help me find this miss in my 305 please!!! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bloodybigblock · 07-22-2013, 08:15 PM

Okay so I have a 305 that is all stock minus a holley carb. When I first got the truck it would sputter at any cruising speed. I fixed the vacuum advance and a vacuum leak under the carb. After that was fixed and I drove it, it has a miss now. I checked for every vacuum leak and they turned out good. The plug wires aren't on the manifolds. I pulled the plug wire for each while running and number 6 was getting spark but didn't change the idle. Compression on that was good and I changed the plug and wire and still doing the same thing. Then changed the rotor button and cap with no luck still. I do know at higher rpm you cant tell it has a miss and you cant really tell at idle, just when you give it gas. Like I said, I cant tell any issue at cruising speed. Anything else I can check. I'm losing my mind. Thanks guys

68gmsee · 07-23-2013, 08:48 AM

No, you ain't losing your mind. A miss is not an easy thing to diagnose unless you're lucky or have expensive equipment to diagnose. Most idle misses are hard to detect at higher rpm's but it could be internal engine related like valve spring, cam or valves themselves. May even be bad gas.

Most of the time I've diagnosed a miss on my old trucks it's turned out to be carb adjustments, ignition or vacuum leak type stuff. One time I had an elusive miss that turned out to be a new defective spark plug. I swapped plugs one by one and found the culprit. Last couple of time have been an intake gasket leak on my 250 engine and an idle air adjustment on my 350.

geezer#99 · 07-23-2013, 09:20 AM

Pull the vac hose off your distributor and plug it and then see what it does. Could have too much advance at low rpm.

motornut · 07-23-2013, 10:34 AM

have you blocked off any hoses?
my 79s large line ,carb base to charcoal cannister was split along the bottom,others too.

hows the fuel pump?
my 78 had rusty gunk between the pump and the Holley carb,removed line and cleaned out.

gas flow ok?
my 79 had a cracked sender/return lines,could see fuel vapours/spitting over the tank

i've also noticed that the small tubes for hoses can loosen up,slightly pull out
(from the stock carb) as we wiggle the hoses on off

07-22-2013, 08:15 PM	#1
bloodybigblock Registered User Join Date: Jul 2011 Location: hanceville alabama Posts: 268	help me find this miss in my 305 please!!! Okay so I have a 305 that is all stock minus a holley carb. When I first got the truck it would sputter at any cruising speed. I fixed the vacuum advance and a vacuum leak under the carb. After that was fixed and I drove it, it has a miss now. I checked for every vacuum leak and they turned out good. The plug wires aren't on the manifolds. I pulled the plug wire for each while running and number 6 was getting spark but didn't change the idle. Compression on that was good and I changed the plug and wire and still doing the same thing. Then changed the rotor button and cap with no luck still. I do know at higher rpm you cant tell it has a miss and you cant really tell at idle, just when you give it gas. Like I said, I cant tell any issue at cruising speed. Anything else I can check. I'm losing my mind. Thanks guys

07-23-2013, 08:48 AM	#2
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: help me find this miss in my 305 please!!! No, you ain't losing your mind. A miss is not an easy thing to diagnose unless you're lucky or have expensive equipment to diagnose. Most idle misses are hard to detect at higher rpm's but it could be internal engine related like valve spring, cam or valves themselves. May even be bad gas. Most of the time I've diagnosed a miss on my old trucks it's turned out to be carb adjustments, ignition or vacuum leak type stuff. One time I had an elusive miss that turned out to be a new defective spark plug. I swapped plugs one by one and found the culprit. Last couple of time have been an intake gasket leak on my 250 engine and an idle air adjustment on my 350. __________________ 68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...

07-23-2013, 09:20 AM	#3
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,524	Re: help me find this miss in my 305 please!!! Pull the vac hose off your distributor and plug it and then see what it does. Could have too much advance at low rpm.

07-23-2013, 10:34 AM	#4
motornut 78K & 79C Jimmys Join Date: Nov 2009 Location: Ottawa Ont CANADA Posts: 7,901	Re: help me find this miss in my 305 please!!! have you blocked off any hoses? my 79s large line ,carb base to charcoal cannister was split along the bottom,others too. hows the fuel pump? my 78 had rusty gunk between the pump and the Holley carb,removed line and cleaned out. gas flow ok? my 79 had a cracked sender/return lines,could see fuel vapours/spitting over the tank i've also noticed that the small tubes for hoses can loosen up,slightly pull out (from the stock carb) as we wiggle the hoses on off __________________ John 1978 GMCJimmy4X4-350/203 1979 GMCJimmy4X2-305/350



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