"Trailer Hitching" ARghhh, Please advise

[bill black]

Hello all, I hope you can shed some light!

1969 Sport Satellite, 383, 4 speed, 3.55 rear, 255/60/15 rear 215/60/15 front, 650 double pumper, Edelbrock RPM manifiol, "lumpy" cam (don't know specs), headers..


My car "trailer hitches" on light acceleration and deceleration and it's driving me freakin nuts. The problem is most apparent when I decelerate in 1 st or second and follow that up with immediate and light acceleration: The car jerks back and forth until it has some speed in it and then it's just fine. The car also "trailer hitches" when I decelerate only. For example, If I am cruising at like 20 mph in 2nd gear and take my foot off the gas it will start jerking back and forth: the only way to get it to stop is to push the clutch in.
Ripping through the gears full throttle it works fine.



Does anyone know what causes this phenomena? Can I tune it out or is this just a result of the modifications (cam, intake etc....) and I have to live with it?

I appreciate your advice on this problem, it's a drivability issue and I really want to fix it.

Regards,

Bill B.
l, I hope you can shed some light...

[grouseman]

Is your distributor's vacuum advance cannister hooked up to Ported manifold vacuum? 

[bill black]

It's hooked up to timed vacuum.  I have experimented and Removed the Vacuum advance, and I can't feel any effect of it under any circumstances.  I have tested the vacuum advance and it does work:  it adds advance under high vaccuum conditions.  I put it on the bench and hooked it up to my mighty vac and I can see the arm move after putting about 15 inch of Hg to it.  Less than 15 I don't see it move.

That's the extent of my vacuum advance investigation

[firefighter3931]

Lose the vacuum advance. Adjust the base timing for maximum vacuum and tune the advance curve. Big cams and vac advance don't work together. 


Ron

[grouseman]

Tell us more about your carb, and ignition setup.  It could be either.  But, since it's a light-throttle only phenomenon, I'm thinking about the idle transition slots in the primary bores.  They flow idle mix through them between idle and main venturi operation. 

With your 'lumpy' cam, how far in have you turned the idle speed screw?  I'm guessing it's screwed in quite a bit, so much so that at idle you are using almost all of these idle transition slots.  You should only see 0.050" of the bottom of the slot at the idle setting and if the idle speed screw is really turned in far, the slots are almost fully used up just at idle.  So when you open the throttle lightly, they can't offer anything more - kind of like an inoperative accelerator pump. 

I think the 650 DP has a recessed and hidden screw underneath to open up the secondary plates slightly, so that you can close down the primary plates as much as possible.  This way the idle transition slots AREN'T being used until you actually step on the gas. 

Another way to get the throttle plates closed more is to run FULL manifold vacuum.  It will raise your idle RPMs so you can back out the idle speed screw.  Check with a timing light that it is jumping the full 15 degrees when you plug it in. 

[bill black]

The cam isn't lumpy enough that I had to adjust the idle too far as you suggest:  I checked that.

Remember also that my car does this when I decelerate too.  I.e. if I am cruising along at 25 mph in traffic in second gear and leave off the gas and let the motor slow the car down, it will start jerking back and forth.  I call it trailer hitching, some call it bucking.  Since it does this on deceleration I figured I didn't have to worry about the carb.   It does of course do it on light acceleration too so the bit about the idle transfer slots was a good reply.

The ignition is electronic (MSD).  I don't know what the distributor is, it has a brown cap and very light advance springs.

Timing at ~500 rpm is ~ 15 °BTDC.  34° comes in at about 1500 rpm (yes, the advance is that quick).  If I apply 15 inches of vaccum, the timing will advance another 18° or so. 

With my vacuum advance on a bench, I applied 15 in hg to it and it moves, anything less than that and I can't see any movement:  that's the extent of my vacuum testing.  I ran the car with and without vacuum advance and I can't tell any difference under any driving condition.

[dodge freak]

Maybe you have a vacuum leak. Intake manifold gasket or brake booster or somewhere else. This is hard to guess without seeing and driving it, but thats what comes to my mind.

[grouseman]

What if you coast in Neutral?  Still happen?

[firefighter3931]

Timing at ~500 rpm is ~ 15 °BTDC.  34° comes in at about 1500 rpm (yes, the advance is that quick).  If I apply 15 inches of vaccum, the timing will advance another 18° or so. 

Your idle speed is too slow....you need to bump it up to at least 800 in gear. Idleing at slow engine speeds isn't good  because oil pressure will be too low. You risk wiping out the cam and other nasty complications.

Base timing and total look good allthough the curve is coming in too quick for my likeing. Try a heavier spring to slow down the total having it come in at 2500 and plug off the vacuum advance. I'll bet that fixes the problem.

Ron

[bill black]

Definately not:  If I push the clutch in OR coast in nuetral and no longer use the motor as a brake it doesn't buck.  My driveline is tight with no slop by the way...

Like, let's say you had your wife in the car and you are cruising along at 20-25 in some slow traffic, you wouldn't simply let off the gas to slow down because the car starts lurching back and forth and your wife looks at you like "christ, this thing is a piece of shit"  .  You have to push in the clutch and coast.  The biggest lurching happens when you make a switch from decelerating to accerating.  I.e. you let of the gas to slow down  a bit (lurch lurch) then you poke the gas to speed up (lurch lurch again).   It is most pronounced at slow speeds and in the 1st and 2nd gears.  The drivability problem is hardly noticeable in third and not at all in fourth.  I fourth, you can let off and on the gas in any manner and it is nice and smooth.

[bill black]

RE my advance rate:  I did buy a spring kit (they were all very heavy compared to the light ones I started with).

I chose the lightest springs in the kit and installed them.  My base timing I left unchanged and I measured the new advance rate:  all in by about 3500 rpm.  That's kind of slow I think, but I took it for a test ride any way and it still trailer hitches on light acceleration AND deceleration, most noticeable in the lower gears.

So with a really fast advance curve it bucks and with a really slow advance curve it bucks.

I'll bump up the idle, thanks for the caution Ron.

[bill black]

O.k. guys thanks.  I'll try what I can this weekend.   I'll report back on Monday with some more information hopfully.

Thanks

[bill black]

 hooked up my gauge (manifold vacuum) and went for a ride.

Vacuum is 10 inches at idle.  Vacuum at cruise is 15 inches.

Vacuum during bucking is steady.

At 30-40 mph cruise in 3rd (15 inches), lightly poke the throttle, vacumm drops to 10 and it bucks until I get some speed into it, If I lift off the throttle quickly vacuum goes to 20 and it bucks.    REM it's not a surge or a stumble or a bog, it's a jerking back and forth:  Just like when you are pulling a trailer and the trailer starts jerking back and forth on a hitch ball.

Bench tests show my vacuum advance does nothing unliss vacuum is 15 inches Hg.

[grouseman]

So,

- it bucks with the driveline engaged. 
- it doesn't buck with the driveline disengaged. 

The problem is in the drivetrain somewhere.  Maybe check pinion angles, U-joints, transmission and mounts. 

grouseman

[deputycrawford]

It sounds like a clutch problem or a gear tolerance problem. When was the last time the trans was out of the car? It is definitely in the trans.

[Duey]

It sounds like play in the driveline -- could be from flexplate bolts (hopefully not, this means something is going to break soon) to excess clearance in the diff.  My guess is to take a very careful look at your u-joints and that 833.

Cheers,
Duey

[grouseman]

I meant to say check both the transmission and engine mounts. 

[mikepmcs]

 

[daytonalo]

make sure the trailer is latched down tight !

[Big Sugar]

make sure the trailer is latched down tight !

Good One! LOL.

Ron

[Chryco Psycho]

is there a rear axle brg going bad or out of adjustment & causing the drum to grab the brake shoes ?

[MOPARHOUND!]

Big gap in the thread timeline, June 30th to daytonalo's November 2 post.

Wonder if bill black ever solved his problem??

[daytonalo]

Who is BILL BLACK ?