HELP.......Truck has major shudder.

[Mccoleman830]

I fired up the truck today to discover that my 78 has a major shudder problem. Trying to diagnose the problem has left me clueless. It shudders extremely bad when in reverse, and all drive gears. When placed in park and neutral it still has a slight vibration. I tried to drive it down the street, but while a stop light you have to keep your foot on the gas pedal or it has a violent shake. Any advise where to look at would be great. 1978 GMC K-15 350/350. The only thing ive done lately was get a fill the tank with fuel. I thought it might be bad fuel, but switching tanks from right to left doesn't help at all.

Thanks.

[Stepper562]

try the obvious and most simple thing first like checking how high/low your idle is in park, bad fuel filter if you have one, bad carb, tune up etc...little more ibfo is required in order to diagnose the problem.

[ratrod67]

Sounds like U joint or carrier bearing. Maybe check your transmission mount also.

[Eyegore]

Originally Posted by ratrod67

Sounds like U joint or carrier bearing. Maybe check your transmission mount also.

Add motor mounts also.

[rwhit57]

Could be something as simple as a vacuum line loose. Check things out vacuum lines, plug wires, etc.

[HTX86chevy]

Originally Posted by rwhit57

Could be something as simple as a vacuum line loose. Check things out vacuum lines, plug wires, etc.

x2............... i recently messed with the vacuum lines on mine and it started having the same problem youre having. Check Vacuum lines, i bet you'll solve your prob

[Mccoleman830]

More info........while idling in park my tach reads at 800 rpms. When in drive it drops down to 600 rpms. Also checking out my edelbrock 1400 carb, the left side mix screw doesn't affect engine idle. The right side screw does affect engine idle, if that makes since at all. Adjusting fast idle screw and shudder does not go away.

[68GMCCustom]

are you sure its firing on all 8?

[71swb4x4]

Since you have it at idle you can eliminate the transmission and back.
My "guess" would be that you are missing on a cylinder or more. I would check the distributor/wires/plugs first. Start with the simple things.

[Mccoleman830]

Weird, I just happened to tightened the carb bolts to the intake manifold, and start the truck. After doing that the truck runs perfect. I drove around the block and by the time i got back home it started shuddering again. I'm thinking that maybe I need to re-check all bolts and nuts on the carb and intake. It has been 5-6 years since I've had the intake manifold off. Thanks everyone for your ideas and input.

[71swb4x4]

Might be a vacuum leak if tightening the bolts helped.

[Stepper562]

Best way to check for leaks around the manifold and carb is to spray around it while running. Listen for a change in idle it'll be an obvious change. Also +1 on the vaccum lines. Check for dry, cracked and disconnected lines. Replace if needed.
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[71swb4x4]

Stepper is saying to spray something flammable, usually people spray carb cleaner.

[motornut]

check your fan
my 4 blade fan lost one took out my lines master cylinder to the valve on the front crossmember
vibrated after a lot but maybe it would too just before letting go
surprised me it could do that

[Stepper562]

Originally Posted by 71swb4x4

Stepper is saying to spray something flammable, usually people spray carb cleaner.

Yep sorry about that...carb cleaner is the best and cheapest way to go. Thx for clearing that up.
Posted via Mobile Device

[amodedude1]

an UNLIT (lol) propane torch works too, when the propane gas finds a leak your idle speed will audibly change and will narrow down your search, somewhat safer than carb cleaner. Be very careful with carb cleaner, don't do it right after you drove since as they said, it's flammable.

[rwhit57]

Sounds like it's time for a new carb gasket, some vacuum lines, and general tune-up. Snug the intake bolts down don't tighten them hard. Still sounds like vacuum leak to me.

[irishman999]

Take a look at the harmonic balancer.

[Mccoleman830]

I'm thinking I have the shudder narrowed down to the carburetor. While spraying carb cleaner around the seams and around the back of my Edelbrock 1400 the truck revs slightly higher. My question is, What is this valve in the picture? I believe it goes to the vacuum source for the transmission. When i took carburetor off I noticed it was loose and bent. Is it even worth replacing this unit? I'm going down to the parts store to get a full rebuild kit for the carburetor and hopefully it will take care of this problem.

[motornut]

loose and bent.....fixing that can't hurt
yup looks like the vac line for the tranny