Bushing Carb Throttle Shaft (DCH340) OR ???

[Cardinal Grammeter]

I'm trying to my ducks in a row because when I do this job, my sole daily driver 74 620 will be down.
 

My carb is so bad on the primaries that you can easily see the movement if you rattle the shaft by hand.  (.020-.030 inch-ish).  The reason I want to address this is because the shaft is stiff to turn because of the heavy return springs (take them off and shaft turns easily.)

 

Has anyone done this?   

 

Can't find long pilot back cutting reamers like Carb Junkies sells in industrial supply houses.

 

Use Lathe:  Chuck 8mm rod in tailstock, slide base over rod, dog to keep from rotating, drill from chuck.

 

The base plate is cast iron which is not like muscle car alum bases.  I think a neat fit bushing w/Loctite Green (Sleeve) or Red would keep it put.

EDIT:  It is possible with the plate being cast iron, that the wear will be in the shaft.  I've got a good set of shafts from a later model DCH.

 

NOTE:  I have a good, tight base/shafts from that later DCH - it uses the thick to-body gasket which is different.  QUESTION:  Mod the newer base and use?

 

Material:   Oilite, UHMW, solder bore, ...?

 

The DCH340 is probably an 8 mm bore - the shaft mikes from 7.75 to 7.79 mm so I guess it is 8 mm.  But the 0.2 mm clearance seems like a lot to me.

 

 

Edited August 4, 2018 by Cardinal Grammeter

[Stoffregen Motorsports]

All this effort to bring your stock carb back to life... Is it worth it when a brand new Weber DGV is only $300? The Weber will be so much easier to tune than the stock carb.

[banzai510(hainz)]

I had 2 of these stockcarbs and I just gave them away. But if good I would still use them but once a weber is used they are so much simplier to maintain . All ever goes bad is accel pump on me. tear in diaphrame buy a spare.

 

Cardinal is a stock perfectionist and he will alwasy be.  A weber is not in his sight yet.

[Cardinal Grammeter]

It's the $300 that is the obstacle.

 

This truck was brutally molested, both stripped and cobbled, and I'm compelled to re-"barn find" it - I want that "never been touched, barn find (raccoons living inside) look!"  ... still questing for the loom barrel connector firewall clip too.  NOTE:  But I already have a 5-speed to install when I get a chance and I have 1979 fender protectors to install...  So I'm not totally OCD about it.

 

The thing that impresses me about the Hitachi is how the air bleeds and emulsion tubes are tunable/removable (a moot point) - something unheard of in Muscle Car Carbs.  However they did fall short by not using metering rods instead of the crude power valve.  And yes, the return spring, working backwards through the accel pump link is pure Beevis and Butthead engineering.  The hollow base plate screw confirms that a lot of the features on this carb were afterthoughts and not involved in the original design.

 

[datzenmike]

2 hours ago, Cardinal Grammeter said:

...  So I'm not totally OCD about it.

 

 

 

Uhhh.... oh never mind.

 

2 hours ago, Cardinal Grammeter said:

 

The thing that impresses me about the Hitachi is how the air bleeds and emulsion tubes are tunable/removable (a moot point) - something unheard of in Muscle Car Carbs.  However they did fall short by not using metering rods instead of the crude power valve.  And yes, the return spring, working backwards through the accel pump link is pure Beevis and Butthead engineering.  The hollow base plate screw confirms that a lot of the features on this carb were afterthoughts and not involved in the original design.

 

 

I think the integrated return spring just makes sense rather than having it connected outboard to the intake or elsewhere. The hollow screw is genius for combining a fastener and porting a vacuum signal to the power valve. The only fault is it can be mistakenly replaced with a similar solid one on re assembly. My current carb came to me this way and I noticed it when I had it off. Doesn't affect running for the most part except high speed passing.

[Cardinal Grammeter]

The problem with the return spring is that it puts a thrust load on the throttle shaft due to the angle of the shaft to bellcrank link.  The only thing that can react that thrust load is the throttle plate side.  Mine was seriously worn and had to be replaced since it randomized idle.  I'm pretty sure I posted a pic of it - or it's on my out-of-date PB signature link.  It does the same thing to the accel pump bell crank which wears out the pivot bearing diagonally.   NOTE:  Hum... my plate was thrust worn and I have massive throttle shaft clearance.  No doubt I have a worn out carb.  I wonder how many miles were on it.  Too bad - it looked like someone disassembled a new carb in the basket...

 

The point is that any engineer would have seen these problems but they chose not to correct them.  I guess they needed a return spring and figured the parts would last long enough.  It could also be just the way the Japanese do things when it comes to cars.  I've heard the Germans are another mindset.

 

Regarding the through-bolt - they should have made it another thread size to pre-empt issues - or just even longer.  Yes, it's clever.  The emergency brake cable at the far end is really clever - I never saw anyone actually use the sheath reaction force.  And they get A++ for heater core R&R ease.   

 

Edited August 5, 2018 by Cardinal Grammeter

[datzenmike]

Well all carbs have a return spring on them one way or another. Tell the truth I've never seen a worn out throttle shaft but I hear they exist. Worst thing is to add a spring

[banzai510(hainz)]

I have a helper spring on my weber as time goes thetrottle  cable looses its grease or what ever is insde it.

[Cardinal Grammeter]

Here is a good throttle return spring design on this Chinese carb:  (If I have nothing beneath the DCH cable track, I could fab a downward return spring.)

 

A cheap but effective way to take up slop in a throttle shaft is to simply wrap some UHMW in the clearance - you don't actually need a bushing.  Use washer(s) to keep it contained.

 

I may get a Weber but don't like hearing about adding a spring.

 

EDIT2:  But after seeing this throttle return spring - one that I feel is necessary.  I may be compelled to rig one on my DCH and see if it solves the problems which I think it will.

 

Edited September 22, 2018 by Cardinal Grammeter

[Cardinal Grammeter]

Started the rebush job:

 

8mm shaft is close enough to 5/16 so don't have to monkey with metric.  (5/16 is 0.0025" smaller than 8mm which will improve shaft clearance so is actually a better choice.)

 

Fixtured carb base by chucking 5/16 rod in lathe chuck laying on drill press table - produced nice perpendicular reference.

 

5/16 rod height adjusted to act as a drill stop.

 

Drilled out in 1/64th steps to 3/8.

 

McMaster Carr actually sells Oilite bushings in just about every conceivable size so a 5/16ID, 3/8OD, 1/2long was something like 89¢.

 

Also ordered some UHMW which is a superior plastic for bushings.

 

Waiting for delivery on the McMaster stuff (tomorrow).  Don't know which material to use.

 

Will update when I finish the job.

 

NOTE:  Ever since I put the hollow base plate screw in the right hole, the power valve (now not always open) is operating properly an gas mileage is very good - over 20 mpg around town and I'm confident I could get 30 on a trip.  So I'm not eager to throw the DCH under the bus.  It also is motivating me to install the 5-spd I bought in the Carolinas during the last total eclipse of the sun.

[Cardinal Grammeter]

FINALLY, finished bushing job and idle is rock steady.... Finally!

 

Went with the UHMW which was a PITA to get the bore right.  Turned OD, but with the thin wall (3/8 OD, 5/16 ID) had to go 1/64th over 5/16 drill to get nice fit.  However, when pressed (very lite fit) in casting, bore was again too small.  Drilling dug in and turned sleeve - had to drill backwards more or less eroding the bore bigger.  Final result was achieved.

 

NOTE:  RE: 5/16 Oillite bushing:  Turns out 5/16 is not enough clearance, need the extra couple mils for the 8mm shaft.  Turns out cannot use a standard size metric bushing since the min OD of 11mm is too big for the casting.  (EDIT: NOTE:  could turn down the 11mm with a good chuck but if doing this, get a 3/4" long one for more jaw engagement.)  So that means have to rebore the 5/16.  A sharp 8mm drill would probably be OK but a cheap eBay 8mm reamer would be better.  They are only a buck or two but over $20 from McMaster Carr.

 

Driving my 620 is transformed not having to heel and toe and constantly blip throttle to get it to drop down to idle speed.  Have other repairs to do (rebrake and tailpipe fell off) which I'm putting off so the love can build up from joy of driving now.  I came very close to giving up and buying a Weber but NO CHANCE now especially since the power valve is not always on due to wrong hollow base screw placement (used 2 hollows and got them both wrong!)

 

IDLE MIXTURE SETTING:

 

Not sure how to adjust idle mixture screw.  Can back out 2 turns or more and it keeps increasing idle speed - so I turn down idle screw.  Old school AFB V8 etc. EDIT:  start 1-1/2 out, turn in until rough, back off 1/2 turn.    Definitely want to keep my good gas mileage going and improve if possible.

 

EDIT:  TRANSFER SLOTS:  My thinking of late is when the carb is off, adjust the throttle plates so they are low in the transfer slots.  Then use the mixture screws to set the idle.  If the plates are above the slots, you'll have dribble off the booster venturies which is very bad.  If too low, you'll have a blind spot while the flow gets moving through the slots.  You can sort of adjust by ear, just blip the throttle by hand and note any hesitation.  This test will reveal big differences when plates set poorly.

 

Edited October 6, 2018 by Cardinal Grammeter