L20b Hitachi Carburetor stumbling/no power

[Lowandslow]

I have a 1980 720 with the stock Hitachi carb. I know a lot of people would tell me to get a Weber but I don't have that option at the moment. I have tuned the carburetor and I had it pretty good. I could rev the engine up to 4500 RPM's with no problem. The problem just came on today, I went to pass a truck and as soon as I stepped on the gas the truck lost power and started stumbling. I had to keep tapping on the gas to maintain the current speed I had. The question I have is what could be causing this? I have little experience with carburetors and don't have a clue on what to begin troubleshooting. This happens as so as I go over 3k RPM's. I have to pull over and sit there for a bit. It still idles fine but as soon as I try to accelerate or cruise it shakes and will not gain speed. Hopefully someone can give me some insight on where to begin with this, thanks!

[datsunaholic]

1) Clogged fuel filter

 

2) Failing fuel pump

 

3) Plugged carb main jet.

[datzenmike]

I'm with Doug. Fuel filters are cheap and easy to change.

[Lowandslow]

I forgot to add that before I tuned it it did this one time and I tuned it and it didn't stumble for a couple of months. I will definitely get a new fuel filter today and let you guys know how it goes.

[racerx]

sounds similar to problem I had awhile back but it was the coil.  Around town I was okay, then got on the freeway or speed close to Freeway speed and my Datsun would stumble.  You can ohm out your coil, too. just to check. Find the resistance reading on it first then ohm out.  Go with the cheapest/easiet route like mentioned above before replacing the fuel pump. good luck.

[Lowandslow]

So I was on my way to the auto parts store for the filter and now I'm on the side of the road. There is no power at all and if I rev the engine it dies. If I feather the throttle it goes up, but if I hold it steady it dies. I adjusted the air/fuel mixture screw and a screw to the left of it right behind the spring.

[datzenmike]

Rev it up slightly and hold until it quits. Now look at the front of the carb... there should be a window or a small glass bull's eye with a dot on it in the middle. Does the fuel show in the glass???

 

 

Disregard the yellow writing.

 

If no gas showing you have a fuel delivery problem. This could be anything from an empty tank, a pinched or blocked fuel line, plugged filter*, bad pump or the needle valve in the carb is stuck.

 

* most likely.

 

 

If the carb has gas in it then it's got dirt in the primary jet.

[datsunaholic]

Take the filter off and blow the remaining fuel out of it backwards.  That might clear it enough to get to the parts store.

 

Buy 2.  Always carry a spare in the glove box.

 

 

Messing with the carb mix screws isn't going to help.  They only apply to idle.  They do nothing once the throttle is off idle.  Carb settings are set and leave alone.

[Lowandslow]

Okay I barely made it back home, I had to keep tapping my foot al the way down to full throttle all the way. My friend bought a fuel filter while I was driving home so I will put it on right now. When it was idling I noticed a loud hissing coming from within the carburetor. Not like when you rev it but it seemed to be coming from the jets. I will check the glass as soon as I put in the new filter because the old one looks horrible.

[datzenmike]

You have to check it when it has just stalled. If the carb is empty then this is the reason. 

[Lowandslow]

Okay I was about to try it out but now were headed to the hospital. Having a pregnant girlfriend = a lot of unexpected hospital visits. I will do the carb test and let you guys know what the results are.

 

I just read the post that said the air mixture screws is only for the idle, what adjusts the fuel mixture during acceleration/cruising? If there is something you guys suggest me read about carbs or a good explanation on how they wok I'd appreciate it.

[racerx]

http://forums.nicoclub.com/720-carburetor-reference-pics-t521844.html

 

try this..may help..good luck with th carb and baby.

[racerx]

try this one, too.

 

http://www.oocities.org/mazdarx05/carby1.htm

[Lowandslow]

Thank you so much, I now have something to pass the time with now! I will also be looking into adjusting the chime spring because I noticed that the choke was not fully open when I was looking at the carb today. I had drive. For 15 minutes so I think it should've been fully open by then.

 

Thanks!! Hopefully we'll get outta here tonight!

[datsunaholic]

Nothing adjusts the mixture at acceleration/cruising, at least, nothing adjustable.  It's done by sizing the jets.  Bigger jet = richer, smaller = leaner.   The mixture and speed screws are for idle only, any tuning is done by physically replacing the jets, air bleeds, etc with different sizes.

 

The accelerator pump shoots extra fuel, so it's a burst richer, but that's to compensate for the carb air flow through the jets not reacting as fast as the air through the venturiis. 

 

The choke makes it richer for warming up by restricting air, but that's only for when its cold.  Once its warmed up the choke should be fully open.

[Lowandslow]

Oh alright I see, this sounds a lot more complicated than I originally thought it was. Im really looking forward to actually putting everything km reading/learning from you guys and the forums to use tomorrow morning.

[Lowandslow]

Okay so I replaced the fuel filter and it is doing the same thing. I held the gas pedal until it died and I looked at the little gas window and it was halfway full.

[datzenmike]

Then there is likely dirt caught in and blocking one of the jets mentioned. Restricting the amount of fuel that can get through makes the air/fuel ratio too lean and power will drop off.

[Lowandslow]

How would I go by cleaning the jets?

[datzenmike]

Get a can of carb cleaner, the kind with a straw like the WD-40 cans, so you can direct the spray.

Place a rag under the front to catch the spilling gas.

Remove the three screws holding the front cover on the float chamber. (the thing with the sight glass in it)

Lift away the bezel and the cover plate but have a care... right behind the cover plate is the float hinge and there is a very small brass cylinder to space the float away from the cover. This can easily fall off and if it does, it's gone. Watch for it.

The outer edge of the float chamber has groove with a square shaped 'O' ring to seal against the cover plate. Save it and clean with carb cleaner.

With the cover off and the gas drained, remove the float and other parts off the float hinge and put in a safe place.

There may even be some sediment on the bottom of the float chamber. Try to wipe it out with a cloth.

If you look at the bottom of the float chamber you'll just barely be able to see two holes in the bottom.

The one to the left holds the primary jet.

Try and spray into this hole with the carb cleaner straw to flush out any debris. Might as well do the other (secondary) jet as well while you have it apart)

Give the float chamber a good rinse with the carb cleaner.

Watch when installing the float. You will have to push the fuel inlet needle valve up, in order to slip the float in under it.

Don't forget the small brass cylinder on the hinge pin or the 'O' ring seal.

Secure the cover plate and bezel with the three screws and you're done.

 

Cost $5 for cleaner and 1/2 hour of time. This should work but if not it was cheap and worth the bother. If it doesn't work the carb should be removed so the jet can properly be removed.

 

Wear safety glasses that carb cleaner can spray back or out of the carb in strange places and it REALLY stings in the eyes.

[datsunaholic]

To add to Mike's post, there are 2 plugs under the float chamber, I thing they're 12mm head.  Supposed to have a keeper but that gets left off sometimes.  Taking those off will expose the primary and secondary main jets, though without a mirror you won't be able to see in there.  I've found all sort of crap in there.  It's easier to clean with the carb off, but it CAN be done on-vehicle.

[Lowandslow]

Thanks for the greatly detailed explanation. I'm gonna go get the can and get started!

 

Theoretically speaking, where does the cylinder thing go if it falls.

[Lowandslow]

Okay so I have removed the part with the glass and can now see the float. Do I just pull on the cylinder?

[Lowandslow]

Never mind I got the float out

[Lowandslow]

Okay so I finished up and it ran beautifully!! I got the level of gas in the glass right on the dot. It was a quarter of the way up before thank you guys for all your help!!