#3 Cylinder Missing? L20B Common issue?

[r@+5un620]

I have two 620 pickups one 76 reg and one 79 kc. My issue is this both trucks seem to have this issue however if i put hotter plugs in the problem is solved but then creates another problem. Back fire. I dont understand it i have a basic knowledge of this engine thanks to this site but i havent found anything other than a story or two with the same issue but no solution... Thanks yall for any advice.

-Kyle

[racerx]

Compression check?

 

Did you check the spark plug wire #3?

[datsunaholic]

Sure you have both of them wired right?  1-3-4-2 COUNTER clockwise.  Can't tell you how many I've seen backwards (wired clockwise).  And yeah, they'll run that way.  Badly.

[r@+5un620]

These were the first thing i did i didnt think i tested compression right so i had a shop check for me and they all tested out great i looked at the charts for the spark plugs and they are all correct but this backfiring is down right embarassing its like driving a damn shotgun lol

[banzai510(hainz)]

I swap a spark plug then open the valave cover and ck # 3 lash pads to see if fell off under the rocker arm

 

any key is how you did this?

Mist only just remove the wire at the spark plug so we assume no plag wires wer all removed at once and restalled wrong.

Be specfic.

 

coils changed or new ones added all most be said and type.

[racerx]

What about dist. cap?

[banzai510(hainz)]

I only seen a cap g bad once from another preson and it was the center contack spring broke.

 

My caps are pushing about 9 years old already. I just clean them.

dealer caps and the Standard motor product caps are the better ones

I think th airtex ones on Rockauto for the l16 are good.

all brass contacks.

The Standard Motor Product used to sell a Blue Streak(blu cap) copper contacks ones but seem to be NLA

 

 

you can alwasy put or finger on the end of the #3 plug wire by spark plug and then youll know if it works.

 

I also buy the NGK wire sets NE64 $25 and under depends where you buy. best wires out there.!

[racerx]

Back in the days, my dist. cap went bad on my 76 capri, forgot which cyl. but it was corroded and would miss, plus the wre was messed up.

[banzai510(hainz)]

I had a 78 German 3 liter capri while in Germany in Airforce. Loved that car!!!!!!!! wasnt imported to the usa after the Capri 2s

Lower RS15x7 Recaros. It  was cool!  138hp

[racerx]

Yep..those are rare.  I had the 76 with a 2.8 and had torque. Headers and holley. Lowered. Dang I'm getting old.

[racerx]

Oh crap, snap! ..you were in the USAF, too!  I was stationed in Loring AFB, Maine, Korea, Japan, Riverside and did TDYs in the P.I. Best decision I ever made.

[r@+5un620]

havent checked the cap, but coil and plug wires were brand new last year when i bought the hotter ngk plugs. I will have a buddy check it out for me as im back to college for now... i hope to get in good enough shape to move it down here and make it the dd :angel:  instead of sharing the pos neon with my wife...

[r@+5un620]

plug wires were reinstalled one at a time and then triple checked the missing wasnt as bad as the backfire so i may just run it with old ngks as they looked alright. the backfire gets worse as it comes back to idle. and downshifting to brake makes it seem like a whoooole lot of :poop:   is going to hit the fan...

[datzenmike]

These were the first thing i did i didnt think i tested compression right so i had a shop check for me and they all tested out great i looked at the charts for the spark plugs and they are all correct but this backfiring is down right embarassing its like driving a damn shotgun lol

 

 

plug wires were reinstalled one at a time and then triple checked the missing wasnt as bad as the backfire so i may just run it with old ngks as they looked alright. the backfire gets worse as it comes back to idle. and downshifting to brake makes it seem like a whoooole lot of :poop:   is going to hit the fan...

 

 

So If I get this right.... #3 cylinder is dead unless you run a hotter plug. But then there is backfiring.

 

Backfire as in out the exhaust??? as opposed to through the intake???

 

How did you determine it was the #3 cylinder that was bad?

 

 

It's normal for the L20B to backfire on deceleration... that's why an anti backfire valve was installed. It detects extreme intake vacuum and allows more air in to lower it. There is a small hose connecting it to the intake so it can sense vacuum. Maybe this hose fell off. Some people remove their "emission crap" without knowing what it does and cause these problems.

 

Looks like this and is behind the carb and clipped onto the back of the intake

 

[ggzilla]

No it is not common for #3 to misfire.

[converted_to_datsun]

I vote for the cap and rotor being the problem. I don't think people who switch to webbers have anti backfire valves anymore... And I've never heard of that causing as much back fire as you are talking about. I had problems with backfires and coughing out the carb before and it turned out my coil was too worn out and weak. I got a new coil for 35 bucks and it's been great ever since

[datsunaholic]

The AB valve only operates upon closing the throttle, for a split second.  Doesn't do anything after that. 

 

1) Dirty/wet cap

2) Cracked cap

3) Ignition Timing

4) Valve timing

 

There's more, but those are relatively easy to check.

 

5) On the '76, wobbly dist drive shaft (if it still has points).  I fought a backfire for months, new points, no change.  Shaft wobble was making the points open rather randomly.  Put a junkyard EI dist on and problem went away forever.

[r@+5un620]

@ mike thank you i called my buddy to go check on that antiback fire valve and neither one of them have it so i guess its the cause of the backfiring but tomorrow hes gonna check on switching the caps for the missing cyl i think i may have an extra in a spare box of parts i got with the 79. The only way that i know that cyl 3 was missing by the replacing the hotter plug in the cylinders on at a time leaving the old ones in the other 3. I hate being away from the little bastard i hate city/college life not getting to shoot shit and work on my stuff is driving me insane!

[ggzilla]

Wait ... the anti-backfire valve is only needed if the engine has an operating smog system. If the smog pump has been removed, there is no need for the AB valve.

 

Replacing the #3 spark plug does show that #3 has a problem. However it doesn't mean it needs a hotter plug. Did you try putting an new standard plug in #3? It would probably have the same effect. Just the act of replacing the plug is what temporarily clears up such a problem. But it doesn't solve the problem.

 

 

A #3 misfire is not common. When does it backfire? Under light acceleration? On deceleration such as going down a hill with the brakes on? Or on normal (moderate to heavy) acceleration?

 

Are your power brakes working? Inspect the No. 3 runner on the intake manifold. Is there a missing hose, or a cracked hose connected to a fitting on the No 3 runner?

 

Finally if your exhaust is not good, yes it will backfire lightly on de-celeration.

[r@+5un620]

Hmm... that makes me nervous because the smog is gone on the 76 and is fully functional on the 79 minus the rotted hose on the backside that i am guessing led to the loss of that valve. This spring break I am going to do a complete and full tune up i downloaded the how to keep your datsun alive book and i am going to go through every single page to ensure she has a long life. I just hope the PO wasnt a full on inbred moron as the place i found it was highly sketchy... around these parts they are known as the mole people... i love the resources and wealth of knowledge this site contains!

[racerx]

maybe try this tool, cheap..

 

http://www.harborfreight.com/catalogsearch/result?q=spark+plug+tester

 

this can check if it's firing or not.