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#1 |
Registered User
Join Date: May 2015
Location: Central Massachusetts
Posts: 495
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350 has bad misfire/shudder over 2000RPM
Just put my '79 C30 service truck on the road recently. After a long 6 months of building it in the driveway, I was pretty darn excited to start working with it... until I discovered a really bad misfire/shudder over 2000RPM when under load. Which brings me here!
Everything has been serviced/gone through on the engine... plug wires, plugs, cap, rotor, distributor all rebuilt with new parts, carburetor rebuilt, vacuum lines all replaced, PCV valve replaced, etc. -Has a Rochester Quadrajet carb -Timing is currently set at 10deg (with dist vacuum like unplugged) -AC R45TS plugs gapped at .045 -Dist power wire is good and getting full current from the fuse block -Truck grounds are good and I added extra cables frame to engine -While running, touched each plug boot with a screwdriver going to ground and nothing arced -Previous owner installed an RV cam (whether it makes a difference...) Took it on a service call this AM and had to turn back. At highway cursing speed (55-60) it was awful. Goosing it will cause the misfire will go away briefly, then come back once you let off. Climbing a big hill, mat the pedal and it goes away for about 20-30sec, then gets really really bad... stumbling all over, won't catch up with itself. The fuel pump did come to mind, but even if the truck is sitting there at an idle and you take off hard, it will still stumble once over 2000rpm or so. Any rpm under that and it's fine. Any thoughts? Have some work scheduled in the next week+/- so would like to remedy this soon. |
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#2 |
Registered User
Join Date: Dec 2014
Location: Euless, Texas
Posts: 511
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Re: 350 has bad misfire/shudder over 2000RPM
I would think more about fuel pump, fuel filter in carb. Just spitballing here could be something else. Just a thought.
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1974 Cheyenne 10 LWB Stock 350 W/HEI /TH350/AC/4 BBL Quadrajet Mopar by Birth Chevy by Choice |
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#3 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,599
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Re: 350 has bad misfire/shudder over 2000RPM
I’m thinking the ‘dreaded flat cam’.
Have you checked all the rockers move the same? Was this a used motor? Or a rebuilt. |
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#4 |
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Join Date: Oct 2009
Location: Buhl, Idaho
Posts: 1,438
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Re: 350 has bad misfire/shudder over 2000RPM
Is the vacuum advance working? should be about 34-36 degrees around 3,000 rpm.
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86 c30 crew cab dually 454/400 4/6 drop SOLD 1-8-11 1981 chevy K10 short Bed Scottsdale SOLD 10-26-2020 67 GMC Stepside Straight six/3 on the tree "The Peach" 2020 Silverado Double Cab LT Z71 5.3 8 speed Daily Driver K10 build http://67-72chevytrucks.com/vboard/s...d.php?t=671934 The Peach build http://67-72chevytrucks.com/vboard/s...d.php?t=733903 ![]() |
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#5 |
Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,540
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Re: 350 has bad misfire/shudder over 2000RPM
Possible vacuum leak around intake gasket or carb base gasket. Do the spray with carb cleaner test.
Does it still have the exhaust riser valve? Those can stick shut and force half the exhaust through the intake crossover which kills higher rpm performance and can overheat the carb. Does it still has the original Y pipe? Some of those had double walls and the inner wall would collapse, mostly blocking the exhaust, Also killing performance. Check the rubber lines from the tank(s) to the frame. If those are original, they are probably crumbling away and letting air into the fuel system. Same for rubber lines from dual tank valve to frame if equipped. Even if hoses recently replaced, mice sometimes like to sample taste the hoses, also letting air in. Nice looking service truck. |
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#6 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,599
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Re: 350 has bad misfire/shudder over 2000RPM
Thinking more on it, might not be cam.
Could be fuel pressure problem. Too much pressure in your case. Who rebuilt the carb? |
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#7 | ||||
Registered User
Join Date: May 2015
Location: Central Massachusetts
Posts: 495
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Re: 350 has bad misfire/shudder over 2000RPM
Quote:
Quote:
Quote:
Riser valve is still there, though I think the PO cut the butterfly out because you can almost make a full turn with the bell-crank. Y-pipe is gone. Currently has dual straight pipes with glass packs, no crossover pipe. Gas tank lines have all been re-done too (and filter). Quote:
I rebuilt it. Very familiar with carburetors, but this was my first time rebuilding a Q-jet, so wouldn't be surprised if something didn't turn out right... Going to check for vacuum leaks and fuel pump functionality today... |
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#8 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,599
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Re: 350 has bad misfire/shudder over 2000RPM
If the clip on the needle is on wrong or not used it can cause flooding like that.
Info in here on that. http://www.crankshaftcoalition.com/w...4MV_carburetor |
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#9 |
Just here to tinker
![]() Join Date: Jun 2008
Location: Holland, MI
Posts: 3,692
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Re: 350 has bad misfire/shudder over 2000RPM
Vacuum advance is working but does mechanical advance move freely? When your foot is into it, no vacuum will be there to advance so it will be relying on the weights. Because you say it is only over 2K RPM under load, this makes me think it is not advancing. But check your fuel system first like everyone else stated.
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78 C10 SWB Diesel #1 (wrecked)http://67-72chevytrucks.com/vboard/s...1978+silverado 78 C10 SWB Diesel #2 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=650751 60 GMC Suburban (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=445526 60 GMC 1000 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=298235 67 GMC 1500 (sold) http://67-72chevytrucks.com/vboard/s...d.php?t=803695 86 Suburban(summer unit) http://67-72chevytrucks.com/vboard/s...d.php?t=810697 88 Suburban 4x4(winter unit)https://67-72chevytrucks.com/vboard/...25#post8900725 |
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#10 | ||
Registered User
Join Date: May 2015
Location: Central Massachusetts
Posts: 495
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Re: 350 has bad misfire/shudder over 2000RPM
Quote:
Quote:
-So, fuel pump is good. -Turns out the carburetor wasn't bolted down tightly, never re-checked after I installed and ran it. -No vacuum leaks around the manifold or carb base (after tightened) -Set the carb idle screws by the book with a tach and it sounds much smoother at idle and starts better After that, it still misfired when shifting up, even going down hill. So I pulled a plug wire and hooked up one of those inline spark testers. When running it jumped the 3/16 gap, but the spark was orange at best. Leaning towards weak spark now... Never replaced the coil when I rebuilt the distributor, did everything but... dang! Going to pick one up tomorrow. |
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#11 |
Registered User
Join Date: May 2016
Location: Willow, Alaska
Posts: 877
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Re: 350 has bad misfire/shudder over 2000RPM
Difficult to start when hot is also a symptom of tight valves. If you still have the older style heads without hardened seats, you may have valves receding into the heads.
Also, to somewhat mirror one of the suggestions above, aside from checking for flat cam lobes, also check the lobe that drives the fuel pump for wear. |
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#12 |
Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,540
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Re: 350 has bad misfire/shudder over 2000RPM
Might double check that the plug wires are in proper firing order. Some swapped pairs run surprisingly well at idle and near idle speeds.
Check that all the screws on the carb top are tightened. There are many and it is easy to skip one. Especially the pair inside under the choke flap. Run it for a bit and check plugs for color. Might be able to find the problem cylinder(s). |
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#13 |
Registered User
Join Date: Jul 2007
Location: Toppenish, WA
Posts: 15,500
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Re: 350 has bad misfire/shudder over 2000RPM
The 454 in my Dually is doing pretty much the same thing and I need to get it sorted out. It started on the way back from the dump one day and spits back when I accelerate.
Carb bolts coming loose isn't something you usually think about though. I figured the cam went flat on it. The smoke from the fires is so bad here that I haven't been wanting to stay outside long enough to fool with it.
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Founding member of the too many projects, too little time and money club. My ongoing truck projects: 48 Chev 3100 that will run a 292 Six. 71 GMC 2500 that is getting a Cad 500 transplant. 77 C 30 dualie, 454, 4 speed with a 10 foot flatbed and hoist. It does the heavy work and hauls the projects around. |
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#14 |
Registered User
Join Date: Jun 2014
Location: CO
Posts: 738
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Re: 350 has bad misfire/shudder over 2000RPM
Pour some Carb/fuel injector Cleaner down vent tube in carb. You could have some carbon or debris in carb bowl. I did this and idle smoothed out nicely
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79 Chevy 2wd BIG10 350 TH400 |
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#15 | |
Registered User
Join Date: May 2015
Location: Central Massachusetts
Posts: 495
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Re: 350 has bad misfire/shudder over 2000RPM
Quote:
Thanks for all the help and suggestions guys. Luckily it was just a bad ignition coil. Replaced it this AM and that 350'll wrap up tight now with no misfire. Glad it wasn't cam or valves related, time's been too short lately for major engine work! |
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#16 |
Registered User
Join Date: Jan 2007
Location: vernon b.c.
Posts: 3,022
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Re: 350 has bad misfire/shudder over 2000RPM
How did you figure out it was the coil. I would never of thought of looking there and how would the coil do that. I thought coils worked or didn,t
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#17 | |
Registered User
Join Date: May 2015
Location: Central Massachusetts
Posts: 495
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Re: 350 has bad misfire/shudder over 2000RPM
Quote:
It does make sense, I run into that with Kohler engines and Gravelys. The old Kohler K series will idle fine on a weak spark, but try to put a load on or rev up and it falls on its face. Gravely's with Wico magnetos will start and run fine, throttle up okay, but one sudden load and it immediately stops firing and dies. Some will run but have choppy intermittent fire (like the 350 at high RPM's). |
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#18 |
Just here to tinker
![]() Join Date: Jun 2008
Location: Holland, MI
Posts: 3,692
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Re: 350 has bad misfire/shudder over 2000RPM
Glad you got it solved. Great work!
__________________
78 C10 SWB Diesel #1 (wrecked)http://67-72chevytrucks.com/vboard/s...1978+silverado 78 C10 SWB Diesel #2 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=650751 60 GMC Suburban (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=445526 60 GMC 1000 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=298235 67 GMC 1500 (sold) http://67-72chevytrucks.com/vboard/s...d.php?t=803695 86 Suburban(summer unit) http://67-72chevytrucks.com/vboard/s...d.php?t=810697 88 Suburban 4x4(winter unit)https://67-72chevytrucks.com/vboard/...25#post8900725 |
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