Timing Chain - smoothing out idle

[620owner]

Trying to smooth out the L20B engine in my 620. Occasional stumble at idle.

 

1) Eliminated a Vac leak - I found that there was a small leak at the base of the carb. (Used my propane tank with a hose and found a leak at the base of the front of the carb) Those two front carb nuts are a son of a bitch. (Ground down two 12mm wrenches to get to them)  

 

2) Adjusted Air and idle screws. - Warmed up engine with a Vacuum gauge attached and adjusted the air/fuel to fasted RPM/Highest Vacuum - about 4 turns out.... adjusted idle to 850 RPM then went back and forth to get best vacuum at 850rpm. I left it there and didn't back it off 50 RPM as in the manual.

 

3)  Adjusted Valves- I believe it was .008 and .010 There is a slight tick in one lifter now but shouldn't be a problem until I revisit.

 

4) Set the timing - Just a little below TDC maybe 5 degrees.

 

5) The choke is working as it should closing when cold and open fully after warmed up.

 

6) Checked compression all cylinders were even I believe about 115 (Don't have the result sheet )

 

Symptoms:

1) Now the engine runs great for about 30-45 minutes  of driving and then starts to stumble at idle. 

2) Noticed last night that with the headlights on it seemed to miss more often. Maybe due to the load on the alternator or maybe voltage is falling off and effecting spark ? 

3) I did hear what I thought might have been the chain slap when I reved the engine hard a couple times. (Engine just turned 40,000)

 

I must say the engine runs perfect really smooth and good power for the first 30 minutes of driving. Then the ruff idle starts up. The still engine runs great except at idle.....

 

 

Appreciate your advise !

 

 

 

 

[Roadster-ka]

Still has points in distributor?

[G-Duax]

More oil pressure would halp the timing chain flop issue, since the tensioner relies on pressure to do most of the work.

But the compression sounds low also.....

Engine up to temp, more oil will get by the rings, and valve seals could play with a smooth idle.

[Doctor510]

Timing needs to be about 10* BTDC

[banzai510(hainz)]

I would more adv timming like 7-12 degs

 

one needs to ck the timming once every few months as it does changed as the dist wears/ weights inside ect....

[620owner]

Replaced the distributor with electronic New cap, wires and plugs. Also changed the coil to a 1979 coil that had electronic distributor. I'll double check the compression reading's I'm probably not remembering them correctly, might have been 145.

[620owner]

Now on timing... You guys are saying 10 degrees before top dead center. Now that's with vacuum advance disconnected on distributor ?

[racerx]

How's your fuel, dirty, have you checked your fuel filter.  You can disconnect the hose to the distributor but I tried both and did not see any difference.

[racerx]

Forgot to mention your compression at 115 is very low.  Did you check when engine was at operating temp.?  Did you make sure your throttle was wide open by sticking something in the butterflies to make sure or having someone open it up by pulling on throttle, what kind of compression tester did you use.  I found out through trial and error that the screw in type is much more accurate.  The last time I did it with a plug in type, the compression was like 140, then screw in type was close to 170.

[datzenmike]

Stock timing on the L20B is 12. Depending on gas quality it may be adjusted lower but 4 degrees is way too retarded. Engine will lack power and run warmer. Set timing BEFORE making final carb adjustments as advancing will increase the idle speed. You must have the idle low enough so that the centrifugal advance is not in operation and the throttle should be closed enough that there be no vacuum advance. See below.

 

Vacuum advance is drawn from a port on the side of the carb above the throttle plate. If the idle is properly set there is no vacuum present and ambient air pressure is there. When the throttle is opened the throttle plate rises above the advance port and intake vacuum is applied to it. You can pull the vacuum advance hose off the distributor if you like but there should be no difference. No, you do not have to plug this hose as it's not a vacuum leak to the intake.

 

 

 

Poor idle after 30-40 min could be the engine not warmed up. Make sure timing and carb settings are done with temp gauge at the half way position. If engine is slow to warm up, change the thermostat. If stuttering idle after 30 min set the mixture and idle speed then.

 

Another thing to do is clean the EGR valve. It should be shut tight at idle and if leaking will run poorly at idle. Just remove and wire brush deposits off.

[620owner]

Thanks for the timing remarks, I'll revisit and and re-adjust the carb.

 

I put 93 octane in it. I just had the tank chemically cleaned and flushed. It looked like a mirror inside. I replaced all the rubber lines and filter. I also took the front off the carb and sprayed it out with carb cleaner and reset the float. The gas level is smack dab on the dot. The Hitachi carb is brand new. The vapor tank had been removed, I replaced it and also reconnected the air pump. I wanted all to be back to original. All seems good except for this damn idling issue.

 

I guess it could be gas still, I haven't run but maybe a quarter tank out making these changes.

[racerx]

Have you sprayed brake cleaner around carb, carb plate, intake and any where else where vacuum is at? 

[banzai510(hainz)]

just adjust the timming . I bet that and the mixture will take care most of the stumble.

It did mine.  L series dist do change over time.  I have gone thru 3-4 already in my truck already.

 

 

I have only done a compression test once in my life on a L motor.

[DanielC]

I am not familiar with L-20-B engine, but on a L-16, you set the timing below 750 RPM.  Go above that RPM, and centrifugal advance starts coming in, and the timing cannot be set accurately.  Disconnecting, and plugging the vacuum advance also removes another variable.   Running premium gasoline, also has the same affect as retarding the timing.   850 RPM sounds too high to set the timing to me.

 

Make sure it is the idle stop screw on the carb that is actually holding the throttle position when the throttle is closed.  If the linkage, or cable is holding the throttle position, that is way less accurate than the idle screw on the carb.

 

Check the valve clearances, and set the engine ignition timing.  I use the recommended timing as a guide, advance the timing until it slightly knocks, and then retard the timing 2 to 3 degrees.  After everything else is set, then with a completely warm engine, you can adjust the carb.

[datzenmike]

With 93 you should be able to easily max the timing at 12.

 

Engines before the 70s used to run and idle on intake vacuum advance. Trouble is it produces emissions so carbs were changed to ported vacuum so that there is none at idle but there is above and every where else. All engines LOVE advance at idle because there is none from the vacuum advance. Any time you want to prove this, set the advance properly to 12 and then turn the distributor towards advance... engine will speed up markedly from improved combustion efficiency. Basically engines today are retarded at idle.

 

Running at 12 degrees will improve everything about your engine running.

 

 

Gas won't be the problem as it's the same gas all the time. If it happens 30 min. later you have to ask... what's different after 30 min than earlier?

 

For one the engine is thoroughly warmed up. So check and see when the heat gauge reaches it's highest point. If 30 min that's almost 3Xs to long, so suspect the thermostat being stuck part open.

 

The L20B also has a Thermal Vacuum Valve (T V V) which closes a vacuum leak in the vacuum advance and the EGR line. Until this valve closes, there can be no vacuum advance or EGR. The T V V  senses coolant temp so again if the engine is slow to warm up the advance and performance will be less. The T V V will have little to do with idle.

 

Choke and fast idle should shut off in under 10 min. Check that the choke plate is FULLY vertical after about 10 min. If not this needs to be fixed.

 

The air filter snorkel has a vacuum operated valve that mixes cold outside air with warmed air from the exhaust manifold and is regulated by a temperature controlled valve inside the air filter housing. This set up tightly controls the air temp going through the carb to about 100F and allows the engine operate normally through a wide range of outside weather and temperatures. The most noticeable thing is that it eliminates carb icing.

[HOGIE]

It's a simple this but after I cleaned my fuel tank I went through a fuel filter really fast because of things that had been loosened up. 0.02