Stock Hitachi carb suddenly running super-rich

[rundwark]

Took the 510 for a drive this weekend, about 40 miles each direction (longest drive since I got the car, it hasn't had much action in the past couple of years).

 

On the way back, after getting of the highway, noticed how the car was running weirdly. I got lots of popping noises – gas burning in the exhaust – when letting go of the throttle, in every gear, at any speed. Idle was suddenly super-lumpy too and there's more black/blueish smoke out the exhaust and more gasoline smell than I'm used to. My conclusion is that it's running super-rich all of a sudden.

 

Before, it ran nicely while hot, and too lean when cold. When cold, it would very easily lean out when applying the throttle from idle (only a couple of quick pumps of the accelerator would get it past this "dead spot" when completely cold), and the engine would misfire when putting load on the engine at low revs (sub 3000rpm). So I thought I'd maybe just have to adjust timing and the accelerator pump may have been somewhat worn out.

 

Right now, it starts super-easily and doesn't exhibit these problems anymore, but it's smoky and lumpy, and pops when easing of the throttle.

 

So, removed the air cleaner and started messing with throttle and idle adjust screws, which had no effect on the problem. I'm wondering if messing with the idle mixture screw would do anything, but after searching here I understand that would not affect mixture under throttle. From reading more (e.g. http://community.ratsun.net/topic/52330-l20b-hitachi-carburetor-stumblingno-power/?p=856076 ) it seems that nothing is supposed to affect mixture, so I'm getting the sense something defective.

 

Does this sound familiar to anyone? Is it time to rebuild/replace the carb, or are there simpler things I can try first?

 

Also, and sorry for the stupid question, can someone tell me where to find the idle mixture screw on this thing?

 

Thx in advance!

 

oics:

 

 

 

 

 

[datzenmike]

 

Idle mixture to the right. Idle speed is left behind that linkage.

 

If the exhaust is belching black smoke and running rich, this must be corrected before bothering to set idle mixture.

 

When running look at the sight glass on the front... where is the gas level?????

 

Be sure the valve lash is set properly

Be sure the points and clean and properly gaped.

Be sure the timing is set properly

 

Now you can set the idle mixture. This adjustment only affects idle. Has no affect on anything else.

Set a reasonable low  idle speed and turn the mixture in and out to find the fastest smooth setting and turn idle down. Turn mixture in till quality drops off. Turn out until quality drops. Pick a position half way between these two where it idles smoothest. Turn idle speed back down. Repeat as many times as needed till you cannot improve the idle quality. Done

[LenRobertson]

A couple of ideas on the carb suddenly running too rich: Maybe the choke isn't opening all the way after the engine is warmed up. Do you have a manual control choke, rather than an electric/automatic choke? I don't see an electric choke heater in your pics. In some of your pics, it looks like the choke butterfly may be partly closed, but I can't be sure. If it is a manual choke and the bracket for the cable is loose somewhere, I suppose it might let the choke jiggle closed.

 

More likely though would be a piece of crud or something in the needle and seat above the carb float which would allow the gas level to get too high in the float chamber and flood over into the engine. So checking the gas level through the window in the front of the carb is what I would do first. There should be a white line etched on the glass at the correct gas level. Watch it for awhile at different engine speeds to see if the level stays fairly consistently near the line. The carb float (if the hollow brass type) can get a pinhole, get gas inside, and no longer float. Replacement floats can still be purchased I think.

 

Or it could be something inside the carb could have some loose, but not as likely as a gas level or choke problem.

 

Len

 

 

[rundwark]

Thanks Len!

 

The choke is indeed manual and it does re-open completely. I tried this a couple times and it consistently opens all the way back up.

 

With the engine off, the fuel level sits perfectly at the line, I'm about to turn it on and see if it stays there with the engine on and at different speeds. To do this, I removed the air cleaner and disconnected the hoses that attach to it:

- 1 from the air pump to,

- 1 from the valve cover,

- and one other small one, I think to what's called the valve flow guide.

 

Little worried to run my engine like this, but I'm sure it's fine if it's only for a short while to check some stuff…

[rundwark]

Just ran it and varied the throttle, the float chamber fuel level remains perfectly stable.. Weird.

[Stoffregen Motorsports]

 

It looks like that linkage rod isn't hooked up. See the linkage piece in the throttle shaft with nothing attached to it? Hmmmmm.

 

Is your choke plate flapping open/shut?

[rundwark]

Thanks for the extensive advice Mike!

 

So, the screw labeled 3 is the idle mix one, or did I misunderstand you?

 

 

I'll go through the steps you outlined and report back.

- Dirk

[banzai510(hainz)]

numbered wrong!

far right is the mixture adjust

left is the speed adjust , look close it linked to the tthrottle shaft/linkage.

 

 

I would ck your points and condensor and distributor shaft for looseness also.

2nd set of points??????? maybe disconnect the 2nd set and se eif runs better with the main set.

 

 

I never actayully seen a 510 with all the emission still on it. I wonder its it soemthing related there

[datsunaholic]

That pic numbering is all kinds of screwed up.

 

#1 is idle speed adjust.

 

#2 is Idle Mix

 

#3 is an assembly bolt (holds the middle part of the carb to the base)

 

#4 is a carb hold down nut & stud.  Holds the carb to the manifold.

[rundwark]

It looks like that linkage rod isn't hooked up. See the linkage piece in the throttle shaft with nothing attached to it? Hmmmmm.

 

Is your choke plate flapping open/shut?

 

That was it! Found what it was supposed to connect to and hooked it back up. Things are back to normal. Here's a pic with it reconnected:

 

 

And thanks banzai510 and datsunaholic for the corrections :)

[banzai510(hainz)]

Stoffregen Motorsports =winner

Banzai,Datsunaholic=losers

[Stoffregen Motorsports]

A picture is always worth a thousand words.

 

Glad to help.

[Dats Right]

hey rundwark wondering if you know what model carb u have i think i have the same one, is it hitatchi dcj-328?