Need help backfire in 3rd

[George521]

I need a little advice I picked up this beautiful 521 a few months ago and it's in great shape with only 67K+ miles. The only issue I am having with it is that it miss fires and backfires if I am in 3rd to long and go to accelerate. I don't think it's vacum leak because every other gear is fine. Maybe fuel filter?

 

http://www.flickr.com/photos/91624153@N04/8318950502/

http://www.flickr.com/photos/91624153@N04/8317893189/

[DanielC]

Does your 521 still have the dual point distributor?

Datsun used a system of switches to retard the ignition timing on the 521.  This system only functions when in third gear, part throttle.

 

That is a nice looking truck!

[datzenmike]

Find a deserted road and drive hard through the gears until it begins to 'act up'. The next part has to be done swiftly. Get the clutch in and turn the ignition OFF. What we are trying to do is see if the carburetor fuel bowl has gone dry and to do this, we want a heavy load on the motor and then stop it before it can re fill again. You don't have to stop the truck in a hurry, just the motor, so find a safe place to do this away from traffic.

 

Once stopped remove the air filter so you can see the front of the carburetor. The glass front will have a line across it where the fuel level should be.

 

 

If the carb is dry or low it would indicate a fuel delivery problem. Most likely a clogged fuel filter but could be clogged fuel line, bad pump or the brass screen just above the needle valve in the carb is blocked. Most likely is the filter. If you have never changed it then just go ahead and change it now.

[Dat Lurka]

I've been working out fuel issues for a few months now.

 

A dirty filter will cause your float bowl to run dry at high load and cause you to run very lean / backfire. 

 

Check all fuel lines for cracks or holes. At higher rpms it will start to pull more air than fuel through the lines.

 

This was the line behind my filter.

 

 

Two days ago the soft line from the pump to fuel rail had a hole and was causing me to pull air also. 

[Jayden71]

I had this same problem on my 521. Disconnected 2nd set of points, fixed. Got EI dizzy, even more fixed :thumbup:

[datzenmike]

That's the next thing. There is a third gear switch on the transmission, maybe coincidence?

 

 

Figured fuel filter first to rule that out before getting technical/electrical.

[dat521gatherer]

Very nice truck. Please add some full interior shots. Definitely check the point gap on both sets and fuel level on the window as others have mentioned. Make sure your nuts on the carb base are snug. For me it's always been the points working their way closed a few times a year that causes backfire. 2 of my 521's now have electronic ignition. I cant say there is a performance difference but I dont have to reset and buy points all the time anymore. Set it and forget it as they say.