'78 620 Fuel Feed Problem

[difrangia]

I'm having a problem with what seems to be intermittent loss of fuel. After sitting for at least several hours, she will run OK for 3 or 4 minutes, then fall flat on acceleration for 5-20 seconds; pick back up; fall again; etc. For a little pertinent recent rebuild info, since completing install of new valves I've:

 

1.  Rebuild kit in carb, including clean barrel inlet fitting screen.

 

2.  New mechanical fuel pump.

 

3.  New correct fuel filter.

 

4.  Blow out all fuel lines from carb back to tank.

 

5.  All new rubber fuel lines.

 

She ran OK for about a week after I finished the head work and the above replaced parts and more non-fuel related refurbishments. Pickup set for longer periods before I acquired her from elderly original owner. PO told me that a local garage had flushed the tank. I can't verify how thorough a job they did though.

 

I'm thinking that either a small piece of crap moving is around in a carb passage or jet or I'm picking up some crap in the tank and stopping the fuel supply to the filter and pump.

 

Is there a screen on the pickup tube in the tank? If so, it could be getting blocked after a short time running and the blockage falling away when the vehicle sets a short while, only to start over next time it runs. I did drain several gallons of fuel out when I first brought her home and only got clean fuel out of the tank.

 

Anyone had similar problems? Thanks in advance.

Steve

[datzenmike]

I'm having a problem with what seems to be intermittent loss of fuel. After sitting for at least several hours, she will run OK for 3 or 4 minutes, then fall flat on acceleration for 5-20 seconds; pick back up; fall again; etc. For a little pertinent recent rebuild info, since completing install of new valves I've:

 

.

Sound sort of like a fuel delivery problem. Carb rebuilt, so lots of problems could have been added there.

 

Float not set correctly and fuel level in carb too low. Carb runs out of gas when floored

 

Why was fuel pump replaced? Unless you are sure the pump is bad you may have replaced a good one with a cheap piece of crap from China. If you have the old one... put it back!!!

 

Tank (may) have been cleaned. POs say anything. Maybe it was... maybe it wasn't. You have a new fuel filter, replace it and cut the old one open and examine it for rust and crap. They're cheap and maybe it's plugged.

 

Pull the fuel line off the pump and direct into a pail to catch the gas. Have someone crank the motor. Gas should shoot out into the pail in a strong pulses. If it does... tank, lines, filter and pump are likely OK.

[difrangia]

.

Sound sort of like a fuel delivery problem. Carb rebuilt, so lots of problems could have been added there.

 

Float not set correctly and fuel level in carb too low. Carb runs out of gas when floored

 

Why was fuel pump replaced? Unless you are sure the pump is bad you may have replaced a good one with a cheap piece of crap from China. If you have the old one... put it back!!!

 

Tank (may) have been cleaned. POs say anything. Maybe it was... maybe it wasn't. You have a new fuel filter, replace it and cut the old one open and examine it for rust and crap. They're cheap and maybe it's plugged.

 

Pull the fuel line off the pump and direct into a pail to catch the gas. Have someone crank the motor. Gas should shoot out into the pail in a strong pulses. If it does... tank, lines, filter and pump are likely

 

 

Thanks, Mike. I'm thinking along the same lines of all that you say including the diagnostics. Truck originally had dealer add-on air which included an electric pump which had been replaced recently when the tank was flushed, supposedly because the truck had been stored for a lengthly time. I removed the air bracket and went back to mechanical pump. I retained the original fuel supply manifold at top front of engine including the return line; & to answer what might be the next question, I do have the restricted tube plumbed to the return side, so that is not the problem.

 

When I acquired the 620, I was led to believe that it had a burnt valve from running on old storage gas. Zero compression on #4. When I pulled the cover, #4 intake rocker had come unwound & got between the cam & valve retainer bending a valve. Off with her head; new valves. All the parts that I acquired from classicgarage were old-stock Jap manufacture, to the extent many were in shelf-tatterd boxes.

 

One main question I have is what the arrangement is on the pickup line inside the tank. Is there a screen or filter in there that might be picking up crud after a few minutes of running, which falls away when fuel pump vacum shuts off allowing her to run again for a short bit??

[datzenmike]

There is no screen just a pipe close to the bottom of the tank to suck it dry. The return line is always  a good idea to keep as fuel is constantly circulated past the carb. This supplies cool gas from the tank and prevents stagnant gas between pump and carb from absorbing heat and boiling on very hot days, something the L20B does and why it was installed. BEST of all fuel is circulated constantly through the fuel filter. Any crap falling into the tank when filling or anything the comes loose or rusts eventually is trapped in the filter. Return line vehicles should have very clean tanks. It would be common for vehicles that have sat and possibly rusted to require a filter change a second time right after the first one because of this.

[difrangia]

There is no screen just a pipe close to the bottom of the tank to suck it dry. The return line is always  a good idea to keep as fuel is constantly circulated past the carb. This supplies cool gas from the tank and prevents stagnant gas between pump and carb from absorbing heat and boiling on very hot days, something the L20B does and why it was installed. BEST of all fuel is circulated constantly through the fuel filter. Any crap falling into the tank when filling or anything the comes loose or rusts eventually is trapped in the filter. Return line vehicles should have very clean tanks. It would be common for vehicles that have sat and possibly rusted to require a filter change a second time right after the first one because of this.

 

Thanx, Mike. Answers my questions bout how she's built in-tank.  I have another new filter. I'll put it on this pm as I'm going to be doin' a little diagnostics on her.

[datzenmike]

Cut the old one open.

[datsun 160J SSS]

check the timing too .trust me my truck did the samething

[datzenmike]

Pull the fuel line off the pump and direct into a pail to catch the gas. Have someone crank the motor. Gas should shoot out into the pail in a strong pulses. If it does... tank, lines, filter and pump are likely OK.

Do this (above) next

 

 

 

 

Assuming for a moment that it's the carb....

 

 

Floor it until it starts acting up.

 

At this point, I believe that the carb may have run out of fuel so it's important to do the following as fast as you can to preserve the carb fuel level as it is while it's acting up.

 

You want to stop the motor as fast as possible to prevent it re-filling the carb. So get the clutch in and the ignition OFF together, as quickly as you can and SAFELY pull over. Do this on an empty road.

 

You should now have the carb exactly as it was while driving. Take the air filter housing off and look at the sight glass on the front of the carb. If set properly you should see gas at or near the line (old models) or at or near the dot on the newer ones. If you don't see gas, and as this carb was re-built, possibly the float was bent accidentally on assembly and is set too low.

[difrangia]

Cut the old one open.

 

 

'Phantom in the Works'  slayed. Scalpel not required DZMike. Photo of the replaced filter with less than half a tank ran through it. Happens again the tank comes off for a cleaning. Last tank I cleaned was on the BMW 600 Limousine we had a few years back. I've used several methods in the past on the tank insides including a brutal shaking with gravel, chains, and whatever inside with a little solvent. On the BMW tank I used steel shot for shotgun shells. You can buy a 5 lb bag at sporting goods stores and its washable and re-usable.

 

Thanks for the tips on this Mike & others.

[datzenmike]

Yeah that sure look like the problem.

 

 

My 74 710 did this in 20 miles on the way home after picking it up. It had say since at least 2002 and probably longer. I disconnected the tank end and shook the crap out. This got me another 40 miles, and then home. I put a fresh one in and it was plugged in a week. Took tank out and put two hand fulls of drive way gravel in, shake and empty and a new filter. That was two years ago and a trip to Canby and there is a small amount of sediment in it now

[difrangia]

I'm keepin' an eye on this one. The tank may have to come out yet.

 

By the way, I did score the proper mounting bracket for the filter from member TJ. Much thanks TJ. The bailing wire bracket was a get-by till I could locate the proper one.