Problems resurrecting an L20B

[patricktheofanis]

Hey everyone, this is my first post on the forum. I've got an L20B (long block with W58 head) and FS5W71B 5-speed tranny that I bought off another forum member and I'm running into some problems trying to resurrect it. It hasn't run in 7 or 8 years. My plan is to eventually swap it for the L16 in my '72 521. My goal is just to get enough power to make my truck highway capable. 

 

Here's what I've done so far and where I'm at: 

+ I got the motor with 5 speed transmission on a test stand and hooked up my 521's starter and radiator to it. I also added a mechanical oil pressure gauge and a tachometer. The whole thing is wired to an extra ignition switch I have. 

+ I checked valve lash and the 4 exhaust valves were too loose so I tightened the clearances to cold specs

+ I found TDC (cylinder one exhaust valve at 2 and intake valve at 10) and verified that valve timing is good. The dash is right in the middle of the v-notch

+ With TDC identified, I opened up the distributer, noted the position of the rotor, and I connected the spark plug wires in 1-3-4-2 order

+ I replaced worn out vacuum lines and capped the vacuum line that would go to the carbon canister. I also capped the brake booster vacuum line. 

+ I jumped the red idle cut solenoid wire to the blue choke heat wire and connected this to ignition power. When I turn on the ignition I hear the idle cut solenoid click, so i think I should be getting fuel to the idle circuit. 

+ I primed the fuel pump and turned the thing over to verify the pump works properly. The fuel return line is also clear. 

 

Here's my problem: The engine fired up but the RPMs were very high. It rev'd to ~3300 rpms. I got some penetrating oil into all the moving parts on the carburetor and loosened up the sticky vacuum diaphragm. The engine never calmed down, and at 3k rpm my timing gun showed it was at ~5 degrees before TDC. The photo below shows where the distributer was rotated to:

I thought maybe the valves could be open too far, so I checked those. The choke seems open the right amount:

And I checked to make sure the primary and secondary throttle valves are closed. It's hard to get the camera to focus on them but here are photos:

 

Rotating the distributer CCW to retard the timing brings the fast idle RPM down to ~2000 where I think it's supposed to be, but it never calms down to warm slow idle RPMs. Here's where its at now: 

 

The oil pressure is also a little high at 55 psi. Hopefully that will go down once the engine can idle normally. Initially the motor blew lots of blue smoke, but a new oil filter, new oil, and more running time cleared that up mostly. 

 

I didn't want to just start turning idle mixture and speed screws without a strategy. Am I missing something? How can I get this thing straightened out? 

 

There are other problems to address like oil leaking from the valve cover and water leaking from the water pump near the pulley during test runs. I replaced the cover gasket, but I need to clean up the old mess to verify that its fixed. 

 

Also I want to get some general guidance: My original plan was to get this motor running properly before taking it apart for a basic rebuild and cleaning. Nothing major, just new gaskets and piston rings, and checking innards. I am also considering swapping the Hitachi carb for a Fitech fuel injection setup like Peter Spackman's. Another possibility is a Weber redline 32/36. Should I give up on the Hitachi and move forward with the rebuild, or get the thing working first? 

 

My L16 runs perfectly, so I'm in no rush to swap in this L20B. 

 

Since this is my first post, and I know folks will ask, here's my truck

 

 

 

 

[datzenmike]

A much better TDC is obtained by using the timing scale and the mark on the crankshaft pulley. Make sure you are on the compression stroke and turn only clockwise up to and stop at the 0 (zero) mark. If you overshoot, and that is easy to do, back up 1/4 turn and keep trying. This will assure that all the chain slack is on the 'slack side' and the notch and horizontal line are accurately set.

 

 

Ignition timing should be 12 degrees Before TDC. This will be impossible with the idle at 3,300. At 3,300 check that the timing is about 32 degrees. This will be in the ball park but set accurately once the idle is fixed. Idle should be below 1,000 for this.

 

 

To idle at 3,300 when warmed up there must be air getting past one of the throttle plates. When the choke is operating there is a fast idle cam setting. 1,800 to 2,200. It may be stuck. The plastic cam below has steps that the throttle closes on and rotates out of the way as the choke warms up allowing the throttle to close down to proper idle.

 

 

If not the fast idle cam, you may have to take the carburetor off to be sure both throttle plates are closed with the choke held in the off position. You turned the idle speed screw down? yes?

 

One last possibility is the BCDD (boost controlled deceleration device) It's on the left side of the carburetor at the top in this picture. It has a ring of screws and a rubber cap covering the adjustment screw. It may be set too sensitive and not shutting off. Turn screw counter clockwise to decrease sensitivity and the idle will drop when the throttle is blipped. Keep track how many turns so it can be returned if this isn't the cause.

 

 

 

 

55 oil pressure is perfectly fine and that high because of the 3,300 RPM.

 

I would get this running well first rather than trying to trouble shoot after a rebuild. Then you know that it should run.

[banzai510(hainz)]

thats a nice truck. better keep that in the garage!!!!!!!!

 

521s are not a really a Highway Burner and never will be.IN my opinion. that rear end is a 4.375 so a 5speed is needed but that only helps cut off maybe 500rpms depending where the 5speed comes from. The L20 is not as high a rpm maker.

 

Just my opinion if your a entry level mechanic I would just swap out the L16 if its going bad and install the L20 and just call it good.untill then just drive it.

Edited September 19, 2022 by banzai510(hainz)

[datzenmike]

The J13 powered 520/L521s were worse! 4.875!!!!! Find a 3.70 or 3.889 3rd member from a 720.

[patricktheofanis]

I just noticed that my BCDD is missing a vacuum connection!
 

Can I use the T’d plugged connection off the line to the distributer for this? 

[datzenmike]

I believe the vacuum is connected internally on the later L20B. By later, I'm referring to the W58 head. This was used on the '78-'80 engines.

[patricktheofanis]

Here’s a better look at mine. What should I do with nipple on the top right of BCDD?

 

 

[datzenmike]

I don't think that is the correct carburetor. Maybe from an earlier year. I don't think the '78 and up had the hose and the solenoid (hex thing right beside the  small hose fitting) I doubt the solenoid is connected. If wrong carburetor it would explain the hose not being connected there.

 

Not being connected it can't turn on. But turn the screw inside a couple of turns counter clockwise anyway.

[thisismatt]

Nice truck. Looks like what mine should look like if it hadn't been abused by previous owners 😅

 

Mine does 70mph on the freeway no problem with an L16...not sure why yours wouldn't, but of course the L20 would be an improvement.

[datzenmike]

I think my '71 would do that. Though I almost never would drive it that fast. Took a while to get there. The L20B I put in would get there faster.

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