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#1 |
Registered User
Join Date: Mar 2016
Location: Puyallup, WA
Posts: 235
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Extremely rough idle.
Hi everyone, I've been trying to figure out what is wrong with my 69 c/20. I've got the stock 350 that was rebuilt and 3 days ago it was running fine and all of a sudden it started running like crap at idle. I had it idling at 750-800 and its now idling extremely rough at 500-550. It smooths out at about 1000rpm and up but you can hear a contant faint back fire out the exhaust. I've replaced the cap and rotor, coil, wires, and plugs and nothing has changed. I got a timing light and its at 6 degrees at around 800rpm. I did some math and if the timing was set at 12-13 then if I'm correct and if it jumped a tooth it would be right around 6. I just would like to get some opinions before I go diving into it to put on a new chain.
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#2 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Extremely rough idle.
Bump the timing up to 14.
Plug off the vac pot when you set it. Then turn down your idle down and adjust the mixture screws. |
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#3 |
All about them K’s
![]() Join Date: Jun 2014
Location: Land of Entrapment
Posts: 6,374
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Re: Extremely rough idle.
You can get a decent idea of cam timing by bring #1 up to compression tdc. Pull the distributor cap and look at the rotor. It should be pointing approximately to #1 cylinder. If it's not than you've got issues.
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Tyler '57 3100 http://67-72chevytrucks.com/vboard/s...d.php?t=813888 '72 K20 Cheyenne http://67-72chevytrucks.com/vboard/s...d.php?t=662879 ‘69 K10 SWB http://67-72chevytrucks.com/vboard/s...d.php?t=805206 '98 Silverado LT K2500HD ECLB Vortec 454/4l80E: 6" lift 315/75/16's ‘87 IROC-Z all original 50K mile survivor TPI 305 IROC Blue ‘10 Camaro 2SS/RS Aqua Blue Metallic #93 -version 2.0 ![]() |
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#4 |
Registered User
Join Date: Mar 2016
Location: Puyallup, WA
Posts: 235
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Re: Extremely rough idle.
I know, I am planning on doing that next time I go out to the garage. Just going to take a break from if for a little bit. Was going to be putting new wheels and tires on it in a couple days so after that I'll go back at it.
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#5 | |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Extremely rough idle.
Quote:
That tells you nothing! What if the balancer slipped? And only points at #1 if your timing is zero. If it's advanced or retarded it'll be off quite a bit. |
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#6 |
Custom Sport W/T
Join Date: Nov 2010
Location: Tarpon Springs,Fl & Annapolis,Md. ....................Quaint drinking villages with a fishing problem
Posts: 898
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Re: Extremely rough idle.
You said you have the stock 350 that was rebuilt which in my opinion means a new chain and gears for sure. Unless you have put lots of miles on it since the rebuild you should look elsewhere for the source of the problem unless no new timing set was used. Just my $.02
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#7 |
All about them K’s
![]() Join Date: Jun 2014
Location: Land of Entrapment
Posts: 6,374
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Re: Extremely rough idle.
I had one jump and I did what I mentioned and the rotor was WAY Off... worked for me guess not for you... tore it down and the plastic timing gear had come apart. Maybe I'm having a brain fart but when I brought to tdc I used a screw driver in plug hole, Not the balancer. if the balancer slipped it wouldn't have had any effect on what I was doing. Not trying to argue just stating what I did.
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Tyler '57 3100 http://67-72chevytrucks.com/vboard/s...d.php?t=813888 '72 K20 Cheyenne http://67-72chevytrucks.com/vboard/s...d.php?t=662879 ‘69 K10 SWB http://67-72chevytrucks.com/vboard/s...d.php?t=805206 '98 Silverado LT K2500HD ECLB Vortec 454/4l80E: 6" lift 315/75/16's ‘87 IROC-Z all original 50K mile survivor TPI 305 IROC Blue ‘10 Camaro 2SS/RS Aqua Blue Metallic #93 -version 2.0 ![]() |
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#8 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Extremely rough idle.
Did you know those uncalibrated screwdrivers can be off 20 degrees.
But it matters not! All he needs to do is put some timing in it. |
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#9 |
Registered User
Join Date: Mar 2016
Location: Puyallup, WA
Posts: 235
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Re: Extremely rough idle.
I'd like to also mention there has always been something that sounded like a rattle in the front of the block when idling above 1200 rpm
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#10 |
All about them K’s
![]() Join Date: Jun 2014
Location: Land of Entrapment
Posts: 6,374
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Re: Extremely rough idle.
Ok your right. I'm out of this one.
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Tyler '57 3100 http://67-72chevytrucks.com/vboard/s...d.php?t=813888 '72 K20 Cheyenne http://67-72chevytrucks.com/vboard/s...d.php?t=662879 ‘69 K10 SWB http://67-72chevytrucks.com/vboard/s...d.php?t=805206 '98 Silverado LT K2500HD ECLB Vortec 454/4l80E: 6" lift 315/75/16's ‘87 IROC-Z all original 50K mile survivor TPI 305 IROC Blue ‘10 Camaro 2SS/RS Aqua Blue Metallic #93 -version 2.0 ![]() |
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#11 |
Registered User
Join Date: Sep 2016
Location: Vancouver BC
Posts: 784
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Re: Extremely rough idle.
You sure you don't just have a vacuum leak somewhere? Came on suddenly--smooths out over 1000 rpm--could be a bad vac hose or fitting, or even bad carb to manifold or manifold to head gasket.
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Rick -69 GMC 910 Long Box, 350 -98 Chev Silverado 1500, 350 Vortec 4L60e -08 Mustang GT Convertible |
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#12 |
Registered User
Join Date: Apr 2010
Location: So Cal
Posts: 1,437
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Re: Extremely rough idle.
In Nov you were working on an idea of changing your HEI to an external coil.
http://67-72chevytrucks.com/vboard/s...&highlight=hei I suspect your problems originate from the tach and what you were doing there. Even your HEI wiring leaves a lot to be desired.
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'67 GMC 2500, 292, 4spd, AC |
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#13 | |
Registered User
Join Date: Mar 2016
Location: Puyallup, WA
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Re: Extremely rough idle.
Quote:
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#14 |
Registered User
Join Date: Mar 2016
Location: Puyallup, WA
Posts: 235
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Re: Extremely rough idle.
And I have find of it rodded the engine.... Like going on the freeway going 4k+ rpm a couple times
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#15 |
Registered User
Join Date: Apr 2010
Location: So Cal
Posts: 1,437
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Re: Extremely rough idle.
>>And I have find of it rodded the engine.... <<
In English, that would mean? Try a stethoscope to listen at your noise. Wood dowel, rubber hose or $4 Harbor Freight. ![]() Listen to Alternator bearings, water pump and cam chain cover. Your description of slight backfire might be exhaust leak. Listen along exhaust manifold and valve cover. Remove distributor cap. Rotate crank until distributor rotor moves, then rotate crank in the opposite direction until rotor moves again.
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'67 GMC 2500, 292, 4spd, AC |
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#16 |
Account Suspended
![]() Join Date: Jul 2016
Location: Tacoma, WA
Posts: 6,873
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Re: Extremely rough idle.
You didn't skip a tooth unless you have a very old used up nylon timing gear in it. If its rebuilt it then the timing hasn't changed.
Push the secondary throttle closed, I bet its hung open a little bit and pouring gas in the engine (explains the slight backfire sound), also look down the primary and see if fuel is dripping from the carb venture at idle, there should be no drip at all, if its dripping then you need to close the throttle idle screw and add timing. Add timing for sure...add at least 6 degrees and back off the idle screw. |
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#17 |
Registered User
Join Date: May 2016
Location: Henderson NC
Posts: 975
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Re: Extremely rough idle.
Take the belts off/loose and start it up. If the rattle goes away it's not the timing chain.
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#18 |
Registered User
Join Date: Mar 2015
Location: Seaside,CA
Posts: 142
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Re: Extremely rough idle.
Just thinking aloud here. Did you install a new cam or lifters?
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#19 |
Registered User
Join Date: Oct 2005
Location: Metro Vancouver
Posts: 444
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Re: Extremely rough idle.
x2
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1972 Chevy C10 Custom Deluxe Highlander 1972 Chevy C10 Custom Deluxe Medium Green |
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#20 |
Registered User
Join Date: Mar 2016
Location: Puyallup, WA
Posts: 235
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Re: Extremely rough idle.
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#21 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Extremely rough idle.
Did you bump up your timing yet??
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#22 |
Registered User
Join Date: Mar 2016
Location: Puyallup, WA
Posts: 235
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Re: Extremely rough idle.
Hello everyone. I did some more research and pulled the cap and tested how many degrees my crank would turn counter clockwise until the rotor on the distributor would start moving. It took 12 degrees of the crank untill the rotor would move. This is one sloppy chain. A chain that needs replacing should be no more than 7.
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#23 |
Registered User
Join Date: Jul 2015
Location: Central Coast, CA
Posts: 500
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Re: Extremely rough idle.
Thanks for reporting back!
It is very good etiquette because as you can see lots of people are racking their brains and most of us have been there trying to chase down these gremlins.. Always appreciate seeing the eventual solution. I was going to guess the carb hold down bolts needing to be re-torqued 'cause those gaskets are so thick and the torque is only 60 to 80 inch-pounds and they tend to loosen up a lot in the first running Member Longbox mentioned this already I just noticed.. |
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#24 | |
Senior Member
![]() Join Date: Apr 2016
Location: Cypress, TX
Posts: 3,702
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Re: Extremely rough idle.
Quote:
I've been watching this thread with interest. You mentioned in the first post about the 350 being rebuilt 3 days ago, so this has me curious. Is this a failed new component, or missed during examination of the components, something that would have been replaced with the rebuild, other things were focused on during the rebuild, just plain bad luck, or don't know yet? |
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#25 | |
Registered User
Join Date: Mar 2016
Location: Puyallup, WA
Posts: 235
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Re: Extremely rough idle.
Quote:
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