1986 C20 350/400 wont go past 2500 RPM in Drive

[demian5]

Help - My son has a bone stock 1986 C20. It free revs fine, starts fine idles fine.

When driving, it craps out at about 2500 RPM in 2nd and 3rd gear, like fuel starvation or plug wire miss.
Manual in first gear it will pull till you shift it, no lack of power or miss.
Free rev it will rev great, secondaries open and everything. Runs strong.
Choke pull off is working. No vacuum leaks.
Stock Q jet that we have gone through way before this started happening.
HEI with new cap and rotor after this started happening.
Weights in distributor free.
It does diesel after being warm when shutting off.
Did the spray bottle of water on the plug wires at night, nothing arching (plugs and wires new in the last year R44TS).
Timing at 6BTDC (however when warm, and vaccum line still connected, still 6BTDC).

Any insight? Should there be vacuum on the dist when warm at idle?

[cadillac_al]

I'd say clogged catalytic or weak fuel pump. Check the vacuum to see if it is working normally. If it looks ok then check it while it's dying at 2500 rpm. If the vacuum is ok then it must be a weak fuel pump. Check that pressure too.

[Feel the 'Pane]

X2 on a clogged cat.

After the truck is warmed up, run it in 1st or 2nd gear at the peak power for about 20-30 seconds, then stop & look underneath the truck and see if the cat is glowing a dull red. Easier to see at night.

No glow & it's not the cat. Good luck w/ your investigation!

[Dead Parrot]

No change in timing with vacuum connected at idle usually means you are on ported vacuum. Switch to a manifold vacuum source and see what happens. Should be a manifold source on the other side of the carb front. Stock SBCs usually like a lot of timing advance. Also try bumping the initial timing to 10 or 12 BTDC. Should fix the dieseling problem after you back off the idle screw.

Double check your rubber fuel lines from tank to frame and frame to pump. If bad, can let air into the system.

[kidatheart]

Keep us posted on what you find. Thank you!

[demian5]

This truck has no cats from factory. And its in CA!

Has all the original vacuum switches and lines that are routed correctly still. This is a stock as they come.

Ill figure it out, but it may take some time.

[demian5]

Anyone had a EFE valve get stuck closed and cause this?

[demian5]

Figured it out. The check valve in the factory fuel filter in the Q-Jet was sticking. Went ahead and bought a new filter and pulled that valve out. Glad it was something simple.

[Palf70Step]

Good to hear you got it figured out.

[demian5]

Originally Posted by Palf70Step

Good to hear you got it figured out.

Thanks. When I have an issue, I like to post if I fixed it or not so in case anyone else has a similar issue and is using the search function, they at least have a conclusion.