Truck stalls under load

[Flhftw]

85 k5, 87 gen 1 350 with 70’s double hump heads and Comp cam through a 700r4, with 6al msd and coil in a stock hei

hey guys here’s a long one
A few weeks ago a changed out a bad engine with a used one that had been cammed and it’s been nothing but one headache after the other but yesterday I was able to get the timing valve adjustment and idle dialed in yesterday(or so I think). Truck idles good around 800-900 rpm at 10* btdc and 36* advanced.
One thing I noticed however while adjusting the valves is that the tdc notch is close to 90* before the #1 piston is at the top. I don’t know if this common but on my last engine it was off also and I had to time it by vacuum.
Anyway when I put it in gear the idle drops to around 500 rpm and is close to stalling and sometimes will coming to a stop but drives and advances great with throttle
-First thought is vacuum leak but all my lines and fittings are new and tight
-then there is the timing notch issue
-header burns on some plug wires
-possibly a torque converter issue (I had driven it about 3 miles with improper kick down cable adjustment before I figured it out)
-maybe air/fuel mixture (along with the new engine I installed a ready to run 600cfm Holley carb with all the necessary brackets)
-maybe pcv but it’s new, also have a blocked off egr

I was thinking I would start by painting a mark on the damper where the actual #1 tdc then go about timing from there... I need help with this one

 

[Wes Harden]

What is the cam specs, specifically the lobe separation. What kind of vacuum do you have in park at 800 rpms?

 

[Flhftw]

What is the cam specs, specifically the lobe separation. What kind of vacuum do you have in park at 800 rpms?

If it’s a mutha thumpr like I was told, it should be 107. That’s what I was able to look up.
I haven’t hooked a vacuum gauge up to it yet, I understand bigger cams rob vacuum. What should I be looking for?

 

[Wes Harden]

Yeah you may have to live with a high neutral idle speed to have a reliable in drive idle

 

[Flhftw]

Yeah you may have to live with a high neutral idle speed to have a reliable in drive idle

I would be okay with that but when I increase the idle, the vacuum advance engages and jumps the rpms high. I may need an adjustable advance canister. I have just an accel replacement right now

 

[Wes Harden]

Possibly a higher stall converter

 

[dyeager535]

when I increase the idle, the vacuum advance engages and jumps the rpms high.

Vac advance should be linear, not on or off, unless that carb has ported/timed vacuum ports. If it does, and that's where you have it connected, move it to manifold.

 

[Flhftw]

Vac advance should be linear, not on or off, unless that carb has ported/timed vacuum ports. If it does, and that's where you have it connected, move it to manifold.

I’ve tried both ports and when connected the rpms instantly shoot up until I turn back the idle screw. I’ve seen there are adjustable dizzy vacuum canisters where you can turn a screw in or out to adjust the advance but not sure if that’s something I need

 

[dyeager535]

I’ve tried both ports and when connected the rpms instantly shoot up until I turn back the idle screw. I’ve seen there are adjustable dizzy vacuum canisters where you can turn a screw in or out to adjust the advance but not sure if that’s something I need

RPM's are going to go up because the timing goes up. Adjust the idle RPM back down with the advance hooked up like that, once you ensure that your base timing (vac advance disconnected) is where you want it.

More advance equals better idle. If the majority of the advance is done via vacuum, it will start fine. Too much base advance is going to result in hard starts.

 

[Blue85]

Start with the stuff you know is bad, then move on to looking for other issues.

1. Get the ignition timing figured out. It's not uncommon for timing marks to be off when parts get swapped between engines, as there were (at least) two common timing tab locations. Painting a line on the balancer is good enough to get started, but you might think about buying timing tape or swapping the balancer or timing tab setup to make things easier. The timing tab is more for setting base timing than seeing the whole range of the vacuum can and dial timing lights aren't always accurate.
2. Replace some plug wires with known good ones. You can get smooth idle with bad wires/coil/rotor, but it falls on it's face whenever there's some load.
3. Check for vacuum leaks with a stethoscope, piece of tubing, can of carb cleaner or propane torch. Too cheap and easy not to be sure.

Then we can talk about fuel pressure, ported vacuum, vacuum advance limits and all that other stuff.

 

[JoshHefnerX]

There are 3 sets of timing tab/dampers for sbc. If you don't have a matching set of tab/damper you'll play hell setting the timing. Make sure they're matched, or get an adjustable and make it match.

What's your vacuum at idle?
What kind of carb do you have - didn't see it.

If it's a huge lumpy cam you'll play hell getting it to idle. It may kick off the power enrichment if the vacuum is too low and throw off your a/f ratio.

If your throttle blades are open so far they're allowing the ported vacuum to flow, something out of whack (timing) or you have a bigass cam. If it's that big you may need to drill small holes in the throttle plates or open up the idle air bypasses to keep the throttle closed enough. But you'd only do that as a last resort.

 

[Flhftw]

There are 3 sets of timing tab/dampers for sbc. If you don't have a matching set of tab/damper you'll play hell setting the timing. Make sure they're matched, or get an adjustable and make it match.

What's your vacuum at idle?
What kind of carb do you have - didn't see it.

If it's a huge lumpy cam you'll play hell getting it to idle. It may kick off the power enrichment if the vacuum is too low and throw off your a/f ratio.

If your throttle blades are open so far they're allowing the ported vacuum to flow, something out of whack (timing) or you have a bigass cam. If it's that big you may need to drill small holes in the throttle plates or open up the idle air bypasses to keep the throttle closed enough. But you'd only do that as a last resort.

I haven’t had a chance to work on it since I posted because the neighbors aren’t too happy with me starting it up after work. I’ll be on it Tuesday probably and I can hook up the vacuum gage and put a mark where the actual tdc is, it looks like an aftermarket tab. I have a digital light which makes life a bit easier.
The carb is a 600cfm Holley, don’t have the part number right off hand.
I’m guessing I’ll need to set base timing so the psi is in the green and go from there. I know vacuum is hurting because it’s not enough for the brake booster

 

[Flhftw]

Start with the stuff you know is bad, then move on to looking for other issues.

1. Get the ignition timing figured out. It's not uncommon for timing marks to be off when parts get swapped between engines, as there were (at least) two common timing tab locations. Painting a line on the balancer is good enough to get started, but you might think about buying timing tape or swapping the balancer or timing tab setup to make things easier. The timing tab is more for setting base timing than seeing the whole range of the vacuum can and dial timing lights aren't always accurate.
2. Replace some plug wires with known good ones. You can get smooth idle with bad wires/coil/rotor, but it falls on it's face whenever there's some load.
3. Check for vacuum leaks with a stethoscope, piece of tubing, can of carb cleaner or propane torch. Too cheap and easy not to be sure.

Then we can talk about fuel pressure, ported vacuum, vacuum advance limits and all that other stuff.

Thanks, should be able to get started back on it Tuesday and will update. I have 8mm summit wires on now but some plug boots are pretty melted, I have some 8mm msd wires I have to cut to length and will swap. I could probably invest in some header plugs I saw the other day to clear the headers better.