Diagnosing Engine Fueling or Ignition Problem

[Cardinal Grammeter]

Totally Stock OE 1974 L18 620

 

Frequency: 

 

This problem happened once and then maybe a second time months later, now it occurs every time I drive

 

Symptom: 

 

About 6 months ago:  initially occurred when hard throttle up a big hill.  Engine would surging and shutting down similar to fuel starvation.  No ignition misfiring or back firing.  After shutting engine off, it would restart and I'd be on my way with no issue.

 

A few days ago:  same deal on hill, pulled off, engine would idle, but if I gave it a tiny bit of gas, I could get the revs up maybe 500 pm - anything more and engine would die like no fuel, but back to idle.  I tried to coax it to higher rpm and would not do it.  Then out of frustration, I floored it - the engine then started chugging, picked up speed, cleaned out and revved high with out issue - as if the secondaries were working and the primaries weren't.  I made it to my doctor's appointment, but on the way back, this problem was not going away.  Afraid that I would brake down in

a tunnel, I had it towed home with my AAA.

 

I remember back when I drove points/cond V-8's, when the points needed replacing, there would be a slight surging when cruising at part throttle.  So I thought maybe the points.  I order some from RA, and filed mine down (they were pitted with metal transfer) and reset them to .022"  (they were probably .018" - but this was after filing.  The spec is .022 to .018 I believe.

 

When cold, truck runs fine, but as soon as it starts to warm up, engine surges and you can feel "no gas pedal" as if fuel starvation.  Again, no obvious ignition symptoms or backfiring.

 

I have the new P&C and haven't put them in, but I am not hopeful.

 

CARB BASE?:

 

My latest suspect is carb (OE Hitachi) throttle plate base screws have worked loose.    OR dirt in the carp but I don't believe that one since carb was 100% clean when rebuilt AND there was always a Hastings clear fuel filter present.  NOTE:  the original Hastings had a ton of rust in the bowl and I swapped this out thinking it was perhaps clogged.  But I hooked up a cowl fuel pressure gauge and it maintains a perfect 3.5 psi at all time.

 

TO DO:

 

1) put he points in

 

2) take the carb apart - I DO NOT WANT TO DO THIS !!!!

 

3) OR ??

 

Thanks in advance,

Tom

[flatcat19]

Clean out gas tank.

 

Install new fuel filter.

 

Install Weber.

 

Live long.

 

And prosper.

[banzai510(hainz)]

I would have put a electronic ignition in a while ago as they help trouble shoot cause once set they usually dont go bad. Then one can focus ibn the fuel dilevery side of things.

 

If your sure your points distributor isnt like worn out and the shaft wiggles an you have stock coil and ballast  thenI bet the carb is getting worn out.

 

I wouls also have dist go out of time and have reset it once in awhile.

[Roadster-ka]

If fuel filter doesn't fix, check screen inside fuel inlet banjo fitting on top of carb.

[racerx]

Run it with gas from a can, u can isolate better

[datzenmike]

Just replace the fuel filter. It's quick easy and cheap. If not the fuel filter you have wasted nothing as they should be replaced every 2 years and it's probably due anyway. I think you'll find this is the obvious cause.

 

Cut the filter open and inspect. If full of crap replace the filter more often.

[Cardinal Grammeter]

Filter is replaced (first one when I got truck), 2nd one now.  I always use a clear bowl Hastings GF85 filter so I can see exactly what is in the filter.  And yes, big chunks of rust scale appear shortly after I started running truck - I should have replaced after a month or two, but I didn't since it ran just fine.

 

Recall I have a fuel pressure gauge installed so it's not a clogged anything on the delivery side.  Question is, if the filter has lots of rust in it but not enough to restrict fuel pressure, should it still be protecting carb from contamination?  (I guessing Yes)

 

I think entire ignition is original and stock - coil looks very rust and old, but has good spark - can jump over 1 in if plug wire pulled out of dist cap.  When setting/filing points today, there was no transverse slop at the cam.

 

SUSPICIOUS thing now is that for the first 2-5 min of operation it is 100% OK, but as soon as things start to warm up, the symptom definitely is coming back at part throttle.  Could be related to electric choke starting to open?

 

Regarding throttle plate base screws loosening:  I had to R&R the carb soon after I put it on and I did note those screws were not tight.  I really tightened them the 2nd time, but I wouldn't bet 5¢ they are till tight - should have used blue Loctite. 

 

TO DO TOMORROW:

• Will check banjo on carb - that is easy.
• Points/cond will be replaced tomorrow hopeully...

[datzenmike]

It may have fuel pressure at idle and low speed, but the restriction affects the engine when it's working hard climbing a hill. Just replace the filter.. either it will do nothing or it will cure the problem.

[Cardinal Grammeter]

Filter replacement does not guarantee supply.  I could have leaks in the fuel line or a failing fuel pump.

 

The tough one to diagnose is the leak in the fuel line, but is academic if 3.5 psi is maintained continuously when road tested.   This was not hard to verify with the fuel pressure gauge on the pump outlet.

[datsunaholic]

Issue I've had with Hitachis is a bit of debris ends up in the primary jet.  The way the jet is positioned, stuff can get in there and stay there.  When the engine dies, the debris falls out of the jet and the engine runs again, until the debris gets pulled back into the jet and repeat. 

 

It is possible to pull the jet caps off without removing the carb (they're under the float bowl), and sometimes the debris runs out with the fuel (it WILL drain the bowl when you do that!).  It's just a lot easier to inspect it with the carb off.

[nl320what]

I would take a bit of everyones suggestions to help pinpoint it, sounds very much like carb passage contamination.

 

Get a few cans of Starter Fluid/Big Carb Cleaner which will serve 2 functions: 1 is to test while its doing this dying part throttle, spray in carb while giving it gas to see if it works better and if it does then your carb has blockages. 2 is to pull carb off and pull the fuel bowl window off and blow all the passages outwards from the venturi ports and it should help to blow the stuck crap out the other way it came in.

 

Spain Weber with a pressure regulator is the way to go, and the matchbox dizzy with a turbo style muffler.

[banzai510(hainz)]

I had a crack in the intake fuel line once and would suck air instead of gas thinking it was running out of gas

 

replace the filter anyway but once carb is clogged its too late but it woill cause a restricted gas flow.  I just had this last year and filter didnt look that bad. But the type you got I seem them full of rust and still work as all the particles go down to the bottom

[Cardinal Grammeter]

Thanks for he carb thoughts - I thought a clogged primary where when you turn engine off, the dirt falls out of the jet so when you start engine again, all is fine until ....

 

If carb is clogged, that means fuel filter is not doing its job (and the carb banjo filter.)  That is worrisome...

 

A Weber now would be admitting defeat - I made a decision at the start to find the correct carb for this truck - the previous asshat monkeys had a MonoJet mounted on a screwdriver chiseled wood adapter plate with washers on studs clamping carb flange down.  But... if I stumbled across a deal on one....

[DanielC]

I do not know about 620 trucks, but the fuel pick up line into the gas tank on a 521 was just an open pipe near the bottom of the tank.

If there is debris in the gas tank, fuel pump suction can draw the debris to the fuel pick up, and plug it.  Then when the engine dies, or is shut off, the fuel pump suction goes away, the debris is no longer held to the pick up line, the debris moves away, and the engine runs fine again, for a while.

 

A quick test, when the engine dies, blow air backwards from the fuel filter inlet into the tank.  This dislodges the debris plugging the fuel pick up, and if the engine runs, you need to clean out the gas tank.

[Cardinal Grammeter]

DanielC, to eliminate all considerations on the everything downstream of the pump, I installed a fuel pressure gauge on my cowl.  It maintains 3.5 psi and drops to maybe 3.1 under hard accel up a hill.

 

UPDATE:  I did the completely tuneup:  points, cond, cap, rotor, plugs and it failed in classic fassion:  ran out of power, only enough "fuel" to idle, little gas and "flame out," but secondaries working.  Stopped.  Counted to 10.  Started up and everything was fine. 

 

Will be pulling fuel bowl sight glass plate and flush out with carb cleaner and blow out with compressed air.  I will have white cloth to catch whatever comes out of the bowl for scrutinizing.

[Cardinal Grammeter]

UPDATE:  CARB PHASE 1:

 

Checked the inlet mesh filter and found some debris there and a bunch of particles that were caught in the mesh.  Clearly smaller particles would have made it through.

 

Removed bowl view plate and a one "large" piece (maybe a 1/16" long sliver) came out.

I sprayed carb cleaner every way I could, especially in the bottom passage on the LHS (driver's.)

 

ALL CARB SCREWS AND FITTINGS WERE FINGER TIGHT AT BEST, including the needle seat fitting.

I tightened up what I could, especially the one base plate screw that comes in from the top.

 

TEST DRIVE:

 

First observation was my "bad" accelerator pump was nearly perfect now - evidently there was a vacuum leak.

Seems to run a lot better however I only drove it for two 2-mile jaunts.  More driving today.

 

CONCLUSION:

 

Carb should be removed and everything tightened.  I'm wondering if every threaded part should have blue LocTite...

[datzenmike]

Note also that one of the bottom screws is hollow and must be returned to the same position. This is the vacuum signal from intake to top of the carb for the power valve. My 710 carb had been apart by previous owner(s) and was wrong. It increases the mixture richness and added a noticeable push of torque when the secondaries were working.

[Cardinal Grammeter]

OMG I think I may have missed that since I assembled my carb from a basket of parts (ironically for a 74 manual trans!) 

 

I guess it is hollow at the thread end and then has a radial hole below the head?

 

I know there is one screw from the top and the rest from the bottom - 4 total?

• Parts diagram shows (2) #51 bottom screws.
• Can't find the top screw  (worst exploded view ever)
• Can't find the other bottom screw.

Really academic since you cannot tell which hole which screw goes into.

 

1974 L18 620 Carb Parts Diagram:

 

[Cardinal Grammeter]

UPDATE -or- Like it was Too Good To Be True that my problems were over...

 

3rd outing:  at just a few minutes longer than the first 2 trips, carb started acting up again.

 

While not "running out of gas", now it was running super rich - so much that idle was loading up and would have stopped engine if not riding the gas pedal.  Could also smell gas walking by hood.  Looked, some present on carb, but not clear of leak since carb is not perfectly clean.

 

NOTE:  Up until this "tightening of the screws" I would have sworn I had a bad accel pump.  However not one inkling of a AP problem after the TotS.

 

Going to have to disassemble carb, clean all passages and reassemble.

 

RE:  Loose Carb Bolts:  Need to do something... Blue Loctite?

____________________

 

 

EDIT:  RE:  Loctite:

 

Lots of controversy on internet about using Blue.  Some have had screws break off.  Others say OK.

 

Someone called Henkel (owns Loctite) said for small screws and carb screws to use the Low Strength Purple.

 

So Purple it is.

 

However, I'm considering a method to apply small amounts to the threads - not full drop which immerses their entire engagement.

[Charlie69]

Put a kit in the carb drop the tank and have it hot tanked for cleaning.  Replace all rubber hose in the fuel system clean fuel pickup assembly before installing in tank. replace fuel pump with proper mechanical pump.  Remove fuel pressure regulator put in trash or box for yard sale.  If you install the proper fuel pump you do not need a pressure regulator.

 

Put back together.

 

People when your fuel filter keeps filling up with shit find the source of the problem.  People like to do as little as possible in maintenance to their vehicles and wounder why the vehicle breaks down and is not dependable.

 

My truck looks like shit on the outside and the interior is filthy, but you lift my hood and the engine and mechanical of the truck is clean and maintained.  I am not afraid to drive the truck from Phoenix AZ to Elko NV where I bought it in 2011.

[Cardinal Grammeter]

Time Line suggests Loose Screws the Culprit:  Carb ran fine for almost 2 years, then started acting up a few months ago with the "bad accel pump" symptom which disappeared for a brief moment after I tightened carb screws that were easy to get to.  Loosening screws would take months if not years go work free.  Rust and dirt from tank would happen very quickly as soon as the truck was driven.

 

My mistake was not changing the fuel filter a month after I start driving it - as soon as I saw crud in the filter, I should have changed it and kept changing it until no crud appeared.  NOTE:  the 2nd filter I installed a couple weeks ago is still clear w/o any sediment on the bottom suggesting all the loose stuff in the tank has worked it's way out.

 

Why loose screws?  Either they are backing out OR the gaskets are taking a set.

 

Carb was like new cleaned and assembled with kit not even 2 years ago.  If the gaskets are taking a set, they are garbage.  I've never had loose screws in AFB's and Stombergs - or Holleys for that matter.

 

I'm  NOT using new gaskets for carb tear down and clean out - I don't want to chance gaskets setting.

 

I'm putting Purple Loctite on everything.

 

Fuel Pressure Regulator:  Wasn't aware there was one.  I'm getting 3.5 psi which is correct.  If I remove it, pressure will be higher.  Then I'd have to adjust float for proper bowl level. 

 

Where's Waldo:  Where's the Power Valve #62 (and the Spring-Rod #63 for that matter) on the carb parts diagram?

 

Filters are suspect:  If the filter is working properly, it will keep dirt out of the carb.  This engine was never run w/o a new Hastings filter.  Of course it could be that these Hastings fuel filters do not have a small enough micron rating - who knows what micron specs any of these fuel filters have. 

 

Crud between the filter and the carb should be handled by the carb fuel fitting screen.  I found a pinch of crud when I pulled the fitting the other day - not much, not as much as you'd find under a fingernail.

[racerx]

Never had fuel regulator on my Datsuns or 1st gen celica, even stock carb or 32.36 weber and ran good.

[Cardinal Grammeter]

BASE PLATE SCREW WITH HOLE LOCATION:  The hole is a through hole entire length of screw:  there is a vacuum passage that goes through the screw all the way to the air horn - this passage is parallel and close to the power valve (that would make the location near the fuel bowl. 

 

If you have the air horn off, you can sight through it and see daylight.  If the air horn is attached, you can find it by fishing a wire to see if the hole goes all the way to the AH.

_________________

 

HILARIOUS UPDATE:  I had 2 hollow base plate screws installed and they were both in the wrong location!

 

Carb is off and there is no sign of dirt.  I pulled the jet and the smallest one diameter is humongous compared to the dirt/debris I found in the carb intel fitting filter.

 

Base plate gasket is split at a vacuum hole and no kit available locally (no surprise there), will probably seal with something and reassemble rather than wait for gasket.

________________

 

REBUILD UPDATE:  Yes, there was a plug of debris, but not in the main jet - but the primary air bleed above the emulsion tube.  This is troublesome since to get to that air bleed, you have to go through the holes in the emulsion tube which looked too small to pass that junk.  Further there was nothing stuck in those holes and nothing in the well.

 

Also installed the spring above the acel pump weight - I intentionally left that out because the carb was a basket case and although installed correctly, I'd never seen such an approach.  Weight and Spring?  That is redundant.  Anyhow, acel pump should be better.

 

And with the hollow base screw now sending vacuum to the power valve, will be interesting to see what happens.  However IF (this is big IF) that is the only signal, a block vacuum would be atmospheric pressure or WOT which is full enrichment.  With the vacuum present, it should lean out engine cruising.  But no matter, there should be a difference driving it.

 

Should be testing in an hour. 

 

Install carb tips: 

• Remove the BCDD and gain easy access to both mounting studs under it (this is a HUGE simplification.)

• DO NOT slide carb over studs down to gasket.  In each corner, lay nut on flange and slide over stud just enough to engage nut - then you can spin it on with a finger instead of trying to balance it on top of a stud which takes 2 fingers.  Once you get the hang of this, you can have the carb tightened almost as quick as an AFB.

Back in Service:  Driving around now and all seems to work OK except for the one thing bad on this carb:  the throttle plate shaft is quite worn - so much that the idle mixture screw can be turned in completely or out 3 turns w/o affecting idle appreciably.  But that is a project for another day.

[Cardinal Grammeter]

UPDATE:  Fuel Economy:  Improvement (hopefully)

 

Gas gauge appears to be "broken" :unsure: in that it doesn't seem to be moving much.

 

With the hollow base plate screw now feeding the power valve, I'm hopeful of an increase in mpg since power valve is now not always open.

 

It's going to take a while since it's moving so slowly. 

[datzenmike]

Under light load conditions intake vacuum would pull upward on the vacuum piston compressing the spring leaving the power valve closed. Under heavy throttle the intake vacuum signal lowers and the spring pushes down on the power valve opening it and adding extra fuel. So... light load, closed. Heavy load, open. No vacuum signal permanently closed on.