73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: txchainsawgogi on August 04, 2011, 08:46:11 pm
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1978 chevy 350 5.7L V8. the carb is a 78 rochester qjet 4MV (i believe).
its idling high for the past two or three days now. spark plugs i put in are black after a 30 minute drive. unburned fuel is blackening the exhaust on the DRIVER side exhaust pipe.
*two days ago a plug boot melted through and the spark was arcing against the header pipe. i swapped boots and plugs and it was fine. it ran and idled normal.
*today it turned the new plug black.
*the choke plate sticks open a little and i dont think my thermostat coil is working proper. *EDIT* replaced the vacuum choke pull off. works fine now.
*could the melted arcing plug have damaged my distributor, or the wires themselves? could that cause a plug to not fire as hot as it should, thus sending unburnt fuel out the exhaust and fouling the plug? *EDIT* checked the spark plug wires and distributor cap, normal resistance and no burns or rust etc.
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Since you have header, do as I suggested in the other post. Get a spray bottle with water in it and spray it on each of the pipes coilming from the exhaust ports to see if each cylinder is firing. The water should evaporate almost instantly from the heat. If it doesnt evaporate quickly, its a dead/weak cylinder. Or if you have a 12 volt test light, ground the clip then stick the probe of the test light through the spark plug boot to where the spark plug clips in. if the engine doesnt change noise or run bad its a no sparking cylinder.
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the driver's side row of pipes seem to all evaporate just SLIGHTLY slower than the passenger side. each cylinder seems to atleast be firing, but maybe the spark is just really, really weak? i dont have a 12V test light.
my idle speed is also off. it used to idle at just below 1000rpm in park/nuetral and 550rpm in gear. now its more like 1400rpm/800 rpm.
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can someone please give me a step by step on setting the idle mixture? ive got a vacuum gauge that might work for the job.
also my carb is a 4MV but does NOT have an electric choke. should i convert it to electric?
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not sure, but there should be a video on you tube or there are rebuild books on that carb at the parts store or a book store.
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my idle speed is also off. it used to idle at just below 1000rpm in park/nuetral and 550rpm in gear. now its more like 1400rpm/800 rpm.
Sound to me like a vacuum leak there....
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tried to set my idle mixture... even if i turn in both screws all the way it still idles above 1000rpm. the vacuum gauge says im getting right under 20, even with the scews all the way in, two turns out, or where they were before i touched them.
whats going on?
*edit*
i noticed my distributor cap is cracked. its brand new(ish), so im guessing it happened when my plug boot melted and arced like crazy.
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Spray WD40 on all of your vacuum lines, where they connect, around the base of the carb, and around where the intake bolts to heads. If your rpms go up while spraying, you have a vacuum leak. Report back woth findings.
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Ooooh. I just though of something. Make sure all.of your intake bolts are properly torqued, and not missing. If one of the bolts is missing by the port where the fouled plug is, it can let oil.in the.cylinder and cause it to foul.
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what about the high rpms? i cant get them to lower... i'll go do the wd40 thing.
*update*
wd40'd everything and no jump in rpms. do i need to open the carb up and double check all the internals again?
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opened up the distributor and everything looks normal. no damage found aside from a little crack from the top being tightened down a little tight. doesnt seem to affect anything. checked the plug wires for too much resistance with my multimeter, they look normal too(no internal burns i guess?).
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Grab you throttle on the carb and turn it towards the front of the truck. I had a problem for a while where my throttle wouldnt turn back all the way on its own. And pull the distributor cap off and check the center contact on the rotor, the outside contact on the rotor, and also chexk the inaide of the cap on the prongs for any carbon buildup, chips, missing pieces, etc.
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High idle is too much air or fuel. Spark would lower it. If no vacuum leaks from the base of Carb, intake or vacuum lines then I would think the Carb needs a rebuild or like said the throttle is sticking. I think those carbs have bushings on the flappy things lol and those can cause a vacuum leak
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One way to check for a vacuum at the trottle is to grab it and move the whole thing in a circle. If you hear a whistle, if theres a lot of play, or if the idle changes then there is your problem.
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well it still wont idle correctly, still too high. so i guess i'll tear open the carb AGAIN tomorrow haha.
the throttle definitely isnt sticking, but i saw a DROP of gas come out of the gasket.
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what gasket?
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I know what it is! Did the drop leak out of the passenger side of the carb? For a while, my float stuck at the top and it made my idle really high and ran like crap and ran rich. I beat the crap out of the top and sides of the carb with a screwdriver handle and hit a bunch of hard bumps on the road and it settled down.
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lol i can see it now... well this pothole looks big enough hit it at 50 ;D. if that’s it good, but if fuel can get out air can get in
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the 1/4 inch thick base gasket.
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Idle mixture screws not responding is generally vacuum leak. I'm not sure of anything else that can cause that. It would also cause high idle, so...
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just bought the last felpro base gasket my autozone had in stock... hope its all i need.
so, would a leak in the gasket cause my symptoms? i sprayed wd40 all around it yesterday and saw no sudden increase in rpms... just the steady climb it was kind of already doing.
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yeah would cause high rpms not sure about the fouled plugs but its defiantly a problem. i would think it would cause you to be lean
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replaced the gasket... now instead of idling REALLY high, its only a little high... but i cant get the idle smooth.
its at 1100rpm but choppy, and in drive its at 600/550rpm but choppy.
how do i get the idle smoothed out?
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what im thinking is when you where screwing in and out those screws you messed up the tune. i dont know where to start on tuning that carb but if you search the web it might tell you where to start
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what im thinking is when you where screwing in and out those screws you messed up the tune. i dont know where to start on tuning that carb but if you search the web it might tell you where to start
thats exactly what happened. a few days ago when the plug boot first melted and dipped the idle speed and made it ride rough, i must have messed with the fast idle cam(which i still know almost nothing about, tuning wise).
as of five minutes ago i finally got it to idle where it used to be (1000rpm in park, 550 ish in gear) BUT its still very rough in DRIVE.
i got it back to drivable thanks in large part to trial and error turning in/out between the fast idle cam, idle mixture screws and the idle screw at the throttle linkage.
now i just need to tune it again.
i dont trust my cars emergency brake enough to tune it in drive so im stuck waiting for my girlfriend to get home. bummer.
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with the engine running (at idle ) and the choke fully open, look down the carb and see if
you can see any raw fuel dripping . If so, open up the carb and check for proper float
adjustment. If it's a fiber float, make sure it's not saturated with fuel, causing it to not
float and properly the needle valve. Also make sure the needle valve and seat do
not have any debris in them. Make sure the retaining clip that holds the needle valve
to the float is properly attached and not causing the float to hang open.
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i got it where it needs to be, its just a matter of movin' the screws around some more. the idle speed in drive is very choppy and eratic.
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Did you replace the fouled plug(s)?
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yeah all new plugs, all gaped correctly. new base gasket...no vacuum leaks.
one thing though, when doing a vacuum gauge tuning of the idle mixture screws:
what do i cap off, and where do i attach the vacuum gauge?
i had it attached to the port right under the choke pull off(passenger side)?
i had the front facing port capped and the vacuum advance capped.
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well its the end of my long weekend, with a full week of driving to work ahead... will the truck be ok even with the low idle?
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the real thing to worry about is if it stalls and you dont want it to be to lean
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After it warms up...it starts idling high AGAIN. It was low two days ago...but when it gets hot...boom 700rpm/1300rpm (drive/park).
Also it dieseled yesterday like its rich but the exhaust is the cleanest looking I've ever seen it.
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dieseling=timing usually.
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dieseling=timing usually.
X2
Advanced too far.
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oh yeah...got it. This high idle thing has me stumped AND p.o. big time.
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new symptom! its no longer carbon fouling the plugs, now the plugs are showing signs of the mixture being too lean. the idle is (usually) within the ballpark range of where it should be but its a very rough, sluggish idle.
if youre sitting at a stop light it'll noticeably shake and shimmy while the rpms dip around the 550 mark. idle in park is fine. driving conditions are fine.
the other odd thing i noticed is a slight bit of grey smoke on start up that doesnt happen everytime, and is so light that its hard to pinpoint where its coming from. looks like its coming from the under the front of the engine, possibly by the harmonic balancer.
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Check the plugs from both sides of the motor, if they are both lean burning, screw each mixture screw in 1/4 turn. Then check pkugs again.
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I thought turning it in made it leaner?
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Turn the screws in, gives less air, makes it richer. Turn screws out gives more air, makes it leaner.
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http://www.ehow.com/how_7763497_tune-quadrajet.html
love step number 5 but you need to start from scratch
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when that person says "plug the tach into the vacuum port on the carb" do they mean a vacuum gauge?
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I hope so lol