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DCH-340 DCH340 Identification, Adjustment, Issues, etc.


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I looked in my '79 200SX Factory Service Manual which shows exploded views of Calif. and non-Calif carbs, and neither show springs over the air bleeds. I'm not sure how carefully those diagrams were made. One diagram shows the primary main jet in the picture and in the parts list, but the number for the part is missing from the drawing. Easy to figure out what is what, but makes me wonder how closely the diagram was proofed.

 

The other odd thing about those little short springs over the air bleeds is you have a 340 with one of them and I have a 340 with two. I'm not sure, but it is possible the 340 presently on my 510 had at least one. Since they don't fit anything else in the carb, where did they come from? And they are sized to the air bleed hole. Dunno.

 

I'll check the gasket tomorrow on the carb on the 510. That is the carb that the screws loosened on. I looked at the gasket on the other 340 I have torn down and it is just as you described, about 1/8" thick and a hard material. I have a new Keyster made in Japan carb kit here, labeled for  620 PU 1974/08- (their typo not mine) that seems to have two thin gaskets for that area. They may have different hole spacing is why there are two. In a sealed bag so I can't tell for sure.

 

I was pondering the two different mountings for the small venturis which I mentioned in a earlier post. I was guessing it was early/late model 340s or something like that. To my surprise, the '79 200SX FSM shows the Cali 340 with the venturi holder held in by a set screw through the side of the carb body (the holder that is loose in my one 340), with a footnote saying not to remove these parts. The non-Cali carb is shown with the small venturi holder held down by two screws down through the top. So more Hitachi trivia for you - both style venturi holders in the same model year. Oh, and also a chart of "General Specifications" shows venturi diameters for the Cali carb (both manual trans and auto) as 24/31mm primary/secondary, while the non-Cali is shown as 23/30mm. I think this spec refers to the diameter of the main venturi/carb bore down below the small venturi.

 

I never intended a week or so ago to know as much about Hitachi 340s as I do now! :lol: And still so much to learn.

 

Len

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I checked my L20 carb w/standard screw down booster venturis and checked under the other brass plug and yes, there was another of those short springs sitting on top of the "down in the bore" jet.  So this carb had those short springs on top for both barrels.   And yes, it is NOT in the parts manual and NOT in the carb rebuild kit.

  • EDIT:  WRONG!  I was just looking at the Beck kit instructions (see as follows) for the o-ring boosters and while it looks like a sealing washer, items 83 and 93 are springs!  So there they are.  Now all we need is the explanation.  Keeping the jets down in the bore if they get loose is all I can think of.

I am just amazed at the sophistication of the DCH-340.  And I really get a kick how simply they describe its operation in the FSM in just a couple small paragraphs.  However, understanding the BCDD is a real mental challenge.

 

The BCDD adds fuel during high vacuum engine braking.  I wonder how much that lowers the fuel economy?  Of course when does one high vacuum engine brake?  Once a week?

 

Someone should write like a HP Book about the DCH-340.  But its only an economy carb.  I guess some lunatic could make a 6-pack with them!  Ohhhhh.... that is a Sick Thought!

 

It might be a good blow-through supercharger carb with the foam float.

 

beck%20arnley_162-9553_2.jpg

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OMG, and I thought getting the nuts on the mounting studs was hard.

 

I had to replace the mangled fast idle "lever" on my basket carb reconstruction (74 M/T) so I'm hoping that is why the starting, full choke, fast idle is a bit high - and not some mysterious vacuum leak.  I knew there was an adjustment procedure in the FSM but I thought, "I'll just do it on the engine!"  HaHa.  I'm from the world of AFB's and all you do is go right in from the front while at WOT.  But seriously, why would anyone design a screwdriver adjustment screw that basically pointed down?

 

Using mirrors, I've been able to get a small screw driver in the slot at a terrible angle, but haven't address the lock nut yet.  Hopefully I can get a wrench on it.

 

I was thinking that knowing this in hind site, a possible improvement would be to replace the adjusting screw with an Allen screw and then use a ball end Allen key to adjust.  The ball end may be just enough to deal with the angle.

 

So I'm wondering if anyone has come up with some tricks to do this?

 

 

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The fast idle cam is set by the maker and isn't intended to be, or even need to be adjusted. Carb has to be off the engine to set the fast idle cam.

 

 

 

The fast idle is set by opening the throttle plate to a specific value. A gauge or drill bit can be used for this. For example 1.33mm-1.47mm on a '78 620 L20B manual trans. Open throttle plate and place gauge and let throttle plat close on it. Then the fast idle cam is adjusted to sit on the first step (highest step) of the fast idle cam.

 

With engine fully warmed up, set the fast idle cam on the second step, (one step down from the first) restart engine and the RPMs should be between 1,900-2,800.

 

This adjustment provides the extra air needed for proper choke operation in cold weather. Naturally a properly functioning and adjusted choke is the second half of the equation

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You still need to be able to adjust it on the engine even if doing the FSM setup.

 

The problem is that there are at least 2 kinds of "cable arms" on the throttle shaft:  On allows easy access to the locking nut from the side while the other does not!  The "other" needs something like a stubby socket to actually turn the nut. 

 

As luck would have it, I installed the one with no side access.  Rather than do the swap and drop tiny bits all over the engine, I'm going to take a die grinder with small burr (not quite Dremel small) and open up a window to the lock nut.

 

Still, I am going to make some attempt to see where the cam is set as is - try to get the gauge wire in from the top (should be possible) and see just where I am.  And then there are the cryptic FSM words, "... placing the upper side of fast idling screw on the second step..."  Upper side?  Wonder if they meant "end of the screw which is pointing upward?" 

 

I also believe a long bladed screwdriver can get to the screw w/o too bad an angle.

 

That's all for now... getting near kickoff time... Go Steelers...  (I will get this done this evening.  Truck is running and chomping at the bit to move under its own power.)

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It's set at the factory and doesn't need to be adjusted, ever. Probably why it's placed there. It would be nice to adjust on the engine, yes, but not needed as part of a 'tune up'. It can be checked as part of a rebuild operation.

 

 

 

 

I had to replace the mangled fast idle "lever" on my basket carb reconstruction (74 M/T) so I'm hoping that is why the starting, full choke, fast idle is a bit high - and not some mysterious vacuum leak.  I

 

You say 'too high'. What is too high? 

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Before making a huge effort to turn that hidden screw, I'd be sure the screw is bearing on the lowest of the four steps on the fast idle cam (maybe you've already checked this). There is a lot of junk hung in the area of the fast idle cam and something might be preventing the cam from going to the low idle setting. There is a bent rod that goes from the cam up to the choke linkage and one of my FSMs shows a little adjustment screw up at the choke shaft that seems to control the travel of that arm. But I haven't read enough to understand how all that works. The  Hitachi 340 I have here in the house I can look at also has a screw with spring around it that goes down through the cast iron carb base and bears against an arm on the throttle shaft. It appears to be the idle adjustment screw to use when the carb is on the engine. It is directly below the accelerator pump arm so would be on the valve cover side when the carb is on the engine. It wouldn't be easy to see on the engine either. But maybe your Hitachi doesn't have this screw.

 

Len

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I have no tach on it but I'm guess its running at least 3000 rpm.  It's way to high for a fast idle start up.  Even if the factory called for this speed, I would lower it.

 

I also don't know if I have the right choke bimetal spring installed.  There was a wide one (maybe 3/8") in the basket parts while the ones in the L20B carbs were about 1/4 inch.  The wide one slid over the slotted pin in the choke housing and resided OK axially.  The narrow one had all this axial slop in the slot - it could rub against the ceramic heater or almost slide off n the other direction - perhaps they staked it somehow.  The wide one had fewer turns.  But wider means it generates more load per delta T so you would design with fewer turns.  I used the wide one.)

 

As far as the fast idle cam working properly, that part of the linkages has been carefully scrutinized.  I found the severely bent up fast idle adj. screw arm after I was trying to figure out when the cam over-travel stop (little sheet metal tab under one of the vacuum secondary diaphragm housing screws) could not be adjusted properly.  So I've been through that linkage thoroughly.  I even made sure I have the little washers on both sides of the links - the thick one at the bend, and the thin one at the clip or cotter.

 

I'm getting read to head into the garage and I'm going to see if I can swap the cable arm out without having to disconnect the accel pump link.  That connection has the spring and tiny parts that I know I would fumble working on the engine.

 

EDIT:  I just got the cable arm off the throttle shaft from carb now on my L18.  Laying the two pieces side by side, it is obviously clear that the large 1" square "shield" (the center of which has the "slot" that slides over the parallel flatted throttle shaft) is intended to block adjustment of the fast idle! 

 

The part I'm going to use has just enough material to handle the throttle shaft and the accel pump rod - this part allow easy access to the adj screw lock nut from the side.

 

Glad I decided to change this arm.  I was using the wrong one.

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Engines always seem to be revving higher than they seem.  I took my L20B to 5K last summer (on a tach) and I thought it was going to explode. 

 

The 1,900 to 2,800 RPM is so that there is enough air for the increased rich mixture from the choke being on when very cold. If too low a fast idle, the engine will blow black smoke, foul the plugs and idle poorly. This 'fast idle' seldom gets this high when the engine is cold. If sitting in the driveway with the choke on it will warm up and climb this high, but if driving or if you give it even a small tap on the gas it will drop down a step or even off. 

 

 

 

When the choke is off, the fast idle cam drops out of the way by it's own weight.

 

Cam rotates clockwise, down the steps and then off the last big jump.

 

carbZ24idlestepLg.jpg

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I was looking at my cam and there are 4 "lifts" available.  I wonder if the "highest" lift is actually just a dimension of the stamping and that the other 3 "lifts" are what are to be used and appear to be precision notches which are broached or machined from the stamped part.  This would mean the highest lift is not to be used.

 

There is another adjustment made by bending the finger (you can see the end of an attached spring to it) on the RHS of your plastic cam which establishes the position of the cam relative to the choke.  By bending this finger, you could make the fast idle work on all 4 lifts or just two or just one. 

 

The FSM talks about the "second step" which could actually be the 3rd lift if you interpret "step" as changing from one lift to the next, e.g. "you encounter the first step when you go from the first lift to the second lift.

 

Who knows.

 

Now with the "correct" arm installed, adjusting the fast idle should be a moot point. 

 

But my next issue is that there is no 12v at the electric choke.  

 

... EUREKA!  And I know why!  I don't have the voltage regulator hooked up and since the Heater Relay (very misleading, has nothing to do with the Body Heater) makes the contact to the choke when there is alternator output (only reason I can think of why the YELLOW wire powers the relay in the Heater Relay, right?)

 

Actually, that is kind of dangerous since if you lose your alternator, the automatic choke will cool down and really mess with engine operation.

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It's been a long day and I'm starting to fade, so I'll just post a couple of quick things. The FSM for my' 79 200SX has a drawing for the fast idle cam with the four steps numbered. 1st is the tallest and 2nd is the first "step down". Then 3rd and finally 4th being the lowest notch.

 

I don't have a 620 FSM, but have one for a '74 710. It seems like the carb section in the 710 manual is kind of skimpy compared with the one for the '79 SX. The SX manual has two carb exploded views and several other pieces of info lacking in the '74 book. I don't think the '79 carb is a lot more complicated so maybe Nissan found that mechanics needed more details on Hitachi carbs.

 

I'm sure learning a lot about carbs from all this discussion. Just the sort of knowledge to go along with my love of steam engines and film cameras. :lol: About the only thing worthwhile in the present century is computers. Otherwise give me the '70s or earlier!

 

Len

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From what I could tell from the wiring diagram, the contacts in that heater relay get their power from a fuse powered by "Ignition" from the Ignition Switch.

 

Also, there are errors on both my 73 and 74 620 FSM:  At the Heater Relay, they show (2) LW wires.  They should be a LW and a WL.  Lots of errors in the FSM.

 

I'm thinking I'll eliminate the Heater Relay - I really don't like the idea of losing the alternator and then having the choke cool down.  Frankly, the only time the Ignition Switch is on Ignition is when your running the engine.  I suppose you could have the Ignition On if you wanted to run the wipers with the engine off.  The real way to run the choke is from an oil pressure switch, not the alternator.

 

For now, simplest solution:  put the darn Voltage Regulator in - I mean like I just bot one from RockAuto.

_________________________

 

As far as the fast idle screw adjustment and cam timing adjustment, that's just part of tuning.  NOTE:  Bending that finger to change the cam "timing" is equivalent to simply adjusting the choke housing - at least to a point.  If the fast idle is running off 3 or the 4 steps, leaning out the choke will let you run it off 1 step if you wish.  However, you can only go in that direction.  If you need the 4th step (highest), and the linkage is "maxed out," you'll need to bend that finger.

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There is a Standard PS-64 oil pressure switch which is a SPDT 3-spade.  Use one side for idiot light and other side for relays to choke, electric fuel pump, etc.  Downside is that oil pressure switches are prone to failure - especially if you use silicone brake fluid which is another story.

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Well, vacuum leak does not appear to exist now.  No explanation.  I had a vacuum gauge on the manifold and it was almost in the "green" range,  it was in an adjacent red region, can't remember if low or high, but I'm thinking low by a couple inches.

 

Now the fast idle is a bit on the high side but not crazy high.

 

With the choke set at the middle mark, on a 50 deg day, it comes off the fast idle cam too quickly and the engine is too cold for the idle setting.

 

So that means richen up the choke a mark.

 

Then if the fast idle is a bit high, I'll adjust it down a bit.

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101_0099.jpg

 

Fast idle is set by placing a wire gauge in the throttle bore and letting the throttle plate close on it. That's what the adjustment screw and lock nut is for. It's set at the factory and is unlikely to change. Manual fast idle is anywhere between 1,900 and 2,800 RPMs. This gives the proper amount of air for the amount the choke is on. If you manually turn it down below this you won't get the proper idle as it warms up. Basically there is no 'too fast, fast idle' it is where it is. Again, do you have a tach or is this just a guess??? Engines always seen to be revved higher than they really are.

 

101_0100.jpg

 

 

 

As the choke comes on, a linkage drops, allowing the fast idle cam to drop of it's own weight. As the choke warms up the linkage lifts up pulling the fast idle cam with it. If you should feel that the fast idle is too fast for the choke setting, bend the linkage with pliers to slightly shorten it. This will mean the choke has to be on more to get the fastest idle step and comes off sooner.

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Actually, that is a good point.

 

There are 2 things going on:  choking and fast idle.  They are related to each other by linkage.

 

As you said, this is established by the wire gauge.

 

There are two events

  • when the choke opens enough to come off the fast idle cam and go to the idle setting.
     
  • how much choke is needed at cold start

I haven't touched the fast idle adj. and have been enrichening the choke to get the engine to start and take throttle w/o a blind spot.  I've gone 2 steps rich and it seems to like that.  I was surprised that engine likes the choke tightly closed at cold start.

 

One can get pretty OCD about getting choke, fast idle, and cam timing set just right.  I'm calling it quits here.  Lots more problems to solve.

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  • 2 weeks later...

ISSUE:  No accelerator pump discharge with new piston:

 

New blue silicone rubber piston sticks and does not slide down bore so you get NO pump. Either the piston is poorly designed of there is a friction problem, but the spring backing the piston which actually pushes it, is NOT strong enough to force it down the bore.

New red silicone rubber one works!

 

EDIT:  There is a reason the accelerator pump piston does not move:  the wrong nozzle check valve parts were used!  See my next reply:

Edited by Cardinal Grammeter
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  • 2 years later...

Accelerator Pump Nozzle Check Valve Types

 

There are at least two types:  Call them the SMALL and LARGE Bore.

 

Both types used the same part types:  

  • Ball as a check valve
  • Follower or Weight on the ball
  • Spring on the Follower
  • Threaded Plug

While parts list calls out a "Weight", it is more like a Follower that allows the Spring to bear on the Ball without binding AND allowing fuel to flow around the Follower.

 

How is the SMALL bore different from the LARGE bore:

  • The Spring is much smaller in diameter
  • Both the Follower and Plug have protruding pins that the spring fits over thereby keeping it positioned on the bore axis.  (The LARGE bore Follower is a plain Cylinder and the Plug has a flat bottom.)

MY PROBLEM was that I was assembling my carb from a basket of parts and used the LARGE bore parts in my carb's SMALL bore.  While the parts fit, the Follower becomes a piston which actually seals rather effectively and keeps any fuel from passing.  So NOTHING gets to the pump nozzle.  And so the Accelerator Pump Piston does not move!

 

EDIT:  Found a pic on Ratsun of what I call the SMALL bore setup:  100_0075.jpg

 

 

Edited by Cardinal Grammeter
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Yes, there is a check ball (A)  in the vertical piston and another (B) in the vertical nozzle bore.

 

When the piston goes down, A seals and B opens to allow the fuel shot.

When the piston goes up, A opens to allow fuel to refill piston bore while B closes to keep the fuel topped off in the nozzle bore.

 

Originally I think I had just the ball in the nozzle bore which actually works.  Might have had a spring in there too.  Then I learned that there is a "weight" (or follower) in there, so I had one in my loose parts.  So I used it not realizing it was from a Big Bore carb.

 

My carb was totally disassembled and somewhat intermixed with other DCH340 variant parts.  A challenging puzzle. 

Edited by Cardinal Grammeter
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  • Cardinal Grammeter changed the title to DCH-340 DCH340 Identification, Adjustment, Issues, etc.

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