Jump to content

Age old AFR decision


Recommended Posts

I have finally begun to get my engine into a good condition but now I gotta decide how to tune it. Right now I believe my A14 is running very rich just bye the smell and the fact that upon removal of the sparks they are completely coated in unburnt fuel. My sparks are set to 0.045 and im trying to figure out what afr I should aim for and/or spark gap in order to get a snappy response but still be fuel efficient to daily drive. Any suggestions?

Link to comment
  • Replies 6
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

First be sure the engine is running at it's best. You can check but EI systems run 0.038" to 0.041" plug gap usually. 0.045" is too wide and harder for the spark to jump this gap.  Reduce yours to 0.040" or less. Does your A14 burn oil? How many miles can you get before needing to refill a quart? If excessive it may be oil you're seeing on the plugs. Have the valve lash and the ignition timing checked and set.




What carburetor are you using? Stock?  Because it shouldn't be running rich or lean with the stock parts. This might indicate that something is wrong with it.


1/ When warmed up for at least 20 minutes, take the top off the air filter and look at the choke plate. It should be fully vertical or open. 


Like this...



NOT like this...



The choke forses a rich mixture when the engine is cold but it should shut off or be fully open once warmed up. Does it open fully???


2/ Is the air filter dirty. It would have to be extremely dirty to restrict flow (just like a choke) but check it.


3/ Look under the air filter at the front of the carburetor. There is a small sight glass that shows the fuel level inside. Is the level more or less in the center of the window when running?

Link to comment

Ok so originally it did seem to be burning oil but now it doesn't, and the plugs are definitely dosed in fuel not oil now. Also according to the manual I should be running 0.043 to 0.051 which is quit large but its what they recommend, air filter is new, and yes its the stock carb with all stock parts. But I will check my choke cause Im yet to check the warm vs cold

Link to comment

If the gap is wide it's harder to fire the plug. If the spark can't make the jump it will try an easier path to ground. Old and compromised insulation on the wires, distributor cap, rotor and even the coil may allow this. If there is miss firing the unburnt fuel will build up. I would reduce the gap to 0.040" for now till sorted out. You can go larger later but find a reliable source for this gap information, I've never heard of above 0.028" to 0.040" from Nissan.


If you have never run the engine to full temperatures and never long enough then the choke has probably been on most of the time. Spark plugs are self cleaning but only when the engine reaches operating temperatures and for long enough.

Link to comment

Well after '77 a high output Electronic Ignition (EI) is used. Most Factory Service Manuals (FSMs) say to run 0.038" to 0.041". It's a good idea to keep your wire insulation, cap, rotor and coil clean and in good condition.


It needs a good long drive to self clean the plugs.


Stoichiometric or perfect amounts of fuel and air for complete combustion is about 14.7 a/f reading, but you really need a gauge and a wide band 02 sensor for exact readings. An a/f gauge gives you readings at all times from idle to full throttle. You can get close by 'reading' the plugs but a lot of work. They will look light tan and never clean white porcelain. 

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.