Hitachi rebuilt carb stumbles

[flatbed620]

I need help! So I rebuilt a Hitachi carb, installed it on my 620 with the L20 motor, it runs really nice, except it stumbles when you hit the gas or punch i.t. Gets a little better once it warms up, but is still there. Had a weber on before, worked really well, but took that off to put on a different truck. I tried adjusting the mixture screw, but no change to the stumble. Anyone have a clue as to what I should check? This is my third hitachi rebuild, and the others worked great, don't know what to check.

[datzenmike]

Accelerator pump. Look down the carb and pump gas (engine off) you should see a strong squirt of gas.

 

There are two small BBs in this system... one near the discharge nozzle and one directly below the accelerator pump under a very fine spring. They act as one way valves. These are easy to loose when taking apart.

 

Anyway find out if there is a strong squirt and if not, why.

[flatbed620]

Thanks again DatzenMike, the bb's were replaced on the rebuild, I thought it might be the accelerator pump, so I switched that out with a new one, and the bb was still in there. There are two different settings on the accelerator pump shaft that you can set the pin in, and 3 holes on the end of the accelerator pump bracket, I tried changing those around, is there a right and wrong setting for those? I will check to see if I am getting a good flow through those holes and reply.

[mike]

Check for a vacuum leak and/or check if you have the vacuum lines for the factory air filter hooked up correctly. If you have the vac lines plugged off on the carb you will have a high idle and rough acceleration. I cant seem to get a hitachi to run right without using the factory air filter and having all the vacuum lines installed correctly... Maybe someone has some tips if this is the case with yours...

 

Also check the harness that goes to the choke/anti diesel solenoid and the BCDD, there is a connection right at the clip on the firewall that can come loose and cause numerous low idle speed issues as well.

[flatbed620]

I do have the all the vacuum plugged off, because it had a Weber previously. Thanks for the input, will check it out

[Rays74]

Sometimes you can "tune" the stumble out of these carbs by juggling the accel pump discharge weight assy buried under a threaded plug (under the top half of the carb). Sounds confusing I know, BUT I did rebuild one recently that exhibited the same type stumble you are mentioning.

Look at the exploded view here

http://www.carbureto...om/expvw29.html

Where I am talking about is part #60, 61, 62 and 63.........see how they are assembled under that threaded plug? Ok that's the discharge for the accel pump stream, sometimes there may even be a lite spring sitting between the plug and that brass dowel #62. If you aren't getting enough of a shot and the motor is coughing or bogging under heavy throttle, you can try removing the spring (not in the diagram) or the brass dowel to see if that helps. I have pulled that dowel and used a second ball bearing as a weight before with good results too. What you are doing here is removing a restriction (emission related perhaps?) from the accel pump shot to allow a little more to be injected in the intake during demand (quick throttle).

Also....as Mike said above, be 100% sure the ball is down in the bottom of the accel pump well AND the spring is orientated so that it puts a little pressure on that ball via the leg that extends horizontally across the well :) Been there.......blah blah.

 

Good luck! These carbs are all i use, I like them a lot.

--Ray

[doomdatsun]

did you clean the fuel jets correctly (could be clogged), if your weather is cold then as soon as you start the engine don't ever never touch the throttle, wait about 10 to 20 minutes until the engine is hot then try throttling it, those hitachi carbs are good, had lots of trouble with them, but in the end they worked fine, for stumbling I would either think its a clogged jet or bad fuel lines (dirt, etc), use fuel tank additives like stp injector cleaner ( with your tank about 1/4 full) and try to run the car, also when you mentioned the accelator pump adjustment holes in the lever arm, that would count for the travel of the pump, and if set to activate the pump (pump pushed down by the spring force) it will add fuel in the idle circuit (happed to a corrolla engine I was working on)without touching throttle, so its a rich mixture + cold engine = clogged jets, put the lever in the furthest hole from the pump (the beginning of the arm) to lift the pump to the max height, then check your idle for vaccum leaks and stalling, test after that for high speeds.

[cdub42]

i have the same problem. it usually in low rpms under load when accelerating. ive never been able to fully chase it out. ive rebuilt my hitachi and the problem stayed the same even. ive added this thread to my lurk list....

[loungin112]

i have the same problem. it usually in low rpms under load when accelerating. ive never been able to fully chase it out. ive rebuilt my hitachi and the problem stayed the same even. ive added this thread to my lurk list....

Common problem it seems. mine is the same. Warms up to a nice idle at about 900rpm, revs nice in neutral, but put it in gear and it stutters. Curiously, if i leave the choke out about a quarter the stutter is gone. pretty sure it is running rich at that point though. Im following this as well.

[banzai510(hainz)]

I got 2 Hitachis and might be selling them to Ted soon from a L20 620

 

I could never rebuild them right myself(Rebix qube)

[lilhslr]

Seems like I have a similar problem as well. Been fighting with it for weeks. Adjusted the accelerator pump lever arm to the hole farthest from the pump and that seems to have helped a bit. There's still a bit of a stutter when it's cold but after it warms up, the stutter is almost gone completely. carb could probably due with a complete clean and rebuild, but I'd probably only make it worse.

[datzenmike]

Common problem it seems. mine is the same. Warms up to a nice idle at about 900rpm, revs nice in neutral, but put it in gear and it stutters. Curiously, if i leave the choke out about a quarter the stutter is gone. pretty sure it is running rich at that point though. Im following this as well.

The choke out and runs better is a clue. Choke out = richer = runs better. I would say your main jet is partly clogged or an orifice is partly clogged and it's running lean.

[loungin112]

The choke out and runs better is a clue. Choke out = richer = runs better. I would say your main jet is partly clogged or an orifice is partly clogged and it's running lean.

Sigh.....just reading the comments about tearing these carbs down makes me dread doing the same. Another thought, this car appears to originally be from texas...and is now in colorado. Maybe i should look into jets for higher altitude. Doubt the previous owner ever did anyhting to address the change in altitude.

 

Thanks.

[datzenmike]

Higher altitude is thinner air. Less air needs less fuel so if anything it will be already running slightly richer than needed.

[Fat510]

mine is doing this too now, i think it may be a vacuum leak when i sprayed the linkage with a silicone type lube and the idle dropped significantly.

 

is it weird that only one (left) side sprays gas? (if standing in front of car)

[datzenmike]

The right or secondary side is closed, only the left or primary barrel opens when stepping on the gas. When the throttle is opened the engine isn't spinning fast enough to produce a vacuum at the venturi to draw fuel in and will suddenly go lean. The accelerator pump squirts raw gas into the primary to help transition from the idle circuit to the primary barrel. Sounds normal to me.

 

If spray changes your idle check the bolts are tight on the carb.

[Fat510]

The right or secondary side is closed, only the left or primary barrel opens when stepping on the gas. When the throttle is opened the engine isn't spinning fast enough to produce a vacuum at the venturi to draw fuel in and will suddenly go lean. The accelerator pump squirts raw gas into the primary to help transition from the idle circuit to the primary barrel. Sounds normal to me.

 

If spray changes your idle check the bolts are tight on the carb.

 

 

Bolts were loose. thank you based god that it was not the accelerator pump

 

thanks mike :thumbup:

[ericsb210]

Along with Dmikes info, I have found that with carbs in the past that if I do a thorough cleaning of the carb body, like an overnight soak in Perrymans, the throttle shaft will attain a vacuum leak. I believe that is from clearing off years of varnish that has built up. So magically the carb develops a stumbling issue that it did not have prior to the rebuild. 

 

Do as Fat510 did and spray around the throttle linkage and see if you get an increase in idle speed. If you do, check your mounting bolts first and then spray again, if the problem still persists put a shield between the base and the throttle linkage and repeat the spray, If the idle again increases you have found your problem.

 

Only solution for this is to have your throttle shaft sleeved. 

[ericsb210]

Dmike, is it possible to sticky a "how to" for rebuilding a Hitachi carburetor? Maybe even a diagnostic chart.

 

Obviously all of our Datsuns are fed fuel via a carburetor, but maintaining and rebuilding a carburetor is definitely a lost art.  There is always a current topic in all of the forums asking what is wrong with my carburetor and it seems that most of the senior members grow tired of repetitive questions.

 

Just a thought, anyhow.