Pulling my hair out

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gumper

gumper

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Butte, MT
I am having a problem with my Duster I cannot find for the life of me. 1970 Duster, 410 on toilet injected alcohol, 727 with 4800 stall spragless converter, 8.75 with 4.30s, Caltracs at 3/4 to 1 turn preload and shocks between 7-9 depending on the track. What’s happening is as soon as I let go of the transbrake it cuts off 3 times. When I foot brake it cuts off the same but only once. I have readjusted the suspension several times, checked the whole ignition system (@yellow rose), richened the fuel, leaned the fuel, unhooked the tach, unhooked the Digital Delay shift box, and chased all the wiring. The tricky part is it ONLY happens on the track. It will not do this on the return road, or in my concrete driveway. It’s something related to the hard shock of the launch, and it happens real quick.
 
That looks like you are planting the tire, then it bounces back, then you hit it again. I think that is a chassis issue. I’m going to watch it a few more times.

If that’s what it is, the only fix I know is to make the bump (compression) on the shock much stiffer so the tire can’t bounce back once it’s hit, or you have to stiffen up the rebound (extension) so it doesn’t hit the tire so hard.

I’ll watch it a few more times and watch the front end. Doesn’t sound like a miss to me.
 
Have you tried another ignition box? I run a Digital 6 and had a pop right after the gear changes at the track only. After changing the plugs, wires, coil, pick up, reluctor, rotor and cap the pop was still there. I reset the rev limiter to 0 and the pop went away.
 
justinp61 said:
Have you tried another ignition box? I run a Digital 6 and had a pop right after the gear changes at the track only. After changing the plugs, wires, coil, pick up, reluctor, rotor and cap the pop was still there. I reset the rev limiter to 0 and the pop went away.
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No, the ignition box with coil and distributor was just tested and passed fine.
 
yellow rose said:
That looks like you are planting the tire, then it bounces back, then you hit it again. I think that is a chassis issue. I’m going to watch it a few more times.

If that’s what it is, the only fix I know is to make the bump (compression) on the shock much stiffer so the tire can’t bounce back once it’s hit, or you have to stiffen up the rebound (extension) so it doesn’t hit the tire so hard.

I’ll watch it a few more times and watch the front end. Doesn’t sound like a miss to me.
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The shocks are on on full tight with 1/2 turn preload there. This is the exact same setup I’ve been using since 2017. 12.5 pounds of pressure in the tire.
 
If you watch the video in slow motion you can hear the motor cut out before it unloads the tires.
 
gumper said:
The shocks are on on full tight with 1/2 turn preload there. This is the exact same setup I’ve been using since 2017. 12.5 pounds of pressure in the tire.
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Remind me what you have for shocks. I’m going to watch the video some more.
 
yellow rose said:
Remind me what you have for shocks. I’m going to watch the video some more.
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Calvert 9-way adjustables. Same thing I’ve been using for a decade except the current shocks are only two years old. The rear axle isn’t moving much. If I set them at 5 it will drive the rim into the asphalt when hit with power.
 
Ok, I slowed it down to .25 and it’s almost pulling the real tires off the ground. I still don’t think that’s a miss. I think the engine is seeing load, then it unloads and gains RPM when it’s unloaded and the loses RPM again when it gets loaded. That’s what it looks and sounds like.

Shocks are parts that wear out. Whatever shocks they are may be used up or if they can be rebuilt, they may need rebuilding.

I’m going to go back and watch it again to pay attention to the front end. It looks like it’s topping the shock out or hitting the travel limiters
 
dirty white boy said:
could it be the hard launch causing a a fuel delivery issue?? or the launch causing ignition to lose ground??
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I thought it was an ignition issue, but I’ve chased every wire in the car. The ground goes directly to the battery. The power comes right from the battery. The ignition that turns the box on comes directly from the switch.
 
gumper said:
I thought it was an ignition issue, but I’ve chased every wire in the car. The ground goes directly to the battery. The power comes right from the battery. The ignition that turns the box on comes directly from the switch.
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Doesn't sound electrical to me... usually electrical will have more of a "snap" to it.
 
gumper said:
Calvert 9-way adjustables. Same thing I’ve been using for a decade except the current shocks are only two years old. The rear axle isn’t moving much. If I set them at 5 it will drive the rim into the asphalt when hit with power.
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Ok. That’s one problem I have with shocks like that. You can’t separate the bump and rebound. When you change one setting you change them both, rather than being able to control the hit and then keep the tire planted once it gets hit.
 
yellow rose said:
Ok. That’s one problem I have with shocks like that. You can’t separate the bump and rebound. When you change one setting you change them both, rather than being able to control the hit and then keep the tire planted once it gets hit.
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The front shocks are 90/10 Calverts. I’ve never had this problem. At the last race last year everything the same it was doing this.



DD685DB4-A1AE-4113-9EEB-7CBD9B64470A.png
 
yellow rose said:
Any chance they changed the track prep or something?
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I’ve been at two different tracks this year with the same result. I hate just throwing parts at a problem. I’m fine changing shocks if that was the problem, but they’ve never been a problem before. Talking $240 a shock for double adjustable QA1s that may not help the issue. It’s all very strange.
 
Only does it once while foot brakes and three times off the transbrake... When is the last time the transmission was gone through and what does the fluid look like in it? It's acting like the transbrake is trying to reactivate after launch.
'
 
It’s hard to disagree with @yellowrose here. Have you tried different launch rpms in the two step? I would try to increase wheel speed on the hit and see if I could slip the tire a little. Maybe add 5 psi in the tires?
 
The car does bounce 3 times it’s just really hard to tell if the bounce is causing the sputtering or the sputtering is causing the bounce.
 
I would have a GoPro focused on the leafs during a launch. That might reveal something not seen otherwise. Tell us about your fuel system, tank to pump. Is the tank sumped?
 
TT5.9mag said:
I would have a GoPro focused on the leafs during a launch. That might reveal something not seen otherwise. Tell us about your fuel system, tank to pump. Is the tank sumped?
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too fast to be anything but electrical , short of some sort , bet it doesn`t do it when free reving in the driveway ?
could even be a short in the battery , coil or ------------something on a hard jerk --we had a cut out like that , only more all the way thru ,on the hemi once when I switched to higher comp pistons , (only thing that was changed in the entire set up , went from a well set up chrysler ignition to a msd 6 and stopped it . Always figured the higher comp. was blowing the fire out of the plugs --but dont know about that ,
was happening to top fuel cars way back when -------
 
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yellow rose said:
That looks like you are planting the tire, then it bounces back, then you hit it again. I think that is a chassis issue. I’m going to watch it a few more times.

If that’s what it is, the only fix I know is to make the bump (compression) on the shock much stiffer so the tire can’t bounce back once it’s hit, or you have to stiffen up the rebound (extension) so it doesn’t hit the tire so hard.

I’ll watch it a few more times and watch the front end. Doesn’t sound like a miss to me.
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Have to agree. It doesn't show a miss when he is doing the burnout.
 
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