very high idle. fouled plugs. idle mixture screws not responding.

[txchainsawgogi]

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.

[1979C20]

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.

[txchainsawgogi]

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.

[txchainsawgogi]

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?

[beastie_3]

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.

[topp]

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....

[txchainsawgogi]

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.

[1979C20]

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.

[1979C20]

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.

[txchainsawgogi]

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?

[txchainsawgogi]

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?).

[1979C20]

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.

[Irish_Alley]

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

[1979C20]

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.

[txchainsawgogi]

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.