She idled great, until she didn't. . . - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Mike Quillici · 03-06-2012, 06:03 PM

I have a 72 Chevy with a 350/350. I drive her on and off, but right now my daily driver is in the shop, so I'm driving my truck to work every day. Today, I was running some errands at lunch, and she was running just fine. I got on the freeway for about 3 minutes, and when i got off, I noticed she was idling really rough, and at low rpms (~500 in gear).

I'm wondering what may have happened in such a short time to cause such a change. Oil in engine and tranny are good. Power steering was very low (off the stick), but I added some. Distributor is locked down tight, so I don't think it's a timing issue. Pulling the PCV hose from the front of the carb really causes it to run rough, as would be expected, so I don't think it's a massive vacuum leak.

When I get home tonight, I will check to see what my idle vacuum is, and spray some carb cleaner in the carb. Does anyone else have any ideas? She seems to run fine off of idle, although the transition is a little sudden.

Thanks,

Mike

**LockDoc** · 03-06-2012, 09:59 PM

-
Make sure a spark plug wire hasn't come off or shorting out. They are easy to spot at night with the lights out.

LockDoc

Mike Quillici · 03-06-2012, 11:19 PM

Spark plug wires are all firmly attached, and are about two years old. I started her up to leave work, and she ran great. By the time I got about 3/4 of a mile down the road (about 2 minutes later) and pulled up at the stop light, she was running poorly again.

When I got home, I unplugged the distributor vacuum hose and hooked up the vacuum gauge. She was only idling at about 400 rpm in park and had a vacuum of 14 in Hg. Now, I realize that the idle is low, but it had been idling at 800 rpm in park earlier that day and all I did was drive it. 14 in Hg also seems low, but I can't remember what it was when I tuned the carb about 6 months ago.

I sprayed in some carb cleaner, but that didn't help. I did notice that if I hit the throttle too hard, I get a screaming sound that resembles a squealing belt, but I think it's coming from around the carb. Might I have a massive vacuum leak?

SuperHerk · 03-08-2012, 02:09 AM

Quote:

Originally Posted by Mike Quillici

I did notice that if I hit the throttle too hard, I get a screaming sound that resembles a squealing belt, but I think it's coming from around the carb. Might I have a massive vacuum leak?

I had a truck make a similar sound once, and the issue was not enough fuel getting to the carb. Maybe a clogged fuel filter or a worn out fuel pump?

dan42 · 03-07-2012, 12:31 AM

Not to jack the thread, but I have a similar problem - it will idle fine after start up and warm up, and then when it's nice and warm, sometimes it won't idle. I'm thinking I have a intermittent vacuum leak, I can't think of anything else that would cause it to go back and forth like that.

Mike Quillici · 03-07-2012, 03:45 AM

Well, I just went out and tightened down the carb. I probably got about 3/4 of a turn out of each bolt. So, we'll find out tomorrow morning if that helps at all. . .

jhwkns · 03-07-2012, 01:24 PM

a vacuum leak would make it idle faster. Check to make certain that the choke butterfly isn't hanging up. Things like this are hard to diagnose over the www, but you said it runs good cold?

Mike Quillici · 03-07-2012, 01:26 PM

Choke is wide open when warm. Runs fine cold.
Posted via Mobile Device

Mike Quillici · 03-07-2012, 01:38 PM

Also, I've dealt with a sticking choke and my truck running rich before. I'm almost positive that is not the problem.
Posted via Mobile Device

CC69Rat · 03-07-2012, 02:31 PM

*edit* posted on the wrong one .. sorry.

Mike Quillici · 03-07-2012, 02:34 PM

Wow. That's frustrating. I do have a spacer and installed new gaskets about 6 months ago. Hopefully it was just loose and tightening the bolts will do the trick. If not, I'll go for. SW gaskets.
Posted via Mobile Device

treveiger · 03-07-2012, 03:55 PM

Interested
Posted via Mobile Device

Mike Quillici · 03-07-2012, 05:41 PM

Tightening the carb didn't help. . .

crazy longhorn · 03-07-2012, 10:09 PM

Check your vac can , with a vac pump....see if the can is holding its "mud"? crazyL

CC69Rat · 03-07-2012, 10:39 PM

Did the line to the modulator valve come off? Maybe a vacuum leak there? Any fluid in the line, white smoke in the exhaust or problems shifting? It could cause the idle fluctuation if the modulator is / or has gone bad.

http://auto.howstuffworks.com/automa...smission13.htm

Interesting read anyway

Mike Quillici · 03-07-2012, 10:40 PM

On the tranny? I checked that. . .still attached.

Coley · 03-08-2012, 12:39 AM

Check your fuel line for debris/crap and your fuel filter. Sometimes sediment, etc settles when the truck is cold then migrates up and forward and starts to plug up the carb under pressure/speed.
Good Luck
Coley

Mike Quillici · 03-08-2012, 02:11 AM

Fuel pump is new as of about 7 months ago, as is the fuel filter.
Posted via Mobile Device

MagmaJct · 03-08-2012, 07:11 AM

Check the timing anyway. Does it have points? Is it possible the idle screw backed out? I know these seem unlikely causes, but we want to eliminate potential causes.

hjewell2 · 03-08-2012, 08:10 AM

Change carb see what happens. Pull fuel line off pump and check for flow. yup its new things break. Also check filter, yup its new, they plug up because it is doing the job intended for.

After these items are checked and no change, Then you move on to the second of three that make a vehicle run which is electric--air----fuel... I would stab at the electric next

CC69Rat · 03-08-2012, 08:58 AM

Points dizzy or HEI ?

Mike Quillici · 03-08-2012, 01:37 PM

HEI. Yeah. Next step is timing.

CC69Rat · 03-08-2012, 01:46 PM

When you 'got on' the freeway .. did you stand in it?

How many miles are on the 350? Could it be the timing chain is streched (I think someone mentioned this earlier if I recall) .. possibly it jumped time?

Any cracking / popping / coughing in the exhaust? If you pull the time around a little does the idle increase?

CC69Rat · 03-08-2012, 01:47 PM

.. just noticed 188k miles?

stsalvage · 03-08-2012, 11:21 PM

I be checking fuel filter and check fuel pressure.How old is the motor how many miles when was the last time you ran some Sea foam gas treatment in it you might of got some water in the carburetor how many fuel filters do you run on your truck.Best to run 2 and the one that is in your Rodchestor carb.I check your modaul in your HEI if its going bad it can cause that.

One last thing you might of got some carbon onto the plugs you might want to check your gap and the plugs.When was the last time you pull your carb off to check to see if your carb was not flooding out the motor

03-06-2012, 06:03 PM	#1
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	She idled great, until she didn't. . . I have a 72 Chevy with a 350/350. I drive her on and off, but right now my daily driver is in the shop, so I'm driving my truck to work every day. Today, I was running some errands at lunch, and she was running just fine. I got on the freeway for about 3 minutes, and when i got off, I noticed she was idling really rough, and at low rpms (~500 in gear). I'm wondering what may have happened in such a short time to cause such a change. Oil in engine and tranny are good. Power steering was very low (off the stick), but I added some. Distributor is locked down tight, so I don't think it's a timing issue. Pulling the PCV hose from the front of the carb really causes it to run rough, as would be expected, so I don't think it's a massive vacuum leak. When I get home tonight, I will check to see what my idle vacuum is, and spray some carb cleaner in the carb. Does anyone else have any ideas? She seems to run fine off of idle, although the transition is a little sudden. Thanks, Mike __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

03-06-2012, 09:59 PM	#2
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,649	Re: She idled great, until she didn't. . . - Make sure a spark plug wire hasn't come off or shorting out. They are easy to spot at night with the lights out. LockDoc __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

03-06-2012, 11:19 PM	#3
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: She idled great, until she didn't. . . Spark plug wires are all firmly attached, and are about two years old. I started her up to leave work, and she ran great. By the time I got about 3/4 of a mile down the road (about 2 minutes later) and pulled up at the stop light, she was running poorly again. When I got home, I unplugged the distributor vacuum hose and hooked up the vacuum gauge. She was only idling at about 400 rpm in park and had a vacuum of 14 in Hg. Now, I realize that the idle is low, but it had been idling at 800 rpm in park earlier that day and all I did was drive it. 14 in Hg also seems low, but I can't remember what it was when I tuned the carb about 6 months ago. I sprayed in some carb cleaner, but that didn't help. I did notice that if I hit the throttle too hard, I get a screaming sound that resembles a squealing belt, but I think it's coming from around the carb. Might I have a massive vacuum leak? __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

03-07-2012, 12:31 AM	#5
dan42 Registered User Join Date: Feb 2007 Location: Cali Central Coast Posts: 5,191	Re: She idled great, until she didn't. . . Not to jack the thread, but I have a similar problem - it will idle fine after start up and warm up, and then when it's nice and warm, sometimes it won't idle. I'm thinking I have a intermittent vacuum leak, I can't think of anything else that would cause it to go back and forth like that. __________________ '67 SWB Fleetside ZZ4/350/Tremec 5-speed/4link/Scott's IFS - and fun as heck! SOLD Click here to support the board Philippians 4: 6-7

03-07-2012, 03:45 AM	#6
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: She idled great, until she didn't. . . Well, I just went out and tightened down the carb. I probably got about 3/4 of a turn out of each bolt. So, we'll find out tomorrow morning if that helps at all. . . __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

03-07-2012, 01:24 PM	#7
jhwkns Registered Cruiser Join Date: May 2002 Location: Las Vegas NV Posts: 2,230	Re: She idled great, until she didn't. . . a vacuum leak would make it idle faster. Check to make certain that the choke butterfly isn't hanging up. Things like this are hard to diagnose over the www, but you said it runs good cold? __________________ I intend to live forever. So far, so good.

03-07-2012, 01:26 PM	#8
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: She idled great, until she didn't. . . Choke is wide open when warm. Runs fine cold. Posted via Mobile Device __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

03-07-2012, 01:38 PM	#9
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: She idled great, until she didn't. . . Also, I've dealt with a sticking choke and my truck running rich before. I'm almost positive that is not the problem. Posted via Mobile Device __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

03-07-2012, 02:31 PM	#10
CC69Rat Registered User Join Date: Jan 2011 Location: Chattanooga TN Posts: 6,212	Re: She idled great, until she didn't. . . edit posted on the wrong one .. sorry. __________________ Chad 1967 C10 SWB - Project Savannah - 6.0/4L80 Currently underway 1968 C10 SWB - TOTY 2018, 50th Anniversary Tribute Project * Sold * Pride and Joy 1986 Silverado Short Fleet - Scarlet Sold 1985 Silverado Short Fleet Sold 2022 Jeep Gladiator Mojave 2001 Jeep Cherokee XJ 4x4 - Lifted, Built 1992 Jeep Cherokee XJ 4x4 - Lifted 2013 Honda Accord EX-L v6 Coupe 6spd (wife's ride)

03-07-2012, 02:34 PM	#11
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: She idled great, until she didn't. . . Wow. That's frustrating. I do have a spacer and installed new gaskets about 6 months ago. Hopefully it was just loose and tightening the bolts will do the trick. If not, I'll go for. SW gaskets. Posted via Mobile Device __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

03-07-2012, 03:55 PM	#12
treveiger Senior Member Join Date: Jul 2010 Location: South Dakota Posts: 4,093	Re: She idled great, until she didn't. . . Interested Posted via Mobile Device __________________ 1969 Chevy c20(Miss Hackjob)

03-07-2012, 05:41 PM	#13
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: She idled great, until she didn't. . . Tightening the carb didn't help. . . __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

03-07-2012, 10:09 PM	#14
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	Re: She idled great, until she didn't. . . Check your vac can , with a vac pump....see if the can is holding its "mud"? crazyL __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

03-07-2012, 10:39 PM	#15
CC69Rat Registered User Join Date: Jan 2011 Location: Chattanooga TN Posts: 6,212	Re: She idled great, until she didn't. . . Did the line to the modulator valve come off? Maybe a vacuum leak there? Any fluid in the line, white smoke in the exhaust or problems shifting? It could cause the idle fluctuation if the modulator is / or has gone bad. http://auto.howstuffworks.com/automa...smission13.htm Interesting read anyway __________________ Chad 1967 C10 SWB - Project Savannah - 6.0/4L80 Currently underway 1968 C10 SWB - TOTY 2018, 50th Anniversary Tribute Project * Sold * Pride and Joy 1986 Silverado Short Fleet - Scarlet Sold 1985 Silverado Short Fleet Sold 2022 Jeep Gladiator Mojave 2001 Jeep Cherokee XJ 4x4 - Lifted, Built 1992 Jeep Cherokee XJ 4x4 - Lifted 2013 Honda Accord EX-L v6 Coupe 6spd (wife's ride) Last edited by CC69Rat; 03-07-2012 at 10:45 PM.

03-07-2012, 10:40 PM	#16
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: She idled great, until she didn't. . . On the tranny? I checked that. . .still attached. __________________ 1972 Chevy C-10 ~ 188,000 miles give or take



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03-08-2012, 12:39 AM	#17
Coley Registered User Join Date: Oct 2011 Location: Victoria, B.C Posts: 3,794	Re: She idled great, until she didn't. . . Check your fuel line for debris/crap and your fuel filter. Sometimes sediment, etc settles when the truck is cold then migrates up and forward and starts to plug up the carb under pressure/speed. Good Luck Coley __________________ ....for some men, there is experience, skill and effort....for the others...there is visa and UPS LOL 1966 Chevy 1/2 ton (Florida- Red/white) 1972 Chevy 1/2 ton (California- Blue/white) 2005 Chevy Silverado HD2500/Duramax 2000 Dodge Ram 1500

03-08-2012, 02:11 AM	#18
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: She idled great, until she didn't. . . Fuel pump is new as of about 7 months ago, as is the fuel filter. Posted via Mobile Device __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

03-08-2012, 07:11 AM	#19
MagmaJct Registered User Join Date: May 2008 Location: Mesa, AZ Posts: 281	Re: She idled great, until she didn't. . . Check the timing anyway. Does it have points? Is it possible the idle screw backed out? I know these seem unlikely causes, but we want to eliminate potential causes. __________________ I own 3 classic vehicles. 1971 LWB C10 Deluxe. 1974 VW Super Beetle. 1971 VW Fastback. The C10 and the Super Beetle run, the Fastback is just not there yet! Please visit my Super Beetle and Fastback profile on TheSamba.

03-08-2012, 08:10 AM	#20
hjewell2 Registered User Join Date: May 2011 Location: mich Posts: 655	Re: She idled great, until she didn't. . . Change carb see what happens. Pull fuel line off pump and check for flow. yup its new things break. Also check filter, yup its new, they plug up because it is doing the job intended for. After these items are checked and no change, Then you move on to the second of three that make a vehicle run which is electric--air----fuel... I would stab at the electric next

03-08-2012, 08:58 AM	#21
CC69Rat Registered User Join Date: Jan 2011 Location: Chattanooga TN Posts: 6,212	Re: She idled great, until she didn't. . . Points dizzy or HEI ? __________________ Chad 1967 C10 SWB - Project Savannah - 6.0/4L80 Currently underway 1968 C10 SWB - TOTY 2018, 50th Anniversary Tribute Project * Sold * Pride and Joy 1986 Silverado Short Fleet - Scarlet Sold 1985 Silverado Short Fleet Sold 2022 Jeep Gladiator Mojave 2001 Jeep Cherokee XJ 4x4 - Lifted, Built 1992 Jeep Cherokee XJ 4x4 - Lifted 2013 Honda Accord EX-L v6 Coupe 6spd (wife's ride)

03-08-2012, 01:37 PM	#22
Mike Quillici Registered User Join Date: Oct 2009 Location: Sacramento, CA Posts: 161	Re: She idled great, until she didn't. . . HEI. Yeah. Next step is timing. __________________ 1972 Chevy C-10 ~ 188,000 miles give or take

03-08-2012, 01:46 PM	#23
CC69Rat Registered User Join Date: Jan 2011 Location: Chattanooga TN Posts: 6,212	Re: She idled great, until she didn't. . . When you 'got on' the freeway .. did you stand in it? How many miles are on the 350? Could it be the timing chain is streched (I think someone mentioned this earlier if I recall) .. possibly it jumped time? Any cracking / popping / coughing in the exhaust? If you pull the time around a little does the idle increase? __________________ Chad 1967 C10 SWB - Project Savannah - 6.0/4L80 Currently underway 1968 C10 SWB - TOTY 2018, 50th Anniversary Tribute Project * Sold * Pride and Joy 1986 Silverado Short Fleet - Scarlet Sold 1985 Silverado Short Fleet Sold 2022 Jeep Gladiator Mojave 2001 Jeep Cherokee XJ 4x4 - Lifted, Built 1992 Jeep Cherokee XJ 4x4 - Lifted 2013 Honda Accord EX-L v6 Coupe 6spd (wife's ride)

03-08-2012, 01:47 PM	#24
CC69Rat Registered User Join Date: Jan 2011 Location: Chattanooga TN Posts: 6,212	Re: She idled great, until she didn't. . . .. just noticed 188k miles? __________________ Chad 1967 C10 SWB - Project Savannah - 6.0/4L80 Currently underway 1968 C10 SWB - TOTY 2018, 50th Anniversary Tribute Project * Sold * Pride and Joy 1986 Silverado Short Fleet - Scarlet Sold 1985 Silverado Short Fleet Sold 2022 Jeep Gladiator Mojave 2001 Jeep Cherokee XJ 4x4 - Lifted, Built 1992 Jeep Cherokee XJ 4x4 - Lifted 2013 Honda Accord EX-L v6 Coupe 6spd (wife's ride)

03-08-2012, 11:21 PM	#25
stsalvage Scrapper 1 Join Date: Jan 2007 Location: Riverside Calif Posts: 1,649	Re: She idled great, until she didn't. . . I be checking fuel filter and check fuel pressure.How old is the motor how many miles when was the last time you ran some Sea foam gas treatment in it you might of got some water in the carburetor how many fuel filters do you run on your truck.Best to run 2 and the one that is in your Rodchestor carb.I check your modaul in your HEI if its going bad it can cause that. One last thing you might of got some carbon onto the plugs you might want to check your gap and the plugs.When was the last time you pull your carb off to check to see if your carb was not flooding out the motor

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