Rough running with acceleration stumbles and weak performance.

[Gery]

I recently purchased a 1978 620 pickup (L20b with automatic).  It was running quite roughly and barely idling.  

Going through it I discovered and corrected: 

-spark plug. Cables for cylinders 2 and 3 were switched.

- plugs for # 1: 2, and 4 looked clean but #2 was gunked up.  

- no response on the idle screw.  

(Because the car had been sitting for months, the apparent fouling of the idle circuit, and that the hitachi carb was a federal model rather than a California version, I replaced the carb itch a professionally rebuilt one)

-electric fuel pump receiving power and seeming to function

-gas filter clean

-leaky distributor vacuum advance tube

-distributor fine adjustment range all the way to the left, i.e. the distributor at counterclockwise extreme

-timing mark at tic mark at mild rev., with no apparent way to advance it.

-distributor rotor has appropriate play and spring-back

-vacuum advance occurs when tube is sucked

-new battery

-good ground connections

-air cleaner clean

-fresh gas

-Pulling cables from plug 2 or 3 does NOT stall the engine

Nevertheless it continues to:

idle roughly until warmed up

Stumble when reved while in Park

Poor running in gear with very weak idle....often stalling.

 

What am I missing here?  The timing seems off but I am unable to adequately adjust the distributor.  

 Thank you

 

 

 

[datzenmike]

Firing order is 1342 in a counter clockwise direction on the distributor. Check carefully, don't assume. It's most common to get the direction wrong and this will swap plug wires 2 and 3.

 

When warmed up check the choke butterfly is fully vertical and OPEN.

 

Check the new carb mounting bolts are snug (no vacuum leaks) and all vacuum lines are secure.

 

Check that the idle cut solenoid is working. Usually there is a light clicking sound at rear of carb when the ignition is turned on and off. If the solenoid is not working there will be no gas getting to the idle mix screw. PERIOD. It will not idle below 1,200 RPM.

 

 

1/ Set valve lash. Intake 0.010" exhaust 0.012" HOT ENGINE!!!

 

2/ What's the timing set to??????? should be 12 degrees before top dead center. (TDC) This must be done with the idle at or near 750 or mechanical advance will kick in and make this adjustment a waste of time. If throttle is open too far to make it idle, vacuum advance will kick in making this adjustment a waste of time.

 

With 1/ and 2/ set, turn the idle speed down as low as possible. Turn idle mix screw in or out looking for the fastest smooth improved idle position. Presumably the idle went up so turn it down and repeat. There is usually about one full turn with it dropping off in quality on either side of this... just set half way. Continue adjusting as many times as needed till you cannot improve the idle further and the idle speed is around 750-800.  Idle mixture is now set. You may have to do this in order to just get the timing set. Come back after the timing is set and re-do again.

 

 

'78s don't have electric fuel pumps so what is the output pressure of this thing? Look at the sight glass on the front of the carb. Its IMPORTANT that the fuel level while idling is at the dot on the glass.

 

 

Set the engine accurately to TDC on the compression stroke of the #1 piston. Unbolt the distributor pedestal from the timing cover and lift it out. Look in the opening. This is what you should see if the distributor is properly timed to the crankshaft.....

 

. Note the small and large half moon and which side is which. If ok put everything back. Timing is unaffected by doing this. If the distributor is at 11:28 (or whatever) the timing adjustments on the side of the distributor should allow you to set 12 degrees BTDC.

[Gery]

I will do and or redo all of this.  

BTW, because it was originally an a/c equipped truck and the compressor bracket covered the mechanical fuel pump port, it was equipped with an electric fuel pump.  

Gasoline fill the bottom 3rd or 4th of the carb window.

i sprayed starter spray (ether) around all the carb fittings and no increase in revs.

 

okay, the good news is that it is running now in a way that allows driving.  Reving in neutral is vastly improved as is running once moving.  

 

The idle in gear is very tentative....seeming almost to stall at stops.  

The air cleaner is in place but other than being snugged down on the intake, none of the various tubes (other than one to the valve cover) are connected.  I have no idea what some are for (gulp valve, etc.), but hopefully some will at least stabilize performance.  Though I know smog devices don’t do that or else everyone would want them. 

I notice there is no white (bcdd) wire from the carb?  

There’s no dieseling ?

To get the right advances, the distributor adjustment still needs to be pegged to one side.

 

The most pressing issue (to me) is dealing with the weak idle in gear.

Thank you for your expert guidance on getting  me on the road.

 

[datzenmike]

What's the build date on the driver's door jam?

 

Get the timing right and the valve lash then adjust the idle mix. This will usually firm up the low speed running and idle.

 

There are two lock downs for the distributor I think. One may be stopping the other from a full turn adjustment. 

 

The '78 BCDD didn't have a solenoid wire. It resets automatically.

[Gery]

Okay.  With a lot of minor adjustments and readjustments and then readjustments (as per suggestions here), we are running pretty well.  Once the timing was sorted, valve adjustments quieted things down and helped smooth Cold starts are easy to catch, but still need constant pedal gas’s to remain running. Once warmed up, (5 min. In SoCal)) idle is nice.

In hindsight, working through this was a pleasure due to all the help I received here.  THANK YOU...THANK YOU...THANK YOU

BTW, I even squeezed through the smog test...yay!

 

Now, time to start adding some power.

Compression is even at 15 across all cylinders.  

Would an l16 head make sense?

A more aggressive cam?

Discussions here seem to discount any real benefits of adding a header..

I’m not looking for a street racer performance, just awakening the potential and getting some punch.

Suggestions? (I have searched this.....seems to inspire a lot of back and forth resulting in ...at least for me...confusion)

[datzenmike]

Method or tester seems wrong. I would expect 150 and 175 would be very good. Below 120 is a sign of being well worn although it can still run ok if all cylinders are even.

 

Exhaust is under extreme pressure. It's going out no matter what, you can't stop it. Most exhaust systems are simply good enough and making them 'perfect' adds very little... on a stock engine. Once an engine is modified and moves more exhaust, then improvements can help get it out. Generally a very slightly larger exhaust pipe and a lower restriction 'muffler' will be fine. Resistance to flow affects the much higher RPM range, and as most driving is below 3,000 it's something that can wait.

 

You're in California, so if you need to pass emissions testing, you can't just start throwing carbs and cams at it.

[Gery]

Sorry...150 not 15 on the cylinder compressions.  

 

The reason son I mentioned a different head and cam is that I thought I could switch things back, if needed.....and meanwhile enjoy a tad more performance.

[datzenmike]

Ha ha that's a relief.

 

Yes with the time you could swap heads back for the test.

 

 

You have the L20B engine? If so an L16 head would be a bad idea. It's made for a 20% smaller displacement engine, and the valves and ports are smaller.

 

If I had a '78 (with a W58 head) I would look for the '75-'77 U67 head to build with. The W58 has round exhaust ports with steel liners in them. It's an emissions thing but must offer some restriction. The U67 is the same for valves as the W58 but has square exhaust ports which would allow the earlier L16/18 'cast iron header' exhaust manifold. This is pretty close to a header without any of the downsides to having one. Expense, short life, loud, never a perfect fit, they come loose, increased under hood temps, rust...

 

 

Keep your '78 down pipe and have a custom 2 into 1 made and just unbolt the clamp when you swap heads for emissions testing. Here's is the time to increase the pipe diameter to 2" or 2 1/4" with a low restriction turbo muffler. These pipes can stay on the vehicle so if you have a cat incorporate it into this

 

Now you can go to side drafts but you will need an intake to go with this. Expensive. Maybe motorcycle carbs with a home made intake? Or...

 

Your '78 intake will fit and work with this 'header' but you will need to drill two 3/8" holes for coolant flow into it. Then buy a 32/36 Weber?

 

After market cam. Limit the lift to 0.480" and you can keep the stock springs or get a complete package with springs retainers and lash pads. 

 

I think some Z car valves are larger so if rebuilding the head you might consider getting over size seats and valves?

 

Gasket match the intake and exhaust manifolds to the ports on the head.

 

Head can be milled to remove any warp and compression will also be raised.

 

 

[distributorguy]

Did you install new plugs?  Shoot for NGK BP6ES.  I think the number on the box is 7444?

 

Which distributor do you have? point gap?  Timing set at idle to...?

[datzenmike]

It's a '78 so it should have a remote igniter EI distributor. Should have.

[Gery]

Yes, it has an EI distributor.

i will change the plugs today.  They look good (cafe colored), but I want to cover all bases.

Picked up the manifold.  Zeroing in on a head.  

 

[Gery]

Whoa!  I just put in a Weber 32/36.  What a difference.  No srumbles, no hesitations, no roughness.  Good bye, Hitachi!

 

so, I suppose next is the head change and cam.  

[datzenmike]

That's one way to do it. You can port it, replace the guide seals so it doesn't burn oil, lap the valves, swap the cam (but be sure to get the correct lash pads to match)  and do stuff then just swap it on for the cost of a head gasket.