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1979.5, 720 4x4 - High speed Vibration on deceleration or engaging clutch
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Author:  gone2long [ Wed Apr 24, 2019 6:09 am ]
Post subject:  1979.5, 720 4x4 - High speed Vibration on deceleration or engaging clutch
OK gang, here is another challenge.

Background: I am living in the boonies of SE Asia, and I am somewhat mechanically challenged, so the normal fixes don't always apply. I have a 1979.6 Datsun 720 4x4, with a stock, 5 speed manual transmission and I had a new clutch and throw out bearing installed about three years ago. Have only maybe 5,000 miles on her since then.

The truck was recently tuned and it is running well. Around the same time, I added 90 weight oil to the forward differential, which seemed kind of dry when I opened the plug. Mia culpa : ( In any case, after adding the oil I noticed that, when running at high speed in 5th gear and easing off the gas pedal or hitting the clutch pedal on a downshift, I would hear a rattling sound, kind of like a BBBRRRRRRR - as if something was vibrating intensely right under the shifting boot. As soon as i take my foot off the clutch or hit the gas, the sound goes away.

I am wondering what could be causing this sound, and if it might be an indication of something serious. The throw out bearing and clutch seem to be OK, as I have no problem shifting and engaging the clutch at low speed, and I do not hear the sound at low speed. What might be causing it?

Thanks to everyone for all your help in the past.

G2L
Author:  gone2long [ Wed Apr 24, 2019 6:17 am ]
Post subject:  More Details - High speed Vibration on deceleration or engaging clutch
More background: I drove about 100 miles straight after the lube and did not notice the rattle/vibration.

The rattle occurred during another 150 mile drive taken after a new muffler was installed. I suspected the muffler as a problem initially so I looked at it and the exhaust pipe, but it did not appear that there were any holes, broken or loose segments in the muffler or exhaust pipe welds.

The muffler, however, was "home made " by a local shop, so the muffler itself seems like an "X factor".

However, it also seems that there isn't much in a muffler that could be causing a vibrating sound, occurring only on deceleration or engaging the clutch.

What think ye?
Author:  Dave [ Wed Apr 24, 2019 9:11 am ]
Post subject:  Re: 1979.5, 720 4x4 - High speed Vibration on deceleration or engaging clutch
Loose baffle, etc in muffler ? Exhaust pipe / muffler contacting something (body / chassis) as engine moves on its mounts when you back off the accellerator ?
Author:  gone2long [ Thu Apr 25, 2019 4:31 am ]
Post subject:  Re: 1979.5, 720 4x4 - High speed Vibration on deceleration or engaging clutch
Yeah, thanks. Gonna get under there soon and start shaking things around, see what I can figure out.

Best,

G2L
Author:  nick [ Thu Apr 25, 2019 10:21 pm ]
Post subject:  Re: 1979.5, 720 4x4 - High speed Vibration on deceleration or engaging clutch
If it was a loose baffle in the muffler i reckon it would make the sound at standstill by revving the motor but it seems load related. As its only under deceleration or engine braking and not under acceleration it does sound like its contact as the motor twists one way. As Dave said, check the exhaust (especially the muffler) has adequate clearance all around it right from the engine all the way back to the exhaust tip and give it a real firm shake to see if any part of it is loose and wobbling where it could make contact on something. Also check your engine mounts and gearbox mount for cracked rubbers or missing/loose bolts...
Author:  gone2long [ Sat May 04, 2019 6:41 pm ]
Post subject:  Latest Update - Re: 1979.5, 720 4x4 - High speed Vibration on deceleration or engaging clutch
Latest Update:

Went to the muffler shop. Guy rattled the exhaust pipe, muffler, and resonator. With the truck running, he found leaks in the resonator weld, and starting tapping it. His analysis was that it was shot, and all the interior had essentially decayed and that we needed to replace it.

We cut it out and tapped some more. It sounded very hollow and was kind of light, so he took that as a verification that it was shot. However, when I shook it, no rattling sound occurred. Put on a new, home-made one, which was a little heavier and did not make a hollow sound. Sounds fine when idling or running at low speed. Have yet to test it in 5th running at high speed and coming off the throttle or engaging clutch, but I explained those symptoms to him, and he seemed convinced that the resonator being shot was causing them.

Comments anyone? Think he knows what he's talking about?

Will report back after I test her at high speed on the highway. BTW, there are no rubber connectors holding the exhaust system in place. Could not find the parts, so the exhaust is Oki-rigged onto the frame using welded rebar. Not a great solution of course, but the only one available at the time. The vibration from the road and years of use/rust probably caused the resonator welds to leak, so it was probably a good idea to replace it anyway. I'm pretty sure it is the original, dating back to 1979.5.

The latest work cost a total of $23 (Philippine peso equivalent) including the new resonator and the labor. Can't complain at that price.

That's it for now. Thanks for all the great advice - G2L
Author:  nick [ Sun May 05, 2019 3:29 pm ]
Post subject:  Re: 1979.5, 720 4x4 - High speed Vibration on deceleration or engaging clutch
All exhausts should have rubber hangers to reduce noise and vibration in the cab aswell as minimise strain on the exhaust parts when the motor moves on its rubber mounts. You can make them easy enough using strips of 6 or 8mm thick insertion rubber cut into approx 25x50mm and some 4x25mm flat steel bar for weld-on tabs and then bolt them both together. Either that, or cut any old factory hangers off at the wreckers and just re-use those.
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