I was talking about, the whistling has reduced some. I have FINALLY figured out that the whistling is coming from inside my carburetor. It's coming from the butterflies. Now the carb is a couple years old and worn bout still works good. Do any of you know how to stop the whistling permanently? If I can't fix it, I have to get a new carb and I don't have the money for a new one plus it's more difficult to find a smog legal carb here for Cali. I greatly appreciate all the help. :)
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I know you've already sprayed for vacuum leaks, but try spraying directly at the end of the throttle shaft where the shaft enters the base. And, try spraying all the way around the carb directly at the base gasket. Be very specific. Also, is the whistle dampened any at all by installing the air cleaner?
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I have done the directly at the base spray, nothin happens. And no not really, it does change when you pull the top butterfly closed though, sputters. Then you let go and it goes back to normal. The truck runs really good. It just whistles like a no tomorrow! haha
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If you selectively spray directly into the airhorn, does it change?
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Haha yea!
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Not just spraying into the horn in general, but directing the nozzle at specific points (bleeds, etc) with short bursts. Try to determine if the whistle is coming from "inside" or "outside" the carb. If so, any chance the air horn gasket is the wrong one or torn (or missing)? W/o reinventing the wheel, try spraying directly around the pintle shaft of the EGR.
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I have sprayed the EGR and nothing happens. I recently did a rebuild on the carb so I know the gasket is there. I remember putting it all together. I don't know where inside the carb its coming from, I just know its from inside the carb.
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With the engine off, using a good flashlight, look down the throttle bores at the throttle plates. Focus on the "roundness" of the plates and whether they seat in the bores all the way around, especially where they cross the throttle shaft - look for good throttle plate-to-bore contact all the way around their circumference. Are there any gaps? With the engine still off, grab the driver side throttle linkage and open the throttle a few degrees. Wiggle the shaft fore and aft. How much play is there? Can you see the plates wiggling fore and aft in the bores? How much? Did you have any trouble setting the engine idle or with the throttle plates sticking slightly at idle? Do your best to answer all 5 questions.
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hahaha, is this for a grade? i will do this as soon as i finish eating dinner. bout 30 min.
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You already got your 100%! This would be EXTRA CREDIT!!! ;D
Nah! Just trying to figure how much actual wear is in the throttle shaft and at the plate margins. Trying to get my arms around the melody your carb is playing. I'm a bit more familiar with QJets than the Edelbrock AVS. Tryin to diagnose over the phone can be a challenge :o. What I'm really after is to try to isolate the noise to a specific part of the carb. I've had QJets whistle (rarely) from the air bleeds, past the plates, and through the idle passages for various reasons (ie, warped air horns, wrong gaskets, excess shaft wear affecting plate indexing). But, not often, suggesting that circumstances have to be just perfect for it to occur. Sometimes just changing the throttle closing a little bit solves it (as long as the gaskets aren't the issue).
Of course, I guess you could always get earplugs! :P But, it would be a lot more fun to solve it.... 8) Just want to see you get the product you're after... after dinner. :D
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Haha well my carb is a Edelbrock Performer. Sorry if you didn't know that. As for the extra credit: 1, the gap around the plats appears to be equal. Correct me if I'm wrong but if there was no gap then there would be no air fuel mix and truck wouldnt run. with a small gap, air and fuel always pass through while its runnin, creating idle? I think right? 2, the linkage moves fluidly and i can see the plates working together. 3, when we rebuilt it, we had trouple because the plates were sticking so after carefull re-adjucment we got the plates alighned so that everything moves smoothly and nothin sticks. my unlce, who knows so much more than i do, thinks that something is worn so air is slipping past and whistling. not sure tho. how does any of that help?
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Kevin,
The Readers Digest version: On a carburetor, at idle, a metered amount of air flows past the throttle plate (adjusted by the idle speed screw) and air/fuel emulsion flows into the air stream through the idle ports just below the throttle plate. Generally, no fuel flows past the throttle plate at idle. Off of idle, transition slots or holes above the throttle plate are gradually exposed to manifold vacuum as the throttle opens to provide additional fuel/air that mixes with the main air stream until sufficient vacuum tips the main nozzles to start flowing. There are various calibrated air bleeds that work in conjunction with the fuel metering to maintain proper mixing and fuel delivery for any given manifold depression. If the right combination of air pressure/velocity/expansion/resonance occurs, it can do what yours is doing. I think your uncle is absolutely correct! The cause of your problem is wear.
As to where my questions were leading, look at the attached sketch. The new plate is on the left; the worn plate on the right. Gaps caused by wear can alter the air flow. If you get the right combination of "gap" it whistles. The wear you found and compensated in the shaft may well be the cause. In the long run you may have to bush the base. And, if your throttle plates look anything like the drawing at right, the plates will need to be replaced too.
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Just a thought, the intake you put on that was new, it was not a higher than stock one was it ? A higher one might act like a throttle body/carb spacer (vortex effect) and make it whistle.
If you could it would be great to take a video of it so we can hear the whistle you are talking about and post it.
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Well I have so new news. I FIXED IT! I bought a new set of carb stud bolts and installed them. I also found a blow gasket with a worped cover plate. So straightened the plate, sanded it, and reinstalled it to cover the unused port. <---This was causing a vacum leak. With the vaccum leaked fixed and new stud bolts, I thought it would create a different pressure when bolting the carb down. I found that sitting flat on a surfaces, my carb my be slightly worped, only off by thousands of an inch. So with a good base gasket, a fixed leak, new bolts, NOTHING LEAKES! The whistling wasn't there when I started the engine. After some vacum tuning with a vacum gauge, adjusting my mixture and idle, the engine now runs great. Haven't had any problems. I did this yesterday afternoon and since then all is well. I have smooth acceleration, no hesitation, no abnormal noises, idling is great. I may be buying a new Holley carb if my unlce wants to. We are going to see how it continues to run. I'm very pleased with the results. A new carb would be nice but, as of now, this Edelbrock is working very good. I have decided to stop the engine work and continue witht the rest of the truck. It purrs like a MEAN kitten and souns great from the pipes, got a good rumble. Thanks all for your help! She's got quite alot of go go in her and she moves when you want her to! 8)
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That's GREAT news!!! :)
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Vacuum leaks are like trying to find a dead short somewhere in your vehicle. It takes forever to isolate and fix, but in the end you feel like you just triumphed over raging tiger in a roman gladiator arena. ;D
Or your so frustrated that it took you so long you just feel like sticking a piece of dynamite in the gas tank, lighting it and walking away. Guess it depends on your personality.
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heck i would of just cut the mufflers off so it drowns out the noise 8)
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I'm bout ready to chop my catalytic converter. smog.... -__-
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you need something for back pressure
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Thats what my 40 series Flowmaster and eventually when I get them new headers are for. As for now, the Flowmaster and stock mani's work perty dang good. :)
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are you sure it wasnt just you whistling?
^thats the first thing i usually check for.
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I don't whistle often.