Hitachi carb info needed

[albyneau]

Search as I might, both here and on google, there seems to be no definitive rebuild info for for my carb. Also I already looked here at the FSM pdf~ all that's in there is fuel line and throttle linkage junk. Is there a driveability/emissions manual that covers diagnosis and overhaul?

 

Anyway, here's what's goin on~ Cold start works pretty good, and driveablility's fine, up until it's been run about 25-30 minutes or more. THEN it acts like the choke is still on- coughin/sputterin til I kick the secondary in and it straightens back out. If I so much a feather the pedal it comes back. From a stop it seems fine thru the idle circuit, and I can make it go granny style real slowly~ otherwise I've got to mash it into the secondary once again or it'll die.

 

Being unfamiliar with Hitachis form -n- function, my questions (for now) are: 1. The choke appears to be set up so as to pull the valve closed when cold? 2. There's no spring on the choke linkage- so does gravity alone hold it open once warm? 3. What about that mysterious BVDD~ Any testing provedures? I noticed in various pics of them here some have a vacuum line to manifold, while others do not; are these triggered internally from a carb vac source?

 

Today was it's second voyage out into the public domain since I picked the truck up, and in spite of the driveability hiccups I'm pleased with my purchase. I'm gettin about 20mpg~ which is almost twice what my '70 suburban can pull down. If nothing profound comes my way before morning~ I think I'll try removing the choke coil and wire it open, just for grins, see what happens!

[datzenmike]

The round black thing with a blue wire to it is the choke heater and a bi-metal coil that constricts when cold and rotates the choke plate closed for starting. Once started the electric heater warms the bi-metal coil and it expands and unwinds rotating the choke into the open position.

 

Running well when cold with the choke on (rich) and then running like crap with it off tells me that the primary jet is blocked or partly plugged and running lean. Take offs are particularly hard needing lots of revs and slipping the clutch. When the choke is on, the over rich condition masks the problem. Idle is usually fine, as fuel is supplied through a separate idle circuit. Once revved up and into the secondary it also runs fairly well.

 

I don't know which is easier but if you can get the carb off the intake do so. I gave up on my 710 and settled for removing the fuel bowl front cover with the round sight glass in it. If you carefully remove the float and look down to the bottom back of the chamber there are two small holes. The left one goes to the primary and the right to the secondary jets. If you look down under the float chamber there are two 12mm plugs on the outside. Use a socket or box end and remove the left (primary) plug and look up inside at a 45 degree angle you will see a brass jet that leads up into the float chamber. There may be gunk or sediment or whatever, caught in it because the jet hole is very small. I used carb spray from a can with a long straw like from a WD-40 can and sprayed up from below and down from above to clear it out. Watch out that spray shoots out in your face! If you can't clear it you will need to get a suitable screwdriver (or grind one down enough) to reach up inside and unscrew the jet for cleaning. If you do, do not take the secondary out at the same time... lol. I sprayed mine out and it fixed the stuttering on clutch engaugement when warmed up.

[hvyd]

try changing out the accelerator pump. its located at the end of lever and has the rubber boot on it.

[albyneau]

The round black thing with a blue wire to it is the choke heater and a bi-metal coil that constricts when cold and rotates the choke plate closed for starting. Once started the electric heater warms the bi-metal coil and it expands and unwinds rotating the choke into the open position.

 

Running well when cold with the choke on (rich) and then running like crap with it off tells me that the primary jet is blocked or partly plugged and running lean. Take offs are particularly hard needing lots of revs and slipping the clutch. When the choke is on, the over rich condition masks the problem. Idle is usually fine, as fuel is supplied through a separate idle circuit. Once revved up and into the secondary it also runs fairly well.

 

I don't know which is easier but if you can get the carb off the intake do so. I gave up on my 710 and settled for removing the fuel bowl front cover with the round sight glass in it. If you carefully remove the float and look down to the bottom back of the chamber there are two small holes. The left one goes to the primary and the right to the secondary jets. If you look down under the float chamber there are two 12mm plugs on the outside. Use a socket or box end and remove the left (primary) plug and look up inside at a 45 degree angle you will see a brass jet that leads up into the float chamber. There may be gunk or sediment or whatever, caught in it because the jet hole is very small. I used carb spray from a can with a long straw like from a WD-40 can and sprayed up from below and down from above to clear it out. Watch out that spray shoots out in your face! If you can't clear it you will need to get a suitable screwdriver (or grind one down enough) to reach up inside and unscrew the jet for cleaning. If you do, do not take the secondary out at the same time... lol. I sprayed mine out and it fixed the stuttering on clutch engaugement when warmed up.

 

The logic of what you're saying is sound, and maybe I didn't explain myself well enough~ This occurs after a solid half hour of warmed up, on the road time. Also, Mike, the choke coil has no positive means of "locking" onto the choke shaft tab inside the coil housing. Someone in another thread referred to it as a loop~ my bimetal spring just has an arm that sticks up into the path of the choke shaft tab. So as the coil warms/expands, it's actually falling away from the choke tab, and only gravity and vacuum open the butterfly itself. Is that as designed? Seems like a hit-n-miss system if you ask me...

 

Sooo... I guess I'll try the choke in the morning~ if for no other reason than to attempt *dry fixes* first~ and see what happens. It just doesn't seem by the seat of my pants that it's a lean condition~ but that's just one of those things you can't explain thru a keyboard. The carb is new to the truck (as you saw in my other post), but that doesn't mean the old owner didn't drop it in the kitty-litter box b4 puttin it on- so to speak! Sometimes it can take longer to weed out someone else's work than just doin it yourself~ ya know? I'd still like to find some specific (and hopefully factory) info on these carbs, if you or anybody else has a line on some.

 

As far as gettin sprayed in the face goes~ you just haven't LIVED until you've splashed JASCO in your eye!:blink:

[datzenmike]

When you assemble the black round part with the coil inside to the carb you have to engage the coil to the choke shaft. If the previous owner had it apart he may have missed this. When put together properly you can twist the black part and the choke will open and close. This is so you can set the choke to start richer or leaner.

[albyneau]

When you assemble the black round part with the coil inside to the carb you have to engage the coil to the choke shaft. If the previous owner had it apart he may have missed this. When put together properly you can twist the black part and the choke will open and close. This is so you can set the choke to start richer or leaner.

 

Not on this one! Maybe there's a piece missing- or it's not the original coil, or...? But like I said before: the bimetal coil has a tab that sticks up (radially from C/L) and *sort of* engages the choke shaft tab. When it cools if forcibly pushes choke linkage closed. When it heats the only thing keeping the two in contact is gravity and vacuum pulling on the choke butterfly. There is NO mechanical means by which this coil *forces* the choke open when heated. If I make it to Canby I'll show you what I mean. Movin on...

 

Today's results were enlightening. My *dry repair* showed that the choke was not assembled correctly, but there's still part throttle issues. While on the outside it appeared to be a relatively new rebuild, closer inspection indicates it'd been sitting for a while. The picture window was so cloudy (like it was coated with something) I had to rock the truck to see the float level, and the o-ring was dry-cracked badly (but not leaking) in the corners. Pulled it off and found a nice little puddle of fine rust in the bottom of the bowl. Idle mixture and speed screws were also way off, and- lo and behold- a small black paint chip in the primary main circuit. Summed up~ about the only thing that WAS working right on this *rebuilt* carb was the accelerator pump....

 

I still want to figure out how to test my BVDD~ when there's no place to apply vacuum to it.... Anybody have a diagnostic/driveablility/emissions FSM out there???

 

Off for a road test~:cool:

[datzenmike]

...and- lo and behold- a small black paint chip in the primary main circuit.

 

That's all it takes.

 

 

 

I still want to figure out how to test my BVDD~ when there's no place to apply vacuum to it.... Anybody have a diagnostic/driveablility/emissions FSM out there???

 

Off for a road test~:cool:

 

I think you mean the Boost Control Deceleration Device? BCDD. The BCDD supplies extra gas and air on hard deceleration to prevent a lean condition and unburnt hydrocarbons. I find that it delays the the motor from dropping back to idle fast enough. (which is what it's supposed to do) but it's harder to match the RPMs when shifting. The motor is always revved too high when you let the clutch out in the nest gear.... but, whatever.

 

The motor will run fine without it. (as all motors did before these things were added) What I did was take the little rubber plug off the center of the BCDD and turn the adjusting screw counter clockwise. This will set the vacuum signal that triggers the BCDD so high it won't work. (it can always be returned to it's old setting) Now the RPMs will drop to idle as soon as you let off the throttle.