1974 620 L18 Hitachi DCH340-12 16010-B7900 Air Horn Carburetor Gasket: NEED

[Cardinal Grammeter]

I can't find one.  There seems to be a readily available L20B 1975-1979 carb kit which comes with 2 air horn gaskets - I have them, they are different.  Can't find any kits older than 1975.

 

I an considering an epic cut one by hand - some of the widths are 1/32" which is hella tough to do.

 

Looks like mine is some kind of rubber impregnated fiber gasket that is starting to disintegrate shedding black chucks that I've twice had to clean the carb to remove.  Here's a pic of a nasty goober that got into the primary emulsion tube.

 

 

 

The MIDDLE gasket is the one I need.  The other 2 are what's in the 75-79 L20B DCH340 kit.  Who knows - the L18 and 1974 being the Bastard Year might be a special one-year-only application.  That wouldn't surprise me at all, in fact, that might be my bet since the L16 is non-EGR which is probably yet another DCH340.  Keep in mind the 74 manual trans is already a -12 variant.

 

Regarding the middle gasket I have.  It is still INCORRECT.  If you look closely, I had to cut a notch out at the bottom to clear an air bleed.  So I've NEVER found the correct air horn gasket.

 

I'm not even sure I have the correct carb: 

I bot my carb disassembled but appeared new.  It has a "74 MT" tag on it.  No legible ink stamp on body.  I check the # of every jet, bleed, and tube and they were correct.  However, it's possible this might be some Franken-carb obtained from some rebuilder/remanufacturer.

 

Edited March 30, 2022 by Cardinal Grammeter

[bananahamuck]

You need one that looks like the middle gasket?? 

[Cardinal Grammeter]

Yeah, it's the middle one I need.

 

But it's sort of academic to a point, my 620 is my only drivable vehicle so if it's down, I'm car-less - so I have to come up with a solution tomorrow.

 

Cutting a gasket sounds like the only option.  

 

If I do find one, I'll order it just so I have it unless my home cut is really righteous.  

NOTE:  I thought some years ago there was an outfit that specialized in Hitachi parts and many other manu's carbs too.  But didn't find such a site this go round.

[Cardinal Grammeter]

I decided to "reman" the gasket.

 

Cleaned up all the crud around hole bores and ultrasonic cleaned followed with a gentle air blow dry (hehe)

 

Gasket looks pretty good.  

 

Going to use the very nifty, curved bottom Dollar Tree plastic organizer to bin all the various jets, bleeds, springs, balls, etc. for a good carb clean wash and compressed air blow out.

 

Printed a large pic of the gasket to serve as a key to identify which parts are in which bins.  Will double up on bins with part combinations (such as large and small) that are obvious - must do this because not enough bins!

 

EDIT UPDATE - Man am I Lucky (!):   So I'm blowing out the secondary air bleed jet.... and I fumble the pill and it goes shooting off out of my hand.  I hear it hit maybe 3 things.  If you know what my shop looks like, it's probably gone forever....  Long story short... I actually found it!  

This is mind boggling - tempted to take the rest of the day off...   Note:  I'm tired of using "the" - what a total waste of effort.  "...tempted to take rest of day off..." is crystal.

Edited February 17, 2022 by Cardinal Grammeter

[bananahamuck]

Yeah ,, definitely don’t blow air through them carbs till you have positively run magnet into all the holes 🤣

 

. 
when a Hitachi is working correctly a Datsun runs like a brand new sewing machine . 

  My oldest is pretty good with Hitachi type carbs.  He can “rebuild” out in the yard on a piece of plywood.. Some would rather throw it away if choke isn’t working quite right. 
. 

 

[Cardinal Grammeter]

It impressed me how many replaceable bleeds are in the 340.  It would be a great carb to custom tune if parts were available.  (Are they?)  However the BCDD is a PITA and it would have been nice if it wasn't there.  Also the throttle return spring coming from the accel pump bell crank is a really poor design when the push rod is set to max pump stroke - it puts a side load on the throttle plate shaft and causes great side wear on the throttle plates.  This rebuild, I'm going to put the push rod (link actually) in the vertical position so no side thrust is produced.

 

My carb was disassembled in a box - it seems to have all the right jet and bleed numbers, it should be a -12 variant.

 

But the choke and fast idle are out of whack.  Since it's a combination of choke setting, bimetal rate, and fast idle adjustment, it is hard to put a finger on what is wrong.  I am not sure I have the correct choke bimetal housing too.

 

I was tempted to take a L20B fixed setting choke housing/bimetal and see if that might solve the problem.

 

I'm going to bleed over the FSM to see if I can read through the poorly written explanation on how to set the fast idle screw and cam position.

[Ooph!]

Making your own gasket is a little time consuming but should work fine.

I've done business with these people and the parts fit mine

https://www.carburetion.com/products/productdetails.aspx?part=KT-5321D

 

I also bought a bi-metal choke mechanism from them :

Carb parts list on this page:

https://www.carburetion.com/CarbNumber.asp?Number=DCH340

[Cardinal Grammeter]

On 2/17/2022 at 5:00 PM, bananahamuck said:

Yeah ,, definitely don’t blow air through them carbs till you have positively run magnet into all the holes 🤣

 

. 
when a Hitachi is working correctly a Datsun runs like a brand new sewing machine . 

  My oldest is pretty good with Hitachi type carbs.  He can “rebuild” out in the yard on a piece of plywood.. Some would rather throw it away if choke isn’t working quite right. 
. 

 

 

Does he have a Go To place to get parts/kits?  How good was he at solving problems with them? 

 

I've succeeded in solving issues on 3 occasions, but this last go round still not fully solved - a flat spot when on primaries at mid-low rpm and heavy throttle.  Before fix, popped all the time if given too much throttle too quickly, but after cleanout, sill does it but rarely (but definitely "feel" the flat spot).  Low rpm lugging 100% OK and doesn't pop.

[Cardinal Grammeter]

UPDATE:  The Tomco kit has ALL FOUR AIR HORN GASKETS!

 

T's philosophy is rather than have a line of 600 kits many of which are only different by a gasket, they put both (or more) gaskets in the kit so it fits more carbs.  So their line was around 200 kits.  There is only one kit for the DCH340 and it has everything as far as 620's are concerned.

 

https://www.carburetion.com/Products/ProductDetails.aspx?Part=KT-5321D