Here's the situation.

If I'm cruising down the road, and start going down hill, when I fully let off the throttle the car bucks back and forth in a rhythm and theres a slight knocking noise. It's not violent or anything, but I wanna know whats wrong. If I push in the clutch than it goes away, until I let the clutch out again. So its something in the drive train.

If I just keep it at a steady throttle it doesn't do it, only when I'm completely off the gas.

It's just annoying, and I hope it isn't a serious issue. I can live with it, I just wanna make sure that it isn't a bad thing.

Any idears?

 

Slight knocking noise is probably the driveline. Could be excessive play in 3rd member or U-joints.

The bucking could be associated with the jetting in the carb. The transition from throttle to idle system.

 

All 3 of mine (even the original 82 with 20K miles when I bought it in 83) have or had some bucking when decelerating. It should not be horrible, just annoying, like being in 5th gear at 30mph and foot off pedal....

Now I drove an SE that was real loose in the back end and it bucked horribly even under power. I would stop driving THAT if it were my car and it bucked under load to find out exactly what was lose/worn. Yours sounds like what mine have been, fairly normal (in my sample of 3).

The Doc has give you some suggestions that of course are right on the money.

 

Coasting valve, the knock in the drivetrain is due to normal wear that the bucking accentuates.

 

He's running an aftermarket carb, no emissions I presume.

 

Missed that one doc, thanks. Still believe it may be carb related though, that's where I would start.

 

Could it be my timing? Cause I have my vac advanced hooked up to a nipple on the intake manifold, and I was told that that is the wrong place to hook it up.

The nipple in the intake manifold is getting full vacuum at idel.

 

Oh yeah, and on decel the vacuum increases even more. That's a good spot for a vacuum gauge, definitly not good for vacuum advance.

 

I'll disconnect the vacuum line and take it for a spin tomorrow to see if that solves it. There is a nipple on my Weber that is specifically for vac. advanced, Ill hook it up to that...or lock my dizzy.

 

Mine does the same thing. End my car is running perfect recently rebuilt and everything. I just assume its the way of the rotary

 

Nope, that is not how they are supposed to run. Neither my nearly stock 2nd gen, nor the widebody 1/2 bp 12A with different carbs has ever done that.

 

Mine does it too i have a 85 GSL bone stock, new fuids, filters, plugs only 130K on it. It happened to me on the dyno i was in 3rd when at about 3K and holding the gas steady, when it just started bucking VOILENTLY! and almost fell off the machine. We then put tie downs and tried again but to avail it did the same thing, tried giving it gas, just got worse and died, even when i got on the clutch it died. then it wont start for about 20-30 minutes then will start up like nothing wrong till i reenact that moment. and for the record it did it in 2nd and 4rth so gear doesnt matter. i already tried a new fuel filter, and nothing. new tank nothing, pump nothing. im running out of ideas besides the carb. or a spark problem i heard maybe the coils. it has been sitting for 12 years.

 

A worn 3rd-member will also exhibit this when trying to gently reverse uphill from a dead stop. It usually gets worse when the gear oil is warm, too, as from long drives.

 

I think it has to do with the shutter valve that controls the idols as you let the gas go. Mine was doing the same thing. I replaced the shutter valve and it stopped jerking back & forth once you let the gas go.

 

yeah this is not normal, unless you're under/over shifting, mine doesn't do it unless my rpm has dropped more than the gear its in needs, but any manual shift car does that. or maybe mine's screwed up and the normal is to buck a little bit :P lol

 

ive driven plenty manual cars and they are NOT supposed to buck like mines did! it was jerking to pieces. best way i can describe it is like a shark attack, thrashing back and forth.

 

You're missing the deceleration dashpot on the carb. It lets the throttle close slowly to avoid backfire, and to a lesser degree, what you're experiencing.

 

okay but to my knowledge i wasn't decelerating i was holding my foot steady on the gas, maybe accelerating if anything. how would i know? if something was missing? i know my carb needs a rebuild because its 12 years off the road, and my choke cable get stuck open its all rusty. I though of you actually due to the recommendations from the masses. Ill PM you.

 

Mine does it, it seems carb related to me. I'm running a Dellorto DHLA though, don't know if it's a property of that. It's like someone aove described, coasting in gear, off the throttle, but only when I'm near idle. Like under 2,200 RPM like 30MPH in 5th, etc.

 

It might be two things; the motor and tranny mounts are old and worn and when
you use the engine as a brake (which is whats happening when you let off the gas
and keep it in gear) it may buck a bit because of slight differences in compression on
the rotor faces (just my little theory) or its caused by the vacuum advance being
hooked to the intake like Trochoid said.

I have it that way on my Dell and if I get it in just the right spot off idle I can make it
buck like this at will but its easy to control. I know I'm not supposed to use straight
intake vacuum on the advance so lets not debate it but it really makes a huge drivability difference other than this small issue. If the Dell had a real advance nipple I would use it.

 

Rotor_Venom08: What you're describing sounds like your carb's tune is off. My car did the same thing when I had too big of idle jets in my Weber.

 

there's nothing like getting major whiplash by putting it in first gear and quickly pressing and letting go of the gas lol, don't even have to open up the secondaries.