720 Hitachi choke not quite closing

[Telkwa]

Good morning!

I bought a Hitachi feedback carb from a Ratsun member.  For the late-model 720's.  It appears to be in much better condition than my original carb.  

 

However, the choke plate doesn't quite close.

 

 

And the ring-shaped bracket that holds the choke heater in position is attached with two rivets and a screw instead of three screws. 

 

 

A very light touch with a screwdriver on that little tab closes the choke plate.

 

 

So the choke mechanism doesn't resist closing, but it gently drifts back to that slightly open position shown in the first photo. 

 

If I recall correctly the simplest way to get the choke plate to close (if everything else is functioning) was to loosen the choke heater and give it a little twist?

 

If that's correct, I'm thinking that the first thing to do is drill out those rivets.  Then try to tap the original holes, or drill out to the next size that can be tapped, or if that doesn't work replace with some small machine screws and nuts.

 

I want to get rid of the rivets either way.  But I'm not sure that twisting the heater is the first, or the correct, course of action.

[Telkwa]

My drug-addled brain has failed me again.  The plastic choke heater housing is pinned in place.  Why the housing has the words "Rich" and "Lean" printed on the outside when you can't adjust it either way is beyond my reckoning.

 

Troubleshooting takes on a whole new dimension when you have a second device!  I'm going to call the original truck's carburetor Carb #1.  The one I bought from a forum member is Carb #2.

 

Carb #1's choke heater snaps its choke closed with conviction.  So I removed Carb #1's choke heater and placed it on Carb #2.  Carb #2 with #1's choke heater; strong like bull.  Carb #2 with its own heater; limp-wristed.

 

The little tiny springs on the various shafts all seem to work roughly the same.  The bimetallic springs inside the two choke heater housings both seem to push back against applied force with the same vigor.  The only difference I'm seeing is this:

 

 

Notice the resting position of the springs.  The spring in Carb #2 choke heater (on the left) stops pushing back "sooner" than Carb #1.  That seems to make all the difference.  I thought maybe I could pop the spring out of the brass hub and move it 90 degrees, but it resisted that idea and I stopped before breaking something.  I think all I need is to get roughly 20 degrees or so more push from Carb #2's spring.

[Telkwa]

I googled "how to adjust hitachi choke spring" and the first few hits were Ratsun.  Looked to me like some folks have reinstalled their choke heaters wrong so I thought "What the hey" I'm already this far into it. 

I hosed all the little springs and gizmos with some LPS 2.  This is what I got.  With the choke heater off, the tiny springs that exist here and there on just about every shaft should pull the choke wide open.  There's not much force involved; just enough to open the choke if nothing's gummed up.

 

 

Notice that little brass-colored tab in the foreground.  The one inside the casting that accepts the choke heater.  In this picture the choke is wide open.  If you look closely you'll see that the tab is right up against a small stop cast into the carb body.  When the choke is closed this tab swings counter-clockwise to about 10:00 or so.  During installation of the choke heater, you have to catch that tab with the bimetallic spring inside the choke heater.  If you miss it the choke will not close when cold. 

 

 

There's a little plate that goes between the carb body and the choke heater.  The plate is off in the first shot.  It's installed in this shot.  I'm using a pick to hold the choke mechanism closed.  See how that tab inside the choke heater casting is rotated roughly 90 degrees to the left of the first picture?  When installing the choke heater, make sure the bimetallic spring catches that tab, then twist the choke heater counterclockwise until it meets the pin.  Check to make sure the choke is closing and button it up.  In my case, 'buttoning it up' will entail a trip to the hardware store for two small machine screws, washers, and nuts to replace the rivets.

 

UPDATE:  I decided to grind off the small pin that locks the choke heater at a certain rotation.  A few minutes with a carbide bit and it was gone.  Twisting the choke heater another 15 degrees or so made the choke plate snap all the way shut.  I rotated the choke heater clockwise until the choke plate wouldn't shut all the way, then rotated it counter-clockwise until it snapped shut more vigorously than I thought necessary, then I picked a spot in between. 

 

WARNING:  It's been said that the wires coming off the back of the carb tend to break because they're too short.  Removing the pin and twisting the choke heater exacerbates the short wire problem.  Depending on how much you twist the heater, it might be a good idea to add a couple of inches to the choke heater wire. 

[nels]

Thanks. I just had this issue, with the addition that the set screw under the fast idle cam was set too tall so the whole arm bound on it and the choke was unable to close. 

 

 

Edit- I couldn't get a picture without taking the carb off, and the weekend got away from me. This is the exploded view of the carb I have out of the Chilton's manual. Is that roughly the same as whats in the 720?

[Telkwa]

Do you have a picture of this?  After reading your post I looked at Carb #2 some more.  The fast idle screw (this screw is upside-down and must be a real mother to work on when the carb's on the engine) is the one that pushes against the white nylon fast idle cam.  Right?  Is that the screw you're talking about?  I can see how setting this screw inward too far would cause the engine to race when on the choke but I don't see how it could stop the choke from closing.

 

ANOTHER NOTE:  It's funny how - OK, it's not that funny - a person can think they understand something but there's almost always more to it.  The bimetallic spring reacts to temperature.  I was working on it yesterday afternoon when ambient temp was in the high 60's F.  This morning, with ambient in the 50's, I could tell a difference.   

 

If you've decided to grind off the pin so you can adjust how hard the choke heater spring pushes the choke plate closed, take temperature into account.  If it's 90 degrees I'll bet you could set it so that the choke plate barely closed and you'd be fine.