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Im stumped and could use some outside perspective

S

SilveryFox

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Location
Central Texas
Diaclaimer: not new to SBC or K5's.
Truck: 86 K5 no computer, 350 (center bolt iron head) mechanical pump, 600CFM carb, stock cam, stock timing, accel Dizzy (brand new) 6psi fuel pressure observed on regulator gauge, headers, true duals, 3.73's on 31's with a 700R/208, stock stall.

Problem: all the way to 80 it runs and accelerates like a scalded dog. However, staying at 80 or even 70 in OD will not happen. Full throttle is 100% required (and the kick down to 3rd). Carb is brand new, dizzy is brand new, fuel pump is brand new.

My last 86 k5 had 3.42 on 31 and had no issues running 90 all day (305/700/208). Even my 1009 can hold 80 on a 6.2/400/3.08.

What am I missing? No computer to interfere, no emissions to alter. Just absolutely no way to stay at freeway speeds here in TX.
 
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TV cable is adjusted wrong most likely.
 
Pretty simple to adjust.
Also, I'm not sure if it would work at all if this happened, but the clip on the end of the cable is plastic and known to break.
They do sell replacement ends at Napa last I knew
 
Is the symptom that it has plenty of power in 1-3, but it's weak in 4th? Or is it caused by the throttle/vacuum change after you stop accelerating? When you drop into 4th gear and the converter locks, what RPM is the engine at? Maybe the secondaries are working well, but the primaries are jetted too lean. Maybe during acceleration you're running at higher RPM where the spark and fuel are correct, then for cruise it's a lower RPM where there's an issue. Just trying to separate if it's based on load, RPM or gear. Is it an Edelbrock or Holley carb?

Being that the distributor is new, did you eliminate the ESC harness that passes through the firewall? I was just wondering if stacking mechanical advance with vacuum advance at cruise is making it ping. The ESC box would be kicking in then. Also, in 84-86 or so, there was a signal from the transmission to open a vacuum valve at cruise and enable the EGR. Maybe a vacuum circuit is messed up and you're getting EGR open when it shouldn't.

Driving squares at 80-90MPH seems crazy to me, but I also don't have any freeways. :dunno:
 
Tv cable is adjusted correctly, that was ensured last month with a feeler gauge. Trans died, new trans, new cable. Adjustment made with pan off to visually verify

The truck has zero emissions or computer control, distributor is Accel.

No tach yet its on backorder
 
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Plenty of power 1-3rd gear, 4th very little.
When I say no emissions, it's a bowtie block, GMPP intake, edelbrock 600 carb, long tube headers, exhaust is verified obstruction free. All computer control is gone only two vacuum lines on the engine for the vacuum advance and power brakes.
 
If you’re not having a transmission slippage issue, it may not be pushing enough fuel at that speed to keep the carburetor working right so it will just lay down
 
How are the shift points otherwise? If the governor is too light (like the Corvette/Camaro version), the shift points are higher, making it downshift more easily. Did this problem start or change with the new transmission?
 
Previous to the trans change, all driveability issues are unknown, governor is a truck unit. The truck is new to me
 
SilveryFox said:
1st gear is a fast engagement at 16-20 , all others are butter smooth, 2nd comes at 30-40, 3rd is 45-50.
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You're describing the shifts by number, not the gears, correct? I mean, 1st gear engages at 0. 45 is about the slowest I would expect OD to engage, at very light throttle. I think the trans will always shift out of OD once you get to about 2/3 throttle. Would you be realistically cruising at that speed down around 1/2 throttle? I know that sometimes in O/D going 60+ I'm only making 10" of vacuum.

They do make aftermarket valves that hold OD at higher throttle positions, but that assumes the shift point is the only issue. https://transpartswarehouse.com/k01...full-throttle-boost-valve-plunger-sleeve.html. Apparently, Corvettes have something like this from factory. Hard to make a car go 150mph without 4th gear. There is also a similar type modification that uses constant pressure, but apparently it sucks. It's unlikely your other trucks had anything besides the common valvetrain.
 
SilveryFox said:
Plenty of power 1-3rd gear, 4th very little.
When I say no emissions, it's a bowtie block, GMPP intake, edelbrock 600 carb, long tube headers, exhaust is verified obstruction free. All computer control is gone only two vacuum lines on the engine for the vacuum advance and power brakes.
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How is your lockup circuit ran?
 
Trans could have a 4th gear issue. Same clutches for 3rd and 4th but maybe internal hydraulic leak/flare. I had an early case 700r4 that did that bad.
 
Fastereddie said:
Trans could have a 4th gear issue. Same clutches for 3rd and 4th but maybe internal hydraulic leak/flare. I had an early case 700r4 that did that bad.
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I've been trying to think of a failure mode that brings on drag/binding in 4th gear and haven't thought of it yet. Do you have more information?
 
Blue85 said:
I've been trying to think of a failure mode that brings on drag/binding in 4th gear and haven't thought of it yet. Do you have more information?
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Only that it was freshly rebuilt by me. Couldn’t understand what I may have done wrong or differently from other rebuilds. Ended up being an internal leak in the case.
Got a later model core from the junk yard with the auxiliary oiling to the rear and swapped all my new parts over. Still running that trans.
 
A bind up in OD would involve another clutch pack applied that should be OFF.

in OVer drive; The band is applied, 3-4 clutches applied, forward clutch pack applied.

If the over runs are applied during OD ... they burn.

IF the low/reverse are applied during OD... they burn
 
If the truck is working normally in 3rd gear, the overrun clutch is disengaged. On the shift to 4th gear, the only thing required is to apply the band. The band apparently is fine since 2nd gear is fine, but it is 2 different sections of the servo responsible. If the servo failed, you would still be in 3rd. So what can cause the overrun clutch re-engage or the forward sprag to not release? These scenarios seem like there would already be evidence in the fluid. I've never heard of application the TCC causing a brake/bind.

Interesting scenario however you look at it.

1713553363298.png
 
Clarification on shifts : 1-2 @ 16-20 2-3 @30-40. 3-4 @ 45-50

On the TV cable, when it was installed and adjusted I dropped the pan, then tried to slide a .001 feeler gauge between the actuator and piston to ensure proper contact at idle as well as visually verify full engagement at WOT. Takes 2 people and it's a pain in the butt, but it's the absolute most effective way to ensure proper adjustment.

Lockup is run through a pressure switch on the trans. Only 4th will lockup with this setup.
It unlocks normally with either brake pedal or 4-3 downshift, lockup in 4th is noticeable for certain. Now if LMC would get the tach set back in stick, that would be something.

I could LS swap this truck in a heartbeat and solve all my issues but that would be easy and what's the fun in that?
 
Engine hp is estimated at 220 hp, what's the possibility I'm just not pushing enough hp for the 3.73's?

My last 86 w/ 305 had upgrades and was closer to 300hp.
 
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