L20 engine pump pedal to start

[Tex alex]

I have 521 truck with a L20b motor and to start it I have to pump the gas pedal at least 5 times to get it to start. It will idle for a bit then it dies. The more times I pump the gas pedal the longer it will idle but it eventually dies. Ideas? 

[Stoffregen Motorsports]

What carb is on it?

 

Carbureted motors need some time to warm up, and depending on the outside temps there, it may take longer to idle steady. Sounds like some choke adjustments are needed.

[Tex alex]

It is a niki carb and today it is in the 60’s. What is a reasonable amount of starts to get it warmed up?

 

[Stoffregen Motorsports]

Two pumps should get it to fire, but it should run after that without too much trouble.

[datzenmike]

Look at the choke plate with the air filter top off and the engine cold. Pump the gas and the choke flap should snap closed. '71 and older had a pull knob manual choke on the dash, later were electric.

 

You have to step on the gas at least once to set the choke. Sub freezing 2-3 pumps and so on.

 

By sound of it the choke is not on. Find out why.

[banzai510(hainz)]

1)Idle jet could be plugged  your just running off the accell pump when you cycle the gas bypassing the idle jet cause maybe jet is plugged. Check your filter if lots of rust. If yes then that could be it.

 

2) adjust the mixture  thew base of the carb  if way off this can give a plugged idle jet symtom

 

3) adjust the timing.

 

make sure carb and intake tight

 

 

later L20 if this a stock carb had a electric cutoff valve on the carb on for the idle jet so it dont have RUN On issues.  Is this a electric choke carb with this cutoff selinoid?????????

 

 

I think 72 or after had these cutoff selinoid . Im sure the stock L20s had them

Edited March 3, 2023 by banzai510(hainz)

[Tex alex]

I took off the top and sure enough the choke was open. I looked around the carb and it is a manual choke that is disconnected. 
 

I adjusted the idle screw and it will idle nicely for about a minute and then dies. If i rev it too much it dies. 

[Tex alex]

Wha

1 hour ago, banzai510(hainz) said:

1)Idle jet could be plugged  your just running off the accell pump when you cycle the gas bypassing the idle jet cause maybe jet is plugged. Check your filter if lots of rust. If yes then that could be it.

 

2) adjust the mixture  thew base of the carb  if way off this can give a plugged idle jet symtom

 

3) adjust the timing.

 

make sure carb and intake tight

 

 

later L20 if this a stock carb had a electric cutoff valve on the carb on for the idle jet so it dont have RUN On issues.  Is this a electric choke carb with this cutoff selinoid?????????

 

 

I think 72 or after had these cutoff selinoid . Im sure the stock L20s had them

what fuel filter should I use? I got this car In Mexico and it has an interfil FGI- 024.

 

thanks

[Stoffregen Motorsports]

Won't run when trying to rev? Can you get it to temperature at all? If so, will it run/rev then?

 

Now it's starting to sounds like a clogged filter or bad accelerator pump or a problem with basic fuel delivery/

 

Is the fuel new?

 

Have you tried disconnecting the hose from the fuel pump and blowing back through the lines to the tank?

[datzenmike]

57 minutes ago, Tex alex said:

I took off the top and sure enough the choke was open. I looked around the carb and it is a manual choke that is disconnected. 
 

I adjusted the idle screw and it will idle nicely for about a minute and then dies. If i rev it too much it dies. 

 

Well connect the choke back up. The carburetor mixes air with tiny tiny droplets of gas. Liquid gas will not burn until it vaporizes. The fuel droplets absorb heat from a warm engine to evaporate. (intake and combustion chamber) When an engine is cold not all the gas vaporizes and the engine won't make enough power to keep spinning. The fix is the choke which forces the carburetor to supply extra gas so that more can evaporate during the warm up and then the choke is shut off.

 

Another thing to look at is the ATC. ( air temperature control) This is a flexible metal hose from the air filter snorkel to the hot exhaust manifold. It allows the carburetor to draw warmed air heated by the exhaust. Again... warmer air helps the cold engine evaporate the gas and run smoother during warm up.

[paradime]

After buying a vehicle with an unknown engine setup and maintenance history, you'll want to go through everything to eliminate potential issues. If the choke was disconnected or missing, that's a good indication to do this sooner than latter. 

 

Congratulations on your new 521!

[Tex alex]

1 hour ago, Stoffregen Motorsports said:

Won't run when trying to rev? Can you get it to temperature at all? If so, will it run/rev then?

 

Now it's starting to sounds like a clogged filter or bad accelerator pump or a problem with basic fuel delivery/

 

Is the fuel new?

 

Have you tried disconnecting the hose from the fuel pump and blowing back through the lines to the tank?

I was looking at the fuel filter and should it be completely filled with gas? When running it is a quarter filled.

[datzenmike]

Long as gas gets to the carburetor it's fine.

[Tex alex]

On 3/3/2023 at 6:30 PM, datzenmike said:

Long as gas gets to the carburetor it's fine.

Just an update it is still cold in DFW Texas we are in the mid 50’s and I can get the engine to idle with the choke on but if I open the choke the engine after it has been idling for five minutes the engine dies. How long should it take for the engine and carb to warm

up?

 https://www.dropbox.com/s/ks0a3os64iv0klh/521Datsun_choke.mov?dl=0

 

there is an incandescent light in the engine bay that when the ignition is on     Has a steady flow but when the engine is running it dims and then brightens up. Don’t know if this means anything.

[banzai510(hainz)]

note 1)Like I said before the idle jet can be plugged if a later model stock carb is installed

or the mixture not set right

 

 

when you pull the choke(or automatic when cold its closed) your bypassing the idle circut and getting into the main jet as the chock flap is crating more vacuum in the carb to suck thru the main jet.

 

One check I do it get it up and running say 22500 rpm and then open the choke and see if the motor stays running at 2500 thats the main jet circut. then if you let off the throttle and goes back to idel and dies then go back to note 1

 

I cant remeber if you have stock carb or weber on here. take a photo of the set up.

 

a leak at the base of the carb will give a No idle also

[Stoffregen Motorsports]

So it's either getting too much air or not enough fuel. Can you rev it up with the choke open?

[RetroRocket]

Check the accelerator pump function of the carb. Before you start it or crank it for the first time of the day, open the engine compartment, ignition off, remove air cleaner, get a flashlight (no matter what time of day it is). Don't move the throttle yet.  Open the choke plate (no matter if it is manual choke or automatic) if automatic there should be some spring pressure wanting to keep the choke plate closed. Prop it open with a chopstick or dowel, or a #2 flat blade screwdriver by using the handle or whatever.

With one hand holding the flashlight, looking down a primary barrel of the carb;, and the other operating the throttle shaft of the carb, give it a full stoke and see if there is a spray of gas droplets. Stop at the end of the stroke for a second and allow it to close and then do it again. If there's nothing then you've diagnosed a carb that needs to be rebuilt. It isn't all that difficult. There are always instructions, specifications for float height & drop, Usually a thick-paper template to set those measurements included, and the bits you need to rejuvenate it. You'll want to compare your accelerator pump diaphram to the new one, or if it uses a piston-cylinder type pump then attention to the pump piston seal. Do the whole thing on a metal splash pan or baking pan with sides on it and have a little magnet ready for the little clips you are going to take off of the carb rods, levers, and linkage. 

If you do get some accelerator pump spray but only a few drops of fuel then it still needs to be refurbished. It depends on price, availability, and quality if the most practical option would be to source a rebuilt unit. In most cases there will be a core charge and shipping the core back can be a pita but they have your money already and you need to spend to get it back.

 

You are referring to engine cold right?              Otherwise there could be vapor-lock issues, etc., which are more common in a warm engine.

 

I'm excluding SU's because they use could enrichment via dropping the fuel nozzles which pulls the jet away from the mixture needle; no accelerator  pumps.

 

Although I made a primer system on my 720 twin-SU for cold starts from an antique airplane instrument panel primer (even one drop of leakage from one of those in the cabin of my 1939 Luscombe 8A was intolerable and unsafe); they are good quality, they are required to be. Just like in my plane I knew how much prime pump to use for what outside temperature range I was starting the engine at and it worked well. There needs to be a check valve in the small caliber line. I think I used oil pressure gauge metal line, no hoses.

 

Be glad you don't have to prop-start every attempt. I did for every attempt because 1939 Continetal 65 hp engines had no provision for either starter or generator cast onto the block. It didn't take long to learn specifically how much prime to use from Big Piney, Wyoming in the dead of winter to the Yucatan Peninsula in the heat of summer, Chicago winter and summer, San Diego (always summer), St. Louis winter and summer and at home in Boulder. I had a legal small battery (about the size of a riding lawnmower battery) to work the position lights and UniCom radio. I had a wind-generator that hung from the fuselage just about at the datum line as I recall. It had a propeller that was about 8" span. It was just squared-off at the back and the prop so I streamlined it using the nosecone from the largest RC airplane I could find. One on each end.

 

Edited April 24, 2023 by RetroRocket
Too wordy.

[datzenmike]

Tex alex was last here over 5 weeks ago, hasn't been back. I guess he fixed his carburetor problem.

[RetroRocket]

I like how minimalist SU's are. There is a certain je ne sais quoi about that feature.  I've wondered about caveman ITB's using SU's. I think it would allow for some resonance tuning of intake airflow, a variant of V.R.I.S. to change plenum volume. The fuel is being intoduced right into the head (like how mpfi is located) so there aren't atomization and mist management issues like car with downdraft manifolds or throttle body injectors do.

 

I have used SM needles and I have turned brass mixture needles to tune out specific lean spots using a drill press to turn needles and applying gentle pinch pressure to fine sandpaper at the distance I want to enrichen. But that was before I had access to wide-band. It would be a lot easier using that to find flat spots. 

 

I'm wondering if a CAD program or 3-D printing could be integrated with a wide-band generated fuel map such that a needle could be machined to the matching needle profile called for to get the needed result based on what rpm and piston spring combinations need tuning for mixture and throttle response.