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Old 01-12-2021, 05:42 PM   #1
steeeeeve
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vibration maintaining cruising speed

hi fellas

68 c10
350
700r4

the truck is running great but if i'm cruising +-60mph with about 1/8 throttle to maintain speed i get a vibration that feels like i'm running over small shoulder bumps. if i let off the gas it goes away, if i give it a touch of gas it goes away.
i can't tell if its the engine or transmission.
any ideas what to look for? or just avoid 1.8 throttle position?
thanks steve

**** i'm thinking it's getting lean between idle circuit to cruise

Last edited by steeeeeve; 01-12-2021 at 06:24 PM.
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Old 01-12-2021, 06:25 PM   #2
Southcity
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Re: vibration maintaining cruising speed

Hard to say what it is and I'm certain others will chime in with suggestions. I usually start with the easy stuff and use the process of elimination.

1. Any balance weights missing from any of your wheels?
2. Check to see that motor/transmission mounts are ok
3. Did it just start happening recently?
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Old 01-12-2021, 06:42 PM   #3
geezer#99
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Re: vibration maintaining cruising speed

Might be a transition spot between idle and power circuits in the carb.
Try in 3rd gear at the same speed to see what it does.
If no vibration try in 3rd gear at the same rpm as it is in o/d.
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Old 01-12-2021, 06:43 PM   #4
1970 CST Short Wide
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Re: vibration maintaining cruising speed

1st thing I would check would be U Joints and Center Support Bearing
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Old 01-12-2021, 06:52 PM   #5
67/Heaven
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Re: vibration maintaining cruising speed

Quote:
Originally Posted by 1970 CST Short Wide View Post
1st thing I would check would be U Joints and Center Support Bearing

+1 I had the same issue and turned out to be U Joints
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Old 01-12-2021, 09:21 PM   #6
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Re: vibration maintaining cruising speed

I would also try putting in neutral, then vary the throttle.
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Old 01-12-2021, 09:27 PM   #7
Corts60
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Re: vibration maintaining cruising speed

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Originally Posted by 1970 CST Short Wide View Post
1st thing I would check would be U Joints and Center Support Bearing
x2
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Old 01-12-2021, 09:44 PM   #8
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Re: vibration maintaining cruising speed

Check if any exhaust equipment is close to anything. At that speed and torque, an exhaust part can lightly touch something and cause a noise. Coasting and harder acceleration could change the light contact and reduce the noise.
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Old 01-13-2021, 08:38 AM   #9
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Re: vibration maintaining cruising speed

Is the truck lowered? It could be the driveshaft angle as well as worn driveline components.
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Old 01-13-2021, 10:46 AM   #10
steeeeeve
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Re: vibration maintaining cruising speed

thanks for all this info guys. i will check the suggestions and look for witness marks.

more info on the truck

new 350 from jegs installed last year
rebuilt 700r4 installed a few month ago from a reputable trans shop
new driveshaft/u-joints (converted to 1 piece during trans swap)
truck is bagged
new tucked exhaust as of about a week ago. shorty hedman headers.

i've noticed it make the rumble if my foot is just slightly on the gas pedal at cruising speed. just a touch more and it goes away.

then during slow speed acceleration i think i can feel the rumble in that sweet spot for just a short instant.
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Old 01-13-2021, 01:11 PM   #11
Rich72C10
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Re: vibration maintaining cruising speed

I have this, I think, but in my case it is right around 40mph. It's like a harmonic distortion/low rumble and what you describe with giving it a bit more gas alleviates it - though if the speed limit is 40mph that is not much of a solution ha!

I suppose the same suggestions apply, even though I don't have a 700r4 but a TH350 (with 2 piece drive shaft) ? I wished I had a 700r4!
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Old 01-13-2021, 05:35 PM   #12
steeeeeve
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Re: vibration maintaining cruising speed

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Originally Posted by Rich72C10 View Post
I have this, I think, but in my case it is right around 40mph. It's like a harmonic distortion/low rumble and what you describe with giving it a bit more gas alleviates it - though if the speed limit is 40mph that is not much of a solution ha!

I suppose the same suggestions apply, even though I don't have a 700r4 but a TH350 (with 2 piece drive shaft) ? I wished I had a 700r4!
you could always let off and go a bit under the speed limit. ha! not an option

but ya. I could hold the pedal in that position and have it make that rumble strip noise for miles and miles then just give it just a touch more gas and it goes away. so weird.
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