carb problem on a 1980 720 l20b

[Lowandslow]

I had posted this problem before on the 720 section but am having a slightly different problem and since you guys might habe a little bit better experience witb the l20b.

 

So the problem I'm having is stumbling and no power on acceleration once the choke opens up. When the choke is closed I have perfect cold idle at 1500 rpms and once I kick it off cold idle it idles at 800. I can also accelerate and cruise when the choke is closed.

 

I had this problem about a year or so ago and it turned out that the jets were clogged. I have since had the carb rebuilt with everything except for a new float and jets. I have a new fuel filter that is 2 weeks old, I recleaned the jets through the float chamber with carb cleaner, new gaskets on the base, new fuel lines and blew out the hard lines. I am completely stumped at this point. I also bought a new ign. Coil since my old one had resistance that was way over the limit.

 

Any help would be grwatly appreciated.

 

Some background on the truck is it has an l20b with a 5 speed and 186, xxx on the odometer. It has a hitachi dch 340.

 

Thanks again!!

[Lowandslow]

Also there is fuel in the bowl and it is slightly past the dot.

[ggzilla]

Carb passage is clogged.

 

Check operation of accelerator pump. Spray out all passages.

[datzenmike]

You should do this stuff yourself. Your info is all second hand and relies on someone else's work. If a garage rebuilt it, it may have been given to the lowliest minimum pay worker to do. No one born after '70 would have much experience with carburetors, most '85 and on vehicles were all EFI. You probably know as much about them as they do. All you need is a clean table with space, newspapers, a few tools, digital camera, a premium carb rebuild kit and a few undisturbed hours. It's NOT rocket science

 

 

Not uncommon for a clean carb to immediately plug up again (or it was never cleaned in the first place) from a hole in the filter or trapped dirt in the inlet line. I've 'rebuilt' carbs 3 times till I finally got all the dirt flushed out.

[Lowandslow]

Ggzilla: Should gasoline be coming out every time I press the gas pedal even when the truck is off? When it is running I've had my Dad rev itup and it looks like it just pours out. Is that how it should look? I only have experience with efi and turbos so I dont know what fuel coming out of the jet should look like.

 

Datzenmike : I bought the kit and float and I brought it with me to the shop. I had them show me all of the old parts, I know I shouldve done it myself but felt overwhelmed when I saw all the parts. All of the information is first hand besides the rebuilding of the carb.I set the float, I cleaned the jets and replaced the gaskets and fuel filter. Is there a special wrench for getting to the nuts on the base? I'm only getting about a quarter turn each time because of the space I have.

[ggzilla]

Agreed.

 

Sometimes the tank needs drain & rinse too. Water drops can go right through a new filter.

[datzenmike]

You may have to grind down a spanner for getting at those nuts. It's a pain I know. Probably closer to 1/16 of a turn.

 

The squirt of gas is the accelerator pump working. It's tho help with the transition from the idle circuit to running on the primary barrel circuit. At idle when you step on the gas, the carb opens and the intake vacuum drops. To make things worse the RPMs are low and there isn't much air passing through to create a vacuum at the venturi.The venturi vacuum is what sucks gas into the primary barrel. The engine will go severely lean. (just air) To help get past this, the accelerator pump dumps raw gas into the carb throat to richen up the mixture. Without it the engine will fall on it's face.

[Lowandslow]

Okay thanks for letting me know how it works, I didn't know what it did past helping with acceleration. I will make one today and see how it goes because like I said it has no power when accelerating so I was thinking maybe it wasn't tight enough or a leak around the base maybe?

[datzenmike]

A leak around the base would affect the idle. Once above the idle the primary barrel has to mix fuel and air. If there is a blockage not enough fuel will be allowed in and it will be lean and unresponsive.

 

Other things that could cause poor acceleration...

 

Timing retarded.

Faulty ignition... broken bad wires, cracked dizzy cap, carbon tracked rotor.

Valve lash too tight.

[Lowandslow]

Okay so I have never adjusted the valves so I don't think the lash should be too tight. The cap and rotor are new. Last time I checked the timing it was at 10 btdc. What would be the reasoning behind the truck running great at full choke but dying at open choke? It doesn't even idle. Would that indicate a lean in a/f ratio? I have the idle mixture screw at 2 turns out

[datzenmike]

Valves never checked or adjusted then you can't say they they shouldn't be too tight. They normally get tighter with wear.

 

The choke forces a rich mixture. If you are running too lean then this will mask the problem. When the choke shuts off the problem is there.

[Lowandslow]

Oh okay I thought they loosened up with wear and such but I will check em out for sure then. Thats what I thought about the choke situation. I just removed the fuel inlet above the fuel bowl and whem I did this I removed the filter inside of the eyelet and on doing so I also removed the fiber glass filter and found the hard part of the eyelet packed with old hose. I had to go to work so when I come back I will update on the running of the truck!!

[Lowandslow]

Okay so I started it up today and it idles great until the choke started to open. I bought a rebuild kit today and will try and tackle it. Wish me luck guys. If the rebuild doesn't get the issue looks like ill be buying a new carb soon. Once again thanks for all of the advice!!

[Lowandslow]

Okay so I finished rebuilding it an hour ago. I installed it back on the truck but the float appears to not be rising with the fuel level. The fuel bowl is full but the float is still all the way down. I removed the flow and stuck it in a bucket full of water and it floated perfectly. I think its time to purchase a new carburetor. I'm thinking a weber since I'm finding them a cheaper than a hitachi. Any input on any of this?

[datzenmike]

Easy to take the front off the carb without removing the whole thing.The float may be on wrong or the needle valve stuck. Did you loose the very small 1/8" brass tube that sits on the hinge pin between the glass and the float arm???

[ggzilla]

Yes, very common to get the float hinge part misaligned or missing.

[Lowandslow]

 

Heres what my fuel bowl looks like without the float. I dont think i lost any parts, but if u did where would i get another one? 

[datzenmike]

From another old carb. Or make something is the simplest/easiest.

 

 

That post sticking out just below the needle valve is the hinge. The small brass spacer fits over the post and is about 1/8" long. It just centers the float under the needle valve. Very easy to loose when taking apart. Almost anything will do to replace it that is similar.

[Lowandslow]

Okay cool, so it goes over the hinge and does it go all the way to the glass? Does it just go from the little grove on the hinge to the glass?

[datzenmike]

It just prevents the float from shifting towards the glass and keeps it under the needle valve. A sort of spacer between float and glass, not a tight fit. Anything would do.

[Lowandslow]

I went to the junkyard to see if by any chance they had an l20b carburetor but they had all been stripped of their carbs. :(

 

Fortunately tho, I looked inside of a solid looking 720 and I found that it had the dashboard with a tach!! I asked how much and the guy said 40 dollars!!

 

So after all this ill have to make my own spacer but got a dash with a tach out of it.

[datzenmike]

I should have mentioned that all the 720 carbs would also have it.

[Lowandslow]

So I met up with a fellow Datsun owner in my hometown. He has a 620 that he recently pulled the hitachi and installed a weber. I got the hitachi and hopefully fixes my idle problem.

 

I love the huge fuel glass!!

 

 

 

 

 

 

Its raining today so hopefully ill have some clear skies tomorrow and the base gaskets should be in tomorrow as well

[datzenmike]

What year was it off of???

 

Two things... that is a really old style metal choke cover. I've only seen them on much earlier L16s. The '74 620s with an L18 motor had them. Also I think the L2B didn't have a full glass front. It was changed to a small round glass covered hole.

 

This carb may work on an L20B.

[Lowandslow]

The truck it came off of was a 73 620 with a l20b. The l20 has a small glass hole for rhe sight. So far it seems pretty close to my hitachi so hoping for another cheap fix!!