Jump to content

Diagnosing Engine Fueling or Ignition Problem

Recommended Posts

Wouldn't no vacuum be the same as full throttle or "as assembled" where the long air horn spring pushes the power valve assembly (bottom of fuel bowl) open?  Without a vacuum signal to the air horn piston, it isn't going to move from that position.


Now when I start the truck, it is much leaner and you have to blip the gas pedal a lot to get the revs up to pull out.


This is exacerbated because I lowered the fast idle setting which evidently with a always on power valve resulted in really high rpm.  Now that is not the case and I need to increase the fast idle adjustment probably to where it was to start with.

Link to post
  • Replies 41
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Yeah I guess it would run richer than needed at part and full throttle. Correcting this would improve your mileage.


Fast idle is only on when the choke is on, or should be. Once warmed up the fast idle and choke are both out of a job. Idle is solely controlled by the idle circuit speed screw and the mixture screw.


Power valve supplies fuel to the primary venturi only under heavy loads. It may have 'available' but does not affect idle when there is not enough air passing through the venturi.

Link to post
  • 1 month later...



New fuel filter, carb taken apart and does the same thing.  It conked out about a 1/4 mile from home just now.  Lots of fun driving back on secondaries and engine screaming rpm.


This is really pissing me off.   I've never had so much trouble with a carb.

Edited by Cardinal Grammeter
Link to post

It worked OK for a couple years and now this dirt thing - what changed?  (Assuming this failure is dirt in air bleed again.)


Is there anything inherently wrong with the Hitachi?  The only issue I can see is how it has a large offset return spring load that wears out the sides of the throttle plates - they made the carb do the throttle return spring job rather than having a stand alone return spring system - one of the reasons I hate throttle cables.


Is the Weber is that trouble free a carb?  But I would HAVE to use the stock air cleaner - it's too iconic - that Baby Blue on the Datsun engine...


EDIT:  OMG!!!:  ($45 though....)




But then I don't know where this mysterious dirt is coming from.  Small carb = Small orifices = Sensitive to dirt.


NOTE:  Have you seen the Auxbeam "Chinese" Hitachi Knockoffs being sold for $90-100 shipped?  (Manual choke though...)



Past Weird Carb Obstructions I have encountered:


PLASTER:  Fixed a gas pressure washer where the fuel inlet passage in carb was solid with something white and hard - like plaster.  Had to remove about a 1/2" length, 1/8" diameter of it.  WHERE did THAT come from?  Washer was like new with NO hours on it.
Maybe something like that in carb?
SOLDER:  I had a "new" AVS carb that just didn't work right.  ASS HATS evidently lost a soft plug near the idle mixture screws, so they thought, "I'll just pour some solder down there to seal it and it'll look like new!"  Finally deduced that was what happened when I could see some solder blocking part of one of he transfer slots.
Again, something like that with this carb?
Edited by Cardinal Grammeter
Link to post

Webers always work better than an Hitachi for the very reason that they are new and the Hitachi is 40 years old.

Webers are no different than any other carb and will also wear out.

Hitachi's are much more complex so more to go wrong.

Hitachi's are over all a better driving carb in all conditions, weather, climate, emissions, mileage and drive ability.

Webers are simpler (less to go wrong) and have a slight performance edge

Link to post

I guess the $254 deal on eBay is the way to go.  Although I like to buy used, maybe not such a good idea on a carb.


MYSTERY:  The first "clog" I found an air bleed completely block w/debris.  While I was happy to find "something", I didn't think it through:

  • A clogged air bleed would simply change the fueling curve - in fact, it would richen things up.  It would NOT limit the primary to only fuel idle.
  • A clogged air bleed would have gotten the debris NOT from the fuel, but from the air outside the carb since it is a bleed which allows air to be drawn into the carb.

This is very confusing.


I'm going to tear down the carb on white paper and ultrasonically clean the body and check passages as best I can.

Link to post

What rick rat said, M Klotz did a aircleaner mod and looks perfect however the Weber carb willl slight be raised depending on what weber adpater one uses. Theres the tall one like 2.5 in and the base 1/4 plate ones which are cheap and soem can crack if over tightened.  So if tall adapter used aircleaner may hit the hood.. But Weber makes a short aircleaner like 1.75 in tall. which will work

Link to post

Nothing wrong with the stock carb if they still work. With trucks I change out the fuel filter more often as the tank is outside(more prone to rust and fuel line drainage into tank unlike a datsun 510


webers are EZ and simple but they are Louder. Stock is nice and queit if still work but most are 40years old and worn out by now esp the accel pump diaphrames

Edited by banzai510(hainz)
Link to post
8 hours ago, Cardinal Grammeter said:

Hey, are the new Webers alcohol proof?

Been using  webers for years, they usually come sober, just don't let them start drinking and they will be fine



Must you overthink everything?????

Edited by Rick-rat
Link to post



Primary MAIN JET HARPOONED by WHALE SIZED RUBBER SLIVER 0.350" long, 0.010" at the ends, and .050 in the middle - not totally unlike Anne Elk's Theory.  (Thoughts of Hoplites, Roman Weaponry, and Ben Hur came to also mind ...)


Where could it have come from???   It definitely is RUBBER - it stretched when pulled.


I checked the fuel bowl view plate gasket and the corners were sharp.  I also noticed the floor of the fuel bowl had casting skin starting to crack and lift, so I broke off all the loose pieces and scraped smooth.  Now I'm paranoid of carb guts coming loose and wreaking havoc.


BTW, LOCTITE PURPLE is the perfect strength for carb bolts!


NOTE:  Let's see if I can get the 4 stud nuts on w/o dropping them in the grass ...


NOTE2:  I'm also going to bench test the accelerator pump.  I think it is weak although replaced recently.  (A new carb kit had a blue rubber pump which stuck to the bore and did not slide - the pump assembly overstroke spring gave way giving the impression externally that it was actually working.)

Edited by Cardinal Grammeter
  • Like 1
Link to post

Good hear u got fixed. But if u do eventally buy a weber,BUY A REAL WEBER FOR THE REAL PRICE!!

i had a knockoff weber,fell aparts.

Real dgv elec choke version,choke house is whiteish in color. Black is chinese crap,so is empi.

Link to post

Thanks,  only thing really left on the carb is getting the

  1. Cold Idle set back up since I lowered it a lot when carb was running w/o hollow base screw and power valve was wide open 24/7, and
  2. Right Electric Choke Spring in it.  I have one in an L20b carb that is fixed and you cannot adjust it - so it isn't that one.  And then I think I had another.  They are very different:  lots of turns vs almost few turns.  I remember comparing them in a pan of hot water - I don't remember what I found or what I chose to use.  If anything, I need more choke 2-5 min after start.
Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.